Absorption and luminescence properties of [2.2]paracyclophane due to strong transannular interaction

The significant bathochromic shift of luminescence and long-wavelength absorption bands of [2.2]paracyclophane compared with the corresponding bands for molecules containing non-conjugated benzene chromophores was examined. The effect was due to strong interaction between π-orbitals of closely spaced benzene rings of the molecule that resulted in formation of an excited state of lower energy that was new in principle. It was concluded that the fluorescent state of the macrocycle and excimers formed by alkylbenzenes were of the same nature. The lowest electronic excited state of [2.2]paracyclophane appeared not only in emission but also in absorption. This explained the lack of an anomalous Stokes shift for it. The stronger shift of the macrocycle fluorescence band compared with the excimer band of alkylbenzenes was due to the fact that the highest occupied π-orbital of [2.2]paracyclophane was perturbed and located higher on the energy scale than for the chromophores of alkylbenzenes and polystyrene. (authors)