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Picosecond relaxation dynamics in polydiacetylene-pTS
Volume 139, number 5
CHEMICAL PHYSICS LETTERS
4 September 1987
PICOSECOND RELAXATION DYNAMICS IN POLYDIACETYLENE-pTS B.I. GREENE, J. ORENSTEIN, R.R. MILLARD I and L.R. WILLIAMS * AT&T Bell Laboratories, Murray Hdl, NJ 07974, USA
Received 16 March 1987; in final form 15 June 1987
Time-resolved absorption spectroscopy as well as transient grating measurements were performed on poiydiacetylene-pTS. Singlet exciton lifetimes of 0.8 f 0.2 ps at 300 K and 1.1+ 0.2 ps at 5 K were measured. We propose that the observed non-radiative relaxation proceeds through a conformationally relaxed singlet state which lives for roughly 1.Oand 3.4 ps at 300 and 5 K, respectively. Data pertinent to the formation of triplet excitons are also presented and discussed.
1. Introduction Polydiacetylenes have come under intensitve investigation due to their unusual optical and material properties [ l-31. Significantly, some polydiacetylenes, unlike polyacetylene, can be prepared as highquality single crystals. The morphology and crystal structures for many polydiacetylenes have been well established by X-ray crystallography [4]. In this paper we present picosecond time-resolved optical data for the most extensively studied polydiacetylene, polydiacetylene-PTS. We suggest that an ultrafast structural relaxation facilitates an observed x 1.O ps singlet exciton decay, and discuss this hypothesis in the context of other previously studied polyene systems.
2. Experimental Measurements were performed with 70 fs light pulses derived from a 10 Hz amplified CPM dye laser system [ 51. Excitation pulses were either at the laser fundamental at 1.97 eV, or the laser second harmonic at 3.94 eV. White light “continuum” pulses were utilized as a probe. Samples consisted of thermally polymerized bulk ’ Also at: Berkeley, CA, USA. ’ AT&T Bell Laboratories, Ph.D. Scholar. Also at: MIT, Cambridge, MA 02 139, USA.
single crystals of poly-2,4-hexadiyn-1,6:diol bis(ptoluene sulfonate), hereafter referred to as PDA-pTS. High-quality optical surfaces for diffraction experiments could be exposed by peeling layers off of bulk crystal with adhesive tape. Single-crystal flakes were utilized in spectroscopic transmission measurements, where the optical surface quality was less critical. Samples were mounted on a cold-finger of a continuous transfer liquid-helium cryostat which could maintain sample temperatures between 5 and 300 K. The polarization of all light beams was parallel to the molecular chain axis. Two types of experiments were performed. The first measured excited state absorption induced in the sample by either 1.97 or 3.94 eV pulses. Induced absorption could be observed at energies below the onset of strong ground state absorbance, i.e. between 1.Oand 1.8 eV. The second type of experiment measured the decay of an optically induced grating on the surface of the crystal. Refractive index gratings were created by interfering two temporally coincident pump pulses at 1.97 eV on the sample at an angle of 18’ with respect to each other. A third, temporally delayed white light pulse was diffracted off of the grating, and detected with either a spectrograph/ OMA detection system or alternatively with a photodiode through a 10 nm fwhm spectral notch filter centered at 1.97 eV. For all experiments, laser beam spot sizes on the sample were roughly 1 mm*. Excitation energies for the transmission measurements were typically 5- 10
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
381
pJ per pulse at 3.94 and 1.97 eV. The diffraction measurements utilized excitation pulses roughly ten times less energetic.
