A Spectroscopic and Computational Study of Dyes Based on Carbazole and TPA Donors, and Indane-Based Acceptors

Spectroscopic and computational analyses were employed to investigate a series of donor-acceptor dyes connected by a fused dithiophene (dctp) 4,4-dihexyl-4H-cyclopenta2,1-b:3,4-b'. The donor units employed were triphenylamine (TPA) and carbazole (Cbz), while the acceptor units included Indane-based 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (InOCN) and 1H-indene-1,3(2H)-dione (IndO). A charge-transfer (CT) transition was evident in the electronic absorption spectra between 17,300 and 15,000 cm-1. Each dye exhibited a singlet state of CT emission across all of the dyes. Emission spectra were observed from 14,700 to 13,100 cm-1. Resonance Raman spectroscopy (RRS) was utilized to experimentally validate the time-dependent density functional (TD-DFT) findings. Furthermore, the results reveal that CT in 2-((6-(9-ethylcarbazol-3-yl)-4,4-dihexylcyclopenta2,1-b:3,4-b'dithiophen-2-yl)methylene)indene-1,3-dione (PX134) and 2-(2-((6-(9-ethylcarbazol-3-yl)-4,4-dihexylcyclopenta2,1-b:3,4-b'dithiophen-2-yl)methylene)-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (PX135) occurs from the donor-π system to the acceptor, whereas in ((6-(4-(diphenylamino)phenyl)-4,4-dihexylcyclopenta2,1-b:3,4-b'dithiophen-2-yl)methylene)indene-1,3-dione (PX137) and 2-(2-((6-(4-(diphenylamino)phenyl)-4,4-dihexylcyclopenta2,1-b:3,4-b'dithiophen-2-yl)methylene)-3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (PX138), it proceeds from the donor to the π-acceptor units. These compounds exhibited distinct solvatochromic behavior in their absorption and emission spectra. These variations are analyzed using Lippert-Mataga (LM), McRae (MR), and Weller plots, which indicate changes in dipole moments between the ground and excited states. The linear slopes derived from the LM, MR, and Weller plots suggest a well-defined CT process with minimal specific solvent interactions. Density functional theory (DFT) modeling also suggests a significant level of planarity across all four dyes, and the experimental Raman spectra closely match the calculated Raman spectra. Compared to IndO groups, malononitrile units induce a greater CT character in the spectral emission behavior of each molecule. Variable temperature (VT) study reveals a blue shift with the increase in temperature across all of the dyes, indicating no evidence of aggregation.