J. Phys. Chem. 1995, 99, 7324-7329
7324
Temperature and Pressure Dependent Optical Dephasing of Pentacene in Triclinic and Monoclinic p-Terphenyl Bruce J. Baer and Eric L. Chronister* Department of Chemistry, University of California, Riverside, Califomia 92521 Received: June 30, 1994; In Final Form: November 1, I994@ Temperature-dependent (4.3- 10 K) photon echo measurements are presented for pentacene doped p-terphenyl at variable high pressure (0-18 kbar). The homogeneous electronic linewidth of pentacene in monoclinic p-terphenyl is observed to narrow at high pressure due to an increase in the pseudolocal phonon frequency. In contrast, pressure induced changes in the linewidth of pentacene in triclinic p-terphenyl appear to occur largely due to changes in the pseudolocal phonon lifetime. In addition, softening of the pseudolocal phonon frequency is observed near the polymorphic phase transition pressure of 5.5 kbar and photosite specific dynamics are observed at high pressure.
Introduction High pressure is a well established experimental parameter with which to probe the nature of molecular excited states, the shift in vibrational and electronic energy levels, and intermolecular interactions in condensed phases. We have recently focused on the use of high pressure as a means of probing nonradiative relaxation processes such as intemal conversion, intersystem crossing,2and vibrational relaxati~n.~.~ In a previous paper we showed that high pressure can be used to significantly alter the intersystem crossing rates of pentacene in triclinic p-terphen~l.~ Pressure dependent photon echo measurements have also been utilized to probe the anharmonicity of pseudolocal phonons associated with a guest molecule in a dilute mixed crystal.6 Pressure induced narrowing of the electronic homogeneous linewidth has been observed in doped glasses and mixed crystals resulting from pressure-induced increases in phonon freq~encies.~-~ The homogeneous dynamics of pentacene doped naphthalene and pentacene-doped p-terphenyl have been of particular interest.10-21 Although mixed molecular crystals can be grown with relatively narrow electronic absorption bands, the lowtemperature linewidths are typically inhomogeneously broadened even at ambient pressure. Thus there have been relatively few pressure dependent homogeneous linewidth studies of molecular solids at high p r e ~ s u r e . ~However, -~ the photon echo technique can be used to obtain homogeneous dephasing even in the presence of pressure induced static inhomogeneities, as demonstrated by previous high-pressure photon echo results for pentacene in naphthalene.6 In the present study we utilize photon echo measurements of pentacene in p-terphenyl over a wide range of pressures (0-18 kbar) and at variable low temperatures (4.3-10 K). Earlier photon echo work on the pentacene in p-terphenyl system at ambient pressure has shown that electronic dephasing can occur by scattering from pseudolocal phonons14 and that the dephasing time equals twice the fluorescence lifetime below 2 K (Le., T2 = 2T1).11 In addition, the intemal conversion and intersystem crossing rates of pentacene in both p-terphenyl and naphthalene have previously been determined by using optical free induction decay,20,21three-pulse stimulated echo,ls optical nutation, and laser-acoustic diffraction measurements.21 More generally, studies on the effect of con~entration~~ and optical
* To whom correspondence should be addressed. Abstract published in A d l m c e ACS Abstrucrs. May 1, 1995.
density16on the photon echo decay rate have demonstrated that measurements need to be performed at very dilute dopant concentrations. Absorption spectra of single pentacene molecules in crystalline ~ - t e r p h e n y l have ~ ~ - ~revealed ~ pressure-induced red shifts but no detectable change in the linewidth at the low pressures utilized.24 It should be noted that the single molecule absorption lines are typically red shifted far from the band center by defects and/or by internal electric field gradients. Thus, the dynamics and pressure shifts of the red shifted single molecules can be quite different from those of the majority of molecules with absorption frequencies nearer the band center.25 Pentacene in p-terphenyl is considered an ideal mixed crystalline system due to the near perfect substitution of a pentacene guest molecule in a host p-terphenyl crystal. Under ambient conditions, p-terphenyl has a monoclinic crystal structure (space group P21/a) with two equivalent molecules per unit Below 190 K at ambient pressure, p-terphenyl has a triclinic crystal structure (space group P i ) with four inequivalent molecules per unit cell,27yielding four distinct sites for a pentacene guest molecule to reside. At higher pressures the triclinic-monoclinic phase transition occurs at lower temperatures, such that above a pressure of 5.5 kbar only the monoclinic phase e ~ i s t s . ~ For , ~dilute ~ pentacene (< mol of guest/mol of host) the low temperature absorption and fluorescence spectra for the triclinic host crystal display four electronic origins, while only a single origin band is observed for the monoclinic host crystal (Le., at pressures above 6.5 kbar~).~,~~ In general, the electronic and vibronic dynamics of pentacene are sensitive to the particular crystallographic site in which the molecule resides within the triclinic p-terphenyl crystal structure.2J8~20.21~29-31 Furthermore, the pressure shifts of the pentacene photosite absorption frequencies and fluorescence lifetimes are also site specific in triclinic p-terphenyL2 Experimental Section The details of the high-pressure technique^,^^ laser ~ y s t e m , ~ and optical measurements have been described previously.2 Single crystal samples of dilute pentacene in p-terphenyl ( < 5 x mol/mol) were grown from the melt under pressure in the gasket aperture of a Memll-Bassett diamond anvil cell.32 The high pressure samples were prepared by fist growing dilute mixed crystals in a Bridgman furnace at ambient pressure, which were then loaded into a diamond anvil cell, pressurized, remelted
