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J . Phys. Chem. 1985,89, 3238-3243
Plcosecond Reorientatlonal Dynamlcs of Resorufin: Correlations of Dynamics and Llqutd Structure Eva F. Gudgin Templeton, Edward L. Quitevis,t and Geraldine A. Kenney-Wallace* Lash Miller Laboratories, University of Toronto, Toronto, Canada M5S 1 A1 (Received: December 21, 1984) We describe the reorientational dynamics in polar fluids of resorufin, a rigid aromatic probe molecule, whose anionic site permits several specific intermolecular interactions to be investigated. Using the novel technique of difference-frequency multiple modulation picosecond absorption spectroscopy, we report the first measurements on the ground-state dynamics of resorufin in a series of protic alcohols, aprotic fluids, and the binary system of propanokwater. Whereas the reorientation times 7rOtin the alcohols can be well described by Debye-Stokes-Einstein equation with stick boundary conditions, the 7rOt data in acetonitrile, dimethylformamide, and dimethyl sulfoxide show faster reorientation times and quite different behavior from hydrodynamic expectations, even if slip boundary conditions were invoked. Finally, resorufin in formamide, water, and water:propanol systems shows considerable deviation from the 7, pattern established in the alcohols. A model is proposed in which the lifetime of the local and site-specific intermolecular interaction between resorufin and its nearest-neighbor solvent molecules is compared to the cooperative relaxation time of the overall three-dimensional or two-dimensional network of solvent interactions typical of hydrogen-bonded systems. We conclude that both long-wavelength hydrodynamic effects and the nature and behavior of the first solvation shell influence rotational dynamics.
Introduction An understanding of the mechanisms for rotational relaxation of molecules in polar liquids is fundamental not only to the development of a microscopic theory of liquid molecular dynamics but also to the predictions of solvent effects in nonequilibrium ionic systems, the description of transition states, and solution phase reaction dynamics. A study of such motions can also give information on the molecular nature of solute-solvent and solvent-solvent interactions, and how their relative magnitudes influence the macroscopic properties of the system. Furthermore, differences in the rotational dynamics of dye molecules in the ground and excited states can yield information on polarizability and dipole moment changes which occur upon electronic excitation. While a considerable number of data have been reported on the picosecond reorientational motions of different dye molecules in various with a view to testing the predictions of Debye-Stokes-Einstein formalisms and more recent arguments on boundary conditions for angular momentum t r a n ~ f e r , ~we ” focus here on the dynamics of a rigid probe dye molecule whose specific ionic interaction sites can be exploited to investigate correlations between dynamics and local or continuum liquid structure. This motivation arises from a previous study7 in which we concluded that in some cases linear correlations observed between rotational reorientation times (7r0t)and macroscopic properties such as viscosity (1)arose because T~~~and 9 were both linked to a third variable, namely, the liquid structure. In the event that a solute molecule slightly perturbed, rather than accomodated to, the solvent liquid structure, then the rotational motion of the guest molecule was indeed governed by the dynamics of the host fluid and would even reflect the same temperature dependence. In this study we explored the influence of both the interactions and the solvent structure for a given size probe molecule, and then investigated the same probe in a binary system for selective solvation effects as the packing structure was changed. The dye resorufin (Figure 1) is configurationally simpler than most previously studied dyes, and thus interpretation of the rotational motion is made easier, due to a well-defined van der Waals volume and no internal rotations. The absorption and emission spectra in methanol are shown in Figure 1. Both its ground-state ~ excited-state ) reorientation time (7*rot) reorientation time (T ~ and can be measured, using visible picosecond pumpprobe laser absorption spectroscopy and picosecond fluorescence depolarization techniques, respectively. No measurements of ground-state 7r,,t Present address: Department of Chemistry, Texas Tech University, Lubbock, TX 79409. * E.W.R. Steacie Fellow.
0022-3654/85/2089-3238$01.50/0
have been performed to date but a preliminary report on T * has ~ ~ appeared.* A complete characterization of this dye for possible use as a standard of reorientational motion would involve measurements for the ground and excited states of the various ionic forms. In this paper, we have investigated the ground-state rotational motion of the resorufin anion, while future work will focus on the 7*r0tand the behavior of resazurin, its nitroso analogue.
