Solvent De pend ent Ultrafast Ground State Recovery Dynamics of Triphenylmethane Dyes

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1 Jour nal of the Chi nese Chem i cal So ci ety, 2000, 47, Solvent De pend ent Ultrafast Ground State Recovery Dynamics of Triphenylmethane Dyes Yutaka Nagasawa*, Yoshito Ando and Tadashi Okada De part ment of Chem is try, Grad u ate School of En gi neering Sci ence and Kyokugen, Osaka University, Toyonaka,Osaka , Ja pan We have stud ied ground state re cov ery dy nam ics of triphenylmethane dyes, bril liant green and mal a chite green, by a sin gle wave length pumpprobe spec tros copy at 635 nm with a timeresolution of 33 fs. The re cov ery of the ground state dependeded on sol vent vis cos ity and also on sol ute mo lec u lar size. We ob served a pla teau or a rise com po nent in the sig nal which in di cates an in ter me di ate state. The rise time also showed sol vent vis cos ity de pend ence even in the ultrafast time do main, al though no sol ute de pend ence was ob served. It is sug gested that the lack of sol ute dependece in di cates that only small conformational change is in v olved in the ini tial pro cess. IN TRO DUC TION It is of a great in ter est among chem ists as to how the chem i cal re ac tions de pend on inter and intramolecular dy nam ics. Re cent prog ress in the in ves ti ga tion of the sol va tion dy nam ics have shown that sol va tion can be devided into two ma jor pro cesses, the ultrafast in er tial pro cess which oc curs within 100 fs and the dif fu sive pro cess which oc curs in a slower time scale. 13 In er tial pro cess is in duced by the small an gle free ro ta tion of a few sol vent mol e cules within the first sol va tion shell. Gaussi an func tion is gen er ally used to rep re sent in er tial pro cess and multiexponential func tion is used for dif fu sive pro cess. Re cent ex per i men tal re sults show that the in er tial pro cess is rather sol vent in sen si tive while the dif fu sive pro cess strongly re flects the sol vent char ac ter is tics. 24 Triphenylmethane (TPM) dyes are fa mous for their sol vent vis cos ity de pend ent ex cited state life times. 510 It is gen er ally be lieved that the ro ta tion of the phenyl groups around the cen tral car bon atom is the or i gin of the vis cos ity de pend ent radiationless de ac ti va tion. The aim of this study is to un der stand what hap pens when a chem i cal re ac tion be comes ex tremely fast and reaches the subpicosecond time do main. The ground state re cov ery of TPM dyes can be ex tremely fast in some fluid sol vents. We would like to know how a bulk sol vent prop erty such as vis cos ity will af fect ultrafast re ac tion. In ultrafast time scales, a chem i cal re ac tion may be come sovent in sen si tive like an in er tial re sponse or if it is strongly cou pled to a cer tain intramolecular vi bra tion, the re ac tion may oc cur co her ently. In this study, we have mea sured ground state re cov ery dy nam ics of TPM dyes, bril liant green (BG) and mal a chite green (MG), by a pumpprobe spec tros copy at a cen ter wave length of 635 nm with timeresolution of 33 fs. Since the pi co sec ond re cov ery dy nam ics of these mol e cules have been ex ten sively in ves ti gated, our ma jor at ten tion was aimed at the ultrafast ini tial pro cess. EX PER I MEN TAL The de tails of the homemade cav itydumped Kerr lens modelocked Cr: for ster ite la ser will be re ported else where. 