Weak vibrational coupling in a large van der Waals complex: Fluorescence spectroscopy of perylene/naphthalene

Abstract

Fluorescence excitation and dispersed fluorescence spectra are reported for 1:1 and 2:1 complexes of naphthalene with perylene under supersonic jet conditions. Confirming preliminary results, the fluorescence spectra of the 1:1 complex following excitation of an ag (in-plane) mode at 353 cm−1 and its first overtone show unusually weak vibrational coupling. Although excitation of combination levels of 3531 with out-of-plane modes at 74, 79, and 93 cm−1 gives rise to emission which is predominantly ‘‘relaxed,’’ the residual ‘‘unrelaxed’’ component indicates a significant degree of mode-selective vibrational coupling. It is notable that the vibrational coupling for 3532 excitation (i.e., at ≈700 cm−1) is substantially less extensive than for excitation into the 3531 combination bands nearly 300 cm−1 lower in energy. A similar comparison has been made between a second ag mode, at 550 cm−1, and a perturbed b3g (out-of-plane) mode, at 540 cm−1. In this case, the data indicate a difference in coupling, which is less obvious than for the 353 cm−1 case, but still indicates a significant dynamic difference in the picosecond domain. Higher-energy excitation is seen to give rise to a large amount of spectral broadening, to ≈700 cm−1, yet without any appreciable shift. This indicates that a single equilibrium conformation is present. Comparison with data for uncomplexed perylene and for other perylene complexes at a similar excitation energy (1300–1400 cm−1) suggests that the broadening is due to vibrational coupling involving combinations and overtones of Franck–Condon active low-frequency modes in perylene/naphthalene. Fluorescence excitation data for 2:1 complexes show that the three-band combination of the 1:1 complex at 74–93 cm−1 is replaced by a single, strongly Franck–Condon active mode at 62 cm−1. The corresponding ground state frequency is about 56 cm−1 and, overall, this mode shows harmonic behavior. Like the 1:1 complex, the fluorescence spectrum following excitation into v=2 of the out-of-plane mode, at ≈125 cm−1, shows little evidence of vibrational coupling. At and above 353 cm−1, perylene/(naphthalene)2 shows extensive vibrational coupling, since no structure persists in the fluorescence spectra. A comparison has been made between the naphthalene and some other 1:1 complexes of perylene. At 353 cm−1, the Ar1 complex shows less vibrational coupling than perylene/naphthalene, although the presence even of an argon dimer generates a greater degree of coupling. The benzene complex shows extensive IVR at this energy, by comparison with the weakly coupled naphthalene case. Further, while the naphthalene/perylene out-of-plane modes show predominantly resonant Franck–Condon emission profiles, up to excitation energies of 170 and 124 cm−1 for the 1:1 and 2:1 complexes, respectively, perylene/benzene shows extensive IVR even for 70 cm−1 excitation. These data strongly indicate that intermolecular modes, especially weakly hindered internal rotation, are responsible for the pronounced differences in the degree of vibrational coupling.