Chudoba, C.

Dreyer, J.

Elsaesser, T.

Kummrow, A.

Nibbering, E.

Stenger, J.

Zachariasse, K.

English

MPG.PuRe

20 August 1999 .Chemical Physics Letters 309 1999 357–363www.elsevier.nlrlocatercplett /Excited state structure of 4- dimethylamino benzonitrile studiedby femtosecond mid-infrared spectroscopy and ab initiocalculationsC. Chudoba a, A. Kummrow a,), J. Dreyer a, J. Stenger a, E.T.J. Nibbering a,T. Elsaesser a, K.A. Zachariasse ba Max-Born-Institut fur Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Strasse 2A, 12489 Berlin, Germany¨b Max-Planck-Institut fur Biophysikalische Chemie, Spektroskopie und Photochemische Kinetik, 37070 Gottingen, Germany¨ ¨Received 13 April 1999; in final form 24 June 1999AbstractCombining femtosecond transient vibrational spectroscopy and high-level calculations is a powerful tool in thedetermination of excited-state structures. Striking differences in the experimental vibrational pattern of the locally excited .  .  .states of 4- dimethylamino benzonitrile DMABN and 4-aminobenzonitrile ABN are explained on the basis of molecular .structures obtained from ab initio complete-active-space self-consistent-field CASSCF calculations, giving evidence for astrong sensitivity of the molecular structure on modest changes in the substituents. The 4.0 ps charge-transfer time forDMABN in acetonitrile is resolved for the first time by tracking the downshifted C[N stretching mode. q 1999 ElsevierScience B.V. All rights reserved.1. Introduction .Charge-transfer CT reactions are among the mostimportant processes in chemistry and biology. In- .tramolecular charge transfer ICT in electronicallyexcited states is conveniently studied with spectro-scopic techniques. Over the past decades 4- .  .dimethylamino benzonitrile DMABN, Fig. 1 hasreceived continuous interest from experimentalistsw x w x1–9 and theoreticians 10 alike. The strikingfeature of this donor–acceptor compound is the oc-currence of dual fluorescence in polar solvents re-flecting the existence of two distinct excited-state) Corresponding author. Fax: q49-30-63921429; e-mail:kummrow@mbi-berlin.deintermediates. The normal fluorescence band L -b.type has been assigned to a moderately polar locally .excited state LE , whereas the red-shifted anoma- .lous band L -type has been shown to originateaw xfrom a highly polar CT state 2–9 . In contrast, .4-aminobenzonitrile ABN, Fig. 1 exhibits only nor-w xmal fluorescence regardless of solvent polarity 6–8 .Despite major experimental efforts by means of aw xvariety of absorption and emission techniques 1–9 ,conclusive evidence for excited-state structures incondensed phase is still lacking. Rotationally re-solved gas-phase spectroscopy has provided someindication of structural changes in the LE states ofw xABN and DMABN 11–13 . Further information onLE structures has been obtained by ab initio CI-sin-w x w xgles 14 and semi-empirical calculations 15–17 .0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. .PII: S0009-2614 99 00711-3( )C. Chudoba et al.rChemical Physics Letters 309 1999 357–363358 .Fig. 1. Molecular structures for DMABN and ABN in ground S0and LE states. Bond lengths are given in pm as obtained from .  .CASSCF 4,4 r6-31G d calculations.Direct access to structural changes upon excita-tion is gained from vibrational spectroscopy. The .  .phenyl-amino n PhN and cyano n CN stretchingmodes are expected to play a decisive role in distin-guishing different models for excited-state structuresby reflecting the most prominent geometry changes.Three different models have been proposed forthe structure of the CT state besides exciplex forma-w x  .tion 18 . According to the twist ICT TICT model,the amino group in the CT state has a perpendicularw xconformation with respect to the benzene plane 2–4 .The amino group is decoupled from the benzonitrilemoiety and thus the phenyl–amino bond is expectedto expand leading to a lower vibrational frequency .for the n PhN mode. The pseudo-Jahn–Teller model .assumes the formation of a planar CT PICT statew xwith quinoidal conformation 6–9 . The phenyl–amino bond gains partial double bond character en- .tailing an increased n PhN vibrational frequency. .Finally the rehybridized ICT RICT model impliesthe conversion of the cyano triple bond into a bentw xdouble bond 19 . According to calculations the .n CN vibrational frequency is expected to be ob-y1 w xserved in the area around 1600 cm 19 . . w xNanosecond infrared IR 20 and resonance Ra-w xman 21,22 experiments on DMABN have beenreported. From nanosecond studies, a downshift of120 cmy1 was observed for the cyano stretching .mode n CN , which is attributed to the formation ofthe CT state. Technical difficulties and an insuffi-cient time resolution prevented the determination ofabsolute band intensity changes and their temporalevolution. Consequently, femtosecond time resolu-tion is essential to separate the LE and CT spectraand to avoid excessive accumulation of triplet statepopulation.Here we present the first femtosecond time-re-solved UV pump mid IR probe experiments onDMABN and ABN. Transient IR spectra are ob-tained for the LE states of ABN and DMABN aswell as for the CT state of DMABN. In order toconnect the experimental observations with structuralchanges, the IR spectra are modeled by ab initioCASSCF calculations. This allows the identificationof the LE structures and provides the possibility torule out certain CT structures.2. Experimental sectionThe third harmonic of a regeneratively amplifiedTi:Sapphire laser served for excitation 270 nm,y2 .  . ;10 mJ cm . Mid-infrared MIR pulses ;30y2 .mJ cm were generated by parametric generationand amplification followed by difference frequencyw xmixing in a AgGaS crystal as described earlier 23 .2The MIR pulses were tuned between 4.2 and 8.3mm. The pump–probe cross-correlation width variedbetween 270 and 370 fs. In Fig. 2, the time-in- .tegrated cross-correlation function is shown dotswhich was determined by probing the MIR absorp-tion of an electron-hole plasma generated with the270 nm pulses. .Fully deuterated acetonitrile CD CN Aldrich3was used for experiments in the frequency range of1270 to 1690 cmy1, whereas undeuterated aceto-nitrile has lower absorption in the frequency range of2000 to 2230 cmy1. Both pump and probe pulse( )C. Chudoba et al.rChemical Physics Letters 309 1999 357–363 359Fig. 2. Top: Change of absorbance as function of pump–probe .delay time for DMABN in acetonitrile open circles ; the responsefunction for silicon is used to determine the time resolution 320.fs, dots . Bottom: spectrum of IR absorbance change of ABN at 1ps delay derived from individual transients.were focused into the free streaming jet thickness,.  y3 .300 mm of the sample solution 5=10 M . Thefrequency-resolved absorption changes in the MIRwere measured as a function of the time delaybetween pump and probe. A typical example is givenin the top of Fig. 2 for DMABN. Differential absorp-tion spectra were derived from such individual tran-sients recorded for different spectral positions Fig..2, bottom for ABN . The molar extinction coefficient« IR of the excited states was derived from theE Xmeasured change of the absorbance DOD by« IR s« IR qN DODrN 1 .EX 0 Awhere « IR is the extinction coefficient of the ground0state, N is the Avogadro constant, and N is theAdensity of excited molecules, which was calculatedfrom the intensity of the pump pulse and the UVabsorption of the jet.3. Computational methodsGeometry optimizations and vibrational analyseshave been performed by means of the CASSCFr6- .31G d method implemented in the program packagew xGAUSSIAN 94r98 24 . In order to keep the compu-tational effort in reasonable limits, the active spacehas been restricted to the two highest occupied and Y .  