during the past few decades , 
 processable organic semiconductors , 
 such as small molecules and conjugated polymers , have been intensively 
 studied because of the easy tenability of optical and electrical properties 
 by chemical synthesis . they are attractive for their potential applicability 
 in the fabrication of different types of electronic devices , e.g. , 
 light - emitting diodes ( leds ) , photodiodes ( pds ) , photovoltaic cells 
 ( pcs ) , or field effect transistors ( fets ) . although remarkable 
 progress has been made , the development of highly efficient and long - term 
 stable optical and electrical devices is still a challenge . thus , 
 it is necessary to design and synthesize processable conjugated polymers 
 with expected properties such as a low band gap , a broad absorption 
 range , a high charge mobility , and appropriate molecular energy levels . although polymers exhibit many valuable properties 
 suitable for the construction of optoelectronic devices , interest 
 in low molecular weight semiconductors is growing due to their advantages 
 of a well - defined molecular structure and molecular weight without 
 any distribution and relatively easily purification . among many low 
 molar mass materials developed for optoelectronic applications , especially 
 interesting are compounds that readily form glasses above room temperature 
 and which are called molecular glasses or amorphous molecular materials . thus , they combine the characteristic properties of small molecules 
 with the behavior of polymers , i.e. , the ability to access amorphous 
 solid states . they may form uniform , transparent , and amorphous thin 
 films by either vapor deposition or spin - coating methods in contrary 
 to low molar mass compounds , which have a strong tendency for crystallization . , molecular glasses may show high thermal stability 
 and high glass transition temperatures ( tg ) which can be easily tuned by a subtle structural modification and 
 can result in various tg values . among 
 the currently investigated classes of materials , tetracarboxylic diimides have aroused a lot of attention due to the electron - deficient core 
 leading to n - type ( electron - transporting ) semiconducting properties . although impressive progress has been made in 
 developing electron - rich character ( p - type ) organic materials , semiconductors 
 capable of electron transport are still challenges in the field of 
 organic electronics . diimides considering the imide 
 unit structure are divided into two kinds , that is , compounds with 
 five- and six - membered imide rings , that is , phthalic diimides and 
 naphthalene- and perylenediimides , respectively . a large number of 
 either naphthalene or perylene tetracarboxylic diimides have been 
 synthesized and investigated , and many of them have achieved high 
 electron mobilities and air stability . naphthalene- and perylenediimides are studied as 
 acceptor molecules in organic solar cells . a study has 
 been devoted to examining the linker effect at the n - position in a 
 core - substituted diimide and such compounds given power conversion 
 efficiencies between 0.01 and 0.48% . the authors used a phenyl linkage 
 between thiophene and diimide through the n - position , which is also 
 the scope of this paper , resulting in a 0.43% power conversion efficiency . researchers at the university of washington studied the effect of processing additives on 
 the performance of naphthalenediimide acceptors , showing that when 
 used with proper hole- and electron - blocking layers , naphthalenediimide - bearing 
 device structures can reach a power conversion efficiency of 1.50% . 
 among perylenediimides there are two prominent studies , one of which 
 was conducted by bazan et al . and was 
 a comparative study where they used 7,7-(4,4-bis(2-ethylhexyl)-4h - silolo[3,2-b:4,5-b]dithiophene-2,6-diyl)bis(6-fluoro-4-(5-hexyl-[2,2-bithiophen]-5-yl)benzo[c]thiadiazole ) ( p - dts(fbtth2)2 ) as the donor molecule and compared the acceptor 
 behavior of pc71bm and perylenediimide ( pdi ) . although 
 pc71bm showed higher efficiency of 7% , pdi showed a comparable 
 power conversion efficiency of 3% , suggesting that solution - processable 
 small molecules are promising candidates for photovoltaic applications . 
 in the second study , yao and co - workers investigated the thienyl - linked pdi dimer and its photovoltaic properties , 
 showing a 4.03% power conversion efficiency . on the other hand , five - membered 
 imide rings , that is , phthalic diimides , are rather occasionally investigated 
 as semiconductors . nevertheless , they show completive 
 power conversion efficiencies among their solution - processable small - molecule 
 acceptor counterparts . a detailed study was dedicated to the effect 
 of different groups at the core part of phthalic imide , resulting 
 in a power conversion efficiency as high as 2.89% . having phthalic 
 imide moieties connected to benzothiadiazole via a vinyl bridge , bloking 
 et al . showed that a power conversion efficiency of 2.54% is achievable 
 with open circuit voltage as high as 0.96 v. compounds with a diimide structure are also investigated as potential 
 photoluminescent materials . the chromophores used define the color 
 of emitted light . changing the structure of the compound allows emission 
 in the range from blue to red to be obtained . sauve and co - workers showed naphthalene 
 imides with thiophene which emit red light with a quantum yield of 
 0.56 . in our previous works we showed azomethine diimides emitting 
 blue , green - blue , and green light . recently , research 
 efforts have been focused on the development 
 of alternating compounds consisting of segments of different electron - donating / accepting 
 ( d / a ) properties and resulting in a low band gap . by selection of the 
 donor and acceptor units , the compound architecture enables modification 
 of the energy levels of the highest occupied molecular orbital ( homo ) , 
 lowest unoccupied molecular orbital ( lumo ) , energy gap , and charge 
 transport ability . among various d  a 
 conjugated systems , azomethine diimide - based compounds from small 
 molecules and polymers constitute a promising family of materials 
 whose properties can be explored in ( opto)electronic devices . azomethines 
 due to the presence of an imine bond ( n = ch ) 
 are isoelectronic to their carbon analogues , making them suitable 
 alternatives to conventional conjugated materials . one can find a detailed review article by iwan and sek covering a wide range of azomethine and poly(azomethine ) 
 structures used in photovoltaic devices having power conversion efficiencies 
 ranging between 0.0002% and 3.9% . dingemans and co - workers described liquid crystalline poly(azomethine)s 
 as hole transport materials in solar cell devices and found a power 
 conversion efficiency of 0.12% , the highest one obtained among other 
 devices due to the presence of a thiophene moiety . a recent study 
 was also conducted with azomethines with different substituents such 
 as benzothiazole , thiophene , and bithiophene on their electrical and 
 photovoltaic properties . moreover , fluorene azomethines 
 emitting blue , green , and red light with quantum yields of ca . the continuation of the effort in synthesis of new 
 azomethine diimides along with polymers which form alternating electron - donating  electron - accepting 
 systems is presented . the idea of preparation of compounds with both 
 imine and diimide moieties was presented in our previous works . the objective of this work was to synthesize new azomethine diimides 
 end - capped with thiophene , bithiophene , and ( ethylenedioxy)thiophene 
 rings and polymers and study their chosen properties important from 
 the point of view of their potential applications in organic optoelectronics . 
