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Rotational relaxation of rod like molecules: Diphenylacetylene in various solvents
journal of MOLECULAR
LIQUIDS ELSEVIER
Journal of Molecular Liquids, 65166 ( i 995) 42 i - 4 2 4
Rotational Relaxation of Rod Like Molecules: Diphenylacetylene in Various Solvents. Yoshinori Hirata, Yasuhiko Kanemoto, Tadashi Okada, and Tateo Nomoto* Department of Chemistry, Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka 560, Japan and tDepartment of Chemistry, Faculty of Education, Mie University, Tsu, Mie 514, Japan Abstract By using fluorescence anisotropy decay and polarized transient absorption spectrum measurement techniques with a picosecond time resolution, we have studied the rotational relaxation of 4-[[4-(dimethylamino)phenyl]ethynyl]-benzonitrile in various solvents. Our aim is to clarify the solvent-solute interactions revealed as a solvent dependence of the rotational relaxation time of solute molecules. Among the solvents we used, ethanol shows characteristic behavior compared with other polar solvents. The rotational relaxation time in ethanol seem to be close to that calculated on the basis of the slip boundary condition, while in other polar solvents, the stick boundary condition can reproduce the experimental value. The results suggests that the strong hydrogen bonding between the solvent molecules allows the solute to rotate more freely within the solvent cage than in other polar solvents. I. Introduction Rotational diffusion of molecules in liquids should be quite important to establish the microscopic description of chemical reaction dynamics in the solution phase. Sometimes, geminate process was suggested to be governed by rotational diffusion rather than a translational diffusion. It may play an important role in the radical dimer formation process on the geminate encounter of p-aminophenylthiyl radical pair. 1 Reorientation relaxation of a solute molecule is usually measured by monitoring either the time dependent fluorescence depolarization or the transient absorption of a polarized light pulse. Rotational diffusion of dye molecules in various solvents has been investigated extensively. Although many of them aimed for the elucidation of solvent-solute interactions including hydrogen bonding, there are ambiguities which come from the electronic relaxation and geometrical change of solute molecules. In order to simplify the situation and extract the information about the rotational diffusion of solute molecules, it is necessary to reduce the flexibility and complexity of the molecular structure, which cause the change of the direction of the transition dipole moment within the molecular frame. Considering these factors, we have chosen 4-[[4-(dimethylamino)phenyl]ethynyl]-
H3C/
I benzonitrile as a probe molecule. This molecule has a rigid rod-like structure and the transition moment between the ground and fluorescence states (Sx state in nonpolar 0167-7322/95/$09.50 9 1995 Elsevier Science B.V. All fights reserved. SSDi 0167-7322 (95) 00890-X
422
solvents and charge separated state in polar solvents) are parallel to the long molecular axis. Although the electronic relaxation and twisting of the phenyl rings which occur on the intramolecular charge separation may give some difficulty, such characteristics should allow us a simplified view of the rotational relaxation phenomena. Boundary condition of the rotational motion in the solution phase should reflect the solvent-solute interactions. The solvents used in this work is found to be devided into two rOUpS. One is slip and the other is stick. In ethanol, the rotational relaxation time was und to be shorter than those estimated for aprotic solvents with a similar viscosity. The viscosity dependence of the rotational relaxation time in various solvents will be discussed. II. Experimental The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time--resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere? For the time-correlated single photon counting measurement of which the response time is about 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti:sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm; 10 ps FWHM) was used to measure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. 3 All the solvents of spectro-grade were used without further purification.
