Linear and V-shaped carbazole-based molecules functionalized by cyano acceptors and diversified donors: Synthesis, single- and two-photon related photophysical properties

A series of novel linear and V-shaped carbazole-based molecules functionalized by cyano acceptors and diversified donors (carbazole, diphenylamino, dimethylamino, methoxy, and ferrocene) were synthesized and characterized by hydrogen-1 nuclear magnetic resonance, carbon-13 nuclear magnetic resonance, Fourier transform infrared spectroscopy, and high-resolution mass spectrometry. Their linear and nonlinear optical properties including UV–visible absorption, single-photon excited fluorescence, two-photon absorption, and frequency up-converted fluorescence, were systematically investigated in various solvents. The time-resolved photoluminescence studies using time-correlated single photon counting revealed their fluorescence lifetimes in THF. The solvent polarity exerts little effect on their absorption properties, while the emission properties (maximum fluorescence wavelength, Stokes shift, fluorescence quantum yield) display solvent polarity dependencies. Donor-acceptor functionalities and molecular dimensionality are considered to be the crucial structural factors that influence their optical properties. Pumped by a femtosecond laser (690–990 nm, 80 MHz, 140 fs), all the target molecules emit intense frequency up-converted fluorescence except FC1 and FC2 which contain one or two ferrocene donors. The two-photon absorption cross-sections in THF are in the range of 631–3014 GM. The density functional theory calculations were also conducted to unravel their electronic structures and to further understand the structure-property relationships. To our knowledge, this study is the first to be concerned with the single- and two-photon related photophysical properties of these D-π-A-π-D′ and D′-π-A-π-D-π-A-π-D′ carbazole-based molecules (D = donor, A = acceptor, π = conjugated bridge).