11 The os cil la tor was ex cited by a di odepumped Nd: Vanadate la ser with CW power of 8 W (Co her ent, Compass106410W). The cav itydumping ef fi ciency was about 30% and the dump ing rate was 100 khz. The cav itydumped beam was fo cused into a 4 mm LBO crys tal to con vert the fun da men tal fre quency of 1270 nm into the sec ond har monic at 635 nm. The con ver sion ef fi ciency was 20 to 30% and the sec ond har monic pulse en ergy was about 4 nj. The autocorrelation trace of the sec ond har monic pulse was mea sured by a 0.3 mm BBO crys tal with the same setup used for the pumpprobe ex per i ment. The mea sured FWHM of the pulse was 30 to 35 fs. To the best of our knowl edge, this is the short est pulse ever gen er ated from a cav itydamped Cr: for ster ite la ser. BG was pur chased from To kyo Chem i cal In dus try and MG was from Exciton. Or ganic sol vents were from Kanto and Wako Chem i cals and used af ter dis til la tion when the pu rity was lower than 99.5 %. The op ti cal path of the sam ple cell was 0.4 mm and the op ti cal den sity of the sam ple was about 1.0. The sam ple so lu tion was cir cu lated while the ex per i ment to avoid op ti cal dam age. The sec ond har monic pulse was di vided into pump and probe pulses by a 50 % beam split ter. The en ergy of the pump pulse was 0.6 nj and that of the probe pulse was re duced to 60 pj by a ND fil ter. The pulses were fo cused into the sam ple cell by a 5 cm fo cus ing lens. The sin

2 700 J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 Nagasawa et al. glewavelength PP mea sure ments were car ried out at a magic an gle con fig u ra tion and the sig nal was de tected by a photo diode and a lockin am pli fier. Ab sorp tion spec tra were mea sured by Hitachi U3500 spectrophotometer. RE SULTS The mo lec u lar struc tures and the ab sorp tion spec tra of BG and MG in var i ous sol vents are also shown in Fig. 1. The spec trum of the ex cit ing la ser pulse is also shown for com par i son. BG and MG are both C 2 sym met ric mol e cules with two N,Ndiethyl and N,Ndimethyl amino sub sti tuted phenyl groups, re spec tively. The ab sorp tion peaks of both dyes showed a slight red shift when the car bon chain of the sol vent was elon gated. The PP sig nal of BG and MG in var i ous sol vents are shown in Fig. 2. It can be clearly seen that the de cay of the sig nal be comes slower in al co hols with lon ger car bon chains and MG de cays faster than BG in all the stud ied sol vents. At the be gin ning of the sig nal, ex tremely fast de cay and quan tum beats can be seen. Af ter the ini tial de cay, the de cay slows down and forms a pla teau and then con tin ues to de cay multiexponentially. The sig nal, F(t), was fit ted by con vo lu tion of the autocorrelation trace, g(t), with a fit ting func tion, f(t), which is a sum of ex po nen tial func tions and an ex po nen tially de cay ing co sine func tion, t F( t) g( t ') f ( t t ') dt ', f ( t) Aiexp( t / i) A exp( t / )cos( t ). i The fit ting pa ram e ters are listed in Ta bles 1 and 2. The av er age fre quency of the ob served quan tum beats were 220 and 226 cm 1 for BG and MG, re spec tively. Rise com po nents, rise1,2 (neg a tive am pli tude), and de cay com po nents, 13, were nec es sary to fit the data. In wa ter to 1propanol, three de cay com po nents were nec es sary for BG, while only two were nec es sary for MG. For MG in wa ter and meth a nol, ex tremely weak neg a tive sig nal with long rise times, rise2, were ob served. The sig nal of MG in meth a nol is shown in Fig. 3 with the mag ni fi ca tion of the neg a tive part. A neg a tive sig nal with 14 ps risetime can be clearly seen. The val ues of 2 and 3 of BG in wa ter are larger than those val ues in meth a nol al though the de cay in wa ter looks ap par ently faster in Fig. 2. This is be cause in wa ter, the in ten sity of 3 is con sid er ably weaker than (1) Fig. 1. Ab sorp tion spec tra of (a) mal a chite green and (b) bril liant green in var i ous sol vents and the spec trum of the ex ci ta tion pulse cen tered at 635 nm. Fig. 2. Sin gle wave length pumpprobe sig nals of mal a chite green and bril liant green in var i ous sol vents.