III. Results and Discussion As shown in Figure 1, fluorescence spectrum of I shows a significant solvent effect. In nonpolar solvents, the fluorescence maximum is observed around 370 nm and the absorption and fluorescence spectra show mirror image symmetry. On the other hand, in polar solvents the mirror image symmetry is broken and the broad fluorescence band of which the maximum appears in the long wavelength region is observed. The band maximum shifts to the red with increasing solvent polarity. Measuring picosecond time resolved absorption spectra of I in polar solvents, we observed the spectral change due to the intramolecular charge separation. In polar solvents such as diethylether ether and acetonitrile, immediately after the excitation, the S, ,--$1 absorption band is observed, while the spectrum ascribed to the charge separated state is dominant in the delay time region longer than 10 ps after the excitation laser pulse. The major part of the observed fluorescence in polar solvents seems to be from the charge separated state of I. Because of the short lifetime of $1 in polar solvents, rotational diffusion of $1 should not be significant. It was suggested that the ground and $1 states of I are almost planar and in the charge separated state, two phenyl rings are perpendicular to each other. The geometrical change on the charge separation may not affect the rotational diffusion of the charge separated state significantly because the molecular volume and the ratio of the long/short molecular axis are essentially the same for the $1 and charge separated states. The time dependence of the fluorescence polarized parallel and perpendicular to the pump polarization direction were fitted simultaneously to
I(t) = (I(O)/3)exp(-t/r!)l + 2r(O)exp(-t/ror )
(1)
I(t) = (I(O)/3)exp(-t/r t)l - v(O)exp(-t/ror).
(2)
423
By using nonlinear least squares fitting, the orientation relaxation time (rot), fluorescence lifetime (rf), and the initial value of anisotropy (r(0)) were obtained. The typical results of the fitting to the time dependence of polarized fluorescence of I in n-hexane is shown in Fig. 1. In order to confirm the validity of the data analysis, the fluorescence lifetime was also measured with the magic angle arrangement. 3_. 8
1 ,..,
7.")("
4 "~""~
' ' '
A4i "i,...\ , -",,"Y,.",. \ -~
,
t
i ", ' ', .'. \1;./
.,
i./
400
',..
"-;x \
~,,,,,,,,,,..
500 600 ~,Vavelength [ nm ]
",,(,,:..,.,,..,,:., 700
F i g u r e 1. Fluorescence spectra of I in cyclohexane (6),trans-decalin(8), diethylether (1), ethanol (7), acetone (3), and acetonitrile (4).
"~ 0
I
i
200 ~00
I
0 200 ~00 600 t [ chonnel ]
F i g u r e 2. Time dependence of the polarized fluorescence of I in n-hexane (1 channel = 2.54 ps). The residuals are also shown in the figure.
Except for ethanol solution, a good fitting was obtained for the fluorescence decay curves. In ethanol, even the fluorescence measured at the magic angle shows a complex decay, which should be due to the dynamic Stokes shift. Although we got a rather poor fitting to the biexponential decay function in the short delay times, the estimated fluorescence decay times obtained from the measurements at the magic angle were about 10 and 220 ps. From the fitting of the polarized fluorescence in the long delay times, the rotational relaxation time was obtained to be 124 ps. Reliable value for r(0) was not obtained for this case. The measured rotational reorientation times, initial value of the fluorescence anisotropy, and fluorescence lifetimes in several solvents are listed in Table I. The initial value of anisotropy is very close to 0.4, which suggests that the directions of the transition dipole of the absorption and the fluorescence are the same and the depolarization not due to the reorientation of the molecule can be ignored. Figure 3 shows the plot of the rotational reorientation time versus the solvent viscosity. The straight lines show the reorientation times calculated as rot = C . f~tick " V r # / k B T ,
(3)
where V is molecular volume of solute, 77 is solvent viscosity, and kB is the Boltzmann constant, fstick depends on the molecular shape and is calculated and tabulated by Hu and Zwanzit~.4 The C value is the ratio of the friction coefficient with slip to the friction coefficient ot stick. The probe molecule has been modeled as a prolate ellipsoid with the molecular volume and the ratio of about 70 A 3. Acetone (3) and N,N-dimethylformamide (5) are on the solid line, which was calculated for stick. On the other hand, several aliphatic hydrocarbon solvents (2, 6, 8) and ethanol (7) seem to be
424
on the broken line. The ratio of the slopes of the broken and solid lines is about 0.78, which is slightly smaller than the calculated value from the molecular shape. It is known that the reorientation relaxation times of uncharged molecules are rather close to those estimated under slip boundary condition. The nonpolar solution of I seems to be the case. On the other hand, the stick boundary condition seems to give a better prediction in polar aprotic solvents. This should be due to the strong solvat]on of the large dipole moment produced by the intramolecular charge separation. The orientation relaxation time of I in ethanol was found to be very close to that predicted for slip boundary condition. Important point of our results is that the rotational diffusion in ethanol is faster than that expected for the stick boundary condition. Our results may suggest that the strong hydrogen bonding between the solvent molecules allows the solute to rotate more freely within the solvent cage than in other polar solvents. Table I. Excited-state rotational reorientation times, fluorescence lifetimes (ps), and initial values of the fluorescence anisotropy of I in several solvents at room temperature.
