Insights into the spectral property and electronic structure of di-triphenylaniline modified monothiophene, dithiophene and thienothiophene
Authors: Xing Haowen , Xue Bo , Zhu Shengbo , Su Kehe 
Published: May 2023

Abstract
Density functional theory calculations at PBE38/6-311 + G** level by involving the polarizable continuum model in solvent dichloromethane were employed to explore the geometries, electronic excitations and the associated properties of the donor-acceptor-donor (D-A-D) di-triphenylaniline modified thiophenes of 4,4'-(thiophene-2,5-diyl)bis(N,N-diphenylaniline) (TPA-Th-TPA), 4,4'-([2,2'-bithiophene]-5,5'-diyl)bis(N,N-diphenylaniline) (TPA-ThTh-TPA) and 4,4'-(thieno[3,2-b]thiophene-2,5-diyl)bis(N,N-diphenylaniline) (TPA-TT-TPA). The spectral properties were investigated with the time dependent density functional theory at the same theoretical level by involving 37.5% of the Hartree-Fock exchange energies and with 50% of the local and non-local contributions, respectively for the rest of the energies. It was found that TPA-Th-TPA has one stable structure in 1A electronic state without symmetry, while both TPA-ThTh-TPA and TPA-TT-TPA have most stable structures with C2 symmetry in 1A state. Their vertical absorption spectra were examined with twenty lowest excitations while the emission spectra were equivalently simulated by the vertical transition (from S1 to S0) of the structure of the S1 state. Both the theoretical absorption and emission spectra agree very well with the experiments by absolute wavelengths and their sequence for different compounds. For the absorption with the maximum wavelength and strength, the theoretical wavelengths reproduced the experiments by deviations of only 4.4, 0.6 and 7.3 nm for TPA-Th-TPA, TPA-ThTh-TPA and TPA-TT-TPA, respectively. While the emission peaks have slightly larger deviations by 44.5, 90.5 and 53.3 nm. Detailed features for the next intense peak as well as their peak shoulders were explored. For the electronic properties associated with the S0→S1 transition, the hole-electron, frontier orbital and natural transition orbital analyses supported charge transfer characteristics. The inter-segment charge transfer analyses provided the magnitude of inter-segment charge transfer of TPA-Th-TPA, TPA-ThTh-TPA and TPA-TT-TPA by 67.1, 60.6 and 66.4%, respectively. within which the transfer from donors to acceptor is dominant. In addition to the largest π conjugation of the ThTh group that leads to the largest red shift of the spectra and charge redistribution, TPA-ThTh-TPA has the largest vertical electron affinity energy, electronegativity and global electrophilicity with 2.01, 3.68 and 4.05 eV, respectively. All the molecules have the electrostatic potentials on their S0 and S1 states by approximately 54% of the negative potential region supplied mainly by the lone pair electrons of the S, N atoms and the π electrons of the C atoms. This leads to the compounds being more susceptible to electrophilic reactions. Similar atomic natural charge distributions for the different compounds in their S0 and S1 states were found with the S atom(s) having the most positive (~ 0.42 e) and the N atoms having the most negative charges (~-0.51 e). Small changes of the atomic charge were found in the excitations indicating that the charge transfer does not significantly change the atomic charge distributions.