Relationship between the Molecular Structure and the Absorption Band Shape of Organic Dye

Molecules always show broad absorption band envelopes, and this results from the vibrational properties of bonds. The width of an absorption band can have an important influence on the color of a dye. A narrow band imparts a bright, spectrally pure color to the dye, whereas a broad band can give the same hue, but with a much duller appearance. Typically, half-band widths of cyanine dyes are about 25nm compared to value of over 50nm for typical merocyanine dyes. Thus, cyanine dyes are exceptionally bright. The factors influencing the width of an absorption band can be understood with reference to the Morse curves. The width of the absorption band depends on how closely the bond order of the molecules in the first excited state resembles that in the ground state. We have quantitatively evaluated the "molecular structure-absorption band shape" relationship of dye molecules by means of Pariser-Parr-Pople Molecular Orbital Method(PPP-MO).