1 2 3 4 5 6 7 8
to,. 43 38 63 49 130 102 124 226
Solvent (viscosity(cP)) diethylether ether (0.24) n-hexane (0.32) acetone (0.32) acetonitrile (0.38) N,N-dimethylformamide (0.92) cyclohexane (0.94) ethanol (1.19) trans-decalin (2.13)
200
1"/ 1880 600 1230 750 850 670 224 760
-
7"(0) 0.39 0.39 0.37 0.43 0.38 0.39 0.37
t tt
-
5o
loo
.og
~
13
Oo
1
Visc~sity [(:P ] Figure 3. Viscosity dependence of the reorientation relaxation time of I.
Acknowledgement: The present work was partly supported by Monbusho International Scientific Research Program (05044055) to T. O.
References I. Y.Hirata, Y. Niga, and T. Okada, Chem. Phys. Lett. 221,283 (1994) 2.Y. Hirata, T. Okada, and T. Nomoto, J. Phys. Chem., 97, 9677(1993). 3. T. Nomoto, F. Tanaka, and N. Suzuki, Chem. Express 6, 319 (1991) 4. C. M. Hu. and R. Zwanzig, J. Chem. Phys. 60, 4354 (1974)
LIQUIDS ELSEVIER
Journal of Molecular Liquids, 65166 ( i 995) 42 i - 4 2 4
Rotational Relaxation of Rod Like Molecules: Diphenylacetylene in Various Solvents. Yoshinori Hirata, Yasuhiko Kanemoto, Tadashi Okada, and Tateo Nomoto* Department of Chemistry, Faculty of Engineering Science and Research Center for Extreme Materials, Osaka University, Toyonaka 560, Japan and tDepartment of Chemistry, Faculty of Education, Mie University, Tsu, Mie 514, Japan Abstract By using fluorescence anisotropy decay and polarized transient absorption spectrum measurement techniques with a picosecond time resolution, we have studied the rotational relaxation of 4-[[4-(dimethylamino)phenyl]ethynyl]-benzonitrile in various solvents. Our aim is to clarify the solvent-solute interactions revealed as a solvent dependence of the rotational relaxation time of solute molecules. Among the solvents we used, ethanol shows characteristic behavior compared with other polar solvents. The rotational relaxation time in ethanol seem to be close to that calculated on the basis of the slip boundary condition, while in other polar solvents, the stick boundary condition can reproduce the experimental value. The results suggests that the strong hydrogen bonding between the solvent molecules allows the solute to rotate more freely within the solvent cage than in other polar solvents. I. Introduction Rotational diffusion of molecules in liquids should be quite important to establish the microscopic description of chemical reaction dynamics in the solution phase. Sometimes, geminate process was suggested to be governed by rotational diffusion rather than a translational diffusion. It may play an important role in the radical dimer formation process on the geminate encounter of p-aminophenylthiyl radical pair. 1 Reorientation relaxation of a solute molecule is usually measured by monitoring either the time dependent fluorescence depolarization or the transient absorption of a polarized light pulse. Rotational diffusion of dye molecules in various solvents has been investigated extensively. Although many of them aimed for the elucidation of solvent-solute interactions including hydrogen bonding, there are ambiguities which come from the electronic relaxation and geometrical change of solute molecules. In order to simplify the situation and extract the information about the rotational diffusion of solute molecules, it is necessary to reduce the flexibility and complexity of the molecular structure, which cause the change of the direction of the transition dipole moment within the molecular frame. Considering these factors, we have chosen 4-[[4-(dimethylamino)phenyl]ethynyl]-
H3C/
I benzonitrile as a probe molecule. This molecule has a rigid rod-like structure and the transition moment between the ground and fluorescence states (Sx state in nonpolar 0167-7322/95/$09.50 9 1995 Elsevier Science B.V. All fights reserved. SSDi 0167-7322 (95) 00890-X
422
