Charge-Transfer Formation and Geometry of the Naphthalene−Trimethylamine van der Waals Complex

Abstract

Spectroscopic and kinetic experiments on the naphthalene−trimethylamine van der Waals (vdW) complex in a molecular beam are described. When the complex is vibronically excited, as well as a small amount of fluorescence characteristic of naphthalene, a broad, featureless, red-shifted emission is observed. This is attributed to fluorescence from an excited charge-transfer (CT) state of the cluster. Unlike naphthalene−triethylamine, the trimethylamine complex has absorption transitions with measurable rotational structure, and approximate rotational constants of 490, 500, and 900 ± 50 MHz were obtained. The rise time of the CT emission is ∼60 ps when excited into the S 1 + 455 cm -1, 8(b 1g ) 1, naphthalene transition, and its decay lifetime is 56 ± 3 ns. This lifetime increases as the excitation energy increases to reach 105 ± 5 ns at S 1 + 1131 cm -1, 8(b 1g ) 1 8(a g ) 1, which is the opposite trend to that which is observed in other naphthalene−amine complexes. The geometry of the complex was determined from the rotational constants and by ab initio calculations. The amine was found to lie symmetrically disposed about the naphthalene long axis and above one of the naphthalene rings, with its hydrogen atoms toward the naphthalene. The results are interpreted in terms of the crossing of the initial locally excited (LE) state of the vdW complex, which has mainly naphthalene character, to the CT state.