 in the present paper we show a comprehensive study of the obtained 
 compounds physical properties , such as thermal ( differential 
 scanning calorimetry ( dsc ) ) , optical ( uv  vis , pl ) , and electrochemical 
 ( cyclic voltammetry ( cv ) , differential pulse voltammetry ( dpv ) ) , which 
 were investigated and are discussed in relation to their chemical 
 structure . the optical properties of the thin films were characterized 
 using spectroscopic ellipsometry , and the complex dielectric function 
 was obtained . additionally , homo and lumo levels and eg were calculated theoretically using density functional 
 theory ( dft ) . the synthesized compounds differ in the chemical structure 
 of the diimide core , the kind of end - capping group , and the structure 
 of the moieties between the diimide rings and imine linkages . benzene-1,2,4,5-tetracarboxylic 
 dianhydride ( pyromellitic dianhydride , pmda ) , 1,4,5,8-naphthalenetetracarboxylic 
 dianhydride ( ndta ) , 2,3,5,6-tetramethyl-1,4-phenylenediamine , 
 1,4-naphthalenediimide , 2-thiophenecarboxaldehyde , 2,2-bithiophene-5-carboxaldehyde , 
 toluene-4-sulfonic acid ( pts ) , trifluoroacetic acid ( tfa ) , n , n - dimethylacetamide ( dma ) , n , n - dimethylformamide ( dmf ) , n - methyl-2-pyrrolidinone 
 ( nmp ) , chloroform , and pyridine were purchased from aldrich chemical 
 co. and used as received . n , n-bis(4-amino-2,3,5,6-tetramethylphenyl)naphthalene-1,4,5,8-dicarboximide 
 ( dandi ) was prepared according to a previously described procedure.n , n-bis(4-amino-2,3,5,6-tetramethylphenyl)phthalene-1,2,4,5-dicarboximide 
 ( daphdi ) was prepared according to a previously described procedure . indium tin oxide ( ito ) was purchased from aldrich 
 chemical co. the surface resistance of ito was about 1520 
 /square . pedot : pss ( poly(3,4-(ethylenedioxy)thiophene):poly(styrenesulfonate ) ) 
 and p3ht ( poly(3-hexylthiophene ) ) were also purchased from aldrich 
 chemical co. ntda ( 3 mmol , 0.805 g ) , 1,4-naphthalenediamine 
 ( 15 mmol , 2.3730 g ) , and imidazole ( 23.25 mmol , 1.5 g ) were added 
 to 30 ml of dry pyridine and refluxed under an argon atmosphere . the precipitating 
 diamine was collected by filtration and washed with water and then 
 with hot methanol . . h nmr ( dmso - d6 ,  , ppm ) : 5.92 ( s , nh2 , 4h ) , 6.73 ( d , ar 
 h , 2h ) , 6.95 ( dd , ar h , 2h ) , 7.16 ( t , ar h , 2h ) , 7.75 ( t , ar h , 2h ) , 
 7.61 ( t , ar h , 2h ) , 8.27 ( d , ar h , 2h ) , 8.76 ( s , 4h ) . ftir ( kbr , cm ) :  3438 , 3365 ( nh2 stretch ) , 
 1708 , 1673 ( c = o imide stretch ) , 1630 ( nh2 deformation ) , 1344 ( c  n stretch ) , 769 ( imide ring deformation ) . calcd for c34h20n4o4 ( 548.55 g / mol ) : c , 74.45 ; h , 3.67 ; n , 10.21 . found : c , 74.08 ; h , 
 3.66 ; n , 10.28 . a solution of 3,4-(ethylenedioxy)thiophene ( 1 
 g , 7.1 mmol ) in dry et2o ( 15 ml ) was cooled to 45 c under an argon atmosphere and treated with 1.6 m n - buli ( 12 ml ) . the mixture was stirred for 45 min , dmf ( 2 ml , 25.2 
 mmol ) was slowly added , and the solution was stirred for another 2.5 
 h. the mixture was allowed to warm to room temperature and hydrolyzed 
 with aqueous hcl . the precipitated aldehyde was filtered , washed with 
 water , and dried in vacuum . h nmr ( cdcl3 ,  , ppm ) : 9.91 ( s , 1h ) , 6.79 ( s , 1h ) , 
 4.364.34 ( t , 2h , j = 3.3 hz ) , 4.274.25 
 ( t , 2h , j = 4.2 hz ) . c nmr ( 100 mhz , 
 cdcl3 ,  , ppm ) : 178.9 , 128.3 , 128.0 , 127.7 , 109.9 , 
 64.7 , 63.9 . diamine daphdi ( 0.2553 g , 0.5 mmol ) , monoaldehyde 
 2-thiophenecarboxaldehyde ( 0.2286 g , 1 mmol ) , 2,2-bithiophene-5-carboxaldehyde 
 ( 0.1973 g , 1 mmol ) , or 3,4-)ethylenedioxy)thiophene-2-carboxaldehyde 
 ( 0.170 g ; 1 mmol ) , and 0.2 ml of tfa were added to 3 ml of dma and 
 heated ( 160 c ) under an argon atmosphere . the precipitate was collected by filtration , 
 washed with methanol , and dried in vacuum . yields of azphdis : azphdi-1 , 
 72% ; azphdi-2 , 80% ; azphdi-3 , 19% . h nmr ( cdcl3 ,  , ppm ) : 8.54 ( s , har [ phenylene ] ) , 8.26 
 ( s , ch = n ) . ftir ( kbr , cm ) :  1778 , 
 1727 ( c = o imide stretch ) , 1626 ( ch = n ) , 1372 ( c  n 
 stretch ) , 731 ( imide ring deformation ) . h nmr ( cdcl3 ,  , ppm ) : 8.55 ( s , har [ phenylene ] ) , 8.19 
 ( s , ch = n ) . = n ) , 1371 ( c  n 
 stretch ) , 732 ( imide ring deformation ) . h nmr ( cdcl3 ,  , ppm ) : 8.53 ( s , har [ phenylene ] ) , 8.24 
 ( s , ch = n ) , . ftir ( kbr , cm ) :  2923 , 
 2872 ( c  h aliphatic ) , 1776 , 1724 ( c = o imide stretch ) , 
 1618 ( ch = n ) , 1371 ( c  n stretch ) , 732 ( imide ring deformation ) . diamine dandi ( 0.2803 g , 0.5 mmol ) or dandi-2 ( 0.5 
 mmol , 0.2743 g ) , monoaldehyde 2-thiophenecarboxaldehyde ( 0.2286 g , 
 1 mmol ) , 2,2-bithiophene-5-carboxaldehyde ( 0.1973 g , 1 mmol ) , 
 or 3,4-(ethylenedioxy)thiophene-2-carboxaldehyde ( 0.170 g , 1 mmol ) , 
 and 0.2 ml tfa were added to 5 ml of dma and heated ( 160 c ) 
 under an argon atmosphere . then the reaction mixture was poured into methanol , 
 and the resulting precipitate was filtered off , extracted with boiling 
 methanol ( soxhlet apparatus ) , and dried in vacuum . yields of azndis : 
 azndi-1 , 69% ; azndi-2 , 56% ; azndi-3 , 77% ; azndi-4 , 26% . h nmr ( dmso , 
  , ppm ) : 8.95 ( s , ch = n ) , 8.81 ( s , har [ napthalene ] ) . ftir ( kbr , cm ) :  1715 , 1677 ( c = o 
 imide stretch ) , 1611 ( ch = n ) , 1341 ( c  n stretch ) , 769 
 ( imide ring deformation ) . h nmr ( dmso , 
  , ppm ) : 8.93 ( s , ch = n ) , 8.81 ( s , har [ napthalene ] ) . ftir ( kbr , cm ) :  1714 , 1676 ( c = o 
 imide stretch ) , 1607 ( ch = n ) , 1341 ( c  n stretch ) , 769 
 ( imide ring deformation ) . h nmr ( dmso , 
  , ppm ) : 8.80 ( s , har [ napthalene ] ) , 8.74 ( s , ch = n ) . ftir ( kbr , cm ) :  2921 , 2868 ( c  h 
 aliphatic ) , 1714 , 1674 ( c = o imide stretch ) , 1626 ( ch = n ) , 
 1339 ( c  n stretch ) , 769 ( imide ring deformation ) . h nmr ( dmso , 
  , ppm ) : 8.81 ( s , har [ napthalene ] ) , 8.27 ( s , ch = n ) . ftir ( kbr , cm ) :  2924 , 2873 ( c  h 
