Methodological Survey of Simplified TD-DFT Methodsfor Fast and Accurate Interpretation of UV–Vis–NIR Spectraof Phthalocyanines

摘要

A methodological survey of density functional theory (DFT) methods for the prediction of UV–visible (vis)–near-infrared (NIR) spectra of phthalocyanines is reported. Four methods, namely, full time-dependent (TD)-DFT and its Tamm–Dancoff approximation (TDA), together with their simplified modifications (sTD-DFT and sTDA, respectively), were tested by using the examples of unsubstituted and alkoxy-substituted metal-free ligands and zinc complexes. The theoretical results were compared with experimental data derived from UV–visible absorption and magnetic circular dichroism spectroscopy. Seven popular exchange-correlation functionals (BP86, B3LYP, TPSSh, M06, CAM-B3LYP, LC-BLYP, and ωB97X) were tested within these four approaches starting at a relatively modest level using 6-31G(d) basis sets and gas-phase BP86/def2-SVP optimized geometries. A gradual augmentation of the computational levels was used to identify the influence of starting geometry, solvation effects, and basis sets on the results of TD-DFT and sTD-DFT calculations. It was found thatalthough these factors do influence the predicted energies of thevertical excitations, they do not affect the trends predicted in thespectral properties across series of structurally related substitutedfree bases and metallophthalocyanines. The best accuracy for the gas-phasevertical excitations was observed in the lower-energy Q-band regionfor calculations that made use of range-separated hybrids for bothfull and simplified TD-DFT approaches. The CAM-B3LYP functional providedparticularly accurate results in the context of the sTD-DFT approach.The description of the higher-energy B-band region is considerablyless accurate, and this demonstrates the need for further advancesin the accuracy of theoretical calculations. Together with a generalincrease in accuracy, the application of simplified TD-DFT methodsaffords a 2–3 orders of magnitude speedup of the calculationsin comparison to the full TD-DFT approach. It is anticipated thatthis approach will be widely used on desktop computers during theinterpretation of UV–vis–NIR spectra of phthalocyaninesand related macrocycles in the years ahead.