the specificity of 
 f - f emission signals and long luminescence lifetimes 
 ( s - ms range ) have made lanthanide 
 ions ( ln ) the key elements for a large number of novel 
 advanced applications and technologies . in particular , the ability of some lanthanide ions to exhibit characteristic 
 sharp bands in the near - infrared ( nir ) spectral region has stirred 
 an additional interest for their exciting use in bioanalytical applications 
 and biological imaging , in telecommunications , lasers , oled / led devices , and energy conversion . the er ion , with its emission band centered at 15301540 
 nm , is the best candidate for telecommunications . biological optical 
 imaging suffers from several negative effects caused by the interaction 
 of light with biological media , thereby limiting the sensitivity of 
 detection , penetration depth , and resolution . among these limitations are autofluorescence , high absorption of hemoglobin in tissue in the 
 visible range , and light scattering . as a consequence , nir imaging 
 and bioanalytical applications are gaining attention , and different 
 luminescent probes have been developed . with respect to the design of lanthanide - based luminescent materials , 
 one must take into account the fact that most of the ln ion s f - f transitions are forbidden . consequently , these 
 transitions have low absorption coefficients , and direct population 
 of excited states is only possible with the help of high - power laser 
 excitation sources . however , one practical approach is a  sensitization  
 process also known as the  antenna effect  which involves 
 embedding lanthanide ions into an environment ( either an inorganic 
 matrix or organic surroundings ) with good light - harvesting properties . captured excitation energy is then transferred to the lanthanide ion 
 which , in turn , lights up with characteristic emission bands . different nir luminescent lanthanide molecules 
 and materials have been created based on mononuclear complexes with 
 macrocyclic and acyclic ligands and , more recently , nanoparticles , dendrimers , mesoporous solids , mofs , and 3d-4f heterometallic clusters . quantitative sensitization efficiencies can be achieved ; however , despite numerous efforts , the highest 
 reported overall quantum yields for nir - emitting lanthanide complexes 
 containing c h bonds are inferior : 3.8% for yb , 0.42% for nd , and 0.033% for er . such a large difference compared 
 to visible - emitting lanthanide complexes , with quantum yields greater 
 than 80% for eu and tb , can be explained as follows : small energy gaps between the emitting 
 and first levels of the ground multiplet for nir - emitting lanthanides 
 are easily coupled to the overtone of high - energy oscillations , such 
 as o  h , n  h , and c an 
 enhancement of luminescence efficiency can be achieved by deuteration or halogenation of organic ligands but requires significant synthetic 
 effort . one promising 3d-4f heterometallic platform that provides 
 combined 
 sensitization and protection of ln ions from vibrations 
 is a class of supramolecular structures called metallacrowns ( mcs ) . mcs have a repeating [ m  n  o]n subunit and are named on the basis of their similarities to classical 
 organic crown ethers , which contain a repeating [ c  c  o]n subunit ( chart 1 ) . a [ 12-mczn(ii),quinha-4 ] structure , for example , 
 contains a total of twelve atoms in the ring , four repeating units , 
 zn as the ring metal , and quinaldichydroxamic acid ( quinha ) 
 as the organic ligand . in a similar manner to the classical crown ethers , a central metal atom can be bound to the inward facing oxygen 
 atoms of the mc ring . several mcs have been prepared with different 
 sizes ( e.g. , 9-mc-3 to 60-mc-20 ) and bound metal ions . of particular interest with respect to luminescence 
 properties are mcs that bind central lanthanide ions , of which there 
 are several recent examples . more specifically , ln[zn(ii)mc]s have been shown to 
 exhibit nir luminescence upon uv excitation through the chromophore 
 located on the ligand . in these structures , 
 a single ln ion is sandwiched between two [ 12-mczn(ii),picha-4 ] structures ( picha = picolinehydroxamic acid ) . this  sandwich  is further encapsulated by a [ 24-mczn(ii),picha-8 ] structure . the resulting encapsulated motif , 
 with the formula ln[12-mczn(ii),picha-4]2[24-mczn(ii),picha-8 ] ( ln[zn(ii)mcpicha ] ) , protects the ln ion from interactions with high 
 energy x - h oscillators ( x = n , o , c ) nearby , preventing strong nonradiative 
 deactivation , thereby enhancing the luminescence signal . however , 
 uv absorption of this family of mcs limits the scope of practical 
 applications , especially in the field of biological imaging . in order 
 to shift excitation wavelength toward lower energy and to improve 
 the sensitization efficiency , we report here the design , synthesis , 
 and structural characterization of a series of second generation of 
 mcs with the formula ln[12-mczn(ii),quinha-4]2[24-mczn(ii),quinha-8 ] ( ln[zn(ii)mcquinha ] , quinha = quinaldichydroxamic acid , ln = y , 
 nd , eu , gd , tb , dy , er , yb ) ( figure 1 ) . photophysical 
 properties of nir - emitting nd , yb , and er complexes have been investigated in detail . additionally , 
 in order to gather information about energy transfer processes , ligand - centered 
 luminescence of gd[zn(ii)mcquinha ] and quinha are exploited . x - ray 
 crystal structures of dy[zn(ii)mcquinha ] viewed 
 along the ( left ) a - axis and 
 ( right ) c - axis . fresh hydroxylamine was prepared 
 by combining hydroxylamine hydrochloride ( 12.0 g , 173 mmol ) and potassium 
 hydroxide ( 11.4 g , 173 mmol ) in methanol ( 200 ml ) at 0 c . the 
 solution was stirred for 20 min , filtered to remove potassium chloride , 
 and set aside . meanwhile , quinaldic acid ( 20.0 g , 116 mmol ) and n - methylmorpholine 
 ( 14.0 ml , 127 mmol ) were combined with stirring in dichloromethane 
 ( 300 ml ) . the solution was cooled to 0 c at which time ethylchloroformate 
 ( 12.1 ml , 127 mmol ) was added . the reaction was stirred for 20 min 
 and filtered , and the hydroxylamine solution was added to the filtrate 
 at 0 c . the reaction mixture was allowed to warm to room temperature 
 and stirred for 1.5 h. the volume was reduced to 200 ml in 
 vacuo , and water ( 1 l ) was added to induce the precipitation of a 
 white solid . the solid was collected by filtration and triturated 
 with hot dichloromethane ( 800 ml ) to yield quinaldichydroxamic acid 
 ( 12.7 g , 58.2% ) as a white powder . ] , c10h9n2o2 , 189.1 ; found 189.1 ; calc . for [ m + na ] , 
 c10h8n2nao2 , 211.0 ; found 
 211.1 . h nmr ( 400 mhz , dmso - d6 )  11.51 ( s , 1 h ) , 9.18 ( s , 1 h ) , 8.53 ( d , j = 8.5 hz , 1 h ) , 8.098.04 ( m , 3h),7.84 ( m , 1 h ) , 
 7.69 ( m , 1h ) . c nmr ( 100 mhz , dmso - d6 )  161.7 , 150.3 , 146.0 , 137.6 , 
 130.4 , 129.2 , 128.6 , 128.0 , 127.9 , 118.7 . ( found ) 
 for ( c10h8n2o2)(h2o ) , c : 58.25 ( 58.25 ) , h : 4.89 ( 4.94 ) , n : 13.59 ( 13.65 ) . vis 
 ( meoh ) , max , nm ( log  ) 207 ( 4.4 ) , 238 ( 4.5 ) , 
 300(br ) ( 3.5 ) . quinaldichydroxamic acid 
 ( 200 mg , 1.06 
 mmol ) , zinc triflate ( 385 mg , 1.06 mmol ) , and dysprosium triflate 
 ( 81 mg , 0.13 mmol ) were dissolved in 15 ml of dimethylformamide , and 
 triethylamine ( 296 l , 2.12 mmol ) was added . the solution immediately 
 turned yellow and was stirred at room temperature overnight at which 
 time it was set aside for slow evaporation , producing yellow plate 
 crystals within two weeks . crystals were collected by filtration and 
 air - dried to yield dy[zn(ii)mcquinha ] as a 
 triflate salt ( 29 mg , 8.4% ) . esi - ms , calc . for [ m ] , c160h96dyn32o32zn16 , 1395.8 ; found 1395.7 . ( found ) for ( c163h96dyf9n32o41s3zn16)(h2o)5(c3h7no)3 , c : 41.79 ( 41.81 ) , h : 2.59 ( 2.81 ) , n : 9.92 ( 10.23 ) . esi - ms , calc . for [ m ] , c160h96n32o32yzn16 , 1371.1 ; found 1371.2 . h nmr ( 400 mhz , cd3od )  8.36 ( d , j = 4.7 hz , 8 h ) , 8.34 ( d , j = 4.7 hz , 
 8 h ) , 8.22 ( d , j = 8.5 hz , 8 h ) , 8.09 ( d , j = 8.5 hz , 8 h ) , 7.73 ( d , j = 7.6 hz , 
 8 h ) , 7.52 ( d , j = 8.5 hz , 8 h ) , 7.49 ( d , j = 8.0 hz , 8 h ) , 7.25 ( m , 16 h ) , 7.19 ( d , j = 8.5 hz , 8 h ) , 7.09 ( m , 8 h ) , 6.95 ( m , 8 h ) . , calc . 