3. Results 3.1. Transmissionexperiments When probing for induced absorption in the region LO-l.8 eV, both excitation at 1.97 and 3.94 eV resulted in a promptly decaying ( w 1.Ops) transient, followed by a long-lived ( > 100 ps) signal. However, data taken with 1.97 eV excitatiop showed that the ratio of prompt signal to long-lived signal was intensity dependent. The actual rate of prompt decay was invariant to pump intensity or probe wavefength. Data taken with 3.94 eV excitation showed similar decay kinetics but no intensity dependence of the relative magnitudes of prompt and slow signals. Typical kinetic traces for 1.97 eV excitation are shown in fig. 1. Due to roughly 2.0 ps of spectral dispersion in our probing white-light pulse from 1.0 to 1.8 eV, a fill undistorted transienf spectrum of the 1.O ps lifetime transient was not readily obtainable. For relatively long-lived states, however, full undistorted spectra are routine. We have previously reported spectra taken subsequent to 3.94 eV excitation [ 61. In the present study, similar spectra peaking at 1.4 1 eV were obtained with high-intensity ( > IO9W/cm2) 1.97 eV
-1
0
1
2
3
4
5
DELAY (ps) Fig. 1. Response of PDA-pTS at 300 K excited at 1.97 eV and probed at 1.41 eV. Excitation energies were, from top to bottom, 6x109,3x10qand1.5x10*WfcmZ.
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excitation, however, in this case the absorption band was observed to be 50% broader than that obtained with 3.94 eV excitation, Kinetic data taken at 1.97 eV on several different samples showed some reproducible sample-to-sample variation in the vahte of the prompt decay constant. We report an average single-exponential value of 0.8 Z!I0.2 ps at 300 K for this decay. This lifetime was observed to increase siightly to I. 1 I? 0.2 ps at 5 K. Recently we have obtained hip-quali~ singlecrystal thin film samples of PDA-pTS [ 71. These samples facilitated transmission measurements at pump intensities of roughly 5 x 10’ W/cm2. Virtually no long-lived signal was observed at this intensity. Short-lived signals were observed to decay with time constants identical to those measured at higher intensity on single-crystal flake samples. 3.2. Transientgrating experiments
Traces showing diffraction efficiency versus delay time for two different pump intensities at 300 K are shown in fig. 2. At the higher intensity (6x IO9 W/cm2 ), a prompt decay is followed by a long-lived component. When the intensity was lowered to 6 x 1OS W/cm’, the diffracted signal was observed to decay essentially completely. At intensities below *6x IO* W/cm2, no further changes in temporal response were observed. Fig. 3 displays a semilog plot
0
4
8
DELAY (ps)
12
Fig. 2. Transient grating response at 300 K for 6x 10’ W/cm* pump intensity (upper), and 6 x 10s W/cm2 (lower). Grating was written and probed at 1.94eV.
CHEMICAL PHYSICS LETTERS
Volume 139, number 5
DELAY (ps) Fig. 3. Semilog plot of’low intensity” transient grating response at 300 K.
DELAY (ps) Fig. 4. Transient grating response of PDA-pTS at 5 K (upper); semilog plot (lower).
of the intensity-independent response, indicating a single-exponential lifetime of 2.0 ps. We note that a factor of two enters in this determination due to the quadratic dependence of diffraction efficiency on grating amplitude [ 81. Fig. 4 shows the decay of diffraction efficiency at 5 K. The response displays a double-exponential form, with graphically determined characteristic lifetimes of 1.1 and 3.4 ps #I.
4. Discussion The experimental data presented above provide information on both the singlet exciton lifetime and lt’ While it is technically incorrect to derive double-exponential rate constants by this technique, for sufficiently different rates, the approximation is acceptable.