 aliphatic ) , 1712 , 1675 ( c = o imide stretch ) , 1621 ( ch = n ) , 
 1334 ( c  n stretch ) , 772 ( imide ring deformation ) . diamine dandi-2 ( 0.5 mmol , 0.2743 g ) , dialdehyde 
 2,5-thiophenedicarboxaldehyde ( 0.0701 g , 0.5 mmol ) or 2,2-bithiophene-5,5-dicarboxaldehyde 
 ( 0.1111 g , 0.5 mmol ) , and a pinch of p - toluenesulfonic 
 acid ( pts ) were added to 5 ml of dma and heated ( 160 c ) under 
 an argon atmosphere . the precipitate was collected by filtration , extracted with boiling 
 methanol ( soxhlet apparatus ) , and dried in vacuum . dandi-2 condensed 
 with 2,5-thiophenedicarboxaldehyde resulted in polymer polyazndi-1 , 
 while dandi condensed with 2,2-bithiophene-5,5-dicarboxaldehyde 
 gave polymer polyazndi-2 . yields of polymers : polyazndi-1 , 80% , black 
 solid ; polyazndi-2 , 60% , red solid . ftir ( kbr , cm ) :  1711 , 1667 ( c = o 
 imide stretch ) , 1613 ( ch = n ) , 
 1340 ( c  n stretch ) , 769 ( imide ring deformation ) . ftir ( kbr , cm ) :  
 1715 , 1675 ( c = o imide stretch ) , 1604 ( ch = n ) , 
 1341 ( c  n stretch ) , 768 ( imide ring deformation ) . blends were prepared 
 by dissolving the desired amount of studied compounds and pmma in 
 nmp to form a homogeneous solution ( 1% ( v / v ) concentration of the 
 compound in pmma ) . films cast on glass were dried in a vacuum oven 
 at 90 c for 10 h. proton 
 nuclear magnetic 
 resonance ( h nmr ) spectra were recorded on a bruker ac 
 400 mhz spectrometer using chloroform ( cdcl3 ) as the solvent 
 and tms as the internal standard . infrared ( ir ) spectra were acquired 
 on a perkinelmer spectrum one using kbr pellets . thermogravimetric 
 analysis ( tga ) was done with a tga / dsc1 mettler - toledo thermal analyzer 
 with a heating rate of 10 c / min in a stream of nitrogen ( 60 
 cm min ) . dsc was performed with a 
 ta - dsc 2010 apparatus ( ta instruments , newcastle , de ) under nitrogen 
 using aluminum sample pans at a heating rate of 20 c min . vis absorption spectra were recorded in 
 solution using a perkinelmer lambda bio 40 uv  vis spectrometer . electrochemical measurements were carried out using 
 an eco chemie autolab pgstat128n potentiostat in the three - electrode 
 configuration using a glassy carbon electrode , platinum coil , and 
 silver wire as the working , auxiliary , and quasi reference electrodes , 
 respectively . potentials are referenced with respect to the ferrocene / ferrocenium 
 ( fc / fc ) redox couple , which was used as the internal standard . cyclic and differential pulse voltammetry experiments were conducted 
 in a standard one - compartment cell under an argon atmosphere . bu4npf6 ( 0.2 m ) ( aldrich , 99% ) dissolved in mecn ( carlo 
 erba , hplc grade ) was used as the supporting electrolyte . the nir  vis  uv 
 ellipsometric characterization was done using a woollam m-2000 rotating 
 compensator ellipsometer , which spans an energy range from 0.73 to 
 6.5 ev . the ellipsometric measurements were performed at many different 
 angles of incidence ( 4575 in 5 sequences ) . the 
 measured complex quantity  = ( tan )e can be converted to a complex pseudodielectric function () 
 assuming that the multilayer can be described by a single and uniform 
 dielectric layer using1where 
  is the angle of incidence and 
 a the dielectric function of the ambient , for air 
 taken to be 1 . if the sample under study is just a single homogeneous 
 material , then the pseudodielectric function is the same as the dielectric 
 function , representing the correct material response . if the sample 
 is not a single homogeneous material , e.g. , a thin layer on a substrate , 
 eq 1 can be applied as well , assuming a single 
 homogeneous substrate , but then the result is termed a pseudodielectric 
 function  and indicated by . the dielectric 
 function for the studied polymer films was then obtained by fitting 
 the measured ellipsometric response ,  and  , using a 
 complex model dispersion relation . at first , fitting was performed 
 for the angle of incidence 65. for improving the sensitivity 
 and accuracy , the fitting was cross - checked afterward at other angles 
 ( 4575 ) . the film thicknesses were determined by measuring 
 the layer thickness at four positions with a stylus dektak profilometer 
 before and after doping . the data were fitted using wvase software 
 with the known layer thickness , with the assumption of a negligible 
 roughness , i.e. , small compared to the wavelength , using a generic 
 oscillator . as a result , a preliminary model for the real ( 1 ) and imaginary ( 2 ) parts of the dielectric 
 function was obtained . then the fitting was refined either by adding 
 more oscillators or by a point to point fit and checking the kramers dft calculations were carried out using the gaussian09 package and employing the hybrid b3lyp exchange correlation potential combined with the 6 - 31g(d , p ) basis 
 set . ground - state geometries ( constrained to the c2 symmetry point group ) were fully optimized until a stable 
 local minimum was found and was confirmed by normal - mode analysis 
 ( no imaginary frequencies were present ) . to improve numerical accuracy 
 of the calculations ( possible energy oscillations during scf ) , two - electron 
 integrals and their derivatives were calculated in a modified way 
 employing the pruned ( 99  590 ) integration grid consisting of 
 99 radial shells and 590 angular points per shell . on these ground - state 
 geometries , time - dependent density functional theory ( td - dft ) calculations were performed to find the first 50 excited states 
 employing the same level of theory . photovoltaic cells 
 were fabricated on ito - covered glass slides ( 15 mm  15 mm ) which 
 were cleaned in an ultrasonic bath using organic solvents . next the 
 ito was covered with a pedot : pss thin film by spin - coating and left 
 for 30 min in a vacuum heater at 70 c . after 30 min the active 
 layer , that is , p3ht : azphdi dissolved in chloroform with weight ratio 
 1:1 , was spun over the pedot : pss thin film . at the end photovoltaic 
