J-Type Heteroexciton Coupling Effect on an Asymmetric Donor–Acceptor–Donor-Type Fluorophore

Abstract

The novel donor-acceptor-donor (D-A-D)-type fluorophore with an asymmetric structure is reported. The twisted-induced charge transfer (TICT) luminescence was observed. The degree of charge transfer and radiative rate constant in the luminescence increased simultaneously with increase in orientational polarizability of solvents. In contrast to the numerous CT fluorophore researches, this behavior has never been previously observed. This characteristic behavior reveals the existence of an effective exciton coupling between the CT states in the donor-acceptor-donor-type fluorophore for the first time.

Asymmetric Color‐Changeable Luminophore with Donor–Acceptor–Donor Structure for Solvent and Temperature Sensitive Properties

“…Several stable luminescent conformations are a key point to produce novel organic CT luminophores. Recently, we have reported an asymmetric D–A–D type structure (Figure b, As–H) . As–H demonstrates suppressed delocalization of two D–A excited states and different twisted angles ( θ 1 and θ 2 ; Figure b, Structure–B) based on weakened exciton coupling.…”

“… (a) Symmetric D–A–D‐type structure and excited state structure (Structure–A) and (b) asymmetric D–A–D‐type structure, previous (As–H), novel (As–OM) asymmetric D–A–D compounds, and excited structure (Structure–B and –C).…”

“…NMR peaks at 17.03 ppm (−OH) and 7.00 ppm (−CH) for the cis‐enol proton were observed (Figure S1). Note that no keto signal was observed at around 4.0 ppm . The NMR integration ratio indicated that the enol form dominated (>99%).…”

“…The fluorescence bands were red‐shifted according to solvent polarity (cyclohexane, tetrachloride (CCl 4 ), toluene, chloroform (CHCl 3 ), and dichloromethane (CH 2 Cl 2 )), resulting in a large solvatochromism (Figure a). The vibronic bands at 454, 474, and 512 nm were clearly observed in hexane (Figure b, black line), which originated from mixing with the localized excited states of methoxy‐carbazole and dibenzoylmethane moieties . The solvent‐dependent photophysical properties of As–OM are summarized in Table .…”

“…The luminescence quantum yields dramatically increased with decrease in solvent polarity, up to approximately 50% (cyclohexane, CCl 4 , and toluene), although the luminescence lifetimes were strongly dependent on solvents. These luminescence quantum yields differ from the previous compound (As–H), whose luminescence quantum yields dramatically decreased with a decrease in solvent polarity. Differences between the two originate mainly from variation in the radiative rate constant, which is dependent on solvent polarity (Supporting Information).…”