4 September 1987
the rate of repopulation of the ground electronic state. The first matter to be discussed, however, is the identity and pathways for creation of the long-lived previously spectroscopically characterized metastable state [ 61. Recent work performed on the microsecond time scale has demonstrated the existence of a metastable (r = 50 us) triplet state in PDA-pTS [ 93. This state has been reported to have a spectroscopic signature very similar to what we observe subsequent to excitation with 3.94 eV light [ 61. We therefore assign the 3.94 eV induced long-lived signal in the region 1.8 to 1.0 eV to the triplet exciton. The efficiency of formation of the triplet exciton has been observed previously to drop off signiticantly for excitation energies below x2.4 eV, with no detectable yield at x 2.0 eV [ lo]. Our data indicate that triplets can in fact be generated with 1.97 eV excitation. In this case, however, due to the nonlinear dependence on excitation intensity of the longlived signal, we believe the excitation process to be two-photon in origin. Data in fig. 1 reveal a subquadratic intensity dependence for this signal, which we attribute to a saturation effect [ 111. We observed a broadened triplet-like spectrum subsequent to high-intensity 1.97 eV excitation. This spectral distortion could be due to thermal effects, as by the delay time that a spectrum is typically taken (3 ps), a considerable fraction of the one-photon generated singlet-exciton population has non-radiatively relaxed. The penetration depth for 1.97 eV light is only 100 A, and the resulting temperature increase could be as much as 200’ C. Triplet exciton-exciton interaction might also result in broadened optical spectra. Further evidence for the multi-l.97 eV-photon origin of the triplet signal comes from the transient grating data. Because this is a zero-background technique, considerably lower excitation intensities could be utilized. The diffracted signal results from an induced refractive index grating occurring concomitantly with the spatially periodic excitation of the sample [ 111. In spectral regions associated with strong optical transitions, large changes in refractive index are expected upon excitation. When these excitations relax, the refractive indices revert back to their unexcited, ground state value. Thermal effects on the refractive index are expected to be small com383
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CHEMICAL PHYSICS LETTERS
Fig. 5. Schematic energy level diagram for PDA-pTS depicting proposed reiaxation pathways and characteristic state lifetimes. S, and T, are the singIet and triplet exciton respectively. The absolute position of T, is arbitrary. Es designates the bandgap energy derived from photoconductivity measurements, and may correspond to an as yet uncharacterized excited singlet state.
pared to those created by population changes between electronic states. Data in fig. 2 indicate that as the pump intensity is lowered to roughly 6 x 1O8W/cm2 and below, no metastable signals were observed. Presumably, the long-lived state created with high intensities is the triplet exciton, which under sufficiently low 1.97 eV excitation intensity conditions, has a vanishing low yield of formation. Excitation at 1.97 eV is resonant with the strong spectral absorption feature identified as the singlet exciton in PDA-pTS [ 121. We therefore interpret the initial decay of the transient absorption signal in fig. 1 to the decay of the singlet exciton. The lifetime of this state appears to be roughly temperature independent at 0.8 to 1.1 ps. If the singlet exciton decayed directly to the ground electronic state, the ground state recovery data shown in figs. 2-4 should yield an identical kinetic result. This is clearly not the case. At room temperature, a single-exponential ground state recovery time of 2.0 ps was observed. This result is in good agreement with those performed on the edge of the exciton absorption [ 13 J. At 5 K, this response becomes doubl~x~nenti~, with a 1.1 and a 3.4 ps component. Although not shown, the 5 K transient absorption data revealed no sign of a 3.4 ps decay component, but only a 1.1 ps single-exponential decay. Fig. 5 depicts a schematic energy level diagram and proposed relaxation pathways for PDA-pTS. The salient features of this model are (a) the existence of 384
4 September 1987
