Temperature Dependence of Circular Dichroism and Fluorescence Decay of Pyrene Appended to the Side Chains of Poly-l-glutamine

The temperature dependence of side-chain chromophore orientation and excimer formation in diastereomeric poly[N⁵-(R and S)-1-(1-pyrenyl)ethyl-l-glutamines] (1 and 2) was examined in DMAc using circular dichroic (CD) and fluorescence spectroscopy. In comparison with 1, 2 afforded CD spectra suggesting a better controlled pyrene chromophore orientation. It, however, gave a stronger excimer fluorescence. Assuming that a major portion of excimers are formed in the portions of the polymer where the side chains are disordered, the ratios of ordