Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D 3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine) 3 ] 2 and [Ru(1,10-phenanthroline) 3 ] 2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1 MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1-6.3 D and 5.3-5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solutesolvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.
Lipophilic fluorophores are widely implemented in nonlinear microscopy; however, few existing membrane‐specific probes combine the high brightness of two‐photon excited fluorescence (2PEF) with pH sensitivity. Herein we describe four novel two‐photon excited fluorophores, based on a coumarin 151 core structure, where lipophilicity is induced by a covalently attached phosphazene moiety. Changing the environmental acidity using trifluoromethanesulfonic (triflic) acid leads to profound changes in the linear fluorescence and 2PEF characteristics, due to chromophores’ switching between neutral‐ and protonated forms. We characterize this dependence by measuring the two‐photon absorption (2PA) spectra over the region λ2PA=550–1000 nm, observing 2PA cross sections of σ2PA=10–20 GM, with an associated 2PEF brightness of 10–13 GM, in neutral solutions of both acetonitrile and n‐octanol. Although quantum chemical modelling and NMR measurements show that, at high chromophore concentrations, protonation may be accompanied by a dimerization process, these dimers likely do not form at the lower concentrations used in optical spectroscopy.
Quantitative two-photon absorption (2PA) cross section and 2PA spectrum measurements were used to determine the molecular electric dipole change in the metal-to-ligand charge-transfer transition of ruthenium(II) tris-complexes of 2,2'-bipyridine ([Ru(bpy)3] 2+ ) and 1,10phenanthroline ([Ru(phen)3] 2+ ) in several solvents.
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