Modification of Optical Properties and Excited-State Dynamics by Linearizing Cyclic Paraphenylene Chromophores

Abstract

Cyclic and bent conjugated molecular systems have tunable optical, structural, and dynamical features that differentiate them from their linear counterparts. Examples of such systems are [<i>n</i>]­cycloparaphenylenes (CPPs), which consist of nanorings composed of <i>n</i> para-linked benzene units. Circular geometry and tunability of π-orbital overlaps and bending strains enrich them with unique physicochemical and electronic properties compared to those of the corresponding linear oligoparaphenylenes. Herein, we explore the changes of these properties on alkyl-tethered-<i>p</i>-heptaphenylenes by modifying the methylene tether lengths from 1 to 19 carbons, leading to a gradual linearization of the conjugated backbone conformation. For this purpose, the photoinduced internal conversion processes of different alkyl-tethered-<i>p</i>-heptaphenylenes are simulated using nonadiabatic excited-state molecular dynamics. We found that the greater the strain introduced on the conjugated system, the slower the electronic and vibrational energy relaxation process. All bent <i>p</i>-heptaphenylenes exhibit similar patterns of intramolecular energy redistribution that finally spatially localize the exciton on phenylene units in the middle of the conjugated chain. This behavior is opposite to the random exciton localization previously reported for [<i>n</i>]­CPPs. Moreover, the nonadiabatic S₂ → S₁ electronic transition activates specific collective asymmetric vibrational excitations that promote periodic oscillatory evolution of the excitonic wave function before an excessive energy dissipates into the bath degrees of freedom