Femtosecond two-photon absorption spectra and permanent electric dipole moment change of tryptophan, 2-aminopurine and related intrinsic and synthetic fluorophores

Two-photon absorption (2PA) cross sections for 2-aminopurine (2AP), 7-methyl guanosine (7MG), 6-methyl isoxanthopterin (6MI), isoxanthopterin (IXP), 3-methylindole (3MI) and tryptophan (L-trp) are measured for excitation wavelengths between 550 and 810 nm using two independent methods: femtosecond two-photon excited fluorescence (2PEF) and femtosecond nonlinear transmission (NLT). For all systems we find that in the long wavelength region of the lowest-energy one-photon allowed singlet-singlet transition the 2PA spectral profiles follow the one-photon absorption spectral profiles, thus indicating the strongly dipolar nature of the transition. We use the measured 2PA cross sections to estimate the corresponding permanent electric dipole moment change and find the values to vary in the range 1 to 5 D. These experimental findings are corroborated by quantum-chemical computations.