To design the near-infrared (NIR) dyes, the electronic absorption spectra of arylimidazoleintroduced 3,4,9,10-perylenetetracarboxylic dianhydride and its imide derivatives (PTCAIs) have been theoretically elucidated by using the molecular orbital (MO) calculation. An arylimidazole introduction causes a longer shift of the absorption wavelength (bathochromic shift) by destabilization of the HOMO level. The HOMO level of aryl groups (e.g., benzene and naphthalene) is close to the energy of PTCAI's HOMO; therefore, new orbitals which are characterized as a mixture of each orbital's features are generated by this large orbital interaction. The absorption wavelength shift depends not only on a HOMO level of the aryl group (π-conjugation size), but also on its pattern. With the bathochromic shift of the absorption spectrum, the transition is characterized as more intense of the charge-transfer (CT) property rather than the π-π* excitation on the perylene moiety.
The absorption spectra of 23 indoaniline dyes were calculated on the AM1-optimized structures by means of the INDO/S method. The observed and calculated absorption wavelengths showed good correlation for the main (longest wavelength) absorption bands (observed at 560-670 nm), and there was fair correlation between the observed molar extinction coefficients and the calculated oscillator strengths. The observed absorption wavelengths could be reproduced by the calculation for the subabsorption bands near the visible region (observed at 320-380 nm). A qualitative correlation of the solvatochromic shift with the difference between ground and excited state dipole moments was obtained.
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