photoexcitation of 
 molecules has been of major interest for the 
 last 30 years due to their potential applications in solar energy 
 conversion and storage , chemical sensing , photocatalysis and molecular 
 devices . in this respect , [ cu(nn)2 ] diimine 
 coordination complexes and their photochemical and photophysical properties 
 have been investigated in extensive detail , the first and significant 
 contributions coming from the mcmillin group . these 3d transition metal systems are preferred over well - known 
 ruthenium(ii ) , rhenium(i ) , and osmium(ii ) systems , in that they are 
 economically more viable . the structural aspects of ligands for long - lived excited states 
 in solution , and thus more useful for photoinduced electron and energy 
 transfer , have been investigated . in homoleptic cu(i ) polypyridine 
 complexes , sufficiently bulky ligand substituents at the 2- and 9- 
 positions were found to be required for luminescence , with their lifetimes 
 being ligand , concentration and solvent dependent . [ cu(dmp)2 ] ( dmp = 2,9-dimethyl-1,10-phenanthroline ) 
 is one of the most studied cuprous diimine compounds and its mechanism 
 of generating the mlct state is generally accepted . the cu ground state has a d electron configuration , with a slightly distorted tetrahedral geometry 
 ( c2 ) . absorption of a visible photon promotes an electron 
 from the cu center to the dmp ligands ( to one or spread 
 over both as recently has been suggested ) , generating a franck - cordon 
 mlct excited state with a cu * center . the cu * d center is susceptible to a jahn  teller distortion 
 resulting in an mlct excited state with flattened tetrahedral coordination . the large shift observed between absorption and photoluminescence 
 is consistent with significant structural changes in the flattened 
 mlct excited state that either returns to the ground state via a radiative 
 decay pathway or forms a pentacoordinate complex with strong lewis 
 basic solvents , resulting in exciplex quenching in the ligated mlct 
 state . up until a few years ago , structural information on the mlct 
 state of [ cu(dmp)2 ] was mostly 
 indirect and derived from the strong correlation between accessibility 
 of the copper center to the lewis base and the luminescence lifetimes . in the last ten years , the group of chen et al . has developed 
 and used time - resolved x - ray techniques to study these cu(i ) systems 
 and confirm the formation of an exciplex using extended x - ray absorption 
 fine structure ( exafs ) results and x - ray absorption near edge structure 
 spectroscopy ( xanes ) , providing direct evidence for a five - coordinate 
 species ( by coordination of a solvent molecule or counterion ) upon 
 excitation in both poorly coordinating toluene and strongly coordinating 
 acetonitrile , with the observed interaction being stronger in the 
 latter one . the x - ray absorption spectroscopy ( xas ) pump  probe 
 studies of photoexcited transition metal complexes , with pulsed laser 
 pump and pulsed x - ray probe were performed using a scanning monochromator 
 and a 100 ps time resolution arising from the electron bunch width 
 of the synchrotron source ( advanced photon source ) . the technique 
 is of great current interest , with major current developments being 
 carried out at the aps and sls . each of these facilities utilizes a scanning 
 monochromator maintained at fixed energy with a time series built 
 up by repeat acquisitions at each point in a time and energy 2-dimensional 
 grid , giving experimental acquisition times of up to 40 h ( with a 
 1 khz laser ) , for a good signal - to - noise spectrum . here , we present the first results obtained using an energy dispersive 
 data acquisition approach . first , the x - ray 
 beam is focused so that it interrogates only within the laser irradiated 
 volume . second , the multiplexing intrinsic to an energy dispersive 
 measurement provides the entire xafs spectrum synchronously , and so 
 has the potential to significantly reduce the total acquisition time . this is provisional upon the followin points:(i)the flux of the polychromatic beam 
 for the bandwidth corresponding to the measured spectrum is not significantly 
 less than that of the monochromatic beam.(ii)the gating and repetition rates of 
