Butterfly-shaped donor–benzophenone systems: Probing excited-state dynamics by time-resolved spectroscopy and computational methods

A rational approach was used to design eight benzophenone-based acceptor molecules (Bzp = A) in two structural forms: donor–acceptor (Bzp–D ≡ A–D) and donor–acceptor–donor (D–Bzp–D ≡ D–A–D). The para position of the heterocyclic unit in the Bzp core was functionalised with electron-donating groups — pyrrole, indoline (IND), carbazole (Cbz), diphenylamine (DPA), and phenothiazine (Phen) — to evaluate their impact on the electronic spectral and photoluminescence (PL) properties of the resulting compounds in different organic solvents. The results show that the electron-donating moieties have an important effect on the PL properties of the molecules. Such a structural change leads not only to a red-shift of the fluorescence emission maxima but also to a significant enhancement of the fluorescence quantum yields and a substantial positive fluorescence solvatochromism, particularly evident in compounds Bzp-Cbz, Bzp-BisCbz and Bzp-DPA. The fluorescence solvatochromism is associated to the planar and polar structure of Bzp with an intramolecular charge transfer (ICT) involving the carbonyl group (C O) and the different donor groups. The mechanism of photoinduced ICT was investigated by femtosecond and nanosecond transient absorption (fs/ns-TA) spectroscopy. A combination of theoretical (DFT and TDDFT) and experimental studies has demonstrated that the dihedral angles between donor and acceptor units, together with the singlet–triplet energy gap (ΔE ST ), and the character of the T 1 state, play a decisive role in determining the efficiency of the emission. These parameters were effectively used to shift the emission from a locally excited (LE) to a charge transfer (CT) state. • Functionalization of benzophenone with donor groups enhances fluorescence via intramolecular charge transfer (ICT). • Structural modifications lead to pronounced positive solvatochromism and red-shifted emission. • Emission efficiency is governed by donor–acceptor dihedral angles, ΔE ST values, and the nature of the T 1 state. • Transition from locally excited (LE) to a charge transfer (CT) state is supported by both theoretical and fs/ns transient absorption.