 ( found ) for ( c163h96f9n32o41s3yzn16)(h2o)5(c3h7no)3 , c : 42.42 ( 42.48 ) , 
 h : 2.63 ( 2.90 ) , n : 10.07 ( 10.40 ) . ( 96 mg , 
 28% ) . esi - ms , calc . for [ m ] , c160h96n32o32ybzn16 , 1399.3 ; found 1399.4 . ( found ) for ( c163h96f9n32o41s3ybzn16)(h2o)5(c3h7no)3 , c : 41.70 ( 41.73 ) , h : 2.58 ( 2.74 ) , n : 9.89 ( 9.98 ) . ( 58 mg , 
 34% ) . esi - ms , calc . for [ m ] , c160h96n32ndo32zn16 , 1389.7 ; found 1389.2 . ( found ) for ( c163h96f9n32ndo41s3zn16)(h2o)5(c3h7no)3 , c : 41.94 ( 41.76 ) , h : 2.60 ( 2.76 ) , n : 9.95 ( 10.08 ) . esi - ms , calc . for [ m ] , c160h96gdn32o32zn16 , 1394.0 ; found 1393.2 . ( found ) for ( c163h96f9gdn32o41s3zn16)(h2o)4(c3h7no)4 , c : 42.09 ( 41.84 ) , h : 2.66 ( 3.03 ) , n : 10.10 ( 10.26 ) . ( 39 mg , 
 23% ) esi - ms , calc . for [ m ] , ( found ) for ( c163h96f9n32o41s3tbzn16)(h2o)5(c3h7no)3 , c : 41.82 ( 41.50 ) , h : 2.59 ( 2.71 ) , n : 9.92 ( 9.90 ) . esi - ms , calc . for [ m ] , c160h96eun32o32zn16 , 1392.2 ; found 1391.8 . ( found ) for ( c163h96euf9n32o41s3zn16)(h2o)9(c3h7no)3 , 
 c : 41.27 ( 41.00 ) , h : 2.72 ( 2.90 ) , ( found ) for ( c163h96erf9n32o41s3zn16)(h2o)5(c3h7no)3 , 
 c : 41.15 ( 41.49 ) , h : 2.71 ( 2.86 ) , n : 9.76 ( 10.16 ) . yellow block crystals 
 of dy[zn(ii)mcquinha ] suitable for x - ray analysis 
 were grown by slow diffusion of ethyl 
 acetate into a dmf : pyridine ( 2:1 , v : v ) solution of the triflate salt . a crystal of dimensions 0.22  0.22  0.28 mm was selected 
 for data collection and mounted in the 90 k nitrogen cold stream provided 
 by a cryosystems low temperature apparatus on the goniometer head 
 of a bruker d8 diffractometer equipped with an apexii ccd detector . data were collected with the use of mo k radiation (  
 = 0.71073  ) . the structure was solved by a dual space method 
 ( shelxt ) and refined by full - matrix least - squares on f ( shelxl-2012 ) . the molecule crystallized in the tetragonal 
 crystal system , space group p4 ( no . crystal data : [ c200h136dyn40o32zn16][cf3so3]32(c4h8o2 ) , fw = 5443.32 , tetragonal , p4 ( no . 
 75 ) , a = 30.4975(19 )  , c = 
 13.3192(9 )  , v = 12388.2(18 )  , z = 2 , r1 [ 33073 reflections 
 with i > 2(i ) ] = 0.0415 , wr2 ( all 37814 data ) = 0.1402 , 1506 parameters , 2 restraints . emission and excitation spectra were measured on a horiba - jobin - yvonfluorolog 
 3 spectrofluorimeter equipped with either a visible photomultiplier 
 tube ( pmt ) ( 220800 nm , r928p ; hamamatsu ) , a nir solid - state 
 ingaas detector cooled to 77 k ( 8001600 nm , dss - iga020l ; jobin - yvon ) , 
 or nir pmts ( 9501450 nm , h10330 - 45 ; 9501650 nm , h10330 - 75 ; 
 hamamatsu ) . luminescence lifetimes were determined under excitation at 355 nm 
 provided by a nd : yag laser ( yg 980 ; quantel ) , while the signal was 
 detected in the nir by the aforementioned pmt ( h10330 - 75 ) . the output 
 signal from the detectors was then fed to a 500 mhz bandpass digital 
 oscilloscope ( tds 754c ; tektronix ) and then transferred to a pc for 
 treatment with origin 8 . quantum yields in the nir were 
 determined with a fluorolog 3 spectrofluorimeter according to an absolute 
 method using an integration sphere ( gmp sa ) . i ) n - methylmorpholine , 
 ethylchloroformate , 0 c , 20 min ; ii ) nh2oh , 0 c , 1.5 h ; iii ) 1 eq zn(otf)2 , 0.13 eq ln(otf)3 , 2 eq tea , dmf , 19 c , 
 24 h ( otf = triflate ; ln = dy , er , eu , gd , nd , tb , y , yb ) . the organic 
 framework for these mc structures is based on quinaldichydroxamic 
 acid ( quinha ) , which was prepared in one step from commercially available 
 starting materials through an anhydride intermediate . this synthesis proceeded in moderate yield ( 58% ) but proved 
 less tedious than direct esterification or solid - phase 
 methods . the complete deprotonation of 
 quinha resulted in the formation of a tetradentate dianionic ligand 
 whose charge complements zn in mc assembly . the reaction 
 of quinha with zinc(ii ) triflate , a lanthanide(iii ) triflate , and 
 triethylamine ( tea ) in dimethylformamide ( dmf ) produced a mc of the 
 form ln[12-mczn(ii),quinha-4]2[24-mczn(ii),quinha-8](otf)3(dmf)8-x(h2o)x , where 
 the lanthanide ion is sandwiched between two [ 12-mczn(ii),quinha-4 ] and further encapsulated by a [ 24-mczn(ii),quinha-8 ] 
 ( scheme 1 ) . a highly pure product was crystallized 
 by slow evaporation of the solvent and isolated as thin yellow plates . these crystals were , however , of insufficient quality for x - ray diffraction ; 
 an alternative crystallization method was developed for crystal structure 
