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Optical anisotropy of phosphorescence from photoselected benzophenone molecules in polystyrene matrices
Volume 152. number
1
CHEMICAL
4 November
PHYSICS LETTERS
OPTICAL ANISOTROPY OF PHOSPHORESCENCE FROM PHOTOSELECTED BENZOPHENONE MOLECULES
IN POLYSTYRENE
1988
MATRICES
Gregory W. HAGGQUIST and Richard D. BURKHART Department of Chemistry,University ofNevada-Rena, Rena. NV89557. USA Received 26 June 1988; in final form 10 August 1988
Benzophenone present as a dopant in polystyrene matrices has been photoexcited at 406 nm and 77 K using a polarized beam from a dye laser. Polarization ratios of the phosphorescence from these directly produced trlplet states were measured at delay times from 0.5 to 5.0 ms and at various dopant concentrations. At three different phosphorescence wavelengths the polarization ratios decrease as the dopant level increases. The major mechanism for emission depolarization is, evidently, energy migration. An observed dependence of the polarization ratio upon emission wavelength was tentatively ascribed to preferential coupling of triplet sublevels to upper vibrational states of the electronic ground state.
1. Introduction Investigations on the decay of benzophenone triplets in both poly(methy1 methacrylate) (PMMA) and polystyrene matrices over a range of temperatures have been reported by Salmassi and Schnabel [ 11. Non-exponential decays in certain temperature ranges were attributed to the existence of triplettriplet annihilation competing with first-order decay processes. They concluded that bulk translational motion with an unusually large diffusion coeflicient was the underlying mechanism for the transport of triplets. MacCallum and co-workers [2] have also reported the occurrence of T-T annihilation of several aromatic chromophores in PMMA matrices and suggest that the major mechanism for triplet transport is bulk translational diffusion. Horie and Mita [3] have also studied the decay kinetics of benzophenone triplets in a variety of acrylate and methacrylate polymers and concluded that the decay process includes a contribution from diffusional quenching of triplets by side-chain ester groups of the host matrix. In contrast to these results, it has been known for some time that triplet exciton migration among 1,2benzanthracene (BNZ) molecules can account for all of the decay kinetics observed at ambient temperature for this species when present as a dopant in 56
polystyrene films. The results of conventional phosphorescence and delayed fluorescence spectroscopy for this system are consistent with an energy migration scheme [ 41. Absolute rate constants of triplcttriplet annihilation for BNZ in polystyrene are also in accord with an energy migration mechanism [ 5 1. It may be mentioned in passing that a similar controversy over the nature of transport involving singlet state species has also arisen. In this case the effects of polarized excitation have been examined [ 6,7] but the results have not led to a clear-cut interpretation. Since the benzophenone molecule has been a favorite target species for studies in which bulk translational diffusion is claimed, it was decided to carry out photoselection experiments on this molecule dispersed in polystyrene matrices. The purpose of the work was to investigate the question of whether or not triplet transport by exciton migration is a significant process in these molecularly doped polymers. All of the experiments were carried out at 77 K because of signal detection problems at elevated temperatures. Therefore, it is not possible to address the question of possible diffusive migration at elevated temperatures. On the other hand, because of the relatively large oscillator strength for excitation from the ground state (So) to the lowest triplet state (T, ) of benzophenone [ 8 ] it was possible to pop-
0 009-2614/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
B.V.
Volume 152. number 1 ulate T, directly
by use of a polarized
CHEMICAL PHYSICS LETTERS beam
from
a
dye laser. In this way a photo-selected set of benzophenone molecules in the T, state may be produced and no scrambling of the transition dipoles upon passage through the intervening singlet state is possible. Direct excitation from the ground state to T, does not, in itself, guarantee that the triplets produced will have dipoles oriented collinearly with the electric vector of the exciting beam. This is because there are three triplet sublevels and they may all possess some degree of coupling with the ground state singlet. If preferential coupling to one of the triplet sublevels does occur it would be indicated by a polarization ratio different from unity. In fact it has been observed in earlier studies on crystalline benzophenone at 4.2 K that the polarization ratio for the zero-zero emission component is 2.5 [ 91. In the present case, in which benzophenone is a molecular dopant, a polarization ratio larger than unity has also been observed. It is the change in this ratio with dopant concentration which is of primary interest in the present study. The reason for this special interest in concentration effects is due to the information it gives about the primary mechanism of phosphorescence depolarization. If, for example, the extent of depolarization over a given time period is independent of dopant concentration, then a unimolecular mechanism is suggested such as molecular rotation, translation or internal scrambling among T,, T, and T, triplet components. On the other hand, the observation of a concentration-dependent extent of depolarization signals the operation of a bimolecular mechanism such as energy migration. Of secondary interest was the determination of the polarization ratio as a function of emission wavelength. This ratio has been measured at the wavelength maxima corresponding to each of the vibronic bands of the phosphorescence envelope. The results provide additional insight into the details of the interaction between photophysical events and the distribution of dopant molecules in these amorphous polymer films.