3 Sol vent De pend ent Ultrafast Dy nam ics of TPM Dyes J. Chin. Chem. Soc., Vol. 47, No. 4A, Ta ble 1. Fitting Pa ram e ters, Fre quency ( ), Time Con stant ( i), and Am pli tude (A i) for the Pumpprobe Sig nal of MG in Var i ous Sol vents. Am pli tudes Are Nor mal ized in a Man ner to be 1 When All the Com po nents Are Added In clud ing the Neg a tive One. Sol vent Vis cos ity ( ) and Ab sorp tion Max i mum ( abs ) of MG Are Also Shown /cm 1 /ps (A ) rise1/ps (A rise1) rise2/ps (A rise2) 1/ps (A 1) 2/ps (A 2) 3/ps (A 3) /cp abs/nm H 2O MeOH EtOH PrOH BuOH (1.63) 3.1 (0.01) 0.12 (1.35) 0.52 (1.20) (0.11) 0.39 (2.65) 13.9 (0.04) 0.16 (1.21) 0.61 (2.37) (0.14) 0.89 (3.28) 0.11 (0.51) 1.0 (3.63) (1.32) 0.11 (0.44) 1.8 (1.78) (0.09) 1.3 (0.58) 0.12 (0.43) 1.8 (0.60) 3.1 (0.47) that of 2. Anisotropic de cay of MG in meth a nol and BG in wa ter are shown in Fig. 4 which in di cates that ro ta tional dif fu sion of the sol ute mol e cules can be safely ne glected in the ex per i men tal timescale. The vis cos ity de pend ence of the time con stants 2, 3, and rise1 are shown in Fig. 5. The de pend ence was fit ted by a lin ear func tion. DIS CUS SION Sum mary of our ob ser va tions are as fol lows; (1) ini tial ultrafast de cay of about 100 fs, (2) quan tum beat with a pe riod of about 150 fs, (3) slow down of the rapid de cay which forms the platue, (4) sol vent de pend ent slower de cay, and (5) neg a tive sig nal which was ob served for MG in wa ter and meth a nol. Ta ble 2. Fitting Pa ram e ters, Fre quency ( ), Time Con stant ( i), and Am pli tude (A i) for the Pumpprobe Sig nal of BG in Var i ous Sol vents. Am pli tudes Are Nor mal ized in a Man ner to be 1 When All the Com po nents Are Added In clud ing the Neg a tive One. Ab sorp tion Max i mum ( abs ) of BG Are Also Shown /cm 1 /ps (A ) rise1/ps (A rise1) 1/ps (A 1) 2/ps (A 2) 3/ps (A 3) abs/nm H 2O MeOH EtOH PrOH BuOH (0.07) 0.54 (2.21) (0.50) 0.69 (2.54) (0.11) 0.53 (2.14) (0.34) 0.37 (1.30) 1.3 (1.39) (0.08) 0.75 (1.28) 0.13 (0.43) 0.62 (0.85) 2.5 (0.92) (1.51) 0.11 (0.38) 0.87 (1.30) 4.1 (0.72) (0.09) 1.9 (0.17) 0.12 (0.34) 6.1 (0.74) 629

4 702 J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 Nagasawa et al. The fol low ing dis cus sion will pro ceed ac cord ing to the time scale of the ob ser va tion. Ini tial ultrafast decay The time con stant of the ini tial ultrafast de cay, 1, is in sen si tive to sol vent while the am pli tude is. For MG it is clear that the am pli tude of this fast est de cay be comes stron ger as the sol vent car bon chain gets shorter. It seems to be rea son able to con clude that this pro cess is not in volved in the re cov ery dy nam ics be cause sim i lar phe nom ena are ob served in other nonreactive sys tems. It had been called co her ent spike and treated un fairly as an ar ti fact, though it con tains in for ma tion about sol va tion and intramolecular re lax ation. 3,12 Re cently, it was shown that the ini tial de cay con tains de struc tive mo tion of the intramolecular wavepackets and the sol vent in er tial re sponse, i. e., the small an gle free ro ta tion of a few sol vent mol e cules within the first sol va tion shell. 3 The weak sol vent de pend ence of the de cay time is con sis tent with other ob ser va tions by pho ton echo and op ti cal Kerr mea sure ments in alcoholss. 3,13 The in ten sity be ing stron ger for the sol vents with shorter car bon chains can be ex plained in terms of the stron ger cou pling of the elec tronic tran si tion with the sol vent in er tial re sponse and is also con sis tent with other ob ser va tions. 