 cells voltage ( i  v ) characteristics of the devices were measured using the 
 keithley 2400 sourcemeter . in this paper , the continuation of efforts 
 in the synthesis of 
 materials for potential optoelectronic applications and the new low 
 molecular compounds and polymers bearing imine linkages , diimide units , 
 and thiophene rings is presented . additionally , the chosen properties 
 of the synthesized compounds presented in this paper are compared 
 with those of their analogues , which have been described in our previous 
 work . the 
 new azomethine diimides were prepared from diamine - containing five- 
 or six - membered diimide units , i.e. , daphdi , dandi , and the new dandi-2 . the diamine 
 dandi-2 was prepared from condensation reaction of excess 1,4-naphthalenediamine 
 with 1,4,5,8-naphthalenetetracarboxylic dianhydride ( ntda ) . the diamines 
 daphdi and dandi-2 were applied for condensation with three aldehydes , 
 i.e. , 2-thiophenecarboxaldehyde , 2,2-bithiophene-5-carboxaldehyde , 
 and 3,4-(ethylenedioxy)thiophene-2-carboxaldehyde . thus , 
 azomethine diimides end - capped with thiophene , bithiophene , and ( ethylenedioxy)thiophene 
 units were prepared . these compounds form a block of electron - accepting ( a ) and electron - donating 
 ( d ) systems along the polymer . chemical structures 
 of synthesized azomethine diimides and poly(azomethine 
 imide)s . in our previous paper we have 
 reported azomethine diimides obtained 
 from dandi and 2-thiophenecarboxaldehyde or 2,2-bithiophene-5-carboxaldehyde 
 and the corresponding polymers . thus , 
 the properties of the prepared azomethine diimides can be compared 
 from three points of view : ( i ) the chemical structure of the diimide 
 core , ( ii ) the type of end - capped group of the compounds , and ( iii ) 
 the structure of the moieties between the diimide rings and imine 
 linkages . the assigned chemical structures of all synthesized 
 compounds were 
 identified through the data from h nmr and ftir spectroscopy 
 measurements . exemplary h nmr and ftir spectra of azphdi-1 
 and azndi-1 are presented in figure 2 . the absence of the residual amino ( nh2 ) and 
 aldehyde ( cho ) groups together with the appearance of a band 
 typical for imine bonds ( hc = n ) was confirmed by ftir 
 and nmr spectra . in ftir spectra of the investigated compounds , in 
 each case the characteristic band for the hc = n stretching 
 vibration was detected in the spectral range of 16041626 cm ( cf . the exact 
 position of this band in the investigated compounds varies by up to 
 22 cm and is shifted toward lower wavenumbers 
 for the azomethine diimides end - capped with bithiophene rings ( azphdi-2 , 
 azndi-2 ) and also the polymer with such units ( poly(azndi-2 ) . the 
 wavenumber of these compounds is lower than that for the others , which 
 means that the length of the imine bond in azphdi-1 , azphdi-3 , azndi-1 , 
 azndi-3 , and poly(azndi-1 ) is larger than in azphdi-2 , azndi-2 , and 
 poly(azndi-2 ) . the lower wavenumber of the imine group absorption 
 indicates the better conjugation of -electrons with phenyl 
 ring -electrons caused by the influence of the bithiophene 
 unit . compounds with a phthalic diimide core containing thiophene 
 and bithiophene rings exhibited a higher frequency of the analyzed 
 band in comparison with azomethine naphthalenediimides , which proves 
 that the largest decrease of the conjugation takes place . the possible 
 explanation of this effect can be that the -electrons in the 
 double bond are less involved in conjugation . in the case of azndi-1 
 and azndi-2 compounds obtained from dandi described in our previous 
 work , there was a lack of influence of the number of thiophene rings 
 on the wavenumber of the imine bond , which was at 1626 and 1620 cm in low molecular compounds and polymers , respectively . when comparing the influence of the structure 
 of the moieties between the diimide rings and imine linkages , the 
 better conjugation is exhibited in the compounds with additional naphthalene 
 units ( azndi-1 and azndi-2 ) than in corresponding azomethine diimides 
 with a tetramethyl - substituted phenyl ring between the imide and azomethine 
 bond . in the h nmr spectra 
 of azomethine diimides , a singlet signal due to the hydrogen atom 
 present in the imine unit and multiplets of the protons in the aromatic 
 rings were found . a shift of the position of the imine group signal , 
 depending on the chemical structure , was observed . the protons of 
 hc = n appeared in the range of 8.198.95 ppm 
 ( cf . the solubility of the 
 synthesized compounds was qualitatively determined 
 by the dissolution of 2.5 mg of the studied materials in 1 ml of different 
 organic solvents at room temperature and under heating . the solubility 
 data of the azomethine diimides and poly(azomethine imide)s in different 
 organic solvents are listed in table 1 . the qualitative 
 solubility was tested 
 with a 2.5 mg sample in 1 ml of solvent : ( + + ) soluble at room temperature , 
 ( + ) soluble after heating , (  ) partially soluble after heating , 
 (  ) not soluble . azndis were found to be more soluble in strong polar solvents than 
 their five - membered analogues azphdis . comparing the solubility of 
 azndis with that of the corresponding compounds described in our previous 
 work , it was found that replacement of 
 the tetramethylphenyl rings with naphthalene units improves their 
 solubility in nmp and dsmo . it was found that 
 forming stable , amorphous glassy states at / or above room temperature 
 is desirable for hole / electron transport materials . the gradual growth of crystallites of hole / electron transport 
 materials induced by the joule heat can induce phase separation . this 
 results in detachment at the interfaces of the electrode / transport 
 layer and deterioration , which are themselves thought to be important 
 degradation factors , and then failure of stability and durability . thus , a lot of attention is paid to organic 
 low molar mass compounds that readily form glasses above room temperature . 
