2, Montana State University, Bozeman, Montana, United States
In order to develop new chromophores for nonlinear optics applications our group has synthesized and characterized numerous platinum acetylides. Recently, we synthesized a model series of nominally centrosymmetric chromophores trans-Pt(PBu3)2(CC-Phenyl-X)2, where X = diphenylamino(DPA), NH2, OCH3, t-Bu, CH3, H, F, benzothiazole(BTH), CF3, CN and NO2. We collected linear- and two-photon absorption spectra and performed DFT and TDDFT calculations on the ground- and excited state properties of these compounds. 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 S1 state. The corresponding calculated values calculated from nonplanar molecular conformation showed good agreement with the experimental data indicating that the two-photon spectra resulted from nonplanar ground state conformations. We also measured triplet state spectra, including phosphorescence and time-resolved transient absorption spectra. Through DFT and TDDFT calculations we calculated triplet state energy, geometry and descriptors for the location of the triplet exciton on the molecule. The calculated triplet state energies agreed well with measured phosphorescence emission and excitation spectra. The triplet state lifetime was found to be a strong function of the average distance between the triplet exciton and the central platinum atom, showing the importance of spin-orbit coupling effects of the central platinum atom. We found substituent effects in the vibronic envelope of the phosphorescence spectra. The intensity of the Franck-Condon vibronic bands correlated with triplet state bond length changes in the aromatic ring, showing the DFT calculations described the displacement of the triplet state potential energy surface relative to the ground state. The intensity trends suggested the triplet exciton of compounds with electron-donating ligands resided primarily on the ligand's benzene ring. In contrast, the triplet exciton of compounds with electron-withdrawing ligands formed a charge-separated state, with one electron on the benzene ring, while the other electron residing on the variable portion(X) of the ligand. These two spectroscopic studies show success in using DFT methods to calculate the two-photon and triplet state spectra of a series of model platinum acetylides with systematic variation in electronic properties of the ligand. These methods are now being applied to more complex systems.