Microscopic solvation effects on excited-state energetics and dynamics of aromatic molecules in large van der Waals complexes

Abstract

In this paper we report the results of an experimental study of the formation kinetics, excited‐state energetics, interstate electronic relaxation, and intrastate nuclear dynamics in electronically–vibrationally excited states of van der Waals molecules, consisting of a tetracene (T) molecule and rare‐gas (R) atoms. The TR n molecules were synthesized in seeded supersonic jets. Excited‐state energetics and dynamics of TR n molecules were explored by laser spectroscopy in supersonic expansions, interrogating the fluorescence action spectra, the energy‐resolved emission, the relative emission quantum yields, and the time‐resolved emission. Spectroscopic diagnostic methods for the identification and characterization of the chemical composition of TR n complexes involved the dependence of the spectral features on the identity of the rare gas, an intensity conservation rule for the intensities of TAr n and of T, the pressure dependence of the intensity of the bare T molecule, the pressure dependence of the intensity of the spectral features of the TR n molecules, and their order of appearance. We were able to identify the following 13 molecules: TAr n (n = 1,2,⋅⋅⋅7), TKr n (n = 1,2,3,4), and TXe n (n = 1,2), and to assign the spectral features which correspond to the vibrationless S 0→S 1 excitations of these molecules. For TAr1, TAr2, TAr3, TKr1, and TKr2, a single spectral feature corresponding to each molecule was observed, providing evidence against the existence of distinct chemical isomers of these molecules. For TKr3 and TKr4 a multiple spectrum consisting of several bands for each chemical composition was observed, which was tentatively assigned to chemical isomers of these molecules. The TXe1 and TXe2spectra reveal, in addition to a main band, weak satellites which were tentatively attributed to vibrational structure. The red spectral shifts of the vibrationless and the 314 cm−1 S 0→S 1 electronic excitations of all TR n molecules from the corresponding excitation of the bare T molecule are dominated by dispersive interactions, the red shifts for the TR1 (R = Ne, Ar, Kr, and Xe) molecules being proportional to the polarizability of the R atom. The spectral shifts of TR n molecules are not additive per added atom, the violation of the additivity law being attributed to the occupation of geometrically inequivalent sites by the R atoms. To demonstrate the universality of van der Waals molecule formation by large aromatics, we have studied the energetics of T(N2) n (n = 1–3) molecules and obtained preliminary spectroscopic data on T(C6H6) and T(H2O). We have studied, subsequently, microscopic solvent effects on electronic relaxation from the vibrationless S 1 state and from the 314 cm−1 vibrational excitation of this state of tetracene embedded in well characterized TR n complexes. The decay lifetimes τ of the vibrationless S 1 electronic state of TNe1 and TAr n (n = 1,2,...,7) molecules are in the range τ = 17±2 to 34±3 nsec, being close to or somewhat higher than the lifetime τ0 = 19±2 nsec of the electronic origin of the bare T molecule. The lifetimes of the vibrationless level of TKr1, TKr2, TKr3, and TKr4 molecules (τ = 6±1 to 8±1 nsec) and of TXe1 and TXe2 complexes (τ∼1.5 nsec) reveal a dramatic shortening relative to τ0, which is attributed to the heavy atom enhancement of S 1→T 1 crossing. The lifetimes of TKr n (n = 1–4) and of TXe n are practically independent of the coordination number, whereupon the heavy atom enhancement of intersystem crossing in these systems essentially originates from T–Kr and T–Xe single‐pair interactions. We also explored some effects of intrastate nuclear dynamics in the S 1 state of TAr n and of TKr n molecules. We have demonstrated that the 314 cm−1 vibrational excitations of TAr1 and of TKr1 do not result in vibrational predissociation on the (nsec) time scale of the excited‐state lifetime, the reactive channel being presumably closed. The heavy atom effect on the decay lifetimes of TKr n was utilized to search for the onset of vibrational predissociation, which is exhibited at excess vibrational energies of 1250 cm−1 above the electronic origin.