 on the other hand , when the voltage is applied and current is flowing 
 through organic devices , joule heating processes arise . dsc 
 and tga in a nitrogen atmosphere were applied to examine the thermal 
 properties of the obtained azomethine diimides and polymers . formation of 
 the glassy state in synthesized azomethine diimides was investigated 
 by dsc . a strong correlation between the thermal properties of the 
 compounds and their chemical structures has been found . thus , different 
 behaviors of azomethine diimides during dsc heating and cooling runs 
 were observed . the first and second run heating scans of the investigated 
 compounds were carried out with a heating rate of 20 c / min . figure 
 s1 in the supporting information presents 
 the dsc thermograms of exemplary azomethine diimides  azphdi-1 , 
 azphdi-2 , and azndi-1 . the first run heating scan showed glass transition 
 temperatures ( tg ) for compounds azndi-2 
 and azndi-4 at 291 and 192 c , respectively . the results are 
 similar to those obtained for azomethine naphthalenediimides with 
 thiophene ( tg = 195 c ) and bithiophene 
 ( tg = 174 c ) moieties prepared also 
 from dandi and described in our previous work . for end - capped compounds with thiophene ( azphdi-1 and azndi-1 ) 
 and ( ethylenedioxy)thiophene ( azphdi-3 ) moieties , the first heating 
 scan revealed both tg values ( azphdi-1 , tg = 215 c ; azndi-1 , tg = 280 c ; azphdi-3 , tg = 231 c ) and an endotherm indicating melting of the sample 
 ( azphdi-1 , tm = 403 c ; azndi-1 , tm = 377 c ; azphdi-3 , tm = 363 c ) . on the other hand , the dsc curve of 
 az - ndi-3 did not show any transitions . during the second heating , 
 all samples exhibited only the glass transition ( except for azndi-2 , 
 which exhibited both tg and tm ) . t5 and t10 are temperatures 
 at 5% and 10% weight loss , 
 respectively . thus , 
 all obtained azomethine diimides are molecular glasses . in 
 the dsc thermogram of azphdi-2 during the second heating run , apart 
 from the glass transition ,  cold  crystallization ( exothermic 
 peak ) at 242 c and a melting endotherm at 325 c were detected 
 ( cf . figure s1b in the supporting information ) . comparing azphdis with azomethine naphthalenediimides described 
 in our previous work and azndi-4 and 
 considering the diimide structure , compounds with a phthalic diimide 
 unit as the core exhibited higher tg values . the influence of the end - capped group on tg was found to be different for azphdis than for azndis . introduction 
 of naphthalene units between the diimide structure and imine linkages 
 ( azndi-3 ) caused an increase of tg in 
 relation to azndi-4 containing tetramethylphenyl moieties . the compound 
 with naphthalenediimide containing bithiophene rings ( azndi-2 ) exhibited 
 the highest tg , contrary to the compound 
 with a phthalic diimide core ( azphdi-2 ) , which showed the lowest tg . tga analyses 
 were utilized to examine the thermal stability of the obtained azomethine 
 naphthalenediimides and polymers . the registered thermograms of azphdi-2 
 and the corresponding azndi-2 are given in figure 3 as examples . analysis of the tga curves allowed estimation of 
 the mass loss 
 associated with the processes of degradation up to 800 c , and 
 the temperature of the maximum decomposition rate ( tmax ) is evidenced by the differential thermogravimetric 
 ( dtg ) curve . the tga curves of all compounds indicate the existence 
 of two reaction stages , reflected in two peaks in the differential 
 weight loss curve ( dtg ) . only in the case of azndi-2 , azndi-3 , and 
 poly(azndi-1 ) are the two observed degradation steps separated . in 
 this case the first step is connected with a mass loss of around 6% 
 and the second with an about 20% mass loss . the thermal stability 
 of the compounds which were evaluated in terms of 5% and 10% weight 
 loss ( t5 , t10 ) , tmax , and residual weight at the end 
 of the experiment are listed in table 2 . compounds 
 with a naphthalenediimide core exhibited higher t10 , tmax , and residual weights 
 at 800 c compared to azomethine diimides with a five - membered 
 imide ring . all compounds exhibited high thermal stability , with the 
 value of t5 in the range 345468 
 c , and an excellent residual weight at 800 c within the 
 range of 4662% . the photophysical 
 properties such as absorption and emission in the uv  vis range 
 of the studied azomethine naphthalenediimides and poly(azomethine 
 imide)s were analyzed by uv  vis absorption and photoluminescence 
 ( pl ) spectroscopies in nmp and chloroform solutions and in the solid 
 state as blends with poly(methyl methacrylate ) ( pmma ) . pmma has wide 
 applications as an inert solid matrix for various spectroscopic and 
 optoelectronic investigations for applications in oleds , solar cells , and organic switching devices . absorbance spectra of azomethine diimides 
 and polymers dissolved 
 in nmp are depicted in figure 4 . the data 
 from absorption spectra of all studied compounds are summarized in 
 table s1 in the supporting information . electronic absorption spectra of the studied azomethine phthalic 
 diimides showed similar characteristics , i.e. , one main absorbance 
 band . however , in the case of azphdi-1 and azphdi-3 a shoulder around 
 350 nm was seen . both the position and the intensity of the absorption 
 bands depend on the chemical structure of the compound . the position 
 of the absorption maximum ( max ) of compounds end - capped 
 with bithiophenes was significantly shifted to lower energies as compared 
 to that of other compounds with phthalic diimide rings . this confirms 
 better conjugation in this compound as was concluded from ftir and 
 nmr data . in the uv  vis spectra electronic transitions of azomethine 
 naphthalenediimides and polymers are dominated by a strong band of 
 pronounced vibrational structure with three maxima at about 340 , 360 , 
 and 380 nm , ascribed to the  * transition in 
 the naphthalenediimide core . similarly , as was observed 
 in azphdis , the compounds with bithiophene rings exhibited an absorption 
 range bathochromically shifted as compared to that of other materials 
 ( cf . despite the different polarities 
 of the applied solvents , the spectra showed a max position at almost the same wavelength ( cf . therefore , the solvatochromic 
 effect was not observed in the studied compounds , an observation similar 
 to that in the azomethine naphthalenediimides described in our previous 
 works . in the solid state , as blends with pmma , the investigated compounds 
 showed one main absorption band , which was in most cases hypsochromically 
 shifted , along with a change from the solution state to the solid 
 state . to determine 
 the optical properties of thin films prepared using compounds containing 
 phthahlic diimides ( azphdi-1 , azphdi-2 , and azphdi-3 ) and naphthalenediimides 
 ( azndi-1 , azndi-2 , azndi-3 , and azndi-4 ) , spectroscopic ellipsometry 
 was applied . for each compound the complex dielectric function ( 1 + i2 ) and complex refractive index ( n + ik ) were determined . usually for material dissolved in solution , beer s law 
 is used for determination of  (  = 4/0 ) . however , for the thin film , where the layer thickness is 
 much below the coherence length , the fabry the absorption coefficient is described 
 as2where 2 is the imaginary 
 part of the dielectric function , n is the refractive 
 index of the material ,  is the frequency , and c is the speed of light in vacuum . using the above equation , absorption 
 coefficients of the studied materials were calculated , and the resulting 
 curves are presented in figure s2 in the supporting 
 information . for all three compounds a significant absorption 
 above 3.5 ev from a phthalic diimide can be observed . below 3.5 ev 
 a different shape of the curves , due to the different end - capped group 
 attached , can be noticed . interestingly , the value of the absorption 
 coefficient for azphdi-2 is 2 times stronger than those measured for 
 azphdi-1 and azphdi-3 . the reason is a higher conjugation of the end - capped 
 group ( bithiophene ) which should result in better planarization of 
 the molecule . this also results in better overlapping of the  
 orbitals , which also results in a much higher absorption coefficient 
 of the phthalic group in this molecule . these results are in full 
 agreement with the results of uv  vis spectroscopy measured 
 for dissolved materials and with ftir and nmr characteristics . as 
 in the case of the compounds containing phthalic diimides , use of 
 the naphthalenedimiides and different end - capping groups strongly 
 influences the absorption coefficient spectra . for compounds containing 
 thiophene and bithiophene ( azndi-1 and azndi-2 , respectively ) , the 
 absorption coefficients are much higher than for those containing 
 3,4-(ethylenedioxy)thiophene end - capping groups . the data from 
 spectroscopic ellipsometry can also be compared with the results of 
 td - dft calculations in terms of allowed transitions within this group 
 of compounds . the results of these calculations are gathered in table 
 s2 in the supporting information . what is no doubt worth noting is the fact that , in each case , the 
 homo  lumo transition is not allowed , possibly due to symmetry - related 
 selection rules ( oscillator strength equal to 0 ) . moreover , within 
 the first 50 excited states , there are only several cases with significant , 
 nonzero oscillator strength , therefore simplifying the assignment . 
