Can Range-Separated and Hybrid DFT Functionals Predict Low-Lying Excitations? A Tookad Case Study.

The spectral properties of Tookad (Pd-bacteriopheophorbide, Pd-BPheid), an effective photosensitizer that targets mainly prostate tumors, and metal-free BPheid have been studied using time-dependent density functional theory (TD-DFT). The well-established B3LYP functional, which is known to overestimate excitation energies, was included in the study along with recently introduced range-separated and meta hybrid DFT functionals CAM-B3LYP, M06, M06-2X, M06HF, ωB97XD, ωB97X, ωB97, LC-ωPBE, and PBE0 (PBE1PBE). The main focus is the performance of the new functionals in predicting low-lying excitations (>600 nm), to explore their potential roles in drug development for photodynamic therapy. The data suggests that ωB97XD overall performs best for the Qy transition band (the red-most absorption), followed by CAM-B3LYP. LC-ωPBE, ωB97, B3LYP, and PBE1PBE all generated the Qy band far from the experimental position. The error in absorption energy for the Qy band was found to be at most 0.05 eV for ωB97XD, compared to 0.15-0.19 eV for B3LYP. The use of different basis sets used in excited-state calculations was shown to be of less importance as was the use of either B3LYP or M06 in geometry optimizations.