Design and synthesis of donor-acceptor functionalized phenothiazines

Abstract

Phenothiazine is a class of fused heteroaromatic compound, containing sulfur (S) and nitrogen (N) atoms in the central ring (Figure 1). Phenothiazine acts as a strong donor which has been used as a building block to develop optoelectronic materials. Phenothiazine derivatives possess high chemical and thermal stability, low reversible oxidation potential, strong absorption, high luminescence and photo-conductivities. The most common approach for tuning the photophysical and electrochemical properties of the phenothiazine is to link donor/acceptor units directly or by π-spacer at the 3- and 7- positions of phenothiazine (Figure 2). The donor−acceptor functionalized phenothiazines exhibit strong absorption in the near-infrared region and low HOMO−LUMO gap which are used as potential candidates for the applications in dye sensitized solar cells (DSSCs), bulk heterojunction organic solar cells (BHJOSCs), OLEDs, NLOs, OFETs, hole transporting materials (HTMs), sensing, bioimaging, photodynamic therapy etc. Figure 1. The molecular structure of phenothiazine core. In order to improve the photonic and electronic properties of phenothiazines a variety of donors (ferrocene, triphenylamine, carbazole) and acceptors (BODIPY, 1,8-naphthalimide, 1,1,2,2-tetracyanoethylene and 7,7,8,8-tetracyanoquinodimethane) have been introduced to the phenothiazine core. The photophysical and electrochemical properties of the donor−acceptor functionalized phenothiazines were investigated. The main objectives of the current work are: • To design and synthesize donor−acceptor functionalized phenothiazine derivatives for optoelectronic applications. • To synthesize symmetrical and unsymmetrical phenothiazine derivatives by varying the donor/acceptor units in a systematic way. • To study the effect of substitution pattern of donor−acceptor phenothiazines on their photophysical and electrochemical properties. 4 • To fine tune the HOMO−LUMO gap by altering the donor/acceptor strength or π-linker on the phenothiazine core. • To investigate the structural and photophysical properties of the donor−acceptor functionalized phenothiazines via density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations and compare with the experimental data.