 in the case of pyromelitic derivatives ( azphdi-13 ) , the first 
 allowed transitions are due to excitations from phenylene / thiophene 
  thiophene ( azphdi-1 ) and  * transitions 
 on bithiophene ( azphdi-2 ) and edot ( azphdi-3 ) . the relative energy 
 of these transitions corresponds well with structural diversity . in 
 the case of bithiophene , it is highly red - shifted due to a higher 
 conjugation . the higher energy side of the spectra correspond to the 
 rest of the transitions . for azphdi-1 , it covers  * 
 transitions on the pyromelitic core ( 29 ) on the thiophene moiety ( 34 , 
 45 ) and some minor contribution from charge transfer transitions from 
 thiophene to the core and vice versa ( 33 , 49 , respectively ) . for azphdi-2 the higher energy transitions are dominated by  * 
 excitations on bithiophene and the core ( 12 , 15 ) . in case of azphdi-3 , the situation is very much the same with  * transitions 
 on the edot moiety ( 26 ) and the core ( 36 , 38 ) . the remaining ndi derivatives 
 follow a similar pattern ; however , the contribution from the naphthalene 
 units is much more pronounced . in azndi-1 the first allowed transition 
 is mainly a naphthalene  thiophene  * 
 excitation , but the situation changes when moving to highly conjugated 
 species such as azndi-2 and azndi-3 , where the first allowed transitions 
 are located on bithiophene and edot , respectively . the most unifying 
 characteristic however between ndi - based derivatives is the common 
 contribution of the ndi - located  * transitions 
 to the spectrum , which is more pronounced in the case of azndi-4 due 
 to the lack of naphthalene units . these transitions also carry a substantial 
 oscillator strength and give rise to a vibrational structure . a comparison 
 of normalized ellipsometric curves with oscillator strengths from 
 td - dft calculations for azphdi-3 and azndi-1 is presented in figure 5 ( for a full comparison , see the supporting information ) . it can be clearly seen that in terms 
 of allowed electronic transitions the results match the experimental 
 data . in the case of pyromelitic derivatives , the long shoulder on 
 the low - energy side corresponds to the transitions on the end - capped 
 units ( thiophene , bithiophene , and edot ) . for the rest of the molecules , the situation is slightly more complicated . from dft study it seems 
 that the lower lying allowed excited states carry a contribution from 
 the end caps , but the shape of the experimental curve is very similar 
 and may result mainly from the ndi - centered excitations of a superposition 
 of ndi , naphthalene ( phenylene ) , and thiophene moieties . the 
 emission spectra were recorded with different excitation wavelengths 
 ( ex ) in nmp and chloroform solutions as well as 
 in the solid state as blends with nonemissive pmma . the influence 
 of the excitation wavelength on the pl properties , the position of 
 the emission band maximum ( em ) and the intensity 
 of emitted light , is presented in figure 6 . the emission spectral data of the studied compounds are summarized 
 in table s3 in the supporting information . ( a , b ) fluorescence spectra of ( a ) azphdi-1 and ( b ) azndi-1 in 
 nmp , ( c ) pl and uv  vis spectra of azphdi-2 in nmp , ( d ) pl spectra 
 of all compounds in the blend , and ( e ) photographs of azphdi-2 ( left ) 
 and polyazndi-1 ( right ) in blends irradiated with light at 366 nm . all the compounds with naphthalenediimides 
 in the structure ( except 
 azndi-3 ) exhibited a very week emission in chloroform solution , and 
 thus , these results are not discussed in this paper . in the pl spectrum 
 of azndi-1 , a broad emission band with two maxima at an excitation 
 of 340 nm is seen ( cf . the investigated 
 compounds in solution at different excitation wavelengths exhibited 
 a single emission band . compounds containing azomethine phthalic diimides 
 emitted light with lower intensity in chcl3 compared to 
 nmp except the compound end - capped with an ( ethylenedioxy)thiophene 
 unit ( azphdi-3 ) . taking into account the intensity of emitted light 
 in the same solution concentration , photoluminescence of azomethine 
 phthalic diimides azphdi-1 and azphdi-2 was significantly more intense 
 than that of the corresponding azomethine naphthalenediimides azndi-1 
 and azndi-2 . the intensity of light emission of azphdis decreases 
 in the following order : azphdi-1 > azphdi-3 > azphdi-2 . a significant 
 effect of bithiophene end groups on the decrease of the pl intensity 
 was revealed . the emission wavelength of the azomethine diimides and 
 polyazndi-1 in solutions was independent of ex , 
 except for compounds with a bithiophene ring ( azphdi-2 , azndi-2 , and 
 polyazndi-2 ) and azndi-3 . for those compounds , together with an increase 
 of ex , a bathochromic shift in the em position was observed in the pl spectra . all studied compounds emitted 
 blue light , an observation similar to that for azomethine diimides 
 and polymers with a tetramethylphthalic ring between the diimide structure 
 and imine linkages reported in our previous work . all azomethine diimides and polymers emitted light with 
 the highest intensity at ex = 360 nm , except for 
 azndi-2 and az - ndi-3 ( cf . table s3 , supporting 
 information ) . considering the effect of the solvent polarity 
 on the pl spectra , in all cases of compounds with a phthalic diimide 
 unit , a hypsochromic shift of em was found with 
 a decrease in solvent polarity ; thus , they exhibited solvatochromic 
 behavior . such behavior may be evidence of the photoinduced intramolecular 
 charge - transfer character of the excited state in these compounds . it should be noticed that the uv  vis absorption spectra 
 of the investigated compounds overlap slightly with their emission 
 spectra . the emission maxima are strongly red - shifted from their optical 
 absorption maxima ( cf . the stokes 
 shifts , which are important from a practical point of view of potential 
 applications , were in the range of 588516356 cm . additionally , the fluorescence characteristics of the obtained 
 compounds in the solid state as blends with pmma were studied . the 
 normalized pl spectra of the blends excited with a wavelength corresponding 
 to the highest emission intensity in nmp solution are presented in 
 figure 6d . the pl spectra of the compounds 
 in the blends changed , compared with their spectra in nmp solution . 
