Evaluating Donor Effects in Isoindigo-Based Small Molecular Fluorophores

Abstract

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.

Effects of nanoaggregation on isoindigo-based fluorophores for near-infrared bioimaging applications

“…Though broad, the absorption maximum for the amorphous solid-state (AS) of II-EDOT-TPA is positioned at 613 nm with an intensity that is 4.2% less than the previously reported solution state (640 nm, 6 × 10 −5 M in DCM). 7 However, the emission maximum was measured to be 847 nm, with an emission intensity 5.2% greater than that of the solution state (805 nm). Moreover, the excited state lifetime of the II-EDOT-TPA in the AS was 399 ps, which is higher than the instrument response function of 200 ps.…”

“…The excited-state lifetime of solid-state II-EDOT-TPA (399 ps) is longer when compared to II-EDOT-TPA in solution (<200 ps). 7 This increase is presumably due to the restriction of intramolecular rotations and a larger intermolecular π–π distance, which are indicative of the AIE phenomenon. 22…”

“…II-EDOT-TPA was synthesized according to the previously published methods. 7 Additional details regarding the synthesis of PEG-II-EDOT-TPA can be found in the ESI. †…”

“…3,8,9 Previous work in our group focused on the effect of π-heterocyclic units in an isoindigo-based donor-acceptordonor (D-π-A-π-D) small molecule fluorophore to afford a series of four fluorophores where triphenylamine (TPA) was employed as an auxiliary donor unit. 7 The 3,4ethylenedioxythiophene (EDOT) derivative, II-EDOT-TPA, exhibited notable theoretical and experimental results suggesting AIE behavior. The HOMO was primarily localized on the EDOT-TPA donor unit, while the LUMO was found to be localized on the isoindigo acceptor indicative of intramolecular charge transfer (ICT).…”

Five‐Membered‐Heterocycle Bridged Viologen with High Voltage and Superior Stability for Flow Battery

“…[11] The literature survey suggests that further efforts are still needed on the molecule engineering to lower the redox potential of viologen. Heterocyclic compounds like furan and thiophene are acknowledged to be electron-rich aromatic rings, [12] which is supposed to act as an electron donor when inserted between the two pyridiniums and lower the redox potential. Additionally, the extended π-conjugation can lower the Lewis basicity of the reduced viologen and thus, improves its stability in water.…”