4 November 1988
2. Experimental 2.1. Treatment of chemicals Benzophenone obtained from Aldrich Chemical Co. was recrystallized three times from ethanol and was dried at ambient temperature in a vacuum oven. Methanol, 2-methyltetrahydrofuran (MTHF) and benzene were purified using methods described in earlier publications [ 10 1, Polystyrene was purified by a method recently developed in these laboratories expressly intended to remove impurities associated with aromatic carbonyls [ 111. 2.2. Sample preparation All samples were prepared in an oxygen-free nitrogen atmosphere. A solution of benzophenone polystyrene and benzene was placed dropwise onto an optical quartz disk and was gently heated. When the benzene completely evaporated, a second quartz disk was placed on top of the molten solution and the two wafers were pressed together in order to remove any residual gas bubbles. 2.3. Spectroscopic instrumentation A Lambda Physik FL2002 dye laser was pumped with 308 nm light from a Tachisto 401 XR XeCl eximer laser. The dye used, having the commercial designation DPS, has a tuning range from about 390 to 410 nm. In these experiments an excitation wavelength of 406 nm was used to match the absorption band of benzophenone corresponding to the So+TI transition. The dye laser beam, already possessing polarization in the vertical plane to the extent of 1O/ 1, was passed through a sheet polarizer to augment the polarization ratio. The sample was mounted at the end of a cryotip assembly (Air Products and Chemicals LT-3-110) which was enclosed in a vacuum shroud with quartz windows for entrance and exit of light beams. The axis from the center of the sample to the entrance slit of the emission monochromator was oriented at an angle of 45” with respect to the excitation beam. A sheet polarizer in a rotating mount (Melles Griot) made it possible to determine the angle of the PO57
1
Volume 152, number
CHEMICAL
PHYSICS LETTERS
larizer with respect to the laboratory horizontal plane with a precision of 5 0.5 O. The emission monochromator (Spex model 1670) was calibrated for wavelength and was also tested for polarization bias at different wavelengths by monitoring non-polarized fluorescence from fluid solutions of dye molecules. The bias measurements determined in this way were used to correct all of the polarization ratios quoted below. The emission from photoexcited samples was detected by an end window photomultiplier and recorded either on an oscilloscope (Tektronix 2465 ) or else on a signal averager (Nicolet 12/70) after amplification using a Stanford Research Systems, model SR440 preamplifier.
3. Experimental results The emission spectrum from benzophenone in glassy MTHF at 77 K and excited using 406 nm excitation from the dye laser is presented in fig. 1. The starting wavelength was 4 15 nm in order to avoid contamination of the emission from scattered light. The maxima corresponding to the three vibronic components occur at 425,455 and 488 nm. The time response of the light collection system was on the or-
128
1
4 November
I988
der of 100 us and a minimum time delay of 500 us was inserted between the excitation pulse and the monitor event in order to avoid background emission. Polarization ratios were determined by measuring the intensity of the phosphorescence when the emission polarizer was either horizontal (I,) or vertical Z,) . Thus, the ratio is defined by R=l,f Ih. In every case the plane of the excitation beam is vertical in the laboratory frame. It has been our initial intention to try to determine the polarization ratio as a function of time following the excitation pulse, however, at delay times larger than 500 us any time dependence of the polarization ratio was found to be negligible. The widest possible range of concentrations of benzophenone in polystyrene were used in these experiments. There were limitations, however, at both concentration extremes. For samples which were too dilute, the phosphorescence signal was lost in the noise and for those which were too concentrated, there were definite signs of residual crystalline benzophenone in the samples. Four different concentrations were eventually tested as indicated in table 1. The average interchromophore separation distances for each of these samples was also a matter of interest. These were calculated using an equation developed by Chandrasekhar [ 121 and are also indicated in table 1 along with their corresponding concentrations. All of the polarization data, both as a function of concentration and as a function of wavelength, are collected in table 1. A graphical display is presented in fig. 2.