 in comparison with that of the solution , em of the 
 blends was hypsochromically shifted in the range of 440 nm ; 
 however , all blends emitted blue light ( cf . figure 6e ) . the highest differences between em values 
 of compounds in blends and nmp solution were detected in the case 
 of azndis . the suitability 
 of the use of compounds in a given electronic application can be determined 
 by their redox properties , which are frequently investigated by electrochemical 
 methods . in this study , the redox properties of the studied compounds 
 were investigated using cv and dpv . the obtained electrochemical data 
 are presented in table s4 in the supporting information , whereas the cv and dpv voltammograms of azomethine diimides and 
 polymers are collected in figure 7 . cv ( dashed 
 lines ) and dpv ( solid lines ) voltammograms of azphdis 
 ( a ) , cv voltammograms of azndis ( b ) , cv voltammograms of polyazndi-2 
 and azndi-2 ( c ) , and cv and dpv curves of polyazndi-1 and azndi-1 
 ( d ) . the measurements were performed in acetonitrile containing 0.1 
 m nbu4npf6 using a gc working electrode and 
 referenced against the cp2fe couple ( for 
 cv a scan rate of 100 mv / s was used ) . all measurements were carried out on a glassy carbon electrode 
 ( the working electrode ) . for polyazndi-2 , in comparison the platinum 
 electrode was used , allowing recording of up to five separated reduction 
 peaks , which was not possible on a platinum wire electrode . in addition , 
 the reductions peaks were much less visible on the pt electrode . it is expected 
 that at lower potentials platinum becomes a hard lewis acid in nature . probably a stable adduct with the reduced form of c = o is formed 
 ( interaction between pt and a hard o anion ) . diimide 
 a fully reversible reduction process 
 toward the formation of an anion radical according to the mechanism 
 proposed by hsiao et al . compounds consisting of a phthalic diimide structure ( azphdis ) revealed 
 almost identical properties during the reductions ( cf . figure 7a ) . in each case three , fully reversible reduction 
 steps were measured ( peak - to - peak separation 60 mv ) . this 
 clearly shows that the thiophene ring modifications do not affect 
 the position of the lumo . possibly , these may have 
 undergone the polymerization process during oxidation , such as for 
 compounds consisting of thiophenes and azomethine double bonds . the oxidation potentials ( within the range from 0.3 to 0.53 
 v ) are also too low to be assigned to the oxidations of the thiophene 
 ring . the similarity of the properties during the reduction 
 can also be observed in the case of a series of compounds with a naphthalenediimide 
 unit ( azndis ) . first , they are easier to reduce than phthalic diimide 
 analogues  at a potential slightly below 0.7 v. this 
 is another example of the common phenomenon that the anion is better 
 stabilized on a naphthalenediimide than phthalic diimide moiety and 
 therefore is more easily formed . all of the naphthalenediimide red2 values are also similar for all discussed derivatives , i.e. , 
 in the range from 1.13 to 1.30 v. potentials red3 and red4 are also almost alike . the reduction 
 processes are fully reversible ( peak - to - peak separation 60 
 mv ) ( cf . figure 7 ) . a graphical comparison 
 of polymers with the corresponding model compounds under electrochemical 
 measurements is shown in figure 7c , d . as one 
 can see , there is almost no change in the voltammograms of polyazndi-2 
 and its model compound azndi-2 . therefore , any differences 
 in the properties in the solid state are a consequence of different 
 intermolecular interactions ( not intramolecular ) . on the other hand , 
 for azndi-1 and its polymeric analogue ( polyazndi-1 ) , a significant 
 oxidation potential shift ( to the lower potential ) as a consequence 
 of polymerization was observed . quite unexpectedly , it was more 
 difficult for diimide units in the polymer to undergo reduction than 
 those in azndi-1 ( figure 7d ) . moreover , in 
 both cases , the recorded peaks are slightly less apparent  probably 
 due to sterical difficulties in the flow of charge between a polymeric 
 molecule and the surface of the electrode . from the onset oxidation 
 and reduction potentials in the cv and 
 dpv voltammograms , the energies of the homo and lumo levels ( or rather 
 ionization potentials ( ips ) and electron affinities ) can be readily 
 estimated , assuming the ip of ferrocene to be equal to 4.8 
 ev , according to well - established equations . the calculated homo and lumo levels together with the electrochemical 
 energy band gap ( eg ) the experimental cv data are well correlated with 
 the dpv data . 
 considering the influence of the compound structure on the lumo energy 
 level and eg , it was observed that variation 
 of the end groups of the diimide core can be used to tune the homo 
 energy and consequently the band gap of the material while leaving 
 the lumo practically intact . the values of eg of the azomethine diimides were the lowest for compounds 
 end - capped with bithiophene units , an observation similar to that 
 for azomethine diimides described in our former paper . the effect of the diimide structure on eg was studied , and it was found that compounds 
 with a six - membered diimide unit exhibited a lower electrochemical 
 energy gap in relation to azphdis . summarizing , 
 all of the studied compounds reveal very good properties during reduction 
 ( multiple steps and fully reversible ) , and they are promising electron - transporting 
 materials . to investigate the 
 electron density distribution , a set of dft calculations were carried 
 out . the geometries were fully optimized on the presented level of 
 theory in vacuum and then further analyzed . the theoretically calculated 
 homo and lumo energy levels and eg are 
 presented in table 3 , whereas the homo and 
 lumo contours of the investigated azomethine diimides are shown in 
 figure 8 . the homo and lumo orbitals in each case are exclusively space - separated 
 in almost the same part of the molecule . the localization of the lumo 
 on the strongly electron - accepting diimide core is a key characteristic 
 in this class of compounds and is also pronounced in this study . on 
 the other hand , in the case of the homo some minor differences may 
 be found within the studied molecules . in azphdi-1 and azndi-1 the 
 contribution from the thiophene moiety to the homo is small , and therefore , 
 it is located almost exclusively on the tetramethylphenyl and naphthalene 
 moieties , respectively . incorporation of edot instead of thiophene 
 with strongly electron - donating ochh2ch2o groups ( azphdi-3 , azndi-3 , azndi-4 ) induces a higher contribution 
 of this moiety to the homo . finally , exchanging thiophene with bithiophene 
 results in spreading of the homo almost evenly along the naphthalene 
 and bithiophene in azndi-2 and the dominant participation of bithiophene 
 in the homo in azphdi-2 . another important fact is the analysis of 
 the homo / lumo energy levels and their corresponding eg values . according to table 3 , the homo level varies from 5.37 ev for azndi-3 and azndi-4 
 to 5.85 ev for azphdi-1 . systematic comparison reveals that 
 the exchange of thiophene for ( ethylenedioxy)tiophene or bithiophene 
 is followed by a rise in the homo level by 0.2 ev ( azphdi-1 and azndi-1 ) 
 and at the same time lowering of the eg value . the exchange of the diimide core also affects the homo level 
 with a drop in energy by 0.2 ev when moving from six - membered naphthalenediimide 
 to five - membered pyrromellitic diimide . as for the changes in lumo 
 energies , there is virtually no affect of the side aromatic azomethines 
 on the lumo of a specific compound . on the other hand , there is a 
 great dependence on the type of diimide that is used . six - membered 
 rings cause a drop in the lumo level by 0.3 ev and therefore a drop 
 in eg as well . by combining these two 
 effects of interplay between the electron - donating character of the 
 azomethine substituent and the size of the aromatic core , one can 
 easily change the value of eg from 2.15 
 ev ( for azndi-2 ) to 2.89 ev ( for azphdi-1 ) . the comparison of the 
 theoretically predicted homo / lumo values with the experimental data 
 reveals significant differences . these problems may be interpreted 
 in terms of the functional formulation and the effects that lie inside . first , there may be some unfortunate miscalculations due to the sum 
 of several errors which usually cancel each other . second , the differences 
 may be a result of the dynamics of the electrochemistry . the flow 
 of charge also affects the molecule ; therefore , it is sometimes better 
 to think of the measured values in terms of ionization potential and 
 electron affinity . nevertheless , despite the differences in values , 
 the theoretically calculated values follow the pattern of the electrochemical 
 data . the lumo levels for azndi derivatives are 0.3 ev smaller than 
 for azphdis . at the same time the drops and the differences in eg are a direct consequence of these phenomena 
 and the reasons mentioned above . preliminary investigations of obtained 
 model compounds azphdi-1 and 
 azphdi-2 as active layers in solar cells were carried out . the fabricated 
 devices have the following architecture : ito / pedot : pss / p3ht : azphdi / al . the aluminum electrode acts as the cathode ( collecting electrons ) , 
 while ito is applied as the anode for the purpose of collecting holes . 