X.unoccupied a a and b a orbitals in the2 1 .C C point group for non-twisted structures. For2v s .908 twisted conformations TICT the amino nitro-  Y ..gen lone-pair orbital b a has been added. Sin-2gle-point calculations with extended active spaces aswell as results from previously reported calculationsreveal that this small active space is fully sufficientto warrant a qualitatively correct description of theelectronic wave function and thus also of the geome-try. IR spectra were calculated numerically. Frequen-cies were scaled by a factor of 0.9, and the numeri-cally calculated intensities by a factor of 3 for ABN .and 4 for DMABN cf., Fig. 5 . A more detailedpresentation of theoretical results will be given else-w xwhere 25 .4. Results from IR spectroscopyGround-state and transient IR spectra for ABNand DMABN are shown in Figs. 3 and 4, respec-tively. The IR lines are assigned to normal modesaccording to the results of calculated spectra vide.infra . Transient spectra were recorded 1 ps afterexcitation and can, therefore, be assigned to the LEstates. For ABN, only negligible frequency shifts andrelatively small changes in the amplitudes of the IRabsorption bands are observed upon excitation, whichindicates that only moderate structural changes take .place in the excited state Fig. 3 . Temporal profilesof the IR absorption bands do not exhibit any dy-namics on a picosecond timescale. For DMABN, thetransient IR spectrum of the LE state is strikingly .different from that of ABN Fig. 4 . The amplitudes( )C. Chudoba et al.rChemical Physics Letters 309 1999 357–363360 .  .of the n PhN and n CN stretching modes vanish.In addition, a new band appears at 2112 cmy1 with a .time constant of 4.0"0.5 ps Fig. 2, top . A some-what longer time of 6 ps has previously been esti-mated from picosecond fluorescence decay measure-w x y1ments 9 . The band shifts down by 103 cm , in fairagreement with the downshift of 120 cmy1 observedbefore in an experiment with nanosecond time reso-w xlution 20 . .The temporal profile of the n CN absorptionband shows induced transparency in the first 1.5 ps .Fig. 2, top . The center frequency and width of theband are constant within "1 cmy1 for all delaytimes and the ultimate band intensity is slightly .larger as for the n CN mode in the ground state.Moderate changes in the band intensities for the .  .d CH and n Ph modes are observed without fre-3quency shift. The transients do not show dynamics .Fig. 3. Ground-state IR absorption spectrum of ABN solid lineand absorption of excited molecules taken at 1 ps delay assigned .  .to the LE state dots : a measured in undeuterated acetonitrile, .b in acetonitrile-d3. The spectral dynamics are complete after 1ps. Lines are marked according to calculations.Fig. 4. Ground-state IR absorption spectrum of DMABN solid.line and absorption of excited molecules taken at 1 ps delay .  .  .assigned to the LE state dots : a in undeuterated acetonitrile, bin acetonitrile-d3. The peak at 2112 cmy1 , measured at 15 ps .delay, is assigned to the CT state open circles .with 4 ps time constant. This rules out an accidental .coincidence of the frequency of the n PhN in the .  .CT state with those of the d CH and n Ph modes.3No additional bands emerge above 1270 cmy1. Thus, .the extinction coefficient of the n PhN mode in theCT state must be much smaller than in the groundstate, preventing a measurement of the vibrationalfrequency.5. Discussion .The dynamic feature observed for the n CNmode of DMABN allows to gain some insight intothe ICT reaction. The observed 4 ps time constant( )C. Chudoba et al.rChemical Physics Letters 309 1999 357–363 361reflects the dynamics of conversion from the LE tothe CT state. The IR absorption builds up on atimescale noticeable longer than the correlation timeof solvent fluctuations, which is less than 0.5 ps forw xundeuterated acetonitrile 26 . Solvation dynamicsare thus not expected to play a rate-determining rolein this process.Experimental results and CASSCF calculationsfor the ground and LE states of ABN and DMABNare compiled in Fig. 5. The corresponding geome-tries are depicted in Fig. 1. Excellent agreement isobtained for the frequencies of the electronic groundstates with good consistency for band intensities. The .calculated n CN stretching frequency is somewhattoo high, which results from underestimating theC[N bond lengths, as concluded from a comparisonw xwith crystal structure data 27 . This does, however,not affect the relative band shifts of the excited stateswith respect to those of the ground state. The calcu- .lated LE vibrational spectrum of ABN Fig. 5ashows only minor changes in line positions and bandintensities as compared to the ground state, in accor-dance with the experimental observation. The promi-nent feature in the LE IR spectrum of DMABN isthe strong decrease of the band intensities of both the .  .n CN and the n PhN mode. These characteristics .are reproduced by the calculation Fig. 5c . Experi-mentally no additional lines are observed around1500 cmy1. The calculation obviously slightly over-estimates the intensity of IR lines involving methyl  ..group deformation modes d CH in combination3  ..with benzene stretching motions n Ph .In the calculations, the different characteristics ofthe two LE states result from different molecularstructures. Whereas the geometry of the ground statesand the LE state of ABN are calculated to be pyra-midal, the LE state of DMABN is essentially planar .Fig. 1 . A planar structure of the LE state ofDMABN does not support the conclusion, derivedw xfrom time-resolved fluorescence experiments 6–9 , .  .  .Fig. 5. Comparison of experimental and calculated IR band intensities: ABN in the a LE and b ground states; DMABN in the c LE and .d ground states.( )C. Chudoba et al.rChemical Physics Letters 309 1999 357–363362that planarization of the amino group is the crucialreaction coordinate in the reaction of the LE to theCT state. A similar conclusion was already reachedw xby recent ab initio calculations 10 .Computational results have also been obtained forthe three structurally different CT states proposed inthe literature, the TICT, PICT and RICT models. In .accordance with previous calculations, the n CNmode of the CT state in the RICT model is predicted y1 .to be substantially downshifted f800 cm asw xcompared to the ground state 19 . Our experimentalobservation is in clear contradiction with the RICTmodel, which hence can definitely be ruled out asvalid CT mechanism for DMABN. . y1Calculated n CN shifts of y110 cm for theTICT and y37 cmy1 for the PICT model as well asthe intensity changes are both in the same order ofmagnitude as the experimental shift of y103 cmy1.A discrimination between the TICT and PICT reac-tion path could be made by observing the shift of the .n PhN mode frequency. Unfortunately, the IR band .intensity of the n PhN mode in the CT state is toosmall to be detected with the current sensitivity ofthe experimental set-up and will be the subject offuture research.6. ConclusionsTransient vibrational spectroscopy, in connectionwith CASSCF calculations, gives new insight intomolecular structures of excited states. The LE statesof ABN and DMABN exhibit different IR character-istics and thus have different molecular configura-tions, which are investigated on the basis of CASSCFcalculations. This reveals that relatively minorchanges in the nature of the amino substituents havea considerable impact on the excited state structureof the aminobenzonitriles putting the usefulness ofaminobenzonitrile derivatives as model compoundsfor ICT processes in question. Although the RICTmechanism can be ruled out, an unequivocal distinc-tion between the TICT and PICT models requiresadditional experimental information. Coherent Ra-man measurements may provide additional insight inthis respect.AcknowledgementsWe thank Mrs. G. Kordass for measuring linearinfrared spectra. J.D. is grateful for a fellowship ofthe ‘Deutsche Forschungsgemeinschaft’.Referencesw x1 E. Lippert, W. Luder, F. Moll, W. Nagele, H. Boos, H.