4. Discussion 4.1. Effects of variable concentration 32 i
01 584
I,
1
416
I
/
I
448
WAVELENGTH Fig. I. Phosphorescence persed in a polystyrene was used.
58
I
I
/
480
1
I,
512
(nm)
spectrum at 77 K for bcnzophcnonc dismatrix. Dye laser excitation at 406 nm
The theoretical maximum for a polarization ratio is 3.0 [ 13 ] and so even for the most dilute solutions the maximum value is not reached in these experiments. Furthermore, using longer delay times than the nominal value of 500 ps produces no further change in the observed ratios. It seems clear from the fact that the polarizations increase as the samples become more dilute, that an increasing fraction of the emitting species are ones that originally absorbed the
Volume 152, number Table 1 Polarization
I
CHEMICAL
ratios for the phosphorescence
ofbenzophenone
Concentration
in polystyrene
Average separation distance (A)
(MI
0.002 I 0.00030 0.000099 0.000048
19 88
matrices at 77 K
Ratios a)
5.1 9.8 14.2 18.0
‘I Each ratio is an average of five determinations
4 November
PHYSICS LETTERS
425 nm
455 nm
488 nm
0.99
0.82 1.12 1.55 1.54
0.75 0.99 1.02 1.10
1.22 I .46 I Xi3 and the overall precision
polarized excitation light. No dependence on concentration would have been expected if the major mechanism for depolarization were randomization of the emission dipole either by rotational or translational processes. Thus, it is apparent that the major process for depolarization is energy migration in these samples. It is not clear why there is a negligible time dependence of these emission signals at times longer than the preset delay of 0.5 ms. Evidently, any depolarization which occurs takes place very rapidly at short times and then remains essentially constant at longer times. This sort of behavior suggests that a
is k 10%.
certain fraction of photoexcited molecules are sufficiently removed from neighbors so that these excitations remain localized, or trapped, during the time frame of the experiment. The remaining molecules are, evidently, in regions of higher concentration where energy migration and scrambling of the transition dipoles is essentially complete in less than 0.5 ms. To characterize the kinetics of these processes in any more detail it will be necessary to USCsmaller delay times and to use data analyses which take account of the heterogeneous nature of each molecule’s environment [ 14 ] _
1.7
0.7
f
/
1
7
5
0
I
I
‘3
425nm
I
I
11
Average Seporotion + 455nm
,
I
13 Distance
I
15
0
I
I
17
4a9nm
Fig. 2. Polarization ratios determined for benzophenone in a polystyrene matrix at 77 K versus the average separation molecules in the matrix. Symbols indicate the emission wavelength used for the measurement.
distance between
59
Volume 152, number
1
CHEMICAL
4.2. Wavelength dependence of polarization
PHYSICS LETTERS
ratios
An unexpected result of these measurements was the distinct wavelength dependence of the polarization ratios at any given concentration. It will be noted that for the most concentrated sample at 488 nm, the ratio is less than unity within experimental error. Since energy migration is the major mode of dipole scrambling, this result suggests that, under some circumstances, photoselected molecules may preferentially transfer energy to molecules whose transition dipoles are orthogonal to their own. This effect is less clear-cut for the vibronic transition at 455 nm and for the emission band at 425 nm, where the depolarization ratio is unity within experimental error, it appears that true random scrambling has occurred. An alternative explanation for these observations centers upon the coupling between the ground electronic state and the X, y and z components of lowest triplet state which is formed. For example, let us suppose that the electric vector of the excitation beam is nearly collinear with the z molecular axis (the C=O band axis) of a given molecule. The net positive polarizations greater than unity which are observed for dilute samples suggest that a preference for transitions to the z component of the triplet state occurs. In fact, Dym et al. [ 9 ] have shown that it is only the z and y components (y is in the molecular plane) of the triplet sublevels which couple with the ground singlet state of crystalline benzophenone. Furthermore, the z component is the stronger of the two by a factor of 3. If the T, component couples more
60
4 November
1988
strongly to upper vibrational levels of the ground electronic state than does TZ, then a bias would be expected in favor of the corresponding orthogonal y transition. Any selectivity of this sort would most likely be associated with differences in symmetry of the ground vibronic states.
