Nature of the intense near-IR absorption and unusual broad UV-visible-NIR spectra of azulenocyanines: Density functional theory studies

The nature of the broad and intense near-IR (NIR) absorptions of azulenocyanine (H₂Azc) in the range of 720-1300 nm due to the π→π* transitions is clearly revealed on the basis of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. The electron moving between peripheral conjugated systems and the eighteen electron-π-conjugated core due to different types of electron transitions has also been clarified. Because of the small gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all four H₂Azc isomers, their Q absorption bands in the region of 850-1300 nm are significantly red-shifted relative to its isoelectronic analogue naphthalocyanine H₂Nc. Observation of the new absorption bands between Q and Soret bands from 720 to 1050 nm for H₂Azc is considered to be the result of electron transitions from the fused azulene rings to the 18 electron-π-conjugated core. In addition, the introduction of electron-withdrawing or electron-donating substituents onto the periphery of the azulenocyanine ring is revealed to further reduce the LUMO-HOMO gap, leading to a further red-shifted broad and intense NIR absorption band from 900 to 1700 nm for H₂(F₈Azc) and H₂[(NH₂) ₈Azc].