Push-pull conjugated chromene-derivatives for potential bio-imaging applications. Synthesis, X-Ray and DFT studies, one- and two-photon photophysical properties

Dicyanomethylene-4H-benzopyran derivatives dyes containing D-pi-A-type structure have been prepared and fully characterized. The Z configuration of all three structures has been confirmed by SCXRD studies. Dyes 1-6 present a strong absorption in the 450-550 nm region with epsilon values over 43 000 M-1 cm-1 1-4 showed fluorescence properties while 5 and 6 are very weakly fluorescent. The electronic structure of the first singlet excited state S1 was investigated to further explain the evolution of fluorescence quantum yields. The two-photon excitation spectra were recorded with a similar behaviour upon two-photon excitation, with two ab-sorption bands, one in the 800-850 nm region with sigma 2 of 200-300 GM and another one at about 1000 nm with sigma 2 of 300-400 GM. Dyes 3 and 4 have shown interesting properties (both in intensity and FLIM modes) in two-photon excited microscopy.