Acknowledgement This work was supported by the US Department of Energy under grant number DE-FGOS84ER45 107.
References [ 11A. Salmassi and W. Schnabel, Polym. Photochem. 5 (1984) 215. [2]F.E. El-Sayed, F.R. MacCallum, P.J. Pomcry and M.T. Shcperd, J. Chcm. Sot. Faraday Trans. 1175 (1979) 79. [3 1K. Horie and 1. Mita, European Polym. J. 20 (1984) 1037. [4 1R.D. Burkhart, Chem. Phys. 46 (1980) II. [ 5 1R.D. Burkhart, J. Phys. Chem. 87 (1983) 1566. [ 6 ] J.R. MacCallum, Polymer 23 ( 1982 ) I 75. [7] L. Gardette and D. Phillips, Polymer 25 ( 1984) 336. [ 8 ] N. Turro, Modem molecular photochemistry (BenjaminCummings, Menlo Park, 1978) p. 127. [9] S. Dym, R.M. Hochstrasserand M. Schafer, J. Chem. Phys. 48 (1968) 646. [ lO]N.J. Caldwell and R.D. Burkhart, Macromolecules 19 (1986) 1653. [ 111 N.J. Caldwell, G.W. Haggquist and R.D. Burkhart, Pholothem. Photobiol., to be published. [ 12 ] S. Chandrasekhar, Rev. Mod. Phys. 15 ( 1943) 1. [ 13 ] A.C. Albrecht, J. Mol. Spectry. 6 ( 196 I ) 84. [ 141 K.A. Peterson and M.D. Fayer, J. Chem Phys. 85 (lY86) 4702, and earlier papers in this series.
1
CHEMICAL
4 November
PHYSICS LETTERS
OPTICAL ANISOTROPY OF PHOSPHORESCENCE FROM PHOTOSELECTED BENZOPHENONE MOLECULES
IN POLYSTYRENE
1988
MATRICES
Gregory W. HAGGQUIST and Richard D. BURKHART Department of Chemistry,University ofNevada-Rena, Rena. NV89557. USA Received 26 June 1988; in final form 10 August 1988
Benzophenone present as a dopant in polystyrene matrices has been photoexcited at 406 nm and 77 K using a polarized beam from a dye laser. Polarization ratios of the phosphorescence from these directly produced trlplet states were measured at delay times from 0.5 to 5.0 ms and at various dopant concentrations. At three different phosphorescence wavelengths the polarization ratios decrease as the dopant level increases. The major mechanism for emission depolarization is, evidently, energy migration. An observed dependence of the polarization ratio upon emission wavelength was tentatively ascribed to preferential coupling of triplet sublevels to upper vibrational states of the electronic ground state.
