Fluorescence from an H-aggregated naphthalenediimide based peptide: photophysical and computational investigation of this rare phenomenon

Abstract

Fluorescence associated with J-aggregated naphthalenediimides (NDIs) is common. However, in this study an NDI based synthetic peptide molecule is found to form a fluorescent H-aggregate in a chloroform (CHCl₃)–methylcyclohexane (MCH) mixture. An attempt has been made to explain the unusual fluorescence property of this H-aggregated NDI derivative. Time correlated single photon counting (TCSPC) shows that the average lifetime of the NDI based molecule is on the order of a few nanoseconds. It is revealed from the computational study that the transition from the second exited state (S₂) to the ground energy state (S₀) is responsible for the fluorescence as S₁ is a dark state. Such rare violation of Kasha's rule accounts for the unusual fluorescence properties of this type of NDI molecule in the H-aggregated state.