 the detector are sufficiently fast . the flux of the polychromatic beam 
 for the bandwidth corresponding to the measured spectrum is not significantly 
 less than that of the monochromatic beam . this 
 had been achieved for stopped flow experiments at id24 with 
 experiments on cu complexes in solution on a millisecond time scale 
 using ccd - based detectors . also , a high 
 degree of linearity is required to successfully derive difference 
 spectra between light - on and light - off . what is of final importance 
 for energy dispersive experiments is the relatively high concentration 
 of the species under investigation required , since experiments need 
 to be performed in transmission mode , a factor which can clearly influence 
 the type and properties of excited states . in this paper the 
 energy dispersive approach is demonstrated on 
 the well - known [ cu(dmp)2 ] system 
 as was described above and the xanes data obtained . in addition , a 
 larger ligand system [ cu(dbtmp)2 ] ( dbtmp = 2,9-di - n - butyl-3,4,7,8-tetramethyl-1,10-phenanthroline ) having a much longer lifetime compared to the 
 dmp complex ( i.e. , 181 vs 1.8 ns in acetonitrile for degassed samples 
 at 15.5 mm and 20 mm respectively ) was investigated . the synthesis of the [ cu(dmp)2]x ( dmp = 2,9-dimethyl-1,10-phenanthroline , 
 x = pf6 and [ b(c6f5)4 ] ) 
 and the [ cu(dbtmp)2]y ( dbtmp = 2,9-di - n - butyl-3,4,7,8-tetramethyl-1,10-phenanthroline , y = cl ) 
 were carried out according to literature procedures . the experiments were 
 performed at the energy dispersive xas beamline id24 of the esrf , 
 grenoble , france . experiments were performed in the 4-bunch mode , 
 with 10 ma per bunch ( compared to uniform filling mode of 200 ma ) 
 giving 10 photons per single bunch in the bandwidth 
 used in 100 ps with a 700 ns interval . a 10 hz quantel brilliant q - switched 
 nd : yag laser , with a pulse width of 3 ns ( fwhm ) , was used . laser excitation 
 was performed at 450 nm , with an opo power of 12 mj per pulse . xafs 
 measurements , in transmission mode , with a si(111 ) polychromator in 
 a bragg geometry , using a ge microstrip detector ( xh ) consisting of 
 1024 elements on a 50 m pitch , which was rapid enough in its 
 acquisition time to isolate an individual electron bunch by parallelization 
 of the charge integrating preamplifiers ( 128 element xchips ) . the detector was time - windowed ( 500 ns integration 
 time ) around the electron bunch , achieving a better than 100 khz repetition 
 rate . initial synchronization was achieved using a hamamatsu s2383 
 silicon apd which was sensitive to x - ray and visible light linked 
 to a fast oscilloscope . the esrf machine clock signal ( 352.2 mhz ) 
 was used as a timing basis to trigger the xh detector , and the flashlamp 
 and q - switch of the laser to vary the delay between excitation and 
 recording . the x - ray spot was tuned to 5 ( horizontal )  100 ( vertical ) 
 m and its center positioned 15 m from laser entrance 
 window . alternating light - on and light - off measurements were be taken 
 to minimize the effects of beam movement , and providing direct xas 
 difference - spectra . in this experiment we have focused the polychromator 
 around the xanes area and no high quality long - range exafs data , suitable 
 for analysis , has been obtained . the energy dispersive data , as obtained 
 in absorption as a function of pixel number , i.e. position on the 
 detector , was energy calibrated using a reference cu foil . each 
 differential spectrum presented in this paper is an average of 37 
 spectra , each spectrum obtained from 10 pairs of light - on and light - off 
 recordings ( i.e. , 10 difference spectra ) of 100 accumulations ( i.e. , 
 an average of a 100 individual difference spectra ) each . each set of 10 pairs could be repeated at 10 min intervals , with 
 total experimental time of 2 h. the [ cu(dmp)2]pf6 and the [ cu(dbtmp)2]cl were measured as 20 and 15.5 mm respectively 
 in acetonitrile . the solutions were measured in a continuous flow 
 cell with quartz windows for the laser excitation and pyrolytic graphite 
 windows for the x - ray transmission perpendicular to that . the solutions 
 were pumped continuously to refresh the solution from a large container 
 to minimize sample oxidation and radiation damage . the cell path length 