1. Introduction Investigations on the decay of benzophenone triplets in both poly(methy1 methacrylate) (PMMA) and polystyrene matrices over a range of temperatures have been reported by Salmassi and Schnabel [ 11. Non-exponential decays in certain temperature ranges were attributed to the existence of triplettriplet annihilation competing with first-order decay processes. They concluded that bulk translational motion with an unusually large diffusion coeflicient was the underlying mechanism for the transport of triplets. MacCallum and co-workers [2] have also reported the occurrence of T-T annihilation of several aromatic chromophores in PMMA matrices and suggest that the major mechanism for triplet transport is bulk translational diffusion. Horie and Mita [3] have also studied the decay kinetics of benzophenone triplets in a variety of acrylate and methacrylate polymers and concluded that the decay process includes a contribution from diffusional quenching of triplets by side-chain ester groups of the host matrix. In contrast to these results, it has been known for some time that triplet exciton migration among 1,2benzanthracene (BNZ) molecules can account for all of the decay kinetics observed at ambient temperature for this species when present as a dopant in 56
polystyrene films. The results of conventional phosphorescence and delayed fluorescence spectroscopy for this system are consistent with an energy migration scheme [ 41. Absolute rate constants of triplcttriplet annihilation for BNZ in polystyrene are also in accord with an energy migration mechanism [ 5 1. It may be mentioned in passing that a similar controversy over the nature of transport involving singlet state species has also arisen. In this case the effects of polarized excitation have been examined [ 6,7] but the results have not led to a clear-cut interpretation. Since the benzophenone molecule has been a favorite target species for studies in which bulk translational diffusion is claimed, it was decided to carry out photoselection experiments on this molecule dispersed in polystyrene matrices. The purpose of the work was to investigate the question of whether or not triplet transport by exciton migration is a significant process in these molecularly doped polymers. All of the experiments were carried out at 77 K because of signal detection problems at elevated temperatures. Therefore, it is not possible to address the question of possible diffusive migration at elevated temperatures. On the other hand, because of the relatively large oscillator strength for excitation from the ground state (So) to the lowest triplet state (T, ) of benzophenone [ 8 ] it was possible to pop-
0 009-2614/88/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
B.V.
Volume 152. number 1 ulate T, directly
by use of a polarized
CHEMICAL PHYSICS LETTERS beam
from
a
dye laser. In this way a photo-selected set of benzophenone molecules in the T, state may be produced and no scrambling of the transition dipoles upon passage through the intervening singlet state is possible. Direct excitation from the ground state to T, does not, in itself, guarantee that the triplets produced will have dipoles oriented collinearly with the electric vector of the exciting beam. This is because there are three triplet sublevels and they may all possess some degree of coupling with the ground state singlet. If preferential coupling to one of the triplet sublevels does occur it would be indicated by a polarization ratio different from unity. In fact it has been observed in earlier studies on crystalline benzophenone at 4.2 K that the polarization ratio for the zero-zero emission component is 2.5 [ 91. In the present case, in which benzophenone is a molecular dopant, a polarization ratio larger than unity has also been observed. It is the change in this ratio with dopant concentration which is of primary interest in the present study. The reason for this special interest in concentration effects is due to the information it gives about the primary mechanism of phosphorescence depolarization. If, for example, the extent of depolarization over a given time period is independent of dopant concentration, then a unimolecular mechanism is suggested such as molecular rotation, translation or internal scrambling among T,, T, and T, triplet components. On the other hand, the observation of a concentration-dependent extent of depolarization signals the operation of a bimolecular mechanism such as energy migration. Of secondary interest was the determination of the polarization ratio as a function of emission wavelength. This ratio has been measured at the wavelength maxima corresponding to each of the vibronic bands of the phosphorescence envelope. The results provide additional insight into the details of the interaction between photophysical events and the distribution of dopant molecules in these amorphous polymer films.
4 November 1988
2. Experimental 2.1. Treatment of chemicals Benzophenone obtained from Aldrich Chemical Co. was recrystallized three times from ethanol and was dried at ambient temperature in a vacuum oven. Methanol, 2-methyltetrahydrofuran (MTHF) and benzene were purified using methods described in earlier publications [ 10 1, Polystyrene was purified by a method recently developed in these laboratories expressly intended to remove impurities associated with aromatic carbonyls [ 111. 2.2. Sample preparation All samples were prepared in an oxygen-free nitrogen atmosphere. A solution of benzophenone polystyrene and benzene was placed dropwise onto an optical quartz disk and was gently heated. When the benzene completely evaporated, a second quartz disk was placed on top of the molten solution and the two wafers were pressed together in order to remove any residual gas bubbles. 2.3. Spectroscopic instrumentation A Lambda Physik FL2002 dye laser was pumped with 308 nm light from a Tachisto 401 XR XeCl eximer laser. The dye used, having the commercial designation DPS, has a tuning range from about 390 to 410 nm. In these experiments an excitation wavelength of 406 nm was used to match the absorption band of benzophenone corresponding to the So+TI transition. The dye laser beam, already possessing polarization in the vertical plane to the extent of 1O/ 1, was passed through a sheet polarizer to augment the polarization ratio. The sample was mounted at the end of a cryotip assembly (Air Products and Chemicals LT-3-110) which was enclosed in a vacuum shroud with quartz windows for entrance and exit of light beams. The axis from the center of the sample to the entrance slit of the emission monochromator was oriented at an angle of 45” with respect to the excitation beam. A sheet polarizer in a rotating mount (Melles Griot) made it possible to determine the angle of the PO57
1
Volume 152, number
CHEMICAL
PHYSICS LETTERS
larizer with respect to the laboratory horizontal plane with a precision of 5 0.5 O. The emission monochromator (Spex model 1670) was calibrated for wavelength and was also tested for polarization bias at different wavelengths by monitoring non-polarized fluorescence from fluid solutions of dye molecules. The bias measurements determined in this way were used to correct all of the polarization ratios quoted below. The emission from photoexcited samples was detected by an end window photomultiplier and recorded either on an oscilloscope (Tektronix 2465 ) or else on a signal averager (Nicolet 12/70) after amplification using a Stanford Research Systems, model SR440 preamplifier.
