On the nature of the triplet electronic states of naphthalene dimers

Elucidating the photophysical mechanisms within multi-chromophore assembly (MCA) is essential for many key technological and biological processes. Although it has been established that one of the most important photoactivated applications of MCA is intimately linked to efficient intersystem crossing (ISC) to triplet states and the interplay between delocalized/localized triplet excited states, the underlying mechanism between such equilibrium and the observed optical properties remains elusive. Herein, four suitably designed dinaphthyl compounds, covalently bonded in a face-to-face configuration and encompassing the primary possible stacking geometries, were prepared and their triplet state properties investigated by combining transient absorption spectroscopy experiments with quantum chemistry calculations. Our results offer direct evidence of both localized and delocalized triplet states, with the most stable and long-lived triplet state consistently localized on a single naphthalene unit, irrespective of the stacking configuration. Moreover, depending on the stacking geometry, even if localized, the triplet transient absorption spectrum was demonstrated to be significantly different from that of an isolated naphthalene.