Are all charge-transfer parameters created equally? A study of functional dependence and excited-state charge-transfer quantification across two dye families

• Published in: Physical chemistry chemical physics : PCCP 2021-09, Vol.23 (36), p.2583-2597
• Main Authors: Marshburn, Richard Drew, Ashley, Daniel C, Curtin, Gregory M, Sultana, Nadia, Liu, Chang, Vinueza, Nelson R, Ison, Elon A, Jakubikova, Elena
• Format: Article
• Language: English
• Subjects: Charge transfer; Density functional theory; Dipole moments; Dyes; Electronic structure; Exchanging; Excitation; Functional groups; Mathematical analysis; Molecular orbitals; NMR; Nuclear magnetic resonance; Spectra; Textiles; Time dependence
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d1cp03383b

Small molecule organic dyes have many potential uses in medicine, textiles, forensics, and light-harvesting technology. Being able to computationally predict the spectroscopic properties of these dyes could greatly expedite screening efforts, saving time and materials. Time-dependent density functional theory (TD-DFT) has been shown to be a good tool for this in many instances, but characterizing electronic excitations with charge-transfer (CT) character has historically been challenging and can be highly sensitive to the chosen exchange-correlation functional. Here we present a combined experimental and computational study of the excited-state electronic structure of twenty organic dyes obtained from the Max Weaver Dye Library at NCSU. Results of UV-vis spectra calculations on these dyes with six different exchange-correlation functionals, BP86, B3LYP, PBE0, M06, BH and HLYP, and CAM-B3LYP, were compared against their measured UV-vis spectra. It was found that hybrid functionals with modest amounts (20-30%) of included Hartree-Fock exchange are the most effective at matching the experimentally determined λ max . The interplay between the observed error, the functional chosen, and the degree of CT was analyzed by quantifying the CT character of λ max using four orbital and density-based metrics, Λ , Δ r , S C and D CT , as well as the change in the dipole moment, Δ μ . The results showed that the relationship between CT character and the functional dependence of error is not straightforward, with the observed behavior being dependent both on how CT was quantified and the functional groups present in the molecules themselves. It is concluded that this may be a result of the examined excitations having intermediate CT character. Ultimately it was found that the nature of the molecular "family" influenced how a given functional behaved as a function of CT character, with only two of the examined CT quantification methods, Δ r and D CT , showing consistent behavior between the different molecular families. This suggests that further work needs to be done to ensure that currently used CT quantification methods show the same general trends across large sets of multiple dye families. Twenty dyes from the Max Weaver Dye Library were used to benchmark six commonly used DFT functionals to understand the interplay between the errors in the calculated excitation energies and the degree of charge transfer character of the excitations.