Dithienopyrrolobenzothiadiazole and Naphthalimide-Based Donor–Acceptor Triads for Charge Transport and Photocatalytic Synthesis of Benzimidazoles

Abstract

Herein, acceptor–donor–acceptor π-conjugated small molecules, namely, triad 1 and triad 2, consisting of dithienopyrrolobenzothiadiazole and 1,8-naphthalimide, both with and without a thiophene spacer, were synthesized. Their absorption characteristics, redox properties, charge carrier mobilities, and their potential for metal-free photocatalytic transformations were investigated. Triad 1 and triad 2 showed wide absorption spectra in the range of 300–600 nm in solutions and 350–700 nm in thin films, along with narrow electrochemical bandgaps of 2.05 and 1.83 eV, respectively. Futhermore, the charge carrier mobilities of both triads were assessed through space-charge limited current measurements. The calculated hole mobilities for triad 1 and triad 2 were 1.26 × 10–3 and 8.38 × 10–4 cm2/V s, respectively, while the electron mobilities for triad 1 and triad 2 were 6.73 × 10–4 and 4.18 × 10–4 cm2/V s, respectively. Moreover, triad 1 and triad 2 exhibited efficient photocatalytic behavior toward selective metal-free condensation cyclization reactions of o-phenylenediamine/1,2-diamino-4,5-difluorobenzene and aromatic aldehydes with wide substrate scope under blue LED light to give a variety of pharmaceutically relevant benzimidazole derivatives.

A Dimethyldihydropyrene π-Switch for Light-Mediated Dynamic Control of Charge Transportation

“…Photochromic molecules undergo reversible changes in their structure and electronic configuration upon exposure to light at the molecular level which can be used for conduction switching. − The light-driven structural change of the organic photochromic molecules substantially alters their π-conjugation and hence modifies the energy gap between the highest occupied and the lowest unoccupied electronic levels. , This reversible isomerization can enable the control of various properties of the molecule, including its HOMO–LUMO energy-gap and, consequently, its intrinsic electronic conductance. − Although there are reports on the conductance switching of various photochromic systems such as stilbenes, azobenzenes, − spiropyrans, , and dithienylethenes (DTE), , only a handful of reports are available on the T-type negative photochromic systems . The dimethyldihydropyrene (DHP)–photochromic system switches from the rigid planar aromatic form to the step-like cyclophanediene (CPD) form, having a broken π–conjugation upon exposure to visible light, whereas the reverse reaction takes place with UV-light or even under thermal conditions. − This change in π-conjugation between the two isomeric forms of the “π-switch” can be used in stimuli-responsive optoelectronic devices. ,, So far, the synthesis of small molecules semiconductor materials has mainly focused on donor (D)-acceptor (A) conjugated systems, − where the low HOMO–LUMO energy difference makes them suitable as semiconductor systems . In these systems, triarylamines (TAA) have often been used as the donor unit .…”