Rotational band contour analysis in theÃ1A′-[Xtilde]1A′ electronic systems of substituted naphthalenes

Abstract

The 0₀ ⁰ bands of the à ¹ A′-[Xtilde] ¹ A′ electronic systems of 2-fluoro, 2-hydroxy- and 2-aminonaphthalene have been identified and then rotationally analysed by the method of rotational band contour analysis. The effect of 2-substitution on the direction of the electronic transition moment vector in naphthalene is found to be drastic: the vector is oriented by about 70–80° (anti-clockwise in figure 1) by the substituent. However, the orientation is almost unaffected by the nature of the substituent. The 0₀ ⁰ bands are all type A, B hybrids but predominantly type B. Significant differences in the 0₀ ⁰ band rotational contours in the three molecules are reflected in the different values of ΔA, ΔB and ΔC found from the contour analyses. An attempt to interpret the changes of rotational constants in terms of changes in geometry from the ground to the excited state suggest very strongly that the geometry change of the naphthalene rings in all the molecules is similar to that in naphthalene itself but that the carbon-substituent bonds contract in the excited state. The contraction increases from F to OH to NH₂ and is probably quantitatively similar to that suggested previously for the corresponding monosubstituted benzenes.