Experimental Section The experiments were performed with a novel variation of picosecond pumpprobe absorption spectroscopy, namely, difference-frequency modulation spectroscopy (DFMS). This technique involves the simultaneous modulation of pump and probe beams at different MHz frequencies, and detection at the difference frequency using a fast photomultiplier/MHz lock-in amplifier combination. The full details of the DFMS method have been reported el~ewhere.~The main advantages for pumpprobe measurements at the same wavelength are a significantly improved signal-to-noise ratio and over lo3 dynamic range for linearity of the detection system as compared to conventional modulation spectroscopy1°or multiple modulation spectroscopy” which utilizes a kHz chopper, radio receiver, and kHz lock-in amplifier combination in place of the M H z lock-in amplifier. The output train from a synchronously pumped, mode-locked argon ion rhodamine 6G dye laser, generating pulses of typically 20.8 ps, 12-11s interpulse spacing, and 100-mW average power at 600 nm, is split by a 95:5 beam splitter into an intense pump beam and a weak probe beam. Polarization of the pulse train is set by a Glan-Thompson polarizer before the beam splitter. Both (1) P. Madden and D. Kivelson, Annu. Rev.Phys. Chem., 31,523 (1980), also present a summary of the data and theories over the past decade. (2) D. H. WaldeckandG. R. Fleming,J. Chem. Phys., 85, 2614 (1981). and references therein. (3) A. vonJena and H. E. Lasing, Chem. Phys. Lett., 78, 187 (1981), Chem. Phys., 40, 245 (1979). (4) A review of the various hydrodynamic models which have been proposed is provided in J. L. Dote, D. Kivelson, and R. N. Schwartz, J . Phys. Chem., 85, 2169 (1981). (5) .(a) C-M. Hu and R. Zwanzig, J. Chem. Phys., 60,435 (1974); (b) B. Berne in “Advanced Treatise on Physical Chemistry”, Vol. VIIIB, H. Eyring, D. Henderson, and W. Jost, Ed., Academic Press, New York, 1971, p 540. (6) J. T. Hynes, R. Kapral, and M. Weinberg, J. Chem. Phys., 69, 2725 (1978). (7) Stephen A. Rice and G. A. Kenney-Wallace, Chem. Phys., 47, 161 (1980). (8) K. G. Spears, K. M. Steinmetz-Bauer, and T. H. Gray in “Picosecond Phenomena 11”. R. Hochstrasser, W. Kaiser, and C. V. Shank, Ed., Springer-Verlag, New York, 1980, p 106. (9) E. Quitevis, E. Gudgin Templeton, and G. A. Kenney-Wallace, Appl. .. Opi.,’24, 3i8 (1985). (10) E. P. Ippen and C. V. Shank, Top. Appl. Phys., 18, 83 (1977). ( 1 1) B. F. Levine. C. V. Shank. and J. P. Heritage, - IEEE J. Ouantum Ele&on., 15, 1418 (1979).
0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3239
Picosecond Reorientational Dynamics of Resorufin 0.3-
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TABLE I: Ground- and Excited-State Rotational Reorientation Times of Resorufin in Pure Solvents' --60.000
Figure 1. Absorption and emission spectra of the resorufin anion in methanol. Emission in arbitrary units.
pump and probe beams are then each directed into an acoustooptic modulator (Matsushita M120) operating at 15 and 10.24 MHz, respectively, driven by individual signal processors (Matsushita ELF-C12OP1) with amplitude modulations from clock oscillators with TTL outputs at those individual frequencies. To obtain a reference signal at the difference frequency, a double balance mixer is used to obtain sum and difference frequencies with the local oscillator input taken from 10.24 MHz and the rf input from the 15-MHz line. After filtering, a reference signal at the difference frequency only (4.76 MHz) is obtained and fed into a M H z lock-in amplifier (EGG-PAR 5202), whose signal input is from a 1P28 buffered photomultiplier, which records the transient absorption loss and recovery measured by the probe laser beam. The output of the lock-in amplifier is digitized and stored on MINC 11/23 computer, which also controls the electronically driven translation stages for the optical time delay to f l p precision. The pump and probe pulses are focussed into the sample by using a counterpropagating geometry, the pump beam remaining larger than the probe beam over the interaction length in the dye. The polarization of the pump beam is selected by using a polarization rotator immediately before the sample cell. Measurements of the change in transmission (AT) were taken for parallel and perpendicular pump polarization relative to the vertical probe polarization, and also a t 54.7O. The absorption of resorufin at 580 nm is 35000 M-' cm-',and while in principle a good signalmoise could be obtained from conventional techniques,l excessive scattering of stray pump light into the detector is a significant problem leading to high background light levels, which swamp the desired signal. With DFMS we were able to obtain signals at 0.01% modulation depth on the probe beam9 compared to a limit of 0.1% on kHz modulation and 0.05% for other multiple modulation approaches.12 Furthermore, there are no corrections for nonlinearity of the detector, as is necessary when radio receivers are employed. The 580-nm pump pulse depleted the ground-state population of resorufin through the So SI electronic excitation and the probe pulse monitored the recovery of the ground-state population at time delays from 0 to 600 ps. The sample consisted of a solution M in a 1-mm flow cell thermostated at 25 OC, with an optical density of