3. Experimental results The emission spectrum from benzophenone in glassy MTHF at 77 K and excited using 406 nm excitation from the dye laser is presented in fig. 1. The starting wavelength was 4 15 nm in order to avoid contamination of the emission from scattered light. The maxima corresponding to the three vibronic components occur at 425,455 and 488 nm. The time response of the light collection system was on the or-
128
1
4 November
I988
der of 100 us and a minimum time delay of 500 us was inserted between the excitation pulse and the monitor event in order to avoid background emission. Polarization ratios were determined by measuring the intensity of the phosphorescence when the emission polarizer was either horizontal (I,) or vertical Z,) . Thus, the ratio is defined by R=l,f Ih. In every case the plane of the excitation beam is vertical in the laboratory frame. It has been our initial intention to try to determine the polarization ratio as a function of time following the excitation pulse, however, at delay times larger than 500 us any time dependence of the polarization ratio was found to be negligible. The widest possible range of concentrations of benzophenone in polystyrene were used in these experiments. There were limitations, however, at both concentration extremes. For samples which were too dilute, the phosphorescence signal was lost in the noise and for those which were too concentrated, there were definite signs of residual crystalline benzophenone in the samples. Four different concentrations were eventually tested as indicated in table 1. The average interchromophore separation distances for each of these samples was also a matter of interest. These were calculated using an equation developed by Chandrasekhar [ 121 and are also indicated in table 1 along with their corresponding concentrations. All of the polarization data, both as a function of concentration and as a function of wavelength, are collected in table 1. A graphical display is presented in fig. 2.
4. Discussion 4.1. Effects of variable concentration 32 i
01 584
I,
1
416
I
/
I
448
WAVELENGTH Fig. I. Phosphorescence persed in a polystyrene was used.
58
I
I
/
480
1
I,
512
(nm)
spectrum at 77 K for bcnzophcnonc dismatrix. Dye laser excitation at 406 nm
The theoretical maximum for a polarization ratio is 3.0 [ 13 ] and so even for the most dilute solutions the maximum value is not reached in these experiments. Furthermore, using longer delay times than the nominal value of 500 ps produces no further change in the observed ratios. It seems clear from the fact that the polarizations increase as the samples become more dilute, that an increasing fraction of the emitting species are ones that originally absorbed the
Volume 152, number Table 1 Polarization
I
CHEMICAL
ratios for the phosphorescence
ofbenzophenone
Concentration
in polystyrene
Average separation distance (A)
(MI
0.002 I 0.00030 0.000099 0.000048
19 88
matrices at 77 K
Ratios a)
5.1 9.8 14.2 18.0
‘I Each ratio is an average of five determinations
4 November
PHYSICS LETTERS
425 nm
455 nm
488 nm
0.99
0.82 1.12 1.55 1.54
0.75 0.99 1.02 1.10
1.22 I .46 I Xi3 and the overall precision
polarized excitation light. No dependence on concentration would have been expected if the major mechanism for depolarization were randomization of the emission dipole either by rotational or translational processes. Thus, it is apparent that the major process for depolarization is energy migration in these samples. It is not clear why there is a negligible time dependence of these emission signals at times longer than the preset delay of 0.5 ms. Evidently, any depolarization which occurs takes place very rapidly at short times and then remains essentially constant at longer times. This sort of behavior suggests that a
is k 10%.