0022-3654/95/2099-7324$09.00/0 0 1995 American Chemical Society
Optical Dephasing of Pentacene
J. Phys. Chem., Vol. 99, No. 19, 1995 7325
by heating, and regrown at high pressure by slow compression. After crystallization, further compression resulted in considerable inhomogeneous b r ~ a d e n i n g .To ~ ~minimize these effects, all photon echo measurements were performed on samples grown from the melt at the pressure of interest. Pressures were measured by the shift of the R1 ruby line as well as by the shift in the pentacene absorption f r e q u e n ~ y .Temperatures ~~ between 4.3 and 10 K were obtained by using a temperature-controlled (Lake Shore, Model 330) helium flow cryostat (Janis, STVP100). The second harmonic output of a mode-locked Q-switched Nd:YAG laser was used to pump a cavity-dumped, tunable, dye laser with pulse energies of 10 pJ, pulsewidth of 30 ps, at a repetition rate of 800 Hz. For the photon echo measurements, the ‘laser wavelength was set to the red edge of the electronic origin band of interest. Excitation bandwidths of either 1 or 4 cm-’ yielded the same photon echo decay results. The laser pulses were attenuated to about 0.1 pJ, focused to 50 pm, and spatially overlapped in the sample. The photon echo emission was spectrally filtered with a monochromator, detected with a cooled photomultiplier (Hamamatsu R955P) and a lock-in amplifier, digitized and averaged over several scans, and analyzed on a personal computer.
Temperature and Pressure Effects on Electronic Dephasing in Mixed Molecular Crystals
the pseudolocal phonon in the ground and in the excited electronic ~ t a t e . 3 ~ If the frequency and lifetime of the pseudolocal phonon are similar in the ground and excited states (i.e., hdo/kT > l), eq 2 reduces to the uncorrelated phonon scattering result due to scattering from both ground- and excited-state pseudolocal
The quadratic theory summarized above provides a simple model for evaluating electronic dephasing. However, anharmonicities have been shown to be important in the temperaturedependent optical dephasing of pentacene in benzoic acid.42 A useful phenomenological expression for evaluating temperature dependent photon echo measurements of electronic dephasing is given by14
(3
Coupling to pseudolocal modes must be considered when the T2*(T)= T2*(m)exp introduction of an impurity significantly perturbs a l a t t i ~ e . ~ ~ , ~ ~ , ~ ~ When a guest molecule differs sufficiently from molecules of Equation 5 follows directly from eq 3, where A,??corresponds the host matrix, new vibrational modes localized about the to the pseudolocal phonon energy, hw, and T2*(=)= z* [1 + impurity are created. A pseudolocal mode arises if the (dw~*)~]/(dwz*)~. In the slow-exchange limit (dwz* >> l), the frequency of one of these modes mixes with the host acoustic prefactor T2*(=) simplifies to the pseudolocal phonon lifetime phonon band.33 The relevant theoretical models for optical z*. dephasing in molecular crystals are (1) nonperturbative results Given the small change in the configuration of pentacene derived by assuming a quadratic (Le., harmonic) electronupon electronic excitation, one expects the pseudolocal phonon phonon c o ~ p l i n g , ~(2) ~ - weak ~ ~ electron-phonon coupling lifetimes and energies to be similar in the ground and excited (yielding an Arrhenius temperature dependence at low temperelectronic states. For pentacene in naphthalene, Hesselink and atures), exchange t h e ~ r y , ~ ~(3)- ~“uncorrelated l phonon scatWiersma14 have reported values of w o = 18 cm-’, w1 = 13.8 tering” theory,42and (4) Redfield relaxation theory.43 Theories cm-’, z, = 3.5 ps, and z1 = 11 ps. These quantities indicate involving multilevel systems have also been proposed.* the slow exchange limit; however, corresponding values for The total electronic dephasing rate (T,)-l is typically depentacene in p-terphenyl are not available. Nevertheless, if one scribed by separating depopulation and thermally activated assumes the slow exchange limit in analogy with the pentacene dephasing contributions in naphthalene system, then the T2*(=)prefactor yields a measure of the pseudolocal phonon lifetime. 1 - 1 1 From the above equations, a pressure induced decrease in T, 2T, T,* the homogeneous decay rate is to be expected if the pseudolocal phonon energy ho increases with pressure. Furthermore, where TI is the electronic state lifetime and T2* is the thermally temperature dependent photon echo measurements of T2* at high activated pure dephasing time. pressure can be used to investigate the effect of pressure on the The nonperturbative result for thermally activated electronic pseudolocal phonon energy (characterized by AE). dephasing due to quadratic electric-phonon coupling with pseudolocal phonons can be evaluated at relatively low temResults perature (l/zo, l/zl