solvents and charge separated state in polar solvents) are parallel to the long molecular axis. Although the electronic relaxation and twisting of the phenyl rings which occur on the intramolecular charge separation may give some difficulty, such characteristics should allow us a simplified view of the rotational relaxation phenomena. Boundary condition of the rotational motion in the solution phase should reflect the solvent-solute interactions. The solvents used in this work is found to be devided into two rOUpS. One is slip and the other is stick. In ethanol, the rotational relaxation time was und to be shorter than those estimated for aprotic solvents with a similar viscosity. The viscosity dependence of the rotational relaxation time in various solvents will be discussed. II. Experimental The rotational reorientation times of the sample in several solvents at room temperature were measured by picosecond time--resolved fluorescence and absorption depolarization spectroscopy. Details of our experimental setups were described elsewhere? For the time-correlated single photon counting measurement of which the response time is about 40 ps, the sample solution was excited with a second harmonics of a femtosecond Ti:sapphire laser (370 nm) and the fluorescence polarized parallel and perpendicular to the direction of the excitation pulse polarization as well as the magic angle one were monitored. The second harmonics of the rhodamine-640 dye laser (313 nm; 10 ps FWHM) was used to measure the polarized transient absorption spectra. The synthesis of the sample is given elsewhere. 3 All the solvents of spectro-grade were used without further purification.
III. Results and Discussion As shown in Figure 1, fluorescence spectrum of I shows a significant solvent effect. In nonpolar solvents, the fluorescence maximum is observed around 370 nm and the absorption and fluorescence spectra show mirror image symmetry. On the other hand, in polar solvents the mirror image symmetry is broken and the broad fluorescence band of which the maximum appears in the long wavelength region is observed. The band maximum shifts to the red with increasing solvent polarity. Measuring picosecond time resolved absorption spectra of I in polar solvents, we observed the spectral change due to the intramolecular charge separation. In polar solvents such as diethylether ether and acetonitrile, immediately after the excitation, the S, ,--$1 absorption band is observed, while the spectrum ascribed to the charge separated state is dominant in the delay time region longer than 10 ps after the excitation laser pulse. The major part of the observed fluorescence in polar solvents seems to be from the charge separated state of I. Because of the short lifetime of $1 in polar solvents, rotational diffusion of $1 should not be significant. It was suggested that the ground and $1 states of I are almost planar and in the charge separated state, two phenyl rings are perpendicular to each other. The geometrical change on the charge separation may not affect the rotational diffusion of the charge separated state significantly because the molecular volume and the ratio of the long/short molecular axis are essentially the same for the $1 and charge separated states. The time dependence of the fluorescence polarized parallel and perpendicular to the pump polarization direction were fitted simultaneously to
I(t) = (I(O)/3)exp(-t/r!)l + 2r(O)exp(-t/ror )
(1)
I(t) = (I(O)/3)exp(-t/r t)l - v(O)exp(-t/ror).
(2)
423
By using nonlinear least squares fitting, the orientation relaxation time (rot), fluorescence lifetime (rf), and the initial value of anisotropy (r(0)) were obtained. The typical results of the fitting to the time dependence of polarized fluorescence of I in n-hexane is shown in Fig. 1. In order to confirm the validity of the data analysis, the fluorescence lifetime was also measured with the magic angle arrangement. 3_. 8
1 ,..,
7.")("
4 "~""~
' ' '
A4i "i,...\ , -",,"Y,.",. \ -~
,
t
i ", ' ', .'. \1;./
.,
i./
400
',..
"-;x \
~,,,,,,,,,,..
500 600 ~,Vavelength [ nm ]
",,(,,:..,.,,..,,:., 700
F i g u r e 1. Fluorescence spectra of I in cyclohexane (6),trans-decalin(8), diethylether (1), ethanol (7), acetone (3), and acetonitrile (4).
