Optical selection studies of electronic relaxation from the S1 state of jet-cooled anthracene derivatives

Abstract

In this paper we explore the energy dependence of the interstate electronic relaxation rates knr from the S1 manifold of anthracene and seven of its isotopic and chemical derivatives, which were inferred from quantum yield data. Absolute fluorescence quantum yields Y from groups of rotational states within the electronic origin S1(0) and from vibrational states were obtained over the excess energy Ev=0–3000 cm−1 above S1(0) by the simultaneous interrogation of the fluorescence excitation spectra and of the absorption spectra in seeded, pulsed, planar supersonic jets of Ar. Additional information was obtained from quantum yield data of van der Waals (vdW) complexes of these molecules with Ar. The fluorescence quantum yields from the S1(0) of anthracene, 9-cyano-anthracene, and 9,10-dibromoanthracene were found to be independent of the rotational state, providing further evidence for the rotational independence of knr from a single doorway state. From the Y data of the electronic origins and from the Ev dependence of Y we conclude that intersystem crossing (ISC) dynamics of the S1 manifold is dominated by the interplay between two classes of nonreactive coupling and/or relaxation: (i) Interstate coupling, involving the superposition of direct S1→(T1) ISC together with S1→(Tx) →(T1) mediated ISC through a sparse or dense (Tx) manifold of a higher triplet state; (ii) Intrastate coupling within the S1 manifold, which sets in with increasing Ev and which results in intramolecular vibrational energy redistribution (IVR) at high Ev. The dominant role of mediated interstate coupling in ISC dynamics from S1(0) and from low Ev states was inferred from the inverse deuterium isotope effect on the ISC rates, the extreme sensitivity of knr of deuterated anthracene to a single H atom substitution, and to level shifts induced by complexing with Ar, as well as from the three-orders-of-magnitude difference between the knr values from the S1(0) of 9-bromoanthracene and of 9,10-dibromoanthracene. The onset of the mediated ISC is documented by an abrupt drop of Y in the narrow (Ev =617–805 cm−1) energy range for 9,10-dichloroanthracene and by the oscillatory energy dependence of Y vs Ev and the extreme energy sensitivity of Y in the range Ev =157–800 cm−1 of 9,10-dibromoanthracene, which is attributed to near-degeneracies between S1 states and the mediating (Tx) states. These resonance effects can be drastically modified by dispersive level shifts induced by complexing with Ar. At high excess vibrational energies some universal features of the Ev dependence of knr are exhibited. These involve a gradual increase of k n r with increasing Ev at medium energies (Ev =1000–1800 cm−1), which correspond to the intermediate level structure for intrastate coupling and a very weak Ev dependence of knr at high energies (Ev =1800–3000 cm−1), which manifest the effect of statistical intrastate IVR on interstate ISC.