 was 3 mm for x - radiation that was located 15 m from the end 
 of the quartz rod on the laser inlet . the core shells of all atoms ( except hydrogen ) were treated with 
 the frozen core approximation . the valence 
 atomic orbitals were described by triple  slater - type basis 
 set with one polarization function for cu atoms and double  
 basis also with one polarization function for other atoms . for the 
 ground and flattened excited states , the geometry was restricted to 
 the c2 point group and the structural 
 optimization for exciplex was performed without any symmetry constrains . geometry convergence was considered reached when the cartesian length 
 changes less than 0.01  and cartesian gradients fell below the 
 threshold of 0.01 hartree /  . xanes calculations have been performed 
 on the basis of slater - type orbitals calculated self - consistently 
 using adf2010 package . nonrelativistic spin - unrestricted calculations 
 for singlet and triplet states of the complex were performed . a quadruple- intensities of 1s - to - unoccupied 
 states xas transitions were calculated by integration of dipole transition 
 matrix elements between 1s - originated mos and unoccupied mos . integration 
 has been performed on a 3d spatial cubic grid in close proximity to 
 the absorbing cu atom with 132651 points and 0.01  step size 
 in each direction . simulation of the xanes spectral region ( 50 
 ev ) requires integration for 800 lowest unoccupied mos . finally , 
 lorentzian broadening of this discrete spectrum was performed using 
 the energy - dependent arctangent model . this accounted for the finite 
 mean free path of the photoelectron , the core hole lifetime broadening 
 ( 2 ev ) , and the polychromator resolution 1.4 ev ) . the exchange - correlation 
 potential with parametrization by vosko , wilk , and nusair calculated within local density approximation 
 ( lda ) was used . the xanes 
 experiments are performed at significantly higher concentration 
 than conventional photophysical measurements . [ cu(dmp)2 ] is nonemissive under these conditions and we 
 have therefore performed time - resolved transient absorption ( ta ) measurements 
 at the higher concentration ( 20 mm in degassed acetonitrile ) . photolysis 
 of [ cu(dmp)2 ] , as the barf salt 
 ( barf = [ b(c6f5)4 ] ) , at 355 nm clearly leads to bleaching of the ground - state absorption 
 near 455 nm and production of a lower 
 energy transient peak at 520 and 560 nm due to formation of 
 the mlct excited state . the lifetime of this state under 
 these conditions is 1.84 ( 0.1 ) ns . ns - ta spectra of [ cu(dmp)2](barf ) in ch3cn 
 solution ( 20 mm ) after 355 nm laser excitation . inset : kinetics at 
 559 nm of the degassed sample , fitted to an exponential decay . to perform the x - ray experiments , first the [ cu(dmp)2 ] ground state 
 system was measured using normal 
 ed - xas . the spectrum is shown in figure 2a 
 and is identical to the cu k - edge xanes as previously reported ; figure 2b shows the acquired 
 cu k - edge xanes difference spectrum . before excitation ( 5 
 ns ) and 5 ns after excitation , there is no difference spectrum observed 
 confirming the lifetime to be a maximum of 5 ns . at the laser excitation , however , a significant difference spectrum is observed , indicating 
 the difference in species between ground state and after activation 
 ( excited state species ) . by comparison of the obtained data with the 
 data as presented by l. x. chen , reported 
 then to require 40 h using the scanning approach , similar data quality 
 has been derived and the same difference spectrum recorded . it has 
 to be noted here however that recent upgrades at the aps have reduced the acquisition time scale considerably , 
 for similar cu systems to the order of 12 h. moreover , the 
 experiments by chen et al . are performed 
 at 2 mm cu concentration , whereas a concentration of 20 mm was utilized 
 to afford good ed - xas data . the 2 h data acquisition time 
 in the energy dispersive approach as presented here is mainly limited 
 by the laser frequency being 10 hz . in the future , using a khz laser , 
 we should be able to reduce this to a few minutes total data acquisition 
 at most . cu k - edge xanes spectra for ( a ) [ cu(dmp)2]pf6 in ch3cn ( 20 mm ) . ( b ) non - normalized differential 
