Structure and electronic spectroscopy of naphthalene-acenaphthene van der Waals dimer: Hole-burning, dispersed fluorescence, and quantum chemistry calculations

摘要

Electronic spectroscopy of 1:1 van der Waals dimer of naphthalene and acenaphthene has been studied in a supersonic free jet expansion by measuring the laser-induced fluorescence excitation, dispersed fluorescence, and two-color hole-burning spectra. In fluorescence excitation spectrum the dimer exhibits a long progression of an intermolecualr vibration, and similar vibronic structures are observed also in emission spectra from the origin region of the S_1 surface. The excimer formation from the locally excited state appears as a barrier crossing process and excess vibronic energy required to cross the barrier is about 420 cm~(-1). The equilibrium structurea nd binding energy of the dimer the computed by the ab initio quantum chemistry method at the MP2/6-31G and MP2/6-31+G~*//MP2/6-31G levels. A parallel-displaced structure, in which two moleculesa re displaced from a fully overlapping geometry by 1.16 and 0.45 A. respectively, along the long and short molecualr axes, and maintains a vertical separtion of 3.48 A between two molecular planes, is found to be the most stable in the ground state. The BSSE corrected MP2/6-31+G~*//MP2/6-31G binding energy of the dimer is 9.2 kcal/mol. The observed spectral and dynamical characteristics of the mixed dimer are comapred to those reported for the naphthalene homodimer, and the differences are interpreted in terms of geometry and exciton resonance interactions.