Symmetry Breaking in Platinum Acetylide Chromophores Studied by Femtosecond Two-Photon Absorption Spectroscopy

We study instantaneous two-photon absorption (2PA) in a series of nominally quasi-centrosymmetric trans-bis (tributylphosphine)-bis-(4-((9,9- diethyl-7-ethynyl-9H-fluoren-2-yl) ethynyl)-R)-platinum complexes, where 11 different substituents, R = N(phenyl)2(NPh2), NH2, OCH3, tbutyl, CH3, H, F, CF3, CN, benzothiazole, and NO2, represent a range of electron-donating (ED) and electron-withdrawing (EW) strengths, while the Pt core acts as a weak ED group. We measure the 2PA cross section in the 540 810 nm excitation wavelength range by complementary femtosecond twophoton excited fluorescence (2PEF) and nonlinear transmission (NLT) methods and compare the obtained values to those of the Pt-core chromophore and the corresponding noncentrosymmetric side group (ligand) chromophores. Peak 2PA cross sections of neutral and ED-substituted Pt complexes occur at S0 Sn transitions to higher energy states, above the lowest-energy S0 S1 transition, and the corresponding values increase systematically with increasing ED strength, reaching maximum value, 2 300 GM (1 GM = 10 50 cm4 s), for R = NPh2. At transition energies overlapping with the lowest-energy S0 S1 transition in the one-photon absorption (1PA) spectrum, the same neutral and ED-substituted Pt complexes show weak 2PA, 2 30 100 GM, which is in agreement with the nearly quadrupolar structure of these systems. Surprisingly, EW-substituted Pt complexes display a very different behavior, where the peak 2PA of the S0 S1 transition gradually increases with increasing EW strength, reaching values 2 = 700 GM for R = NO2, while in the S0 Sn transition region the peak 2PEF cross section decreases. We explained this effect by breaking of inversion symmetry due to conformational distortions associated with low energy barrier for ground-state rotation of the ligands.
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