 spectrum ( excited state - ground state xanes ) for [ cu(dmp)2]pf6 in ch3cn ( 20 mm ) . this complex contains a bulkier ligand 
 system , retaining the 2,9-substitution 
 of the dmp complex but extending the methyl groups to butyl chains . this substitution restricts deactivation of the excited state by restricting 
 additional coordination to the metal center , meaning that the luminescence 
 lifetime of [ cu(dbtmp)2 ] in deoxygenated 
 acetonitrile at low concentration ( typically 10 m ) is reported to be 440 ns . given the 
 significantly higher concentrations required for the x - ray experiments , 
 we investigated the excited state behavior of the complex by transient 
 absorption spectroscopy ( figure 3 ) . photolysis 
 of [ cu(dbtmp)2 ] in acetonitrile 
 at concentrations between 3 and 15.5 mm all afforded spectra with 
 evidence of ground - state bleaching and production of transient peaks 
 at 524 and 565 nm due to the formation of the mlct state . the lifetime of the 3 mm deoxygenated sample was found to be 299 ns , 
 decreasing to 181 ns at 15.5 mm concentration , significantly shorter 
 than the lifetime observed for the complex [ cu(dtbp)2 ] ( 1.9 s , dtbp = 2,9-di - tert - butyl-1,10-phenanthroline ) 
 with tert - butyl groups ortho to the coordinated nitrogens . on oxygenating the sample by bubbling 
 with air , the lifetime further decreases to only 48 ns at 15.5 mm , 
 a full order of magnitude shorter than the low concentration , deoxygenated 
 sample . this is entirely consistent with both self - quenching at high 
 concentration and quenching of the excited state by oxygen in solution . 
 quenching by anion coordination has been reported for salts on [ cu(dmp)2 ] , but this effect 
 is much reduced for [ cu(ph2-phen)2 ] ( ph2-phen = 2,9-diphenyl-1,10-phenanthroline ) . chloride will subtend a larger cone angle at 
 copper than acetonitrile and so the o - butyl groups 
 in [ cu(dtbp)2 ] may resist its coordination , 
 and so the nature of the concentration quenching is uncertain . ns - ta spectra 
 of [ cu(dbtmp)2]cl in ch3cn 
 solution ( 15.5 mm ) after 355 nm laser excitation . inset : kinetics 
 at 565 nm of the degassed ( circles ) and aerated ( squares ) sample , fitted to exponential kinetics ( solid lines ) . the ground state cu k - edge xanes spectrum is very similar 
 to that 
 of the dmp complex , indicating a similar cu(i ) geometry . cu k - edge xanes differential spectrum ( excited state - 
 ground state 
 xanes ) spectrum for [ cu(dbtmp)2]cl in ch3cn ( 15.5 mm ) . the decay of the excited state of cu(dbtmp)2 ] was followed using the same ede - pump  probe - xafs 
 method , measured at time - intervals of 10 ns , from 10 ns before laser 
 excitation until 100 ns after laser excitation . the excited state 
 signal clearly decreased over this time . to further quantify this , the areas of the first difference - xanes feature at 8978 ev 
 were estimated by fitting a simple gaussian between 8970 and 8990 
 ev , and plotting the area as a function of time , see figure 5 . the reason that the amount of excited state species at t = 0 ns is nonzero is due to the fact that t = 0 
 ns is measured during the laser pump , not at the 
 end of the laser pump , where the amount of excited state species is 
 still rising . the lifetime ( 27 ns ) , as compared to figure 3 , indicates that there was considerable aeration 
 of this solution . differential spectra peak areas for [ cu(dbtmp)2]cl complex in acetonitrile ( 15.5 mm ) as a function 
 of time , after excitation . these have been performed for two 
 models of [ cu(dbp)2 ] ( dbp = 2,9-di - n - butyl-1,10-phenanthroline ) obtained using dft optimization . the 
 first model assumed only flattening distortions of the complex in 
 the mlct state while the second one corresponds to the exciplex state 
 with the solvent molecule coordinating the metal ion . formation of 
 the exciplex for this complex with bulky groups that protect the metal 
 center from the solvent might be considered less probable in comparison 
 with [ cu(dmp)2 ] , since it requires 
 significant reorganization of the ligand . theoretical difference spectra ( red , solid ) between cu 
 k - edge xanes 