 determination , as discussed below . a relatively modest paramagnetic 
 shift was observed in h nmr spectra ( y vs 
 eu ) ( figures s1 and s2 ) , indicating 
 that the protons of the ligand are sufficiently sequestered from interaction 
 with the ln ion . this result is qualitative evidence 
 that the ln ion is far enough from neighboring high energy 
 c  h oscillators that would quench lanthanide luminescence . it was expected that the mc structure would contain eight solvent 
 molecules coordinated to zn atoms of the [ 24-mczn(ii),quinha-8 ] . h nmr data ( ln = y , yb , eu ) confirmed the presence of approximately three 
 dmf per mc and an excess of water molecules ; therefore , we assumed 
 that five water molecules are coordinated to the zn metal 
 ions . the addition of this number of solvent molecules to the calculated 
 chemical composition produced an excellent match to experimental chn 
 elemental analyses . yellow block crystals of dy[zn(ii)mcquinha ] suitable for x - ray analysis were grown by slow 
 diffusion of ethyl acetate into a dmf : pyridine ( 2:1 , v : v ) solution 
 of the triflate salt . the obtained asymmetric unit contains two independent 
 one - quarter cations of [ dy[12-mczn(ii),quinha-4]2[24-mczn(ii),quinha-8](py)8 ] , 
 with the crystallographic 4-fold axis passing through the dy ions . the cation has overall 4-fold ( s8 ) symmetry ( figure 1 ) and can be considered to consist of three layers : 
 a large radius [ 24-mczn(ii),quinha-8](py)8 ring 
 ( figure 2a ) sandwiched between two smaller 
 concave [ 12-mczn(ii),quinha-4 ] rings ( figure 2b ) . figure 2a indicates the alternating 
 bridging that connects the central metallacrown to the two outer ones . the interior dy ion is bonded to four hydroximate oxygens 
 of each of the outer two metallacrowns , giving it an overall square 
 antiprismatic coordination geometry . the coordination geometry of 
 the zn ions varies between the [ 12-mczn(ii),quinha-4 ] and [ 24-mczn(ii),quinha-8 ] , demonstrating a structural 
 flexibility of zn that we intended to take advantage 
 of . the zn ions in the [ 12-mczn(ii),quinha-4 ] moieties have a distorted square pyramidal geometry , and the 
 hydroximate oxygen atoms are coordinated to the dy ion 
 ( figure 2d ) . the zn ions in the 
 [ 24-mczn(ii),quinha-8 ] moiety , however , are octahedral 
 with alternating / absolute stereochemistry and one 
 coordinated pyridine solvent molecule ( figure 2c ) . the two types of mc structures are linked together by the carbonyl 
 and hydroximate oxygen atoms of [ 12-mczn(ii),quinha-4 ] 
 and [ 24-mczn(ii),quinha-8 ] , respectively , as indicated 
 by the color scheme depicted in figure 2c - d . comparison of this structure to the previously reported [ tb[12-mczn(ii),picha-4]2[24-mczn(ii),picha-8](py)8 ] and [ dy[12-mczn(ii),picha-4]2[24-mczn(ii),picha-8](py)8 ] reveals no significant deviation in coordination geometry around 
 the ln ion . this evidence proves our assertion that a 
 change in ligand has no effect on the ln ion s 
 first coordination sphere , thereby presenting a platform with tunable 
 absorptivity through the choice of organic chromophore . h oscillators ( x = c , n , o ) are a major source 
 of vibrational deactivation of lanthanide luminescence . therefore , 
 an important parameter to be gleaned from the crystal structure data 
 is the shortest dy  ch distance , which was found to be 7.0  . this value represents an improvement over our previously reported 
 complexes by 0.3  ( ca . an apparent -stacking interaction 
 between the organic moieties of the [ 12-mczn(ii),quinha-4 ] and [ 24-mczn(ii),quinha-8 ] was observed . 4  apart with an alignment that 
 deviates slightly from ideal parallelism ( ca . this short 
 distance opens the possibility for intramolecular through - space inter- 
 or intra ligand charge transfer ( ilct ) transitions discussed below . the three 
 layers of dy[zn(ii)mcquinha ] are 
 shown with 50% thermal displacement parameters and a partial numbering 
 scheme . ( a ) depiction of the central large crown , [ 24-mczn(ii),quinha-8](py)8 , and the position of the nonbonded 
 central dy . the dashed lines indicate how the central 
 crown bridges to the smaller , capping , concave crowns . ( b ) one of the capping crowns of [ 12-mczn(ii),quinha-4 ] 
 as well as the coordination to dy . coordination geometry 
 of zn in the ( c ) [ 24-mczn(ii),quinha-8 ] and ( d ) [ 12-mczn(ii),quinha-4 ] structures 
