- Email: [email protected]
Solvent and nuclear dynamics in ultrafast intermolecular electron transfer in a diffusionless, weakly polar system
Volume 207, number 4,5,6
CHEMICAL PHYSICS LETTERS
28 May 1993
Solvent and nuclear dynamics in ultrafast intermolecular electron transfer in a diffusionless, weakly polar system Arkadiy Yartsev a*L,Yutaka Nagasawa b, Abderrazzak Douhal a and Keitaro Yoshihara a,b a InstituteforMolecularScience,Myodadi,Okazaki444, Japan b The GraduateUniversity for AdvancedStudies,Myodaiji,okazaki 444, Japan Received 8 March 1993
Femtosecond intermolecular electron (ET) transfer dynamics were studied by time-resolved fluorescence up-conversion technique in contact systems of oxazine dyes in electron-donating solvents. Clearly non-exponential ET time dependence was observed in aniline and explained by the effects of both solvent reorientation and nuclear motion of the reactants. Single exponential processes were measured for nile blue ( Q 160 fs) and oxazine 1 ( 6 280 fs) in N,Ndimethylaniline. The rate of ET is explained to be limited only by ultrafast nuclear relaxation.
1. Introduction Electron transfer (ET) is one of the most important chemical reactions because of its universality in chemistry and biology. Intermolecular ET, analogous to the primary steps in the photosynthetic reaction centers of green plants and bacteria, is of special interest. Dynamical aspects of solvent effects on the course of ET were intensively studied both theoretically [ l-91 and experimentally [ 10-l 81. For ET with a rate comparable to the solvent fluctuation time, motion and structure of the solvent can determine the rate of chemical reaction. It becomes inversely proportional to the longitudinal relaxation time (TV)of the solvent and is often called “solventcontrolled reaction” [ lo- 141. Recently this effect has been experimentally confirmed [ 15-191. However, if the reaction becomes faster than the solvent relaxation times, a new type of reaction mechanism, controlled by inter- and intra-molecular vibrations would make a significant contribution to the reaction. ’ Permanent address: Institute of Spectroscopy, Russian Academy of Science, 142092 Troitzk, Moscow Region, Russian Federation. 544
We have reported ultrafast ET (as fast as = 1013 s-l) between excited dye and weakly polar electrondonating solvent molecules [ 15-l 71. This system has several specific features. ( 1) Electron acceptor molecules (solute) are surrounded by donor solvent molecules and they are in contact. There is no translational diffusion in order to induce ET. (2) The weakly polar solvent system gives less dielectric friction to ET. (3) There is no change of the net charge before and after the reaction [ 161. (4) The rate of ET is x 50 times greater than that of the solvent longitudinal relaxation time. The time scale of ET in these systems falls into that of solvent relaxation and of nuclear motion and the reaction will be severely influenced by these dynamics. It is also the aim of the present study to investigate the role of intermolecular interaction, namely, solvent effects to the reaction as well as the effects of inter- and intra-molecular dynamics.
2. Experimental A synchronously pumped hybridly modelocked dye laser with control of group velocity dispersion (GVD) by a prism pair was made to generate optical Elsevier Science Publishers B.V.
