Evaluating Donor Effects in Isoindigo-Based Small Molecular Fluorophores

Small molecular organic fluorophores have garnered significant interest because of their indispensable use in fluorescence imaging (FI) and optoelectronic devices. Herein, we designed triphenylamine (TPA)-capped donor-acceptor-donor (D-A-D)-based fluorophores having a variation at the heterocyclic donor (D) units, 3,4-ethylenedioxythiophene (EDOT), furan (FURAN), thiophene (THIO), and 1-methyl-1H-pyrrole (MePyr), with isoindigo as the core electron acceptor (A) unit. Synthesis of these fluorophores (II-X-TPA) resulted in four symmetrical dye molecules: II-EDOT-TPA, II-FURAN-TPA, II-THIO-TPA, and II-MePyr-TPA, where TPA functioned as a terminal unit and a secondary electron donor group. Photophysical, electrochemical, and computational analyses were conducted to investigate the effect of heterocyclic donor units on the II-X-TPA derivatives. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations provided insightful features of structural and electronic properties of each fluorophore and correlated well with experimental observations. Electron density distribution maps, overlapping frontier molecular orbital diagrams, and highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) electron transfer indicated intramolecular charge transfer (ICT). Theoretical studies confirmed the experimental HOMO energy trend and demonstrated its crucial importance in understanding each heterocycle's donor ability. Stokes shifts of up to ∼178 nm were observed, whereas absorptions and emissions were shifted deeper into the NIR region, resulting from ICT. Results suggest that this isoindigo fluorophore series has potential as a molecular scaffold for the development of efficient FI agents. The studied fluorophores can be further tuned with different donor fragments to enhance the ICT and facilitate in shifting the optical properties further into the NIR region.