Energetics and intramolecular dynamics of the isolated ultracold tetracene molecule in its first excited singlet state

Abstract

In this paper we report the results of an experimental study of the energetics and intramolecular dynamics of the first electronically excited S1 singlet state of tetracene (C18H12) seeded in supersonic expansions of rare gases. Internal cooling of tetracene in supersonic expansions of Ar down to rotational temperatures TR∼5–7 K and vibrational temperatures TV<50 K can be accomplished at moderately low stagnation pressures p = 100–200 Torr of Ar, Kr, and Xe when expanded through a D = 150 μ nozzle. We have interrogated the fluorescence action spectra, the energy-resolved fluorescence, and the time-resolved fluorescence of the ultracold, isolated, bare, large molecule for excess vibrational energies EV = 0–4000 cm−1 above the electronic origin of the S1 state. The electronic origin of the S0(1A1g)→S1(1B2u) transition located at 22 360 cm−1 exhibits a partially resolved B-type rotational structure, which concurs with the short axis polarization of this transition. We were able to identify nine totally symmetric alg fudamentals and three vibronically induced b 3g fundamentals in the vibrational structure of S1 in the energy range EV = 0–1600 cm−1. The vibrational structures of the S1 state and of the S0 state in the isolated molecule are in good agreement with the mixed crystal data, indicating small medium effects on the low-energy vibrational level structure. Information on the shift and distortion of the potential surface of the S1 state relative to S0 was inferred from frequency changes and from Franck–Condon vibrational overlap factors. We have investigated two classes of nonreactive dynamic processes in the S1 manifold, involving intrastate anharmonic mixing (IAM) and interstate electronic relaxation (ER). IAM was explored by the observation of the onset for spectral congestion in the absorption spectrum which marks the onset of the vibrational quasicontinuum, as well as by the observation of band splitting in the energy-resolved emission spectra which marks the onset of IAM. The threshold for IAM in the S1 state of tetracene is exhibited at EV?1000 cm−1 below the onset of the vibrational quasicontinuum, which occurs at EV?1600–1800 cm−1. These observations provide a firm basis for the identification of three energy regions for IAM in the S1 manifold of this large molecule, i.e., the sparse level structure (EV = 0–1000 cm−1), the intermediate level structure (EV = 1000–1600 cm−1), and the statistical limit (EV≳1800 cm−1). Information concerning ER in the S1 manifold was obtained from the experimental decay lifetimes τ of photoselected states at excess vibrational energies EV = 0–4000 cm−1. Excitations in the sparse vibrational level structure in the range EV = 0–1200 cm−1 results in a retardation of the ER rate relative to its value for the electronic origin; this blocking of the ER rate for low EV is attributed to S1→T1 intersystem crossing, which is characterized by a low electronic energy gap. Excitation in the vibrational quasicontinuum results in the τ values which are practically independent of EV with τ = 4.5±0.5 nsec in the range EV = 2000–4000 cm−1. This observation cannot be reconciled with the notion of effective energy redistribution among all vibrational modes and is tentatively attributed to the effects of selective intrastate anharmonic mixing which involves a subset of the vibrational states.