 of photoexcited and ground state [ cu(dbp)2 ] , compared to the experimental difference spectra of [ cu(dbtmp)2 ] ( black , dashed ) . theoretical xanes spectra of [ cu(dbp)2 ] in the ground state ( black , solid ) and for two 
 models of 
 the excited state with flattening distortions only ( blue , dotted ) 
 and with exciplex complex ( red , dashed ) with significant rocking distortions . as one can see from figure 7 , the theoretical 
 spectra of excited state species look very different . it can readily 
 be seen from the difference spectra ( figure 6 ) that the agreement to experiment is much closer for the flattened 
 structure . this model with flattening distortions has the same features 
 as the ground state spectrum , but all of them are shifted to higher 
 energy due to the changes of the core level as a result of the oxidation 
 state change . moreover maximum a became more intense while peak b 
 is less pronounced on transient oxidation of the cu center . peak p1 
 that was shifted in the previous work now is in better agreement with the experiment . the use of molecular 
 orbitals to calculate xanes allowed us to clarify the origin of this 
 feature . in figure 8 , we have shown molecular 
 orbitals ( mos ) that contribute to the peak p1 in the ground state . 
 they have significant p - character around cu , oriented between the 
 planes of the ligands and deformed by the butyl tails . additionally 
  orbitals of c and n atoms of one of the ligands contributes 
 to these mos ; a similar qualitative interpretation that the 4pz orbital of cu produces a shoulder on the rising 
 edge has been proposed by solomon . an 
 analogous feature exists in the excited state spectrum for the flattened 
 geometry . the negative peak in the difference spectrum within this 
 model is related to the shift of the edge . on the contrary , in the 
 exciplex model peaks the main maximum a calculated for the exciplex is too intense and 
 shifted to lower energy , rather than shifted to higher energy according 
 to the experimental data . as a result in the difference spectrum there is positive peak at relative energy 68 ev that is negative 
 in the experiment . in the difference spectrum for this model 
 one can note a very small positive peak that theory predicts at the 
 relative energy -7 ev . this peak is related to the depopulation of 
 the mo ( figure 6 , bottom ) with significant 
 cu molecular orbitals that 
 contributes to the peak p1 of the [ cu(dbp)2 ] in the ground state ( top and 
 middle ) and to the pre - e dge peak at 7 ev of the flattened 
 model of the excited state ( bottom ) . thus , we can conclude that xanes has sufficient sensitivity 
 to 
 distinguish exciplex and flattening models for the mlct excited states 
 of cu complexes . good agreement between theory and experiment for 
 the flattening model indicates that this model is the most probable 
 for [ cu(dbtmp)2 ] . within this model 
 the angle between the ligands ( angle between two planes formed by 
 the n atoms of the corresponding ligands and the cu atom ) changes 
 from 83 in the ground state to 60 in the excited state 
 ( figure 9 ) . the use of an mo based 
 approach to calculate xafs spectra has allowed us to quantitatively 
 analyze the xanes region and to give a simple interpretation to the 
 first shoulder , p1 , and corresponding peak of the difference spectrum . these results may be compared to those reported on [ cu(r2-phen)2 ] ( r = ph ) . in that case the feature on the rising edge 
 of the absorption edge , which was assigned at due to the dipole allowed 1s-4pz transition , 
 is more pronounced . the steric distortion in the ground state from 
 a pseudotetrahedral geometry is greater in the ground state than in 
 our example . the additional flattening in the mlct states 
 causes little or no increase in relative intensity of this peak , implying 
 that the intensity may be due to additional multiple scattering components 
 involving the phenyl substituents . view of [ cu(dbp)2 ] in the ground 
 ( left ) and mlct ( right ) states showing the angular distortion . in 
 this study , we have shown that the energy dispersive xas data 
 acquisition is suited for fast time - resolved pump  probe experiments 
 down to the ns time scale . the principle was demonstrated at id24 
 of the european radiation facility , looking at the mlct of [ cu(r2-phen)2 ] excited state derivatives . 