 indicates that the two distinct mc rings are linked through the [ 12-mczn(ii),quinha-4 ] carboxylate oxygen ( blue ) and 
 the [ 24-mczn(ii),quinha-8 ] hydroximate oxygen ( red ) atoms . the ln ion is bound to the [ 12-mczn(ii),quinha-4 ] hydroximate oxygen atoms ( purple ) . the ligand 
 quinha exhibits 
 several absorption bands in the uv region with apparent maxima centered 
 at 238 and 300 nm resulting from * transitions 
 located on the quinoline moiety ; the long wavelength absorption trails 
 to 340 nm ( figure 3 ) . upon deprotonation and 
 formation of the ln[zn(ii)mcquinha ] , a new broad absorption 
 band appears in the near - visible region with an apparent maximum at 
 380 nm (   5.5  10 m cm ) and extends to 470 
 nm . this band accounts for the yellow color of ln[zn(ii)mcquinha ] in both solid state and in solution . to ensure 
 that this near - visible absorption band was not due to the electronic 
 structure of the deprotonated ligand alone , quinha was exposed to 
 several equivalents of tea and the absorption spectra remained unchanged 
 ( figure s4 ) therefore , one can reasonably 
 assume , given that discrete zncomplexes typically do 
 not participate in either metal - to - ligand charge transfer ( mlct ) or 
 ligand - to - metal charge transfer ( lmct ) transitions and absorption 
 spectra are independent of the choice of the ln ion , 
 that the near - visible band can be attributed to the presence of an 
 ilct state located at lower energy and caused by ligand charge redistribution 
 which is unique to the quinha ligand . a near - visible zn lmct ( 4s ) transition has been observed for bulk semiconductor 
 zno and zno colloidal crystals at ca . 365 nm ; however , we would expect this transition to occur at much higher 
 energy for the complexes being discussed here . in the case of yb[zn(ii)mcquinha ] , a broad and very weak (  57 
 m cm ) band in the range 9101000 
 nm assigned to an f - f absorption ( f5/2f7/2 ) was detected ( figure 
 s5 ) . ( left ) absorption spectra of ln[zn(ii)mcquinha ] complexes ( 2.04  10 m , ln = gd , nd , yb , er ) and quinha ( 2.42  10 m ) in methanol at 298 k. for the sake of comparison absorption spectrum 
 of quinha is multiplied by 16 times . ( right ) luminescence 
 spectra of gd[zn(ii)mcquinha ] ( ex = 340 nm ) and quinha ( ex = 290 nm ) at 298 k ( black ) , at 77 k without delay ( red ) and with 
 100 s delay after the excitation flash ( blue ) . the energy gap between ground 
 and excited states for gd is very high in energy , and , 
 therefore , it is impossible for the 
 gd ion in the corresponding mcs to be sensitized through 
 energy transfer from the ligand triplet / ilct states . this characteristic 
 can be used to our advantage to determine the energy levels of the 
 mc complexes in the absence of energy transfer to the ln ion ( but with the electronic state of the sensitizer being exposed 
 to the electron deficient lanthanide cations ) . gd[zn(ii)mcquinha ] was exposed to uv excitation through the ilct 
 band at 340 nm at room temperature ( figure 3 ) . we observed a weak broad - band emission with an apparent maximum 
 located at 580590 nm . the position of this band is insensitive 
 to the excitation wavelength which was varied from 290 to 380 nm indicating 
 that the energy is following the same path independently of the excitation 
 wavelength . lowering the temperature to 77 k led to a slight sharpening 
 of the band and a shift of the maximum to an apparent maximum localized 
 between 545 and 555 nm . a further blue shift toward 510 nm was observed 
 upon application of a time delay of 100 s after the excitation 
 flash . this observation is rather unusual because , in general , one 
 would expect a significant red shift of the phosphorescence band compared 
 to the fluorescence band . thus , we can hypothesize that the broad - band 
 green emission of the gd[zn(ii)mcquinha ] might 
 be arising from a charge - transfer state , while emission from the singlet 
 state ( fluorescence ) is quenched by self - absorption . in order to monitor 
 the position of the triplet state in the absence of an ilct state , 
 we investigated emission properties of the ligand quinha alone . upon 
 uv excitation at 290 nm at room temperature , quinha k , a strong green luminescence band 
 centered at 510 nm and a weaker intensity band in the 325460 
 nm range can be observed . the latter band disappears upon enforcing 
 a delay of 100 s after the flash so it can be assigned to short - lived 
 fluorescence , while the green emission is phosphorescence originating 