3 Fig. 3. Sin gle wave length pumpprobe sig nal of mal a chite green in meth a nol and mag ni fi ca tion of the neg a tive am pli tude part. Fig. 4. (a) Anisotropic de cay of mal a chite green in meth a nol and (b) bril liant green in wa ter. Fig. 5. (a) Vis cos ity de pend ence of the time con stant 3 of bril liant green (closed squares) and 3 of mal a chite green (open squares). The slopes ob tained from a lin ear fit ting are 1.9 ps/cp (solid line), and 1.1 ps/cp (dot ted line), re spec tively. (b) Vis cos ity de pend ence of the rise time, rise1, of bril liant green (closed tri an gles) and mal a chite green (open tri an gles). The slopes ob tained from a lin ear fit ting are 0.56 ps/cp (solid line) and 0.49 ps/cp (dot ted line), re spec tively.

5 Sol vent De pend ent Ultrafast Dy nam ics of TPM Dyes J. Chin. Chem. Soc., Vol. 47, No. 4A, Quan tum beat When the la ser pulse is short enough, quan tum beats caused by im pul sivestimulated Raman scat ter ing can be ob served. 14 The quan tum beat of MG is as signed to the Raman ac tive 226 cm 1 breath ing mode of the bonds to the cen tral car bon atom To the best of our knowl edge, there is no re port on Raman spec trum of BG. How ever, the quan tum beats of BG can be safely as signed to the same breath ing mode be cause of the struc tural sim i lar ity. A slightly lower fre quency in BG may be ex plained by the larger mo lec u lar size. The fre quency and dephasing time of the beat did not change sys tem at i cally with sol vent. Thus this breath ing mode is not likely to take part in the radiationless re cov ery pro cess. Platue The pla teau ob served in the sig nal in di cates an in ter me di ate state. The pla teau in PP sig nal was also ob served by Mokhtari et al. for MG and crys tal vi o let by two color PP mea sure ments with 620 nm ex ci ta tion and prob ing the ex cited state ab sorp tion at 390 nm. 18 Since the pla teau is ob served for ex cited state ab sorp tion, this pro cess should be re lated to the ex cited state dy nam ics. Mokhtari et al. at trib uted this ultra fast pro cess to the conformational re lax ation of tor sional modes in the ex cited state. There are also re ports about flu o res cence de cay be ing faster than the ground state re cov ery for MG and crys tal vi o let. 7,19 The pla teau may be orig i nat ing from the ex cited state re lax ation from a flu o res cent state to a nonfluorescent one. The most re cent flu o res cence de cay time of MG in wa ter was 0.54 ps 20 which ex actly matches the lon gest time con stant 0.52 ps ob tained from the PP mea sure ment. There fore, the pla teau is not re lated to the con ver sion from flu o res cent to nonfluorescent state. The ex cited state is likely to be still emissive even af ter the re lax ation in the ex cited state. It may be pos si ble that the tor sional mo tion of the amino sub sti tuted phenyl group may dis turb a res o nance elec tronic struc ture in the ex cited state to give a some what unsymmetric in ter me di ate struc ture in the course of the re cov ery pro cess. The sol vent vis cos ity de pend ence of the rise time, rise1, is plot ted in Fig. 5(b) and a lin ear fit ting gives slopes of 0.49 ps/cp and 0.56 ps/cp for MG and BG, re spec tively. It is in ter est ing to ob serve such a lin ear vis cos ity de pend ence even in the subpicosecond time do main, since mi cro scopic mo lec u lar na ture rather than a sol vent bulk prop erty should be re flected in such a time scale. It should be noted that the rise time of both TPM dyes are al most iden ti cal within the ex per i men tal er ror. The sol ute de pend ence of the rise and de cay time con stants will be dis cussed in the next sec tion. Slower de cays The PP sig nals al ways de cay faster for MG than BG as seen in Fig. 2. This is the ef fect of the size of the amino sub stituent on the phenyl group. A sim i lar ef fect was also re ported for ethyl vi o let and crys tal vi o let and at trib uted to the size de pend ent tor sional mo tion of the phenyl group. 21 In ter est ingly, Ta bles 1 and 2 in di cate that the de cays of BG are more mul ti ple than MG. This may as well be the size ef fect of the amino substituent. The de cay time con stant, 3, is plot ted as a func tion of sol vent vis cos ity in Fig. 5(a). A lin ear fit ting gives slopes of 1.9 ps/cp for 3 of BG and 1.1 ps/cp for 3 of MG. Al though we can not ig nore the pos si bil ity of non linearity ap pear ing in lon ger al co hols, DebyeStokesEinstein model seems to be a rather good as sump tion for the pres ent system. For 3, a re mark able sol ute de pend ence was ob served and was at trib uted to the mo lec u lar size ef fect. 3 is nearly two times lon ger for BG than MG and the slope of the vis cos ity de pend ence was also twice steeper for BG (Fig. 4(a)). How ever, the rise oc curs in a sim i lar time scale for both dyes and the vis cos ity de pend ent slope is also the same (Fig. 4(b)). The lack of sol ute de pend ence in the faster pro cess in di cates that only small conformational change is in volved. Thus the emissive na ture of the ex cited state re mains af ter the ini tial ultrafast pro cess. We would also like to men tion that, at a UPS con fer ence in Tai pei, it was sug gested that only the sol va tion pro cess may be in volved in the rise. Neg a tive signal Very weak neg a tive sig nal of MG in meth a nol is shown in Fig. 3. A sim i lar neg a tive sig nal ap peared for MG in wa ter al though the in ten sity was much weaker. Robl and Seilmeier ob served a sim i lar neg a tive sig nal with much stron ger am pli tude in the PP sig nal of MG at 665 nm. 22 Neg a tive sig nal in di cates an in crease of ab sorp tion which is usu ally at trib uted to the tran sient ab sorp tion of the ex cited state or to some in ter me di ate state. How ever, they at trib uted this sig nal to the vibrationally hot ground state mol e cules. When PP sig nal is mea sured at the red edge of the ab sorp tion spec trum, such sig nal can ap pear when the ex cited state life time is much shorter than the ground state vi bra tional re lax ation time. MG in wa ter and meth a nol have the short est ab sorp tion peak wave length among the mea sured sam ples. There fore, the sig nal from vibrationally hot mol e cules do have a chance to con trib ute to the 635 nm mea sure ment. In many cases when ex cited and ground state ab sorp tion is over lapped, timeresolved ani so tropy does not start from 0.4 be cause two states are likely to have dif fer ent di rec tions of tran si tion di pole mo ment. The

