Vibrational Population Relaxation of Perylene in n-Alkanes. The Role of Solvent Local Structure in Long-Range Vibrational Energy Transfer

Abstract

We have measured the vibrational population relaxation times of the Raman active v7 mode (1375 cm-') and(v7 + v15) combination mode (1733 cm-') of perylene in eight liquid n-alkanes using ultrafast stimulated emission spectroscopy. The vibrational population relaxation time of the perylene v7 mode ranges from -300 to < l o ps depending on the n-alkane solvent chain length, but there is no simple correspondence between alkane length and TI for v7. Energy transfer from the perylene v7 vibrational mode to a specific n-alkane solvent vibrational mode is dominated by long-range resonance coupling. The perylene (v7 + ~1 5 ) combination mode exhibits additional efficient relaxation pathways for different length n-alkanes. These data point collectively to short-range order in the n-alkane solvent surrounding perylene molecule. IntroductionThere have been a large number of recent studies focusing on intermolecular interactions in room temperature liquids because they are an important but ill-understood vehicle for many chemical processes.' Principal among the difficulties associated with understanding intermolecular interactions in liquids is the rapidly changing spatial relationship between neighboring molecules and elucidation of the various pathways in which energy can be exchanged. There are a variety of length scales on which these questions can be addressed, and the information provided by experiments aimed at interrogating the various length scales, taken collectively, can provide insight into local, transient organization in liquids.The advent of lasers capable of producing short pulses of light has allowed the direct investigation of energy and structural relaxation processes in liquids. A common theme in many laserbased experiments on liquid-phase dynamics is the use of a "probe" molecule that absorbs and (sometimes) emits light at wavelengths accessible to pulsed lasers. For almost all of these experiments, there is excess energy remaining in the probe molecule after the information of interest has been extracted, and at least some of this excess energy is stored as vibrational energy. In low-pressure gas-phase experiments, where an excited molecule is comparatively isolated from its neighbors, vibrational energy will dissipate slowly within the molecule to lower energy modes according to the extent of anharmonic coupling between the vibrational modes.2 For probe molecules in liquids, intramolecular relaxation is often less important than direct intermolecular relaxation because the number of collisional interactions with the surrounding medium is large and individual molecules are in closer spatial proximity to one a n~t h e r .~.~ In solution, excess vibrational energy within a molecule can be dissipated directly into the surroundings, sometimes very efficiently. Intermolecular vibrational population relaxation processes include energy transfer from solute vibrational modes into the translational, vibrational, and rota-* Author to whom correspondence should be addressed. @ Abstract published in Advance ACS Ab...

Investigation of the Influence of Solute−Solvent Interactions on the Vibrational Energy Relaxation Dynamics of Large Molecules in Liquids

“…More recently, Blanchard et al have measured the fluorescence dynamics of Pe in n-alkanes of different length using stimulated emission spectroscopy with 10 ps resolution. 28 The rise time of the fluorescence in a wavelength region corresponding to S 1 (V ) 0) f S 0 (V * 0) emission was attributed to the lifetime of the vibrational excited state. Time constants ranging from less than 10 to about 400 ps were reported, but no relationship between alkane length and VER time constant was found.…”

Solvent-assisted intramolecular vibrational energy redistribution of S1 perylene in ketone solvents

“…Many experimental and theoretical studies have been reported on VER in the last decade [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. that IVR is further classified into two processes in solutions; the fast IVR that takes place in tens of femtoseconds to sub picoseconds time scales and the slow IVR in a few picoseconds time scale [5][6][7][8].…”

“…Blanchard et al studied VER of S 0 perylene and S 0 1-methylperylene in various solvents by using picosecond stimulated emission spectroscopy [9][10][11]. They investigated VET taking place in time scales slower than tens of picoseconds.…”

Rotational Dynamics of Nonpolar and Dipolar Molecules in Polar and Binary Solvent Mixtures

“…The dielectric friction can be modeled using continuum theories of NeeZwanzig (NZ) (Nee and Zwanzig, 1970), which treats the solute as a point dipole rotating in a spherical cavity, Alavi-Waldeck (AW) (Alavi and Waldeck, 1991b;1993) model which is an extension of the NZ theory where the solute is treated as a distribution of charges instead of point dipole and the semiempirical approach of van der Zwan and Hynes (vdZH) (van der Zwan and Hynes, 1985) in which fluorescence Stokes shift of the solute in a given solvent is related to dielectric friction. Conversely, the results of neutral and nonpolar solutes deviate significantly from the hydrodynamic predictions at higher viscosities (Waldeck et al, 1982;Canonica et al, 1985;Phillips et al, 1985;Courtney et al, 1986;Ben Amotz and Drake, 1988;Roy and Doraiswamy, 1993;Williams et al, 1994;Jiang and Blanchard, 1994;Anderton and Kauffman, 1994;Brocklehurst and Young, 1995;Benzler and Luther, 1997;Dutt et al, 1999;Ito et al, 2000;Inamdar et al, 2006). These probes rotate much faster than predicted by the SED theory with stick boundary condition and are described by either slip boundary condition or by quasihydrodynamic theories.…”

“…In the two limiting cases of hydrodynamic stick and slip for a nonspherical molecule, the value of C follows the inequality, 0< C ≤ 1 and the exact value of C is determined by the axial ratio of the probe. It is observed that the experimentally measured rotational reorientation times of number of the nonpolar solutes (Waldeck et al, 1982;Canonica et al, 1985;Phillips et al, 1985;Courtney et al, 1986;Ben Amotz and Drake, 1988;Roy and Doraiswamy, 1993;Williams et al, 1994;Jiang and Blanchard, 1994;Anderton and Kauffman, 1994;Brocklehurst and Young, 1995;Benzler and Luther, 1997;Dutt et al, 1999;Ito et al, 2000;Inamdar et al, 2006) could be described by the SED theory with slip boundary condition (subslip behavior). For a homologous series of solvents such as alcohols or alkanes, the normalized reorientation times decreased as the size of the solvent is increased.…”