Exciton Band Structure and Properties of a Real Linear Chain in a Molecular Crystal

Abstract

An experimental study is presented that leads to the elucidation of the whole band structure for the triplet exciton state of 1,4-dibromonaphthalene. We show that the band states are essentially those of a linear chain characterized by nearest neighbor interactions along the c crystallographic axis. Although the crystal is topologically a three-dimensional network, for the practical purpose of energy transfer and trapping the crystal behaves as a set of linear chains. Heavy doping of DBN-h6 (host) with up to 18% DBN-d6 (guest) yielded the expected discrete spectra of a random linear array corresponding to a nearest neighbor interaction of 7.4± 0.1 cm−1 and total bandwidth of 29.6± 0.4 cm−1 for the zero-zero transition. A symmetric mode at 0+520 cm−1 was shown to have an exciton bandwidth of 15 cm−1, and an unsymmetric vibrational level at 0+250 cm−1 was shown to have a vanishing bandwidth in accordance with expectations from weak coupling theory. The quasiresonance interactions of host and guest were significant and were included in establishing the agreement between experiment and calculation. The total bandwidth is estimated to be 90± 20 cm−1. This type of doping has an effect on line shape even at very low concentration and even in undoped crystals the line shape is asymmetrical, perhaps due to impurities and imperfections that have small zero-order shifts from the band center. The intensity of transitions to the carbon-13 isotopic traps is shown to be distributed over the band states in the origin and is clearly visible in the narrow band nontotally symmetric vibrational state. The vibrational analysis of the crystal spectrum (electronic or vibrational states) is shown to be dependent on exciton effects even in the case where no Davydov splitting is observed. Spectra of DBN-h6 (guest, H) in DBN-d6 (host, D) were also studied and resonance multiplets were seen and identified in absorption and emission. We identified ··· DHD···, ··· DHHD···, ··· DHHHD···, and probably ··· DHDHD··· in the phosphorescence spectra at 2°K. We present a brief discussion of the effect of dimensionality on the dynamical features of energy transfer and trapping, and we show experimentally, that heavy doping (d6 in h6) enhances the phosphorescence of the crystal about 60-fold. In addition the existence at 2°K of a Boltzmann distribution over resonance multiplets leads us to propose a long-range trap-to-trap migration as part of the thermalizing process.