THE RATIO OF EXCIMER TO MONOMER EMISSION IN DIESTERS OF 1-PYRENOIC ACID AND 5 GLYCOLS

The ratio of the intensities of excimer and monomer emission, I(D)/I(M), has been measured for Py-COO- (CH2)m-OOC-Py (m = 2-6, Py = 1 -pyrenyl) in solvents of different viscosity, eta. These compounds are dimeric models for the corresponding polymers, selected to eliminate the influence of energy migration on the population of an excimer. The solvents are H(CH2)nOH, n = 1-7, and ethylene glycol. The values of I(D)/I(M) decrease as eta increases, and the rate of this decrease is a function of m. Therefore the qualitative dependence of I(D)/I(M) on m is a function of eta. In the limit as eta --> infinity, I(D)/I(M) is an odd-even function of m for m = 2-5, and the values at m = 5 and 6 are nearly identical. A rotational-isomeric state analysis of the conformations of the diesters can rationalize the appearance of the odd-even effect. This result is insensitive to assumptions about the extent of overlap of the two pyrene ring systems that is required for the production of emission in the excimer band.