“Metal-free bipolar/octupolar organic dyes for DSSC application: A combined experimental and theoretical approach.”
S. Mandal, S. Kushwaha, R. Mukkamala, S. Vijayakumar, I. S. Aidhen, B. Rajakumar, R. Kothandaraman. Organic Electronics 2016, 36, 177-184.

This study evaluated four donor-π-linker-acceptor (D-π-A) dyes experimentally, which either have 1 or 3 triphenylamine (TPA) donor in combination with either pyridine or N-methyl pyridinium ion as acceptor. In this study, by increasing the number of closely spaced HOMOs (by way of adding multiple TPA to the acceptor), the fraction of visible spectrum that was absorbed increased. Through the density functional theory calculations and the relevant experiments, the number of closely spaced HOMOs was shown to be directly proportional to the number of TPA moieties in the dye. N-methyl pyridinium based dyes have shown lesser band gap (2.2–2.5 eV) when compared to its pyridine based counterpart (3.08–3.3 eV) due to its high electron withdrawing nature. Hence, an octupolar dye, 2,4,6-tris((E)-4-(diphenylamino)styryl)-1-methylpyridinium iodide, containing three TPA donors and N-methyl pyridinium ion acceptor was identified as effective light harvesting dye for dye-sensitized solar cells application.