Giant dipole moment in a triad system. Mechanisms of anisotropic photoresponse in the transient dc conductivity of dipolar solutes

A transient dc conductivity method was used to observe formation of a giant dipole moment for the triad molecule MA-ANI-NI (methoxyaniline-aminonaphthalimide-dimethylphenyl-naphthalenediimide-octyl) in toluene. The independence of the dipole moment on excitation wavelength indicates high efficiency of intramolecular energy or (and) electron transfer. The effect of light polarization on the DC conductivity signal caused by a photoinduced increase in solute dipole moment is considered in detail. It is shown that the time variation of the signal includes information about structural anomalies in the angular distribution function of molecular dipoles and depends on light polarization even for zero ground state dipole moment. Nonzero ground state dipole moment and (or) electric field dependence of the charge transfer rate constants give an additional source for an anisotropic photoresponse signal. Analysis of the photoresponse and its anisotropy for the triad gives ground (μ_g), first (μ₁), and second (μ₂) excited state dipole moments as follows: μ_g = 12 ± 5 D, μ₁ = 35 ± 10, and μ₂ = 87 ± 6 D. The lifetime of the giant dipole state is τ = 290 ± 10 ns, and the molecule's rotational time is τ_r = 1.6 ± 0.15 ns.