certain fraction of photoexcited molecules are sufficiently removed from neighbors so that these excitations remain localized, or trapped, during the time frame of the experiment. The remaining molecules are, evidently, in regions of higher concentration where energy migration and scrambling of the transition dipoles is essentially complete in less than 0.5 ms. To characterize the kinetics of these processes in any more detail it will be necessary to USCsmaller delay times and to use data analyses which take account of the heterogeneous nature of each molecule’s environment [ 14 ] _
1.7
0.7
f
/
1
7
5
0
I
I
‘3
425nm
I
I
11
Average Seporotion + 455nm
,
I
13 Distance
I
15
0
I
I
17
4a9nm
Fig. 2. Polarization ratios determined for benzophenone in a polystyrene matrix at 77 K versus the average separation molecules in the matrix. Symbols indicate the emission wavelength used for the measurement.
distance between
59
Volume 152, number
1
CHEMICAL
4.2. Wavelength dependence of polarization
PHYSICS LETTERS
ratios
An unexpected result of these measurements was the distinct wavelength dependence of the polarization ratios at any given concentration. It will be noted that for the most concentrated sample at 488 nm, the ratio is less than unity within experimental error. Since energy migration is the major mode of dipole scrambling, this result suggests that, under some circumstances, photoselected molecules may preferentially transfer energy to molecules whose transition dipoles are orthogonal to their own. This effect is less clear-cut for the vibronic transition at 455 nm and for the emission band at 425 nm, where the depolarization ratio is unity within experimental error, it appears that true random scrambling has occurred. An alternative explanation for these observations centers upon the coupling between the ground electronic state and the X, y and z components of lowest triplet state which is formed. For example, let us suppose that the electric vector of the excitation beam is nearly collinear with the z molecular axis (the C=O band axis) of a given molecule. The net positive polarizations greater than unity which are observed for dilute samples suggest that a preference for transitions to the z component of the triplet state occurs. In fact, Dym et al. [ 9 ] have shown that it is only the z and y components (y is in the molecular plane) of the triplet sublevels which couple with the ground singlet state of crystalline benzophenone. Furthermore, the z component is the stronger of the two by a factor of 3. If the T, component couples more
60
4 November
1988
strongly to upper vibrational levels of the ground electronic state than does TZ, then a bias would be expected in favor of the corresponding orthogonal y transition. Any selectivity of this sort would most likely be associated with differences in symmetry of the ground vibronic states.
Acknowledgement This work was supported by the US Department of Energy under grant number DE-FGOS84ER45 107.
References [ 11A. Salmassi and W. Schnabel, Polym. Photochem. 5 (1984) 215. [2]F.E. El-Sayed, F.R. MacCallum, P.J. Pomcry and M.T. Shcperd, J. Chcm. Sot. Faraday Trans. 1175 (1979) 79. [3 1K. Horie and 1. Mita, European Polym. J. 20 (1984) 1037. [4 1R.D. Burkhart, Chem. Phys. 46 (1980) II. [ 5 1R.D. Burkhart, J. Phys. Chem. 87 (1983) 1566. [ 6 ] J.R. MacCallum, Polymer 23 ( 1982 ) I 75. [7] L. Gardette and D. Phillips, Polymer 25 ( 1984) 336. [ 8 ] N. Turro, Modem molecular photochemistry (BenjaminCummings, Menlo Park, 1978) p. 127. [9] S. Dym, R.M. Hochstrasserand M. Schafer, J. Chem. Phys. 48 (1968) 646. [ lO]N.J. Caldwell and R.D. Burkhart, Macromolecules 19 (1986) 1653. [ 111 N.J. Caldwell, G.W. Haggquist and R.D. Burkhart, Pholothem. Photobiol., to be published. [ 12 ] S. Chandrasekhar, Rev. Mod. Phys. 15 ( 1943) 1. [ 13 ] A.C. Albrecht, J. Mol. Spectry. 6 ( 196 I ) 84. [ 141 K.A. Peterson and M.D. Fayer, J. Chem Phys. 85 (lY86) 4702, and earlier papers in this series.