Ladder-type heterocycles encompassing two B−–N+ dative bonds possess intense green to red emission, large 2PA cross-sections and superb photostability.

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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.

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