a configurationally relaxed intermediate singlet state through which rapid internal conversion is facilitated, (b) the absence of an efficient intersystem crossing process occurring from the singlet exciton S1, (c) the necessity of accessing higher-excited singlet states (via one- or tw~photon abso~tion~ to efficiently form the triplet exciton. Plausibility for a “barrier-free” structural relaxation in PDA-pTS comes from the extensively studied and well-characterized primary photochemical events occurring in rhodopsin. There is now substantial evidence that subsequent to the absorption of light, an essentially temperature-independent (measured down to 4 K in a rigid glass) relaxation process occurs [ 141. The isomerization of the polyene chromophore all-trans retinal to 13-cis retinal has been implicated. A characteristic time constant of 0.43 ps has recently been measured for this isomerization reaction [ 151. The cis-tram photoisome~~ation of 1,3,5,7-octatetraene in n-hexane at 4.2 K has also been well documented [ 161. Picosecond studies on substituted ethylenes and butadienes have provided a qualitative framework which rationalizes these ultrafast structural relaxaaion processes [ 171. Subsequent to photoexcitation, carbon-carbon bond orders are effectively changed whereby what were double bonds in the ground electronic state become single bonds in the excited state. What were potential minima in the structural coordinates corresponding to rotational degrees of freedom about carbon,carbon bonds, become in the excited state, either weakly bound, or local potential maxima. A weakly activated or barrierless structural relaxation then occurs, often resulting in dramatic changes in the molecular absorption spectra. Interruption of delocalized A electron molecular orbitals caused by the out-of-plane twisting of carbon backbone p orbitals can be held responsible for the often dramatic spectral effects. Our data indicate an apparent single-exponential recovery of the ground state at 300 K (fig. 3)) with a two-step process emerging at 5 K (fig. 4). Transient absorption results indicate a roughly temperature invariant m 1.0 ps initial relaxation step. We assign the initial 1.0 ps process to the formation of the structurally relaxed species, designated the “kink” state in fig. 5. The “kink state” is hypothesized to have a sizeable absorbance at 2 eV, greater in mag-
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CHEMICAL PHYSICS LETTERS
nitude than the singlet exciton, but less than the ground state absorbance. Relaxation of the singlet exciton into the kink state therefore results in the partial recovery of the diffracted signal. A temperature sensitive relaxation then follows, presumably dependent on phonon emission, with a characteristic lifetime of = 1.0 ps at 300 K and 3.4 ps at 5 K. Two sequential 1.Ops decays are expected to appear, with our signal-to-noise, as a roughly 2.0 ps single-exponential decay. We note that the process of internal conversion in this system must certainly result in a lattice temperature rise. Simple exponential decay rates would therefore not be expected. However, for the purposes of illustrating qualitative effects in this system, the temperature trends displayed in the kinetic data are certainly reliable. Finally, we point out that our data indicate that triplet excitons appear within 3.0 ps after 3.94 eV or high-intensity 1.97 eV excitation. This corresponds to an extremely rapid intersystem crossing rate, the origin of which, together with the identity of the strongly coupled singlet state(s), is a subject for further investigation.
5. Conclusion Time-resolved transient absorption spectroscopy together with transient grating measurements have provided considerable insight into the primary steps of relaxation subsequent to photoexcitation of PDApTS. A roughly 1.O ps temperature-independent singlet exciton lifetime has been measured. We suggest that structural relaxation to a “kinked” intermediate is responsible for the ultrashort singlet exciton lifetime. The kinked state lives for roughly 1.Ops at 300 K and 3.4 ps at 5 K. No evidence for formation of
4 September 1987
triplet excitons via the singlet exciton was observed. Efficient and rapid ( < 3.0 ps) triplet exciton formation was observed, however, following 3.94 eV excitation.
References
[ 1] M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Oxford Univ. Press, Oxford, 1982).
[ 2 ] J.C.W. Chien, Polyacetylene (Academic Press, New York, 1984).