 taken in conjunction with previous work , these results demonstrate 
 that the lifetime of the emissive mlct state can be increased 
 by sterically bulky groups adjacent to the coordinated nitrogens by 
 suppressing the formation of an exciplex and flattening by jahn  teller 
 distortion . the cu k - edge xas was obtained with a higher signal / noise and in 
 a shorter acquisition time , as compared to step - by - step scanning approaches 
 as reported in literature . here a high quality spectrum was obtained 
 in about 2 h ; this was not limited by the detector , but was mainly 
 due to the low repetition rate of the laser used ( 10 hz ) and dead - times 
 in the experimental protocol . upgrading to a khz laser , as well as 
 upgrades of the esrf source ( i.e. , photon flux ) , should bring the 
 overall acquisition time down to less than 2 min , using this energy 
 dispersive data acquisition methodology . the xas data acquisition 
 time - resolution which can be obtained is limited to the bunch length 
 of the synchrotron source , being 70 ps at the esrf . other 
 ( newer ) sources like the diamond light source will be able to improve 
 on this , with a source bunch length of 40 ps , in standard 
 filling modes , and 5 ps in a low -mode . fast acquisition ( 200 s integration time 
 per x - ray energy point ) 
 has also been reported using a khz laser and measuring time series 
 xafs spectra with a repetition rate of a 24 bunch mode at the aps 
 ( 153 ns ) . this has provided an efficient 
 method of acquisition on a time scale of 100s ns to 1 s with 
 a time resolution of 100 ps . by using an optimized laser repetition 
 rate for the sls ( 520 khz ) , transient xafs of mbco ( 2 mm ) have been 
 obtained in 4.5 h. the principal 
 disadvantage of the dispersive approach is the restriction to a transmission 
 geometry and thus this test experiment utilized relatively high concentrations 
 ( 1520 mm ) . however , this limitation is alleviated with highly 
 stable x - ray sources , and thus the intrinsic gain by multiplexing 
 the x - ray spectrum should still be a very significant real - time gain 
 in kinetic studies of structural changes . these are being initiated 
 at the diamond light source ( i20 ) and the esrf ( id24 ) .
energy 
 dispersive x - ray absorption spectroscopy ( ed - xas ) , in which 
 the whole xas spectrum is acquired simultaneously , has been applied 
 to reduce the real - time for acquisition of spectra of photoinduced 
 excited states by using a germanium microstrip detector gated around 
 one x - ray bunch of the esrf ( 100 ps ) . 
 cu k - edge xas was used to investigate 
 the mlct states of [ cu(dmp)2]+ ( dmp = 2,9-dimethyl-1,10-phenanthroline ) 
 and [ cu(dbtmp)2]+ ( dbtmp = 2,9-di - n - butyl-3,4,7,8-tetramethyl-1,10-phenanthroline ) with the excited 
 states created by excitation at 450 nm ( 10 hz ) . 
 the decay of the longer 
 lived complex with bulky ligands , was monitored for up to 100 ns . 
 dft calculations of the longer lived mlct excited state of [ cu(dbp)2]+ ( dbp = 2,9-di - n - butyl-1,10-phenanthroline ) 
 with the bulkier diimine ligands , indicated that the excited state 
 behaves as a jahn  teller distorted cu(ii ) site , with the interligand 
 dihedral angle changing from 83 to 60 as the tetrahedral coordination 
 geometry flattens and a reduction in the cu 
 n distance of 0.03 
  .