Density Functional Theory Calculations Of The Electronic Structures And Properties Of Indolenium Squaraine Dyes

Squaraine dyes, as an important kind of functional dyes, show a particular opticalperformance, inherent stability and a strong absorption in the visible and near-infraredregion. Introducing substituents in N-position of indole ring or central squarylium ring,can effectively regulate the electronic absorption spectra and other properties ofsquaraine dyes. In order to clarify the the structure-property relationship of squaraineand its derivatives, density functional theory (DFT) and time-dependent densityfunctional theory (TD-DFT) calculations were carried out to systematically simulateindolenium squaraine dye and its two p-aminoazobenzene derivatives.The molecular structures, atomic charge distribution, dipole moment, the frontiermolecular orbitals, UV-Vis absorption spectra and H-NMR of squaraine dye (SQ1)and its two p-aminoazobenzene derivatives (SQ2and SQ3) have been studied withDFT and TD-DFT methods. We discussed the influence of the different position ofp-aminoazobenzene substituent on the electronic structures, charge distribution,molecular orbitals and electronic absorption spectra, and studied the excited stateproperties of dyes and the process of intramolecular electron transfer. Besides, weexplored the photosensitive properties of squaraine dye.Geometric parameters analysis indicates that the N-substituent doesn’tparticipate in the large π-conjugated system of the molecule, and the substituents onthe squaraine ring can increase the degree of π-conjugation of the whole molecule.Charge distribution analysis shows that the introduction of p-aminoazobenzenesubstituent on the squaraine ring can affect squaraine dye skeleton charge distribution.UV-Vis spectrum analysis indicates that p-aminoazobenzene substituent can broadensquaraine absorption range of the UV-visible region. Frontier molecular orbitalanalysis shows that the atoms which have large contributions to the frontier orbitalsare concentrated on the main structure of the squaraine, N-substituent almost makesno effect on the frontier molecular orbitals, so SQ1and SQ3display similarelectron-transfer after excitation, an intramolecular charge transfer state with electronsbeing mainly transferred from the electron rich oxygen atoms to the electron deficient central four-membered ring; for SQ2, HOMO-LUMO excitation results in an increasein the electron density towards the center four-membered ring of molecule fromoxygen atoms and p-aminoazobenzene atoms. These theoretical calculations would behelpful for the design and syntheses of novel derivatives of squaraine dyes.