Molecular and Electronic Structure and Properties of the Single Benzene-Based Fluorophores Containing Guanidine...

The Gibbs energies of protonation (ΔG_p) and the basic photophysical properties for single-benzene fluorophores (SBFs) containing guanidine and/or amino subunits and the changes that occur upon protonation were modeled by the TDDFT approach. The calculated ΔG_p energies for amino SBFs in the S₁ state range from 985 to 1100 kJ mol^-1 which are below the values for guanidines. The protonation of the guanidine-SBFs induces a hypsochromic shift of the absorption and the emission maxima with the Stokes shift of > 100 nm in both cases. Isomerization through the ESIPT process is less probable than in amino-SBFs due to the unfavorable thermodynamics. Still, if it occurs, it leads to a strong red shift of the emission by > 150 nm. Aromaticity indices point to strong antiaromatic character of the examined guanidino-SBFs in the FC region which decreases upon geometrical relaxation and ESIPT. The excited state proton transfer occurs in guanidine-SBF/phenol complexes in the S₁ state, stabilizing CT states and fluorescence quenching.