 from the triplet state . the electronic envelope and vibrational splitting 
 of the phosphorescence spectra of the gd[zn(ii)mcquinha ] and the quinha are nearly identical . therefore , the energy of the 
 triplet state of the ligand , determined from the 00 transition , 
 is estimated to be located between 20 920 and 21 000 cm ( 476478 nm ) . the position of the lowest excited singlet 
 and the ilct states were determined from the intersection of the absorption 
 and emission ( fluorescence ) spectra of quinha and gd[zn(ii)mcquinha ] , respectively , and found to be located at 29 
 850 cm ( 335 nm ) and 21 560 cm ( 464 nm ) . a proposed schematic diagram of the ligand s energy 
 levels with respect to those of yb , nd , 
 and er ions and possible energy migration paths are sketched 
 in figure 4 . an appearance of the ilct state 
 allows shifting excitation toward longer wavelengths ; moreover , ilct 
 states are known to participate in sensitization of lanthanide luminescence . however , it should be noted that the energies 
 of the ilct and the triplet states are very similar in the investigated 
 compounds , which can create additional ways for dissipation of the 
 excitation energy through energy resonance . luminescence spectra 
 were recorded for ln[zn(ii)mcquinha]complexes ( ln = yb , nd , er ) in the solid 
 state and in solution ( ch3oh / cd3od ) using 370420 
 nm photons as excitation . complexes of the visible emitting ln ions ( ln = dy , eu , tb ) did not display 
 any detectable lanthanide - based luminescence , which might be attributed 
 to an insufficient sensitization due to closed positions of the ligand 
 and ln energy levels in case of dy ( f9/2 , 21 100 cm ) and tb ( d4 , 20 500 cm ) or a presence of an additional nonradiative deactivation pathway 
 through ligand - to - metal charge transfer states for eu . the nir emitting mcs exhibit intense lanthanide characteristic 
 emission arising from the f5/2f7/2 , f3/2 ij ( j = 9/2 , 
 11/2 , 13/2 ) , and i13/2 i15/2 transitions for the yb , nd , and er compounds , respectively ( figure 5 ) . excitation spectra of all studied mcs are dominated by 
 ligand - centered broad - bands pointing toward efficient sensitization 
 of lanthanide cations operating through the electronic structure of 
 the chromophoric ligands . excitation spectra of nd[zn(ii)mcquinha ] and er[zn(ii)mcquinha ] in the solid state exhibit characteristic sharp intraconfigurational 
 f - f transitions with a lower relative intensity compared to the broad - band 
 ligand centered transitions . excitation spectra of the mcs in diluted 
 methanol solutions ( 0.1 mg / ml ) match well the corresponding absorption 
 spectra ( figure 5 vs figures 
 s7 and s8 ) , while an expansion of the bands toward longer wavelengths 
 is observed with increasing concentration ( 1 mg / ml ) and for solid 
 state samples . this observation may be a result of saturation effects 
 and/or enhanced intermolecular interactions . ( left ) corrected and normalized excitation and ( right ) emission spectra of ln mc complexes in 
 solid state ( ex = 420 nm , solid traces ) and methanol 
 solution ( 1 mg / ml , ex = 370 nm , dashed traces ) . ( top ) yb[zn(ii)mcquinha ] , em = 980 nm ; ( middle ) nd[zn(ii)mcquinha ] , em = 1064 nm ; ( bottom ) er[zn(ii)mcquinha ] , em = 1525 nm . experimental 
 luminescence decays for all studied mcs are best fitted as monoexponential 
 functions , reflecting the presence of only one emissive lanthanide - containing 
 species . when going from solid state samples to methanol solutions , the observed luminescence lifetimes decrease by 1.55-fold , 
 despite the fact that emission spectra are very similar by their bandwidths 
 and crystal - field splitting , indicating that complexes remain intact / undissociated 
 in solution . nevertheless , obs are 
 comparable to the highest values reported to date for lanthanide complexes 
 in protic solvents . in deuterated methanol , luminescence 
 lifetimes of 150.7(2 ) , 4.11(3 ) , and 11.40(3 ) s were observed 
 for yb , nd and er mcs , respectively . this represents a significant increase , by 3.510-fold , compared 
 to samples in protonated solvent and a 23-fold increase relative 
 to solid state samples . calculations using phenomenological equations 
 ( which must be applied carefully , table 1 , 
 footnote ) for yb[zn(ii)mcquinha ] and nd[zn(ii)mcquinha ] complexes are in line 
 with the structural analysis and confirm that there are no solvent 
 molecules bound to the lanthanide ion in the first coordination sphere . 
