Tuning the color and excited state properties of the azulenic chromophore: NIR absorbing pigments and materials

In this review, we summarize the work carried out at the University of Hawaii on the highly colored, non-alternant hydrocarbon azulene. Through systematic perturbation of the HOMO and LUMO molecular orbitals, we showed that the characteristic azure color can be changed in a manner that covers the entire visible spectrum based on this single chromophore. Some of the modified azulenes exhibit unusual excited state properties: reversal of energy gap of low lying excited states, unprecedented high fluorescence yield, and long fluorescence lifetime for an upper state in condensed media. Highly polarized polyene aldehydes attached to azulene at C-1 as a pi-electron donating end group have been successfully incorporated into bacterioopsin to generate NIR absorbing bacteriorhodopsin pigment analogs-the longest wavelength absorbing analog with gimel(max) = 830 nm. Azulene-containing donor-acceptor and symmetrical dye chromophores were synthesized and their excited state absorption spectra, and other properties determined. Non-linear optical properties were also examined. The azulenic dye chromophores are reverse saturable absorbers and are potentially useful for optical limiting applications. (C) 2003 Japanese Photochemistry Association. Published by Elsevier B.V. All rights reserved.