Structures of perylene complexes with long-chain alkanes and alkyl halides examined by rotational coherence spectroscopy

Abstract

Fluorescence excitation spectra of jet-cooled van der Waals complexes of the planar aromatic hydrocarbon perylene with the n-alkanes pentane, hexane, octane, and decane show in each case a single structural form. Rotational coherence transients observed for these species, spaced by 2.5–4.4 ns, are consistent with the n-alkane chain oriented parallel to the long axis of perylene and placed 3.6 Å above its surface. In contrast, the 1-chloropentane and 1-fluoropentane complexes of perylene both exhibit three conformational isomers (α,β,γ) in the electronic ground state. Rotational coherence experiments have measured the structures of these different species in the S1 state, via 000 excitation, proving the existence of three distinct isomers in each case. Dispersed emission spectra following vibronic excitation at 355 cm−1 (A10) indicate in each case that the γ structure relaxes to the α form. Knowledge of the structures of the different forms provides a basis to identify the photoisomerization trajectories. Rotational coherence spectroscopy (RCS) was also used to examine the structures following vibronic excitation. The n-alkane complexes each revealed at least one prominent recurrence transient, showing that vibronic excitation and subsequent vibrational redistribution processes do not necessarily cause rapid rotational dephasing. Similarly, an appreciable degree of rotational coherence persisted into the nanosecond domain for the α and β isomers of the fluoropentane and chloropentane complexes. On the other hand, loss of the γ form was confirmed via the RCS traces, since there were no recurrences for 355 cm−1 excitation. In this way, RCS measurements following vibronic excitation can provide a means to probe conformational stability.