 therefore , such variations in luminescence lifetimes are the result 
 of enhanced nonradiative deactivation through outer - sphere interactions 
 with solvent molecules . under excitation into the charge - transfer 
 band at 370 nm , luminescence 
 quantum yields vary only slightly between solid state samples and 
 solutions in deuterated methanol for yb[zn(ii)mcquinha ] ( 2.44 vs 2.88% ) , nd[zn(ii)mcquinha ] ( 1.13 vs 1.35% ) , and er[zn(ii)mcquinha ] ( [ 3.64.2]10% ) . the highest reported quantum yields for yb , nd , and ercomplexes containing 
 c  h bonds are < 3.8% , < 0.42% , and < 0.033% , respectively . values for qlnl measured 
 here are the largest values reported to date for nd[zn(ii)mcquinha ] and er[zn(ii)mcquinha ] . the values of qlnl decrease by 436-fold when measured 
 in protonated solvent ( ch3oh ) with the largest impact observed 
 for er[zn(ii)mcquinha ] . this difference can 
 be explained by the influence of outer - sphere o  h vibrations ; 
 the er ion has the highest intrinsic probability among 
 studied ions to be quenched nonradiatively by high - energy vibrations . compared with the family of the previously published nir - emitting 
 mcs , ln[zn(ii)mcpicha ] quantitative characteristics 
 ( luminescence lifetimes and quantum yields ) in the solid state and 
 methanol solutions of ln[zn(ii)mcquinha ] for 
 both nd and yb derivatives are significantly 
 improved ( table 1 ) . moreover , a replacement 
 of picoline- with quinoline - hydroxamic acid allows extension of the 
 absorption up to 450 nm . in order to get additional quantitative 
 information about energy 
 transfer processes in lanthanide complexes , it is crucial to obtain 
 the radiative lifetime ( rad ) , which 
 equates to the lifetime in the absence of any nonradiative transitions , 
 described by eq 1 where qybyb is the intrinsic 
 quantum yield , and sens is the 
 sensitization efficiency of the ligand.1 radiative lifetime is usually ignored by researchers ; however , 
 it is clear from eq 1 that finding ways to modulate 
 this parameter can improve the intrinsic quantum yield , as rad is inversely proportional to qybyb . in the case of yb[zn(ii)mcquinha ] and er[zn(ii)mcquinha ] , luminescence transitions terminate onto the ground level , so the 
 absorption spectrum corresponding to the emission spectrum might be 
 measured and used to calculate radiative lifetimes with the help of 
 the modified einstein s equation ( eqs 2a-2b ) where c is the speed 
 of light in centimeters per second , n is refractive 
 index ( nch3oh = 1.329 ) , na is avogadro s number , j and j are the quantum numbers for the ground and excited 
 states , respectively , ()d is the integrated spectrum of the f - f 
 transition , and m is the barycenter 
 of the transition.2a2b it is known that f - f transitions have very low molar absorption 
 coefficients , and we only could succeed in measuring the absorption 
 spectrum of the f5/2f7/2 transition in yb[zn(ii)mcquinha ] ( figure s5 ) . a value of rad = 0.68(7)ms was found for yb[zn(ii)mcquinha ] , which is shorter than the radiative lifetimes reported 
 for yb(diethylenetriaminepentaacetate ) and yb(dipicolinate ) complexes 
 ( 1.21.3 ms ) or for yb with terphenyl - based 
 ligands , 2 ms , but close to the 
 values found for yb complexes with benzoxazole - substituted 
 8-hydroxyquinolines ( 0.70.75 ms ) which exhibit high quantum 
 yields . using eq 1 and taking into account obs and qlnl of yb[zn(ii)mcquinha ] in methanol ( table 1 ) , we have been able to obtain quite modest values 
 of the intrinsic quantum yield , 2.2(3)% , and of the sensitization 
 efficiency of the ligand , 12(3)% . the latter might be the result of 
 the relatively long distance between the lanthanide cation and the 
 quinha chromophore . nevertheless , in protic solvents the main path 
 of energy loss remains the nonradiative deactivation through o data for 298 k. 
 standard deviation 
 ( 2 ) between parentheses ; estimated relative errors : obs , 2% ; qlnl , 10% . under excitation at 355 nm . the inner sphere hydration 
 numbers 
 were calculated according to the following equations : qyb = 2  ( kch3oh  kcd3od  0.1 ) ( in s ) and qnd = 290  ( kch3oh  kcd3od )  0.4 ( in ns ) . one major advantage of 
 ln based luminescent reporters , 
 as mentioned above , is their excellent photostability . in order to 
 compare the photostability of our system to the behavior of an organic 
 fluorophore , yb[zn(ii)mcquinha ] , nd[zn(ii)mcquinha ] , and an organic cyanine dye ( 3,3-diethylthiadicarbocyanine 
 iodide ) were irradiated at 400 or 610 nm , respectively , and luminescence 
 intensity of the lanthanide signal ( 980 or 1064 nm ) or corresponding 
 organic emission ( 670 nm ) bands were recorded over an extended period 
 of time ( t > 120 min ) . over the course of this 
 experiment , 
 yb and nd luminescence intensity remained 
 unchanged for the corresponding yb[zn(ii)mcquinha ] and nd[zn(ii)mcquinha ] complexes , whereas the 
 nir cyanine dye suffered from more than a 5-fold reduction in luminescence 
 intensity over the same period of time ( figure 
 s9 ) . this result highlights the favorability of this and other 
 luminescent ln based systems over de facto organic chromophores . the goal of this work was to improve the photophysical 
 properties 
 of near - infrared emitting lanthanide - based metallacrowns in order 
