Tuning the Solution Phase Photophysics of Two De Novo Designed Hydrogen Bond Sensitive 9-methyl-2,3,4,9-tetrahydro-1H-carbazol-1-one Derivatives

Two new fluorophores, 6,7-dimethoxy-9-methyl-2,3,4,9-tetrahydro-1H-carbazol-1-one (DMTCO) and 5-methyl-8,9-dihydro-5H-1,3]dioxolo4,5-b]carbazol-6(7H)-one (MDDCO), first of their kind, have been synthesized from the corresponding methoxy and methylenedioxy derivatives of 2,3,4,9-tetrahydro-1H-carbazol-1-one respectively. Comprehensive photophysical characterization of these compounds has been carried out in sixteen different homogeneous solvents and binary solvent mixtures. Both of these compounds are sensitive to solvent polarity, but the sensitivity is much higher in electronic excited state observed by steady-state and time-resolved fluorescence experiments than in ground state studied by UV–vis absorption spectroscopy. The fluorescence spectral shifts are linearly correlated with the empirical parameters of the protic solvents and also the quantitative influence of the empirical solvent parameters on the emission maxima of the compounds has been calculated. The change in dipole moment of the compounds in their excited state has been calculated from the shifts in corresponding emission maxima in pure solvents. A higher dipole moment change of both DMTCO and MDDCO in protic solvents is due to intermolecular hydrogen bonding which is further confirmed by the comparison of their behaviour in toluene-acetonitrile and toluene-methanol solvent mixtures. From structural features, MDDCO is more planar compared to DMTCO, which is reflected better in fluorescence quenching of the former with organic bases, N,N-dimethylaniline and N,N-diethylaniline. Laser flash photolysis experiments prove that the quenching interaction originates from photoinduced electron transfer from the bases to the compounds.