 the regioregular p3ht with a homo level of about 4.8 ev and 
 lumo level of about -2.7 ev was applied as the donor in the photovoltaic 
 devices . two 
 types of heterojunctions , with thicker ( 208 nm ) and thinner ( 150 nm ) 
 active layers , were fabricated . the samples 
 were illuminated with a light source with a power of 1.3 mw / cm , and measurements were made using a keithley 2400 source 
 with a picoamperemeter . from the obtained characteristics of photocurrent 
 density  voltage , the current density jsc , open - circuit voltage voc , and 
 fill factor ( ff ) and conversion efficiency  defined aswere calculated ( plight is the input 
 optical power ) . the obtained i  v curves in the dark and under 
 illumination are presented in figure 9 , and 
 the corresponding photovoltaic performances are summarized in table 4 . j  v characteristics 
 of 
 organic photovoltaic cells : ( a ) ito / pedot : pss / active layer / al ( active 
 layer thickness 208 nm ) , ( b ) ito / pedot : pss / active layer / al ( active 
 layer thickness 150 nm ) . taking into account the photovoltaic 
 parameter , which depends on 
 the chemical structure of the active layer , which determines the homo 
 level of the donor and lumo level of the acceptor and uv  vis 
 properties , that is , the open - circuit voltage ( voc ) , it can be seen that the device comprising p3ht with azomethine 
 phthalic diimide end - capped with thiophene rings showed significantly 
 lower voc . it should be stressed that 
 the device with azphdi containing bithiophene units exhibited a much 
 higher value of voc above 1 v. considering 
 the conversion efficiency , the effect of the chemical structure is 
 less pronounced . it was found that the thickness of the active layer 
 influences the maximal power point and power conversion efficiency . 
 for thinner layers , new compounds consisting of a phthalic diimide or naphthalenediimide 
 core end - capped with thiophene , bithiophene , and ( ethylenedioxy)thiophene 
 units were synthesized . the structure of the resulting compounds was 
 checked using hnmr and ftir spectra , showing the appearance 
 of characteristic signals from typical bonds . thermal properties 
 were determined using differential scanning calorimetry , while thermal 
 stability was determined using thermogravimetric and differential 
 thermogravimetric techniques . all small molecules exhibited properties 
 characteristic of molecular glasses . in the second heating run only 
 glass transition temperatures between 176 and 291 c were found . the compounds have adequate thermal stability for applications in 
 optoelectronic devices ( t5 in the range 
 between 345 and 468 c ) . to determine photophysical properties , 
 the uv  vis and photoluminescence spectroscopic studies were 
 performed in both solution ( in nmp and chloroform ) and the solid state 
 ( in blends with pmma ) . all compounds emitted blue light . the resulting absorption coefficients revealed a strong correlation 
 with the chemical structure of the presented polymers . finally , the electrochemical properties of the studied compounds were 
 characterized using cyclic voltammetry and differential pulse voltammetry . the energy gap obtained from the cv measurement differed between 1.22 
 and 1.89 ev and for each compound was slightly bigger ( by a maximum 
 of 0.19 ev ) than those obtained from the dpv measurement . however , 
 the difference between the experimentally obtained eg and that predicted from the dft calculations was much 
 bigger ( more than 1 ev ) . the strong difference between theoretical 
 and experimental data is because dft calculates eg from the molecular orbitals , while cv measures the ionization 
 energy . the preliminary photovoltaic experiments ( device architecture 
 ito / pedot : pss / active layer / al ) suggested that model compounds end - capped 
 with bithiophene units are better for photovoltaic applications . the 
 obtained results showed that the chemical structure of the organic 
 compound and thickness of the active layer in the device influence 
 the photovoltaic properties of organic solar cells . the conversion 
 efficiency of the fabricated devices was in the range of 0.690.90% . the high value of voc ( above 1 v ) was 
 obtained for an active layer consisting of azomethine phthalic diimide 
 with bithiophene rings .
new symmetrical arylene 
 bisimide derivatives formed by using electron - donating  electron - accepting 
 systems were synthesized . 
 they consist of a phthalic diimide or naphthalenediimide 
 core and imine linkages and are end - capped with thiophene , bithiophene , 
 and ( ethylenedioxy)thiophene units . 
 moreover , polymers were obtained 
 from a new diamine , n , n-bis(5-aminonaphthalenyl)naphthalene-1,4,5,8-dicarboximide 
 and 2,5-thiophenedicarboxaldehyde or 2,2-bithiophene-5,5-dicarboxaldehyde . 
 the prepared azomethine diimides exhibited glass - forming properties . 
 the obtained compounds emitted blue light with the emission maximum 
 at 470 nm . 
 the value of the absorption coefficient was determined 
 as a function of the photon energy using spectroscopic ellipsometry . 
 all compounds are electrochemically active and undergo reversible 
 electrochemical reduction and irreversible oxidation processes as 
 was found in cyclic voltammetry and differential pulse voltammetry 
 ( dpv ) studies . 
 they exhibited a low electrochemically ( dpv ) calculated 
 energy band gap ( eg ) from 1.14 to 1.70 
 ev . 
 the highest occupied molecular orbital and lowest unoccupied molecular 
 orbital levels and eg were additionally 
 calculated theoretically by density functional theory at the b3lyp/6 - 31g(d , p ) 
 level . 
 the photovoltaic properties of two model compounds as the active 
 layer in organic solar cells in the configuration indium tin oxide / poly(3,4-(ethylenedioxy)thiophene):poly(styrenesulfonate)/active 
 layer / al under an illumination of 1.3 mw / cm2 were studied . 
 the device comprising poly(3-hexylthiophene ) with the compound end - capped 
 with bithiophene rings showed the highest value of voc ( above 1 v ) . 
 the conversion efficiency of the fabricated 
 solar cell was in the range of 0.690.90% .