 to broaden the applicability of this class of compounds . in particular , 
 we would like to demonstrate that the versatility of mc strategy allows 
 shifting the excitation wavelength toward lower energy through appropriate 
 design of the chromophore and enhancing the quantitative luminescence 
 parameters without affecting the global mc structure . we have 
 reported here a series of second generation lanthanide 
  encapsulated sandwich mc complexes with the general 
 formula ln[12-mczn(ii),quinha-4]2[24-mczn(ii),quinha-8 ] ( ln[zn(ii)mcquinha ] , ln = y , nd , eu , gd , tb , dy , er , yb ) in which the mc 
 framework is formed by the self - assembled organization of zn ions and a tetradentate chromophoric ligand based on quinaldichydroxamic 
 acid ( quinha ) . in comparison to the previously published lanthanide - based 
 mcs , the change of picoline hydroxamic acid ( picha ) to quinha did 
 not lead to significant changes in the coordination of the zinc and 
 lanthanide metal ions and to the general arrangement of the chromophoric 
 moieties , allowing the system to fully benefit from the unique mc 
 structure . indeed , nonradiative deactivation originating from overtones 
 vibrations often play a key role in the quenching of nir - emitting 
 lanthanide cations in complexes formed with organic chromophoric ligands 
 that transfer energy to the luminescent ion . this type of quenching 
 limits the photonic output of the complexes and their use in practical 
 applications . the unique encapsulated sandwich metallacrown structure 
 allows one to set lanthanide ions at a predetermined and relatively 
 shielded position so that the closest c  h group is located 
 at a fixed and constant 7  distance . photophysical properties 
 of ln[zn(ii)mcquinha ] ( ln = yb , nd , er ) in the solid state confirmed 
 the efficiency of this approach that imply long luminescence lifetimes 
 and the highest quantum yield values reported to date for nd and ercomplexes with c  h bonds . in addition , 
 unique to these particular mc complexes is the important absorption 
 over a broad range of uv visible wavelengths due to ligand - based 
 transitions , including inter- and/or intraligand ct states . the latter 
 may act as an efficient way of sensitization of nir lanthanide - centered 
 luminescence with high overall efficiency (   qlnl ) at 380 nm : 137/1584 for yb , 
 209/742 for nd , and 0.54/19.8 for er mcs 
 in ch3oh / cd3od solutions . however , a comparison 
 of photophysical properties of ln[zn(ii)mcquinha ] in the solid state and in solution indicates that the present mc 
 structure still does not fully protect lanthanide ions from outer - sphere 
 interactions with solvent molecules . nevertheless , compared to the 
 previously published ln[zn(ii)mcpicha ] quantitative 
 characteristics ( luminescence lifetimes and quantum yields ) in solid 
 state and methanol solutions of ln[zn(ii)mcquinha ] for both nd and yb derivatives are significantly 
 improved ( table 1 ) . moreover , a replacement 
 of picoline- with quinoline - hydroxamic acid allows one to extend absorption 
 up to 450 nm . thus , the versatility of the mc strategy opens multiple 
 possibilities to improve the current performance further and make 
 the reported lanthanide mc complexes promising candidates for a number 
 of luminescence applications .
near - infrared 
 ( nir ) luminescent lanthanide complexes hold great 
 promise for practical applications , as their optical properties have 
 several complementary advantages over organic fluorophores and semiconductor 
 nanoparticles . 
 the fundamental challenge for lanthanide luminescence 
 is their sensitization through suitable chromophores . 
 the use of the 
 metallacrown ( mc ) motif is an innovative strategy to arrange several 
 organic sensitizers at a well - controlled distance from a lanthanide 
 cation . 
 herein we report a series of lanthanide  encapsulated 
 sandwich 
 mc complexes of the form ln3+[12-mczn(ii),quinha-4]2[24-mczn(ii),quinha-8 ] 
 ( ln3+[zn(ii)mcquinha ] ) in which the mc framework is formed by 
 the self - assembly of zn2 + ions and tetradentate chromophoric 
 ligands based on quinaldichydroxamic acid ( quinha ) . a first - generation 
 of luminescent mcs was presented previously but was limited due to 
 excitation wavelengths in the uv . 
 we report here that through the 
 design of the chromophore of the mc assembly , we have significantly 
 shifted the absorption wavelength toward lower energy ( 450 nm ) . in 
 addition to this near - visible inter- and/or intraligand charge transfer 
 absorption , ln3+[zn(ii)mcquinha ] exhibits remarkably high quantum 
 yields , long luminescence lifetimes ( cd3od ; yb3 + , qlnl = 2.88(2)% , obs = 150.7(2 ) 
 s ; nd3 + , qlnl = 1.35(1)% , obs = 4.11(3 ) s ; er3 + , qlnl = 3.60(6)102% , obs = 11.40(3 ) 
 s ) , and excellent photostability . 
 quantum yields of nd3 + and er3 + mcs in the solid state and in deuterated 
 solvents , upon excitation at low energy , are the highest values among 
 nir - emitting lanthanide complexes containing c  h bonds . 
 the 
 versatility of the mc strategy allows modifications in the excitation 
 wavelength and absorptivity through the appropriate design of the 
 ligand sensitizer , providing a highly efficient platform with tunable 
 properties .