Transient absorption and vibrational relaxation dynamics of the lowest excited singlet state of pyrene in solution

Abstract

The spectrum of highly excited electronic states of pyrene in n-octane solution and the relaxation processes in the SI singlet state are investigated by femtosecond transient absorption spectroscopy. Three different UV excitation wavelengths (360, 307, and 290 nm) are utilized to excite the molecule at the edge of the SI - SO transition and to higher S, - SO transitions. The buildup of SI population has been observed to be completed in the first 2 ps, with a time constant of 300-400 fs, depending on the excitation wavelength. The evolution of the absorption spectra on a larger time scale is attributed to the vibrational cooling in the SI state; its rate corresponds to an exponential relaxation with time constant of about 4 ps. The evidence of the formation of pyrene excimer at long time delays in highly concentrated solutions is reported. The interaction of an intense short laser pulse with a molecule in solution leads generally to various photoproducts which relax differently through complex relaxation dynamics. The simplest process is the absorption of a single photon bringing the molecule to some excited vibronic level. The excited molecule undergoes then very fast nonradiative decay processes which eventually produce the buildup of an out-of-equilibrium vibrational population, normally in the SI state. Slower radiative and nonradiative processes, the latter involving mostly solutesolvent interactions, then relax the molecule to the ground state. Conformational changes can take place in parallel to the relaxation processes.' With intense light pulses, multiphoton excitation, ionization, and bond breaking may become competitive processes; in these cases new molecular species are produced, and new channels are made available for energy relaxation. The investigation of the excited states, of the possible photoproducts, and of the associated dynamical processes can be performed by means of several spectroscopic techniques such as transient absorption, transient Raman, and IR spectroscopy utilizing ultrafast laser source^.^.^ Transient absorption spectroscopy with subpicosecond resolution in the visible and near-W region is probably the most widely used and certainly the most feasible among these techniques. Transient absorption can be performed with two kinds of experimental configurations. The first one utilizes two monochromatic pulses at fixed wavelength: one (the pump) centered at the SI - SO transition (or at some higher resonance, from which population in the SI state is eventually produced) and the second (the probe) at the S, - SI transition. In this case the rise and the decay time of the transient absorption give information about the mechanism of population of SI and about the relaxation processes of this state toward the ground state. In the second experimental configuration, the S, - SI transitions (and, possibly, other transitions involving a higher starting level) are monitored by looking directly at the transient spectrum recorded at different delay times, using a white light probe pulse.