Photophysical Properties of a Series of Electron-Donating and -Withdrawing Platinum Acetylide Two-Photon Chromophores

To explore spectroscopic structure-property relationships in platinum acetylides, we synthesized a series of complexes having the molecular formula trans-bis(tributylphosphine)-bis(4-((9,9-diethyl-7-ethynyl-9H-fluoren-2-yl)ethynyl)-R)-platinum. The substituent, R = NH2, OCH3, N(phenyl)(2), t-butyl, CH3, H, F, benzothiazole, CF3, CN, and NO2, was chosen for a systematic variation in electron-donating and -withdrawing properties as described by the Hammett parameter sigma(p). UV/vis, fluorescence, and phosphorescence spectra, transient absorption spectra on the fs-ps time scale, and longer time scale flash photolysis on the ns time scale were collected. DFT and TDDFT calculations of the T-1 and S-1 energies were performed. The E-S and E-T values measured from linear spectra correlate well with the calculated results, giving evidence for the delocalized MLCT character of the S-1 state and confinement of the T-1 exciton on one ligand. The calculated T-1 state dipole moment ranges from 0.5 to 14 D, showing the polar, charge-transfer character of the T-1 state. The ultrafast absorption spectra have broad absorption bands from 575 to 675 nm and long wavelength contribution, which is shown from flash photolysis measurements to be from the T-1 state. The T-1 energy obtained from phosphorescence, the T-1-T-n transition energy obtained from flash photolysis measurements, and the triplet-state radiative rate constant are functions of the calculated spin density distribution on the ligand. The calculations show that the triplet exciton of chromophores with electron-withdrawing substitutents is localized away from the central platinum atom, red-shifting the spectra and increasing the triplet-state lifetime. Electrondonating substituents have the opposite effect on the location of the triplet exciton, the spectra, and the triplet-state lifetime. The relation between the intersystem crossing rate constant and the S-1-T-1 energy gap shows a Marcus relationship with a reorganization energy of 0.83 eV. The calculations show that intersystem crossing occurs by conversion from a nonpolar, delocalized S-1 state to a polar, charge-transfer T-1 state confined to one ligand, accompanied by conformation changes and charge transfer, supporting the experimental evidence for Marcus behavior.