[ 3 ] J. Orenstein, in: Handbook of conducting polymers, ed. T.A. Skotheim (Dekker, New York, 1986) p. 1297. [4] V. Enkelmann, in: Advances in polymer science, Vol. 63, ed. H.-J. Canton (Springer, Berlin, 1984) p. 9 1. [ 51J.A. Valdmanis, R.L. Fork and J.P. Gordon, Opt. Letters 10 (1985) 131. [ 61 B.I. Greene, J. Orenstein, R.R. Millard and L.R. Williams, in: Ultrafast phenomena, Vol. 5, eds. G.R. Fleming and A.E. Siegman (Springer, Berlin, 1986) p. 472. [ 71 M. Thakur and S. Meyler, Macromolecules 18 ( 1985) 2341. [8] A.L. Smirl, SC. Moss and J.R. Lindle, Phys. Rev. B25 (1982) 2645. [ 91 L. Robbins, J. Orenstein and R. Superfine, Phys. Rev. Letters 56 (1986) 1850. [ lo] J. Orenstein, S. Etemad and G.L. Baker, J. Phys. Cl 7 (1984) L297. [ 111 B.I. Greene, J. Orenstein, R.R. Millard and L.R. Williams, Phys. Rev. Letters, to be published. [ 121 M.R. Philpott, Chem. Phys. Letters 50 (1977) 18. [ 13] G.M. Carter, J.V. Hryniewicz, M.K. Thakur, Y.J. Chen and S.E. Meyler, Appl. Phys. Letters 49 (1986) 998. [ 141 B. Honig, in: Biological events probed by ultrafast laser spectroscopy, ed. R.R. Alfano (Academic Press, New York, 1982) p. 281. [ 151 M.C. Nuss, W. Zinth, W. Kaiser, E. Kolling and D. Oesterhelt, Chem. Phys. Letters 117 (1985) 1. [ 161 M.F. Granville, G.R. Holtom and B.E. Kohler, Proc. Nat. Acad. Sci. US 77 (1980) 31. [ 171 G.R. Fleming, Chemical applications of ultrafast spectroscopy (Oxford Univ. Press, Oxford, 1986) p. 179.
385
CHEMICAL PHYSICS LETTERS
4 September 1987
PICOSECOND RELAXATION DYNAMICS IN POLYDIACETYLENE-pTS B.I. GREENE, J. ORENSTEIN, R.R. MILLARD I and L.R. WILLIAMS * AT&T Bell Laboratories, Murray Hdl, NJ 07974, USA
Received 16 March 1987; in final form 15 June 1987
Time-resolved absorption spectroscopy as well as transient grating measurements were performed on poiydiacetylene-pTS. Singlet exciton lifetimes of 0.8 f 0.2 ps at 300 K and 1.1+ 0.2 ps at 5 K were measured. We propose that the observed non-radiative relaxation proceeds through a conformationally relaxed singlet state which lives for roughly 1.Oand 3.4 ps at 300 and 5 K, respectively. Data pertinent to the formation of triplet excitons are also presented and discussed.
1. Introduction Polydiacetylenes have come under intensitve investigation due to their unusual optical and material properties [ l-31. Significantly, some polydiacetylenes, unlike polyacetylene, can be prepared as highquality single crystals. The morphology and crystal structures for many polydiacetylenes have been well established by X-ray crystallography [4]. In this paper we present picosecond time-resolved optical data for the most extensively studied polydiacetylene, polydiacetylene-PTS. We suggest that an ultrafast structural relaxation facilitates an observed x 1.O ps singlet exciton decay, and discuss this hypothesis in the context of other previously studied polyene systems.
2. Experimental Measurements were performed with 70 fs light pulses derived from a 10 Hz amplified CPM dye laser system [ 51. Excitation pulses were either at the laser fundamental at 1.97 eV, or the laser second harmonic at 3.94 eV. White light “continuum” pulses were utilized as a probe. Samples consisted of thermally polymerized bulk ’ Also at: Berkeley, CA, USA. ’ AT&T Bell Laboratories, Ph.D. Scholar. Also at: MIT, Cambridge, MA 02 139, USA.
single crystals of poly-2,4-hexadiyn-1,6:diol bis(ptoluene sulfonate), hereafter referred to as PDA-pTS. High-quality optical surfaces for diffraction experiments could be exposed by peeling layers off of bulk crystal with adhesive tape. Single-crystal flakes were utilized in spectroscopic transmission measurements, where the optical surface quality was less critical. Samples were mounted on a cold-finger of a continuous transfer liquid-helium cryostat which could maintain sample temperatures between 5 and 300 K. The polarization of all light beams was parallel to the molecular chain axis. Two types of experiments were performed. The first measured excited state absorption induced in the sample by either 1.97 or 3.94 eV pulses. Induced absorption could be observed at energies below the onset of strong ground state absorbance, i.e. between 1.Oand 1.8 eV. The second type of experiment measured the decay of an optically induced grating on the surface of the crystal. Refractive index gratings were created by interfering two temporally coincident pump pulses at 1.97 eV on the sample at an angle of 18’ with respect to each other. A third, temporally delayed white light pulse was diffracted off of the grating, and detected with either a spectrograph/ OMA detection system or alternatively with a photodiode through a 10 nm fwhm spectral notch filter centered at 1.97 eV. For all experiments, laser beam spot sizes on the sample were roughly 1 mm*. Excitation energies for the transmission measurements were typically 5- 10
0 009-2614/87/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
381
pJ per pulse at 3.94 and 1.97 eV. The diffraction measurements utilized excitation pulses roughly ten times less energetic.