¨ ¨ .Prigge, I. Seibold-Blankenstein, Angew. Chem. 73 1961695.w x2 K. Rotkiewicz, K.H. Grellmann, Z.R. Grabowski, Chem. .Phys. Lett. 19 1973 315.w x3 E. Lippert, W. Rettig, V. Bonacic-Koutecky, F. Heisel, J.A.´ .Miehe, Adv. Chem. Phys. 68 1987 1.´w x4 G. Kohler, K. Rechthaler, K. Rotkiewicz, W. Rettig, Chem.¨ .Phys. 207 1996 85.w x5 L.W. Peng, M. Dantus, A.H. Zewail, K. Kemnitz, J.M. .Hicks, K.B. Eisenthal, J. Phys. Chem. 91 1987 6162.w x6 U. Leinhos, W. Kuhnle, K.A. Zachariasse, J. Phys. Chem. 95¨ .1991 2013.w x7 W. Schuddeboom, S.A. Jonker, J.H. Warman, U. Leinhos, .W. Kuhnle, K.A. Zachariasse, J. Phys. Chem. 96 1992¨10809.w x8 Y.V. Il’ichev, W. Kuhnle, K.A. Zachariasse, J. Phys. Chem.¨ .A 102 1998 5670.w x9 K.A. Zachariasse, M. Grobys, Th. von der Haar, A. Hebecker,Y.V. Il’ichev, Y.-B. Jiang, O. Morawski, W. Kuhnle, J.¨ .Photochem. Photobiol. A: Chem. 102 1996 59.w x10 W. Sudholt, A.L. Sobolewski, W. Domcke, Chem. Phys. 240 .1999 9, and references cited therein.w x11 G. Berden, J. van Rooy, W.L. Meerts, K.A. Zachariasse, .Chem. Phys. Lett. 278 1997 373.w x12 O. Kajimoto, H. Yokoyama, Y. Ooshima, Y. Endo, Chem. .Phys. Lett. 179 1991 455.w x13 V.H. Grassian, J.A. Warren, E.R. Bernstein, J. Chem. Phys. .90 1989 3994.w x  .14 U. Lommatzsch, B. Brutschy, Chem. Phys. 234 1998 35.w x  .15 A. Broo, M.C. Zerner, Theor. Chim. Acta 99 1995 383.w x  .16 A.-D. Gorse, M. Persquer, J. Phys. Chem. 99 1995 4039.w x17 P. Gedeck, S. Schneider, J. Photochem. Photobiol. A: Chem. .105 1997 165.w x18 P.C.M. Weisenborn, A.H. Huizer, C.A.G.O. Varma, Chem. .Phys. 133 1989 437.w x19 A.L. Sobolewski, W. Sudholt, W. Domcke, J. Phys. Chem. A .102 1998 2716.w x  .20 M. Hashimoto, H. Hamaguchi, J. Phys. Chem. 99 19957875.w x21 G.D. Scholes, D. Phillips, I.R. Gould, Chem. Phys. Lett. 266 .1997 521.w x22 G.D. Scholes, I.R. Gould, A.W. Parker, D. Phillips, Chem. .Phys. 234 1998 21.w x  .23 F. Seifert, V. Petrov, M. Woerner, Opt. Lett. 19 1994 2009.( )C. Chudoba et al.rChemical Physics Letters 309 1999 357–363 363w x  .24 GAUSSIAN98 Revision A.5 , M.J. Frisch, G.W. Trucks,H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C.Burant, S. Dapprich, J.M. Millam, A.D. Daniels, K.N. Kudin,M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R.Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J.Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Mo-rokuma, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B.Foresman, J. Cioslowski, J.V. Ortiz, B.B. Stefanov, G. Liu,A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L.Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.Nanayakkara, C. Gonzalez, M. Challacombe, P.M.W. Gill,B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M.Head-Gordon, E.S. Replogle, J.A. Pople, Gaussian, Pitts-burgh, PA, 1998.w x25 J. Dreyer et al., in preparation.w x26 W.P. deBoeij, M.S. Pshenichnikov, D.A. Wiersma, J. Phys. .Chem. 100 1996 11806.w x27 A. Heine, R. Herbst-Irmer, D. Stalke, W. Kuhnle, K.A.¨ .Zachariasse, Acta Cryst. B 50 1994 363.

Excited state structure of 4-(dimethylamino)benzonitrile studied by femtosecond mid-infrared spectroscopy and ab initio calculations.

Excited state structure of 4-(dimethylamino)benzonitrile studied by femtosecond mid-infrared spectroscopy and ab initio calculations.

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