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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2011-01, Vol.115 (3), p.265-273
• Main Authors: Haley, Joy E, Krein, Douglas M, Monahan, Jennifer L, Burke, Aaron R, McLean, Daniel G, Slagle, Jonathan E, Fratini, Albert, Cooper, Thomas M
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy; Electrons; Platinum Compounds - chemistry; Spectrum Analysis
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp104596v

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 = NH 2 , OCH 3 , N(phenyl) 2 , t-butyl, CH 3 , H, F, benzothiazole, CF 3 , CN, and NO 2 , was chosen for a systematic variation in electron-donating and -withdrawing properties as described by the Hammett parameter σp. 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 T1 and S1 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 S1 state and confinement of the T1 exciton on one ligand. The calculated T1 state dipole moment ranges from 0.5 to 14 D, showing the polar, charge-transfer character of the T1 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 T1 state. The T1 energy obtained from phosphorescence, the T1−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. Electron-donating 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 S1−T1 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 S1 state to a polar, charge-transfer T1 state confined to one ligand, accompanied by conformation changes and charge transfer, supporting the experimental evidence for Marcus behavior.