"~ 0
I
i
200 ~00
I
0 200 ~00 600 t [ chonnel ]
F i g u r e 2. Time dependence of the polarized fluorescence of I in n-hexane (1 channel = 2.54 ps). The residuals are also shown in the figure.
Except for ethanol solution, a good fitting was obtained for the fluorescence decay curves. In ethanol, even the fluorescence measured at the magic angle shows a complex decay, which should be due to the dynamic Stokes shift. Although we got a rather poor fitting to the biexponential decay function in the short delay times, the estimated fluorescence decay times obtained from the measurements at the magic angle were about 10 and 220 ps. From the fitting of the polarized fluorescence in the long delay times, the rotational relaxation time was obtained to be 124 ps. Reliable value for r(0) was not obtained for this case. The measured rotational reorientation times, initial value of the fluorescence anisotropy, and fluorescence lifetimes in several solvents are listed in Table I. The initial value of anisotropy is very close to 0.4, which suggests that the directions of the transition dipole of the absorption and the fluorescence are the same and the depolarization not due to the reorientation of the molecule can be ignored. Figure 3 shows the plot of the rotational reorientation time versus the solvent viscosity. The straight lines show the reorientation times calculated as rot = C . f~tick " V r # / k B T ,
(3)
where V is molecular volume of solute, 77 is solvent viscosity, and kB is the Boltzmann constant, fstick depends on the molecular shape and is calculated and tabulated by Hu and Zwanzit~.4 The C value is the ratio of the friction coefficient with slip to the friction coefficient ot stick. The probe molecule has been modeled as a prolate ellipsoid with the molecular volume and the ratio of about 70 A 3. Acetone (3) and N,N-dimethylformamide (5) are on the solid line, which was calculated for stick. On the other hand, several aliphatic hydrocarbon solvents (2, 6, 8) and ethanol (7) seem to be
424
on the broken line. The ratio of the slopes of the broken and solid lines is about 0.78, which is slightly smaller than the calculated value from the molecular shape. It is known that the reorientation relaxation times of uncharged molecules are rather close to those estimated under slip boundary condition. The nonpolar solution of I seems to be the case. On the other hand, the stick boundary condition seems to give a better prediction in polar aprotic solvents. This should be due to the strong solvat]on of the large dipole moment produced by the intramolecular charge separation. The orientation relaxation time of I in ethanol was found to be very close to that predicted for slip boundary condition. Important point of our results is that the rotational diffusion in ethanol is faster than that expected for the stick boundary condition. Our results may suggest that the strong hydrogen bonding between the solvent molecules allows the solute to rotate more freely within the solvent cage than in other polar solvents. Table I. Excited-state rotational reorientation times, fluorescence lifetimes (ps), and initial values of the fluorescence anisotropy of I in several solvents at room temperature.
1 2 3 4 5 6 7 8
to,. 43 38 63 49 130 102 124 226
Solvent (viscosity(cP)) diethylether ether (0.24) n-hexane (0.32) acetone (0.32) acetonitrile (0.38) N,N-dimethylformamide (0.92) cyclohexane (0.94) ethanol (1.19) trans-decalin (2.13)
200
1"/ 1880 600 1230 750 850 670 224 760
-
7"(0) 0.39 0.39 0.37 0.43 0.38 0.39 0.37
t tt
-
5o
loo
.og
~
13
Oo
1
Visc~sity [(:P ] Figure 3. Viscosity dependence of the reorientation relaxation time of I.
Acknowledgement: The present work was partly supported by Monbusho International Scientific Research Program (05044055) to T. O.
References I. Y.Hirata, Y. Niga, and T. Okada, Chem. Phys. Lett. 221,283 (1994) 2.Y. Hirata, T. Okada, and T. Nomoto, J. Phys. Chem., 97, 9677(1993). 3. T. Nomoto, F. Tanaka, and N. Suzuki, Chem. Express 6, 319 (1991) 4. C. M. Hu. and R. Zwanzig, J. Chem. Phys. 60, 4354 (1974)