3. Results 3.1. Transmissionexperiments When probing for induced absorption in the region LO-l.8 eV, both excitation at 1.97 and 3.94 eV resulted in a promptly decaying ( w 1.Ops) transient, followed by a long-lived ( > 100 ps) signal. However, data taken with 1.97 eV excitatiop showed that the ratio of prompt signal to long-lived signal was intensity dependent. The actual rate of prompt decay was invariant to pump intensity or probe wavefength. Data taken with 3.94 eV excitation showed similar decay kinetics but no intensity dependence of the relative magnitudes of prompt and slow signals. Typical kinetic traces for 1.97 eV excitation are shown in fig. 1. Due to roughly 2.0 ps of spectral dispersion in our probing white-light pulse from 1.0 to 1.8 eV, a fill undistorted transienf spectrum of the 1.O ps lifetime transient was not readily obtainable. For relatively long-lived states, however, full undistorted spectra are routine. We have previously reported spectra taken subsequent to 3.94 eV excitation [ 61. In the present study, similar spectra peaking at 1.4 1 eV were obtained with high-intensity ( > IO9W/cm2) 1.97 eV
-1
0
1
2
3
4
5
DELAY (ps) Fig. 1. Response of PDA-pTS at 300 K excited at 1.97 eV and probed at 1.41 eV. Excitation energies were, from top to bottom, 6x109,3x10qand1.5x10*WfcmZ.
382
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CHEMICAL PHYSICS LEmERS
Volume i 39, number 5
excitation, however, in this case the absorption band was observed to be 50% broader than that obtained with 3.94 eV excitation, Kinetic data taken at 1.97 eV on several different samples showed some reproducible sample-to-sample variation in the vahte of the prompt decay constant. We report an average single-exponential value of 0.8 Z!I0.2 ps at 300 K for this decay. This lifetime was observed to increase siightly to I. 1 I? 0.2 ps at 5 K. Recently we have obtained hip-quali~ singlecrystal thin film samples of PDA-pTS [ 71. These samples facilitated transmission measurements at pump intensities of roughly 5 x 10’ W/cm2. Virtually no long-lived signal was observed at this intensity. Short-lived signals were observed to decay with time constants identical to those measured at higher intensity on single-crystal flake samples. 3.2. Transientgrating experiments
Traces showing diffraction efficiency versus delay time for two different pump intensities at 300 K are shown in fig. 2. At the higher intensity (6x IO9 W/cm2 ), a prompt decay is followed by a long-lived component. When the intensity was lowered to 6 x 1OS W/cm’, the diffracted signal was observed to decay essentially completely. At intensities below *6x IO* W/cm2, no further changes in temporal response were observed. Fig. 3 displays a semilog plot
0
4
8
DELAY (ps)
12
Fig. 2. Transient grating response at 300 K for 6x 10’ W/cm* pump intensity (upper), and 6 x 10s W/cm2 (lower). Grating was written and probed at 1.94eV.
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Volume 139, number 5
DELAY (ps) Fig. 3. Semilog plot of’low intensity” transient grating response at 300 K.
DELAY (ps) Fig. 4. Transient grating response of PDA-pTS at 5 K (upper); semilog plot (lower).
of the intensity-independent response, indicating a single-exponential lifetime of 2.0 ps. We note that a factor of two enters in this determination due to the quadratic dependence of diffraction efficiency on grating amplitude [ 81. Fig. 4 shows the decay of diffraction efficiency at 5 K. The response displays a double-exponential form, with graphically determined characteristic lifetimes of 1.1 and 3.4 ps #I.
