Novel Organic Donor-Bridge-Acceptor Dyad and Triad Using 5,10-Dihydroindolo[3,2-b]indole and Naphthalene Diimide: Photophysical Study Based on Ground-State Charge Transfer Interaction

Abstract

Charge-transfer (CT) complexes, pertaining to the interaction between the electron-donating moieties and accepting moieties and resulting in new electron energy levels, have been widely researched due to their unconventional optoelectronic characteristics, such as superconductivity, ambipolarity, and photoconductivity, among others. Because these unique properties mainly depend on the variety and combination of the donors and the acceptors, developing donors and acceptors is essential. Hence, we focused on the new donor 5,10-dihydroindolo[3,2-b]indole (IDID) due to its strong donating ability caused by its two pyrrole rings and various substitution sites, which provide an opportunity to solve molecular stacking and solubility problems. Based on these advantages, we explored the possibility of realizing CT complex characteristics through photophysical approach involving both intramolecular and intermolecular systems. Here, we report a new ground-state CT complex using IDID as a donor (D) and naphthalene diimide (NDI) as an acceptor (A). Experiments are done to determine the difference between the intermolecular and intramolecular systems by synthesizing 2HIDID, 2EHNDI, and an A-D-A type of triad consisting of T4, T6 and T8. The systems showed different CT characteristics. For the control group, a D-A type of dyad, D4, was synthesized. As a result, the triad and dyad showed similar CT bands compared to an intermolecular mixed group, in contrast to the initial expectations that the triad system would differ from the intermolecular system because the IDID and NDI showed 1: 1 stoichiometry, which can differ in terms of the CT bands in 1:2 triad molecules. We can conclude that the molecular stacking between the donor and the acceptor significantly affect the charge transfer characteristics.