Two-Photon Spectroscopy of a Series of Platinum Acetylides: Conformation-Induced Ground-State Symmetry Breaking

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2017-07, Vol.121 (29), p.5442-5449
• Main Authors: Cooper, Thomas M, Haley, Joy E, Krein, Douglas M, Burke, Aaron R, Slagle, Jonathan E, Mikhailov, Aleksandr, Rebane, Aleksander
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.7b04784

With the goal of elucidating electronic and conformational effects on structure–spectroscopic property relationships in platinum acetylides, we synthesized a series of nominally centrosymmetric chromophores trans-Pt­(PBu3)2(CC-Phenyl-X)2, where X = diphenylamino (DPA), NH 2 , OCH 3 , t-Bu, CH 3 , H, F, benzothiazole (BTH), CF 3 , CN, and NO 2 . We collected one- and two-photon absorption spectra and also performed density functional theory (DFT) and time-dependent (TD) DFT calculations on the ground- and excited-state properties of these compounds. The DFT calculations revealed facile rotation between the two ligands, suggesting that the compounds exhibit nonplanar ground-state conformations in solution. TDDFT calculation of the S 1 state energy and transition dipole moment for a nonplanar conformation gave good agreement with experiment. Two-photon absorption spectra obtained from these compounds allowed estimation of the change of permanent electric dipole moment upon vertical excitation from ground state to S 1 state. The values are small Δμ < 1.0 D for neutral substituents such as CH 3 , H, and F but increase sharply to Δμ ≈ 11 D for electron-accepting NO 2 . When in a nonplanar conformation, the corresponding calculated Δμ values showed good agreement with the experimental data indicating that the two-photon spectra result from nonplanar ground-state conformations. Previously studied related chromophores having extended conjugation (Rebane, A.; Drobizhev, M.; Makarov, N. S.; Wicks, G.; Wnuk, P.; Stepanenko, Y.; Haley, J. E.; Krein, D. M.; Fore, J. L.; Burke, A. R.; Slagle, J. E.; McLean, D. G.; Cooper, T. M. J. Phys. Chem. A 2014, 118, 3749−3759 ) show similar dependence of Δμ on the substituents, which allows us to conclude that the excited-state properties of these floppy chromophores are a function of the electronic properties of the substituents, ligand size, and nonplanar molecular conformation.