4. Discussion The experimental data presented above provide information on both the singlet exciton lifetime and lt’ While it is technically incorrect to derive double-exponential rate constants by this technique, for sufficiently different rates, the approximation is acceptable.
4 September 1987
the rate of repopulation of the ground electronic state. The first matter to be discussed, however, is the identity and pathways for creation of the long-lived previously spectroscopically characterized metastable state [ 61. Recent work performed on the microsecond time scale has demonstrated the existence of a metastable (r = 50 us) triplet state in PDA-pTS [ 93. This state has been reported to have a spectroscopic signature very similar to what we observe subsequent to excitation with 3.94 eV light [ 61. We therefore assign the 3.94 eV induced long-lived signal in the region 1.8 to 1.0 eV to the triplet exciton. The efficiency of formation of the triplet exciton has been observed previously to drop off signiticantly for excitation energies below x2.4 eV, with no detectable yield at x 2.0 eV [ lo]. Our data indicate that triplets can in fact be generated with 1.97 eV excitation. In this case, however, due to the nonlinear dependence on excitation intensity of the longlived signal, we believe the excitation process to be two-photon in origin. Data in fig. 1 reveal a subquadratic intensity dependence for this signal, which we attribute to a saturation effect [ 111. We observed a broadened triplet-like spectrum subsequent to high-intensity 1.97 eV excitation. This spectral distortion could be due to thermal effects, as by the delay time that a spectrum is typically taken (3 ps), a considerable fraction of the one-photon generated singlet-exciton population has non-radiatively relaxed. The penetration depth for 1.97 eV light is only 100 A, and the resulting temperature increase could be as much as 200’ C. Triplet exciton-exciton interaction might also result in broadened optical spectra. Further evidence for the multi-l.97 eV-photon origin of the triplet signal comes from the transient grating data. Because this is a zero-background technique, considerably lower excitation intensities could be utilized. The diffracted signal results from an induced refractive index grating occurring concomitantly with the spatially periodic excitation of the sample [ 111. In spectral regions associated with strong optical transitions, large changes in refractive index are expected upon excitation. When these excitations relax, the refractive indices revert back to their unexcited, ground state value. Thermal effects on the refractive index are expected to be small com383
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Fig. 5. Schematic energy level diagram for PDA-pTS depicting proposed reiaxation pathways and characteristic state lifetimes. S, and T, are the singIet and triplet exciton respectively. The absolute position of T, is arbitrary. Es designates the bandgap energy derived from photoconductivity measurements, and may correspond to an as yet uncharacterized excited singlet state.
pared to those created by population changes between electronic states. Data in fig. 2 indicate that as the pump intensity is lowered to roughly 6 x 1O8W/cm2 and below, no metastable signals were observed. Presumably, the long-lived state created with high intensities is the triplet exciton, which under sufficiently low 1.97 eV excitation intensity conditions, has a vanishing low yield of formation. Excitation at 1.97 eV is resonant with the strong spectral absorption feature identified as the singlet exciton in PDA-pTS [ 121. We therefore interpret the initial decay of the transient absorption signal in fig. 1 to the decay of the singlet exciton. The lifetime of this state appears to be roughly temperature independent at 0.8 to 1.1 ps. If the singlet exciton decayed directly to the ground electronic state, the ground state recovery data shown in figs. 2-4 should yield an identical kinetic result. This is clearly not the case. At room temperature, a single-exponential ground state recovery time of 2.0 ps was observed. This result is in good agreement with those performed on the edge of the exciton absorption [ 13 J. At 5 K, this response becomes doubl~x~nenti~, with a 1.1 and a 3.4 ps component. Although not shown, the 5 K transient absorption data revealed no sign of a 3.4 ps decay component, but only a 1.1 ps single-exponential decay. Fig. 5 depicts a schematic energy level diagram and proposed relaxation pathways for PDA-pTS. The salient features of this model are (a) the existence of 384