6 704 J. Chin. Chem. Soc., Vol. 47, No. 4A, 2000 Nagasawa et al. ani so tropy mea sure ments of BG and MG shown in Fig. 4 show that ini tial ani so tropy starts from 0.4. At pres ent, we do not have any strong in di ca tion that ex cited state ab sorp tion is con trib ut ing to the PP sig nal. Two color pumpprobe mea sure ment, which is one of our fu ture plans, should be help ful for such a judge ment. CON CLU SION We have car ried out sin gle wave length PP mea sure ments of triphenylmethane dyes, bril liant green and mal a chite green, and stud ied the sol vent de pend ence of the ground state re cov ery. The ini tial ultrafast de cay and the quan tum beat were sol vent in sen si tive, while the slow de cay com po nents showed lin ear vis cos ity de pend ence. The de cay was al ways slower for BG than MG ow ing to its larger mo lec u lar size. We have also ob served a pla teau in the sig nal which in di cates an in ter me di ate state. A subpicosecond rise com po nent was nec es sary to fit the pla teau and this com po nent also showed lin ear vis cos ity de pend ence. It was in ter est ing for us to ob serve such a vis cos ity de pend ence even in the subpicosecond time scale, since mi cro scopic mo lec u lar na ture rather than a sol vent bulk prop erty should be re flected in such a time scale. More in ter est ingly, the ultrafast rise time did not show any sol ute de pend ence. This ef fect in di cates that only small or no conformational change is in volved in the rise and thus the ex cited state re mains emissive. Ultrafast spec tro scopic stud ies of TPM dyes have a po ten tial to give valu able in sights to the mi cro scopic na ture of the sol vent and sol ute dy nam ics. Fur ther in ves ti ga tion us ing other TPM dyes and mea sure ments of two color pumpprobe sig nals and flu o res cence de cay are planned as fu ture pro jects. AC KNOWL EDGE MENT This re search was sup ported by a GrantinAid for Spe cially Pro moted Re search (No ) from the Min is try of Ed u ca tion, Sci ence, Sports, and Cul ture of Ja pan and also from Sumitomo Foun da tion. Y.N. wishes to thank Drs. I. V. Rubtsov of JAIST, K. Torizuka of ETL, and Z. Zhang of NEDO for many sug ges tions for con struct ing the Cr:F la ser. Re ceived Jan u ary 21, Key Words Ultrafast dy nam ics; Triphenylmethane dye; Mal a chite green; Nonradiative tran si tion; Pumpprobe. REF ER ENCE 1. Rosenthal, S. J.; Xie, X.; Du, M.; Flem ing, G. R. J. Chem. Phys. 1991, 95, de Boeij, W. P.; Pshenichnikov, M. S.; Wiersma, D. A. J. Phys. Chem. 1996, 100, Joo, T.; Jia, J.; Yu, J.Y.; Lang, M. J.; Flem ing, G. R. J. Chem. Phys. 1996, 104, Passino, S. A.; Nagasawa, Y.; Flem ing, G. R. J. Chem. Phys. 1997, 107, Förster, Th.; Hoffmann, G. Z. Physik. Chemie. NF, 1971, 75, BenAmotz, D.; Jeanloz, R.; Har ris, C. B. J. Chem. Phys. 1987, 86, Ippen, E. P.; Shank, C. V.; Berg man, A. Chem. Phys. Lett. 1976, 38, Madge, D.; Wind sor, M. W. Chem. Phys. Lett. 1974, 24, Sundström, V.; Gillbro, T. J. Chem. Phys. 1984, 81, Nagasawa, Y.; Ando, Y.; Okada, T. Chem. Phys. Lett. 1999, 312, Nagasawa, Y.; Ando, Y.; Okada, T. Appl. Phys. B, in press. 12. Cong, P.; Yan, Y.J.; Deuel, H. P.; Si mon, J. D. J. Chem. Phys. 1994, 100, Kinoshita, S.; Kai, Y.; Ariyoshi, T.; Shimada, Y. Int. J. Mod. Phys. B 1996, 10, Rosker, M. J.; Wise, F. W.; Tang, C. L. Phys. Rev. Lett. 1986, 57, Nel son, K. A.; Wil liams, L. R. Phys. Rev. Lett. 1987, 58, Aleksandrov, I. V.; Bobovich, Y. S.; Vartanyan, A. T. Opt. Spectrosc. 1977, 42, Angeloni, L.; Smulevich, G.; Marzocchi, M. P. J. Raman Spec , Mokhtari, A.; Fini, L.; Chesnoy, J. J. Chem. Phys. 1987, 87, BenAmotz, D.; Har ris, C. B. Chem. Phys. Lett. 1985, 119, Yoshizawa, M.; Suzuki, Kubo, A.; Saikan, S. Chem. Phys. Lett. 1998, 290, Vogel, M.; Rettig, W. Ber. Bunsenges. Phys. Chem. 1987, 91, Robl, T.; Seilmeier, A. Chem. Phys. Lett. 1988, 147, 544.