Volume207, number 4,5,6
CHEMICALPHYSICSLETTERS
pulses at different wavelengths at 76 MHz. Using a combination of rhodamine 6G and DGDCI/DQOCI [ 201 and spatial filters between a second prism and end mirror, 70 fs pulses were generated at 583,605, and 6 15 nm with an output power of = 100 mW. With a combination of sulforhodamine 101 and DQTCI [ 211 generation of 50 fs pulses at 675 nm was achieved. Fluorescence decays at the different polarizations of emission were measured by a conventional fluerescence up-conversion system using a 0.3 mm BBO crystal (type I). Equalization of GVD ‘in both branches of an up-conversion system and GVD precompensation by a prism pair was made to achieve the smallest’width of cross-correlation. This was measured as a noncollinear second-harmonic generation on a BBO crystal with residual beam after sample excitation instead of fluorescence. The narrowest cross-correlation corresponded to 80 fs at 600 and 60 fs at 675 nm. Fluorescence decay measurements were repeated 1O-50 times for obtaining a single decay curve in order to avoid long term instabilities of the dye laser during accumulation. Analysis of measured kinetics was performed by the “Global Unlimited”’ fitting program. One of the advantages of this program is the possibility to perform a rigorous error estimation procedure. The parameter being examined is allowed to vary discretely in some interval, while all other parameters are adjusted to minimize 2’. The cross-correlation was chosen as a system response function in almost all fittings. Nevertheless the effects of fluorescence signal broadening by the dispersive optics elements may be important since the dispersion precompensation by a prism pair was optimized for the wavelength of the dye laser. GVD for different wavelengths, at which the fluorescence decays were measured, was not entirely compensated, leading to effective broadening of the system response function. Different divergence of the dye laser and the fluorescence may also give some response function broadening. Consequently, the results of fitting with cross-correlation as a response function show an upper limit for fluorescence quenching time constants, NiIe blue perchlorate (NB) and oxazine 1 perchlorate (OX 1) were obtained from Exiton and used without purification. Aniline (AN) and N,N-dimethylaniline (DMA) were vacuum distilled immedi-
28 May 1993
ately before use. The concentration of NB and OX1 in the electron-donating solvent was varied from 0.5 x 1Om4to 2 x 1Om4M. Dye solutions were bubbled with nitrogen to eliminate a possible effect of solvent oxidation. All the measurements were made at room temperature ( 24 ’C ) . A flowing sample cell made of quartz with 1 mm optical path length was used to minimize thermal effects on the fluorescence decay. To record one fluorescence decay shown in the tigures, it took 5 h for OX1 /AN and 20 h for OX l/ DMA.
3. Results In fig. 1 fluorescence decay of OX 1 in AN is shown. This was obtained upon the excitation at 605 nm, 25X1 O3
20
VI E
15
?3 10
5
-2
a
2
6
YJ 0
Time&
0.2
0.4
0.6
1.0 112 114 1:6 1:il
WPS ‘ZJps Fii 1. (a) Fluorescencedecay of oxazine 1 in aniline on a linear scale.(W) Experimental data points and (0) instrument response function. The results of rigorous error analysis for (b)
fast and (c) middle time components are also shown.
547
Volume 207, number 4,5,6
28 May 1993
CHEMICAL PHYSICS LETTERS
which corresponds to the blue part of the absorption spectrum. The non-exponential decay character is obvious. To estimate the ranges of temporal changes in the decay we fitted the curve as a sum of exponents. Reasonable fitting can be achieved by the sum of three exponents. Fastest (320 fs, 43%) and middle ( 1.3 ps, 48%) components are defined well, as can be seen in the insets b and c of fig 1. The fitting is not so sensitive to the weak third component ( > 10 ps, -C10%) but inclusion of this component gives much better fitting. We also fitted decay curves by assuming Gaussian distributions of the first and second exponents. The best result was obtained with very good agreement in lifetimes (0.33 and 1.3 ps) in the case of a surprisingly narrow width of distribution for the first decay component as small as a few femtoseconds. Fluorescence decays of OX1 in AN measured in a wide wavelength region (680,700,725,775, and 830 nm) displayed the same time behavior within the present experimental accuracy (fig. 2). A very sim-
ilar non-exponential time behavior at 670,690, 705, 725, 775, and 830 nm was observed for NB in AN. Under excitation upon various polarization angles with respect to that of the laser beam, no deviations of the decay curves were found from the results in the case of excitation at the magic angle. Fluorescence decays of OX1 in DMA (fig. 3) and NB in DMA (not shown ) upon excitation at 605 nm can be well fitted by a single exponential dependence. Since the ET rate in DMA for both OX 1 and NB is quite high, results of the fitting procedure are very sensitive to the system response function. Using the cross-correlation function as a response function, the time constants of 280 and 160 fs were obtained for OX1 and NB, respectively. Taking into account possible distortion of the system response function in the way of expansion of the cross-correlation and considering the value of x2 as a criterium of fitting, 8000
: : ’
a
I
:’ ’
.! *i : .. : :i ‘0
6000
: . , I
. .....” . .-_ . *“.I. ”
.__ .
h
0
-0.5
I
I
I
0
5
10
Time
I ps
Fig. 2. Fluorescence decay curves of oxazine 1 in aniline on a logarithmic scale measured at 680,700, 725,775, and 830 nm. Data were normalized at a delay time of 500 fs.
548
1.o Time/ps
0.5
1.5
2.0
2.5
:: .
;: 1 o-2
0.0
Fig. 3. (a) Fluorescence decay of oxazine 1 in N,N-dimethylaniline on a linear scale. (I ) Experimental data points and ( 0 ) instrument response function. The result of rigorous error analysis is shown in insert (b) .
Volume 207, number 4,5,6
CHEMICAL PHYSICS LETTERS
a lifetime as short as 150 fs was found in the case of ET for OX1 in DMA.
4. Discussion Fluorescence of a laser dye, nile blue perchlorate (NB), is strongly quenched in electron donating solvents and shows an ultrafast decay, which can be interpreted as ET from solvent to dye. In order to prove the presence of ET we have undertaken sub-picosecond transient absorption measurements and observed the reaction products (solvent cation and NB neutral radical) [ 161. To assign the fluorescence decays as the real characteristics of ET, one should consider possible contributions to the obtained kinetics by intramolecular vibrational relaxation and dynamic solvent Stokes shift. ( 1) In a time range from subpicosecond to a few picoseconds, no effect of vibrational relaxation or solvent shift can be seen, since the same decay shape was found in all the observed wavelengths from the blue to red edge of the possible fluorescence spectrum. Additionally, no measurable deviations from single exponential time behavior due to intramolecular vibrational relaxation were observed at 100-300 fs in DMA (fig. 3). (2) To prove this conclusion we further measured fluorescence decays with excitation at 675 nm, which is close to the O-O transition. In AN non-exponential decays were observed and almost the same multi-exponential decay components were obtained by deconvolution as in the case of excitation at 605 nm. The fluorescence of OX1 in DMA also gave a similar single exponential decay to that obtained at the 605 nm excitation. (3) In addition to our results, thermalization of ground and excited states was reported to be as fast as x50 fs for the same oxazine dye, cresyl violet, in transient spectroscopy with 60 fs pulse excitation and 10 fs pulse probing [ 221. With these three observations, one can consider that ET takes place without much pex-turbation from ultrafast intramolecular vibrational relaxation. The non-exponential fluorescence decays measured for these dyes in AN have to be related to the nature of ET itself. Such behavior can be explained using the idea of two-dimensional reaction potential surfaces, suggested by Sumi and Marcus [ 8 1. In this
28 May 1993
approach it is assumed that a diffusive (Brownian) motion along the solvation coordinate gives reorientation of the solvents, whereas overcoming of the activation barrier occurs by the nuclear motion along the classical vibrational coordinate. The former is limited by the solvent relaxation time, and thus is usually slower than the latter. Non-exponential ET will be observed when the following two situations are realized simultaneously. ( 1) Potential energy barrier crossing for ET reaction induced by nuclear motion is faster than the solvation process. (2 ) The energy barrier heights are different at different orientations of the solvent. In this case the reaction will disturb a thermal distribution of reactant orientations, which will give rise to nonexponential dynamics. The result of decay fitting using Gaussian distributions of the time constants could also be understood with the model of static non-exponentiality. The fastest component may be related to the presence of some “best” orientation of reactants with stronger interactions, ET being controlled only by nuclear motion, If the “best” orientation is not formed or if it is disturbed, ET becomes slower due to weaker reactants interaction. The slower part of the fluorescence decay reflects a variety of distributions of the ET rates and/or solvent reorientation dynamics. We have calculated longitudinal relaxation time rL of AN from literature data of Debye relaxation times (ru) and dielectric constants [23]. Two values of z, were reported [ 241, probably due to rotation of total molecular orientation and a partial (amino group) orientation. At room temperature two relaxation times were obtained to be 1.2 and 15 ps. These two relaxation times are close to the middle ( 1.3 ps) and slow ( > 10 ps) component of fluorescence decay in AN which may mean that they are depending on solvation process. The above values are not in entire agreement with the absence of solvent Stokes shift in our experiments. Direct measurement of the solvation time of AN is in progress by observing the dynamic Stokes shift using coumarin dyes as a probe molecule, a preliminary result giving a fastest component of x 3 ps for AN [ 25 1. The fastest component ( s 350 fs) will probably represent only the potential-barrier crossing step along the nuclear coordinate from the best solvent orientation. 549
Volume 207, number 4,5,6
CHEMICAL PHYSICS LETTERS
ET with a simple single-exponential decay in the order of 100 fs was observed for the dyes in DMA. The exponential dynamics much faster than solvent fluctuations may come from very low-energy barrier or barrierless processes. The ionization potential for DMA is lower than AN, i.e. 7.12 eV for DMA and 7.68 eV for AN [ 261. If the energy barrier heights do not change so much with solvent orientations, no effect of solvent is expected and a single exponential ET dynamics will be realized. ET is determined mainly by nuclear motion and not by salvation process. Generally, ultrafast ET can be induced by molecular vibrations (intra- and/or inter-molecular vibrational motions). In conclusion, ultrafast intermolecular electron transfer of oxazine dyes in weakly polar electron-donating solvents is observed, ET in DMA showed a single exponential behavior with a rate constant much faster than the solvation process and ET was explained to be caused by nuclear motions. ET in AN, on the other hand, showed clear non-exponential behavior, which was explained by combined effects of nuclear motion and solvation processes.
Acknowledgement The authors are grateful to Professor P.F. Barbara, G.R. Fleming, J. Jortner, Y. Haas and H. Sumi for fruitful discussions and multiple suggestions. This work was supported in part by a Grant-in-Aid for scientific Research on New Program (OlNP0301) by the Ministry of Education, Science, and Culture of Japan.
550
28 May 1993
References [ 11H.A. Kramers, Physics 7 ( 1940) 284. [2] [3] [4] [5]
L.D. Zusmann, Chem. Phys. 49 (1980) 295. J.T. Hynes, J. Phys. Chem. 90 (1986) 3701. 1. Rips and J. Jortner, J. Chem. Phys. 88 (1988) 818. M. Sparpaglioni and S. Mukamel, J. Chem. Phys. 88 (1988) 3265. 161I. Rips, J. Klafter and J. Jortner, J. Chem. Phya. 94 ( 1990) 8557. [ 71 J. Jortner and M. Bixon, J. Chem. Phys, 87 ( 1988) 167. [8] H. Sumi and R.A. Marcus, J.Chem. Phys. 84 (1986) 4894. [93 W. NadlerandR.A.Mareus, J.Chem. Phys.86 (1987) 3906. [lo] E.M. Kosower and D. Hupper, Chem. Phys. Letters 96 (1983) 433. [ 111 E.M. Kosower and D. Huppert, Ann. Rev. Phys. Chem. 37 (1986) 127. [ 121 M.A. Kalow, T.J. Kang and P.F. Barbara, J. Phys. Chem. 91 (1987) 6452. [ 131J.D. Simon and S.-G. Su, J. Phys. Chem. 92 (1988) 2395. [14]R.M. Neison, GE. McManis, M.N. Golovin and M.J. Weaver, J. Phys. Chem. 92 (1988) 3441. [ 151 T. Kobayashi, Y. Takagi, H. Kaudori, K_ Kemnitz and K. Yoshihara, Chem. Phys. Letters 180 (1991) 416. [ I6 ] H. Kandori, K. Kern& and K. Yoshihara, J. Chem. Phys. 96 (1992) 8042. [ 171 K. Yoshihara, A. Yartsev, ,Y. Nagaaawa, H. Kandori, A. Doubal and K. Kentnitx, in: Ultrafast Phenomena 7, in press. [ 181 G.C. Walker, E. Akesson, A.E. Johnson, N.E.Levinger and P.F. Barbara, J. Phys. Chem. 96 (1992) 3728. [ 19) F. Pollinger, H. Heittele, M.E. Michel-Beyerle, C. A&es and H.A. Staab, Chem. Phys. Letters, in press. [ 201 M.D. Dawson, T.E. Boggess, D.W. Garvey and A.L. Smirl, Opt. Commun. 60 (1986) 79. [21] M.D. Dawson, T.E. Boggess,D.W. GarveyandA.L. Smirl, IEEE J. Quantum Electron. QE-23 ( 1987) 290. [22] C.H. Brito Cruz, R.L. Fork, W.H. Knox and C.V. Shank, Chem. Phys. LettersI32 ( 1986) 34 1. [ 231 J. Bhattacharyya, A. Hasan, S.B. Roy and G.S. Kastha, J. Phys. Sot. Japan 28 ( 1970) 204. [ 2419. Bagchi, D.W. Oxtoby and R. Fleming, J. Chem. Phys. 86 (1984) 257. [ 251 Y.Nagasawa, A. Yartaev, A.E. Johnson, K. Tominaga and K Yoshihara, to be published. [26]O.B.Nagy,S. DurireandJ.B.Nagy,Tetrshedron31 (1975) 2453.
CHEMICAL PHYSICS LETTERS
28 May 1993
Solvent and nuclear dynamics in ultrafast intermolecular electron transfer in a diffusionless, weakly polar system Arkadiy Yartsev a*L,Yutaka Nagasawa b, Abderrazzak Douhal a and Keitaro Yoshihara a,b a InstituteforMolecularScience,Myodadi,Okazaki444, Japan b The GraduateUniversity for AdvancedStudies,Myodaiji,okazaki 444, Japan Received 8 March 1993
Femtosecond intermolecular electron (ET) transfer dynamics were studied by time-resolved fluorescence up-conversion technique in contact systems of oxazine dyes in electron-donating solvents. Clearly non-exponential ET time dependence was observed in aniline and explained by the effects of both solvent reorientation and nuclear motion of the reactants. Single exponential processes were measured for nile blue ( Q 160 fs) and oxazine 1 ( 6 280 fs) in N,Ndimethylaniline. The rate of ET is explained to be limited only by ultrafast nuclear relaxation.
1. Introduction Electron transfer (ET) is one of the most important chemical reactions because of its universality in chemistry and biology. Intermolecular ET, analogous to the primary steps in the photosynthetic reaction centers of green plants and bacteria, is of special interest. Dynamical aspects of solvent effects on the course of ET were intensively studied both theoretically [ l-91 and experimentally [ 10-l 81. For ET with a rate comparable to the solvent fluctuation time, motion and structure of the solvent can determine the rate of chemical reaction. It becomes inversely proportional to the longitudinal relaxation time (TV)of the solvent and is often called “solventcontrolled reaction” [ lo- 141. Recently this effect has been experimentally confirmed [ 15-191. However, if the reaction becomes faster than the solvent relaxation times, a new type of reaction mechanism, controlled by inter- and intra-molecular vibrations would make a significant contribution to the reaction. ’ Permanent address: Institute of Spectroscopy, Russian Academy of Science, 142092 Troitzk, Moscow Region, Russian Federation. 544
We have reported ultrafast ET (as fast as = 1013 s-l) between excited dye and weakly polar electrondonating solvent molecules [ 15-l 71. This system has several specific features. ( 1) Electron acceptor molecules (solute) are surrounded by donor solvent molecules and they are in contact. There is no translational diffusion in order to induce ET. (2) The weakly polar solvent system gives less dielectric friction to ET. (3) There is no change of the net charge before and after the reaction [ 161. (4) The rate of ET is x 50 times greater than that of the solvent longitudinal relaxation time. The time scale of ET in these systems falls into that of solvent relaxation and of nuclear motion and the reaction will be severely influenced by these dynamics. It is also the aim of the present study to investigate the role of intermolecular interaction, namely, solvent effects to the reaction as well as the effects of inter- and intra-molecular dynamics.
2. Experimental A synchronously pumped hybridly modelocked dye laser with control of group velocity dispersion (GVD) by a prism pair was made to generate optical Elsevier Science Publishers B.V.
Volume207, number 4,5,6
CHEMICALPHYSICSLETTERS
pulses at different wavelengths at 76 MHz. Using a combination of rhodamine 6G and DGDCI/DQOCI [ 201 and spatial filters between a second prism and end mirror, 70 fs pulses were generated at 583,605, and 6 15 nm with an output power of = 100 mW. With a combination of sulforhodamine 101 and DQTCI [ 211 generation of 50 fs pulses at 675 nm was achieved. Fluorescence decays at the different polarizations of emission were measured by a conventional fluerescence up-conversion system using a 0.3 mm BBO crystal (type I). Equalization of GVD ‘in both branches of an up-conversion system and GVD precompensation by a prism pair was made to achieve the smallest’width of cross-correlation. This was measured as a noncollinear second-harmonic generation on a BBO crystal with residual beam after sample excitation instead of fluorescence. The narrowest cross-correlation corresponded to 80 fs at 600 and 60 fs at 675 nm. Fluorescence decay measurements were repeated 1O-50 times for obtaining a single decay curve in order to avoid long term instabilities of the dye laser during accumulation. Analysis of measured kinetics was performed by the “Global Unlimited”’ fitting program. One of the advantages of this program is the possibility to perform a rigorous error estimation procedure. The parameter being examined is allowed to vary discretely in some interval, while all other parameters are adjusted to minimize 2’. The cross-correlation was chosen as a system response function in almost all fittings. Nevertheless the effects of fluorescence signal broadening by the dispersive optics elements may be important since the dispersion precompensation by a prism pair was optimized for the wavelength of the dye laser. GVD for different wavelengths, at which the fluorescence decays were measured, was not entirely compensated, leading to effective broadening of the system response function. Different divergence of the dye laser and the fluorescence may also give some response function broadening. Consequently, the results of fitting with cross-correlation as a response function show an upper limit for fluorescence quenching time constants, NiIe blue perchlorate (NB) and oxazine 1 perchlorate (OX 1) were obtained from Exiton and used without purification. Aniline (AN) and N,N-dimethylaniline (DMA) were vacuum distilled immedi-
28 May 1993
ately before use. The concentration of NB and OX1 in the electron-donating solvent was varied from 0.5 x 1Om4to 2 x 1Om4M. Dye solutions were bubbled with nitrogen to eliminate a possible effect of solvent oxidation. All the measurements were made at room temperature ( 24 ’C ) . A flowing sample cell made of quartz with 1 mm optical path length was used to minimize thermal effects on the fluorescence decay. To record one fluorescence decay shown in the tigures, it took 5 h for OX1 /AN and 20 h for OX l/ DMA.
3. Results In fig. 1 fluorescence decay of OX 1 in AN is shown. This was obtained upon the excitation at 605 nm, 25X1 O3
20
VI E
15
?3 10
5
-2
a
2
6
YJ 0
Time&
0.2
0.4
0.6
1.0 112 114 1:6 1:il
WPS ‘ZJps Fii 1. (a) Fluorescencedecay of oxazine 1 in aniline on a linear scale.(W) Experimental data points and (0) instrument response function. The results of rigorous error analysis for (b)
fast and (c) middle time components are also shown.
547
Volume 207, number 4,5,6
28 May 1993
CHEMICAL PHYSICS LETTERS
which corresponds to the blue part of the absorption spectrum. The non-exponential decay character is obvious. To estimate the ranges of temporal changes in the decay we fitted the curve as a sum of exponents. Reasonable fitting can be achieved by the sum of three exponents. Fastest (320 fs, 43%) and middle ( 1.3 ps, 48%) components are defined well, as can be seen in the insets b and c of fig 1. The fitting is not so sensitive to the weak third component ( > 10 ps, -C10%) but inclusion of this component gives much better fitting. We also fitted decay curves by assuming Gaussian distributions of the first and second exponents. The best result was obtained with very good agreement in lifetimes (0.33 and 1.3 ps) in the case of a surprisingly narrow width of distribution for the first decay component as small as a few femtoseconds. Fluorescence decays of OX1 in AN measured in a wide wavelength region (680,700,725,775, and 830 nm) displayed the same time behavior within the present experimental accuracy (fig. 2). A very sim-
ilar non-exponential time behavior at 670,690, 705, 725, 775, and 830 nm was observed for NB in AN. Under excitation upon various polarization angles with respect to that of the laser beam, no deviations of the decay curves were found from the results in the case of excitation at the magic angle. Fluorescence decays of OX1 in DMA (fig. 3) and NB in DMA (not shown ) upon excitation at 605 nm can be well fitted by a single exponential dependence. Since the ET rate in DMA for both OX 1 and NB is quite high, results of the fitting procedure are very sensitive to the system response function. Using the cross-correlation function as a response function, the time constants of 280 and 160 fs were obtained for OX1 and NB, respectively. Taking into account possible distortion of the system response function in the way of expansion of the cross-correlation and considering the value of x2 as a criterium of fitting, 8000
: : ’
a
I
:’ ’
.! *i : .. : :i ‘0
6000
: . , I
. .....” . .-_ . *“.I. ”
.__ .
h
0
-0.5
I
I
I
0
5
10
Time
I ps
Fig. 2. Fluorescence decay curves of oxazine 1 in aniline on a logarithmic scale measured at 680,700, 725,775, and 830 nm. Data were normalized at a delay time of 500 fs.
548
1.o Time/ps
0.5
1.5
2.0
2.5
:: .
;: 1 o-2
0.0
Fig. 3. (a) Fluorescence decay of oxazine 1 in N,N-dimethylaniline on a linear scale. (I ) Experimental data points and ( 0 ) instrument response function. The result of rigorous error analysis is shown in insert (b) .
Volume 207, number 4,5,6
CHEMICAL PHYSICS LETTERS
a lifetime as short as 150 fs was found in the case of ET for OX1 in DMA.
4. Discussion Fluorescence of a laser dye, nile blue perchlorate (NB), is strongly quenched in electron donating solvents and shows an ultrafast decay, which can be interpreted as ET from solvent to dye. In order to prove the presence of ET we have undertaken sub-picosecond transient absorption measurements and observed the reaction products (solvent cation and NB neutral radical) [ 161. To assign the fluorescence decays as the real characteristics of ET, one should consider possible contributions to the obtained kinetics by intramolecular vibrational relaxation and dynamic solvent Stokes shift. ( 1) In a time range from subpicosecond to a few picoseconds, no effect of vibrational relaxation or solvent shift can be seen, since the same decay shape was found in all the observed wavelengths from the blue to red edge of the possible fluorescence spectrum. Additionally, no measurable deviations from single exponential time behavior due to intramolecular vibrational relaxation were observed at 100-300 fs in DMA (fig. 3). (2) To prove this conclusion we further measured fluorescence decays with excitation at 675 nm, which is close to the O-O transition. In AN non-exponential decays were observed and almost the same multi-exponential decay components were obtained by deconvolution as in the case of excitation at 605 nm. The fluorescence of OX1 in DMA also gave a similar single exponential decay to that obtained at the 605 nm excitation. (3) In addition to our results, thermalization of ground and excited states was reported to be as fast as x50 fs for the same oxazine dye, cresyl violet, in transient spectroscopy with 60 fs pulse excitation and 10 fs pulse probing [ 221. With these three observations, one can consider that ET takes place without much pex-turbation from ultrafast intramolecular vibrational relaxation. The non-exponential fluorescence decays measured for these dyes in AN have to be related to the nature of ET itself. Such behavior can be explained using the idea of two-dimensional reaction potential surfaces, suggested by Sumi and Marcus [ 8 1. In this
28 May 1993
approach it is assumed that a diffusive (Brownian) motion along the solvation coordinate gives reorientation of the solvents, whereas overcoming of the activation barrier occurs by the nuclear motion along the classical vibrational coordinate. The former is limited by the solvent relaxation time, and thus is usually slower than the latter. Non-exponential ET will be observed when the following two situations are realized simultaneously. ( 1) Potential energy barrier crossing for ET reaction induced by nuclear motion is faster than the solvation process. (2 ) The energy barrier heights are different at different orientations of the solvent. In this case the reaction will disturb a thermal distribution of reactant orientations, which will give rise to nonexponential dynamics. The result of decay fitting using Gaussian distributions of the time constants could also be understood with the model of static non-exponentiality. The fastest component may be related to the presence of some “best” orientation of reactants with stronger interactions, ET being controlled only by nuclear motion, If the “best” orientation is not formed or if it is disturbed, ET becomes slower due to weaker reactants interaction. The slower part of the fluorescence decay reflects a variety of distributions of the ET rates and/or solvent reorientation dynamics. We have calculated longitudinal relaxation time rL of AN from literature data of Debye relaxation times (ru) and dielectric constants [23]. Two values of z, were reported [ 241, probably due to rotation of total molecular orientation and a partial (amino group) orientation. At room temperature two relaxation times were obtained to be 1.2 and 15 ps. These two relaxation times are close to the middle ( 1.3 ps) and slow ( > 10 ps) component of fluorescence decay in AN which may mean that they are depending on solvation process. The above values are not in entire agreement with the absence of solvent Stokes shift in our experiments. Direct measurement of the solvation time of AN is in progress by observing the dynamic Stokes shift using coumarin dyes as a probe molecule, a preliminary result giving a fastest component of x 3 ps for AN [ 25 1. The fastest component ( s 350 fs) will probably represent only the potential-barrier crossing step along the nuclear coordinate from the best solvent orientation. 549
Volume 207, number 4,5,6
CHEMICAL PHYSICS LETTERS
ET with a simple single-exponential decay in the order of 100 fs was observed for the dyes in DMA. The exponential dynamics much faster than solvent fluctuations may come from very low-energy barrier or barrierless processes. The ionization potential for DMA is lower than AN, i.e. 7.12 eV for DMA and 7.68 eV for AN [ 261. If the energy barrier heights do not change so much with solvent orientations, no effect of solvent is expected and a single exponential ET dynamics will be realized. ET is determined mainly by nuclear motion and not by salvation process. Generally, ultrafast ET can be induced by molecular vibrations (intra- and/or inter-molecular vibrational motions). In conclusion, ultrafast intermolecular electron transfer of oxazine dyes in weakly polar electron-donating solvents is observed, ET in DMA showed a single exponential behavior with a rate constant much faster than the solvation process and ET was explained to be caused by nuclear motions. ET in AN, on the other hand, showed clear non-exponential behavior, which was explained by combined effects of nuclear motion and solvation processes.
Acknowledgement The authors are grateful to Professor P.F. Barbara, G.R. Fleming, J. Jortner, Y. Haas and H. Sumi for fruitful discussions and multiple suggestions. This work was supported in part by a Grant-in-Aid for scientific Research on New Program (OlNP0301) by the Ministry of Education, Science, and Culture of Japan.
550
28 May 1993
References [ 11H.A. Kramers, Physics 7 ( 1940) 284. [2] [3] [4] [5]
L.D. Zusmann, Chem. Phys. 49 (1980) 295. J.T. Hynes, J. Phys. Chem. 90 (1986) 3701. 1. Rips and J. Jortner, J. Chem. Phys. 88 (1988) 818. M. Sparpaglioni and S. Mukamel, J. Chem. Phys. 88 (1988) 3265. 161I. Rips, J. Klafter and J. Jortner, J. Chem. Phya. 94 ( 1990) 8557. [ 71 J. Jortner and M. Bixon, J. Chem. Phys, 87 ( 1988) 167. [8] H. Sumi and R.A. Marcus, J.Chem. Phys. 84 (1986) 4894. [93 W. NadlerandR.A.Mareus, J.Chem. Phys.86 (1987) 3906. [lo] E.M. Kosower and D. Hupper, Chem. Phys. Letters 96 (1983) 433. [ 111 E.M. Kosower and D. Huppert, Ann. Rev. Phys. Chem. 37 (1986) 127. [ 121 M.A. Kalow, T.J. Kang and P.F. Barbara, J. Phys. Chem. 91 (1987) 6452. [ 131J.D. Simon and S.-G. Su, J. Phys. Chem. 92 (1988) 2395. [14]R.M. Neison, GE. McManis, M.N. Golovin and M.J. Weaver, J. Phys. Chem. 92 (1988) 3441. [ 151 T. Kobayashi, Y. Takagi, H. Kaudori, K_ Kemnitz and K. Yoshihara, Chem. Phys. Letters 180 (1991) 416. [ I6 ] H. Kandori, K. Kern& and K. Yoshihara, J. Chem. Phys. 96 (1992) 8042. [ 171 K. Yoshihara, A. Yartsev, ,Y. Nagaaawa, H. Kandori, A. Doubal and K. Kentnitx, in: Ultrafast Phenomena 7, in press. [ 181 G.C. Walker, E. Akesson, A.E. Johnson, N.E.Levinger and P.F. Barbara, J. Phys. Chem. 96 (1992) 3728. [ 19) F. Pollinger, H. Heittele, M.E. Michel-Beyerle, C. A&es and H.A. Staab, Chem. Phys. Letters, in press. [ 201 M.D. Dawson, T.E. Boggess, D.W. Garvey and A.L. Smirl, Opt. Commun. 60 (1986) 79. [21] M.D. Dawson, T.E. Boggess,D.W. GarveyandA.L. Smirl, IEEE J. Quantum Electron. QE-23 ( 1987) 290. [22] C.H. Brito Cruz, R.L. Fork, W.H. Knox and C.V. Shank, Chem. Phys. LettersI32 ( 1986) 34 1. [ 231 J. Bhattacharyya, A. Hasan, S.B. Roy and G.S. Kastha, J. Phys. Sot. Japan 28 ( 1970) 204. [ 2419. Bagchi, D.W. Oxtoby and R. Fleming, J. Chem. Phys. 86 (1984) 257. [ 251 Y.Nagasawa, A. Yartaev, A.E. Johnson, K. Tominaga and K Yoshihara, to be published. [26]O.B.Nagy,S. DurireandJ.B.Nagy,Tetrshedron31 (1975) 2453.