4 September 1987
a configurationally relaxed intermediate singlet state through which rapid internal conversion is facilitated, (b) the absence of an efficient intersystem crossing process occurring from the singlet exciton S1, (c) the necessity of accessing higher-excited singlet states (via one- or tw~photon abso~tion~ to efficiently form the triplet exciton. Plausibility for a “barrier-free” structural relaxation in PDA-pTS comes from the extensively studied and well-characterized primary photochemical events occurring in rhodopsin. There is now substantial evidence that subsequent to the absorption of light, an essentially temperature-independent (measured down to 4 K in a rigid glass) relaxation process occurs [ 141. The isomerization of the polyene chromophore all-trans retinal to 13-cis retinal has been implicated. A characteristic time constant of 0.43 ps has recently been measured for this isomerization reaction [ 151. The cis-tram photoisome~~ation of 1,3,5,7-octatetraene in n-hexane at 4.2 K has also been well documented [ 161. Picosecond studies on substituted ethylenes and butadienes have provided a qualitative framework which rationalizes these ultrafast structural relaxaaion processes [ 171. Subsequent to photoexcitation, carbon-carbon bond orders are effectively changed whereby what were double bonds in the ground electronic state become single bonds in the excited state. What were potential minima in the structural coordinates corresponding to rotational degrees of freedom about carbon,carbon bonds, become in the excited state, either weakly bound, or local potential maxima. A weakly activated or barrierless structural relaxation then occurs, often resulting in dramatic changes in the molecular absorption spectra. Interruption of delocalized A electron molecular orbitals caused by the out-of-plane twisting of carbon backbone p orbitals can be held responsible for the often dramatic spectral effects. Our data indicate an apparent single-exponential recovery of the ground state at 300 K (fig. 3)) with a two-step process emerging at 5 K (fig. 4). Transient absorption results indicate a roughly temperature invariant m 1.0 ps initial relaxation step. We assign the initial 1.0 ps process to the formation of the structurally relaxed species, designated the “kink” state in fig. 5. The “kink state” is hypothesized to have a sizeable absorbance at 2 eV, greater in mag-
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nitude than the singlet exciton, but less than the ground state absorbance. Relaxation of the singlet exciton into the kink state therefore results in the partial recovery of the diffracted signal. A temperature sensitive relaxation then follows, presumably dependent on phonon emission, with a characteristic lifetime of = 1.0 ps at 300 K and 3.4 ps at 5 K. Two sequential 1.Ops decays are expected to appear, with our signal-to-noise, as a roughly 2.0 ps single-exponential decay. We note that the process of internal conversion in this system must certainly result in a lattice temperature rise. Simple exponential decay rates would therefore not be expected. However, for the purposes of illustrating qualitative effects in this system, the temperature trends displayed in the kinetic data are certainly reliable. Finally, we point out that our data indicate that triplet excitons appear within 3.0 ps after 3.94 eV or high-intensity 1.97 eV excitation. This corresponds to an extremely rapid intersystem crossing rate, the origin of which, together with the identity of the strongly coupled singlet state(s), is a subject for further investigation.
5. Conclusion Time-resolved transient absorption spectroscopy together with transient grating measurements have provided considerable insight into the primary steps of relaxation subsequent to photoexcitation of PDApTS. A roughly 1.O ps temperature-independent singlet exciton lifetime has been measured. We suggest that structural relaxation to a “kinked” intermediate is responsible for the ultrashort singlet exciton lifetime. The kinked state lives for roughly 1.Ops at 300 K and 3.4 ps at 5 K. No evidence for formation of
4 September 1987
triplet excitons via the singlet exciton was observed. Efficient and rapid ( < 3.0 ps) triplet exciton formation was observed, however, following 3.94 eV excitation.
References
[ 1] M. Pope and C.E. Swenberg, Electronic processes in organic crystals (Oxford Univ. Press, Oxford, 1982).
[ 2 ] J.C.W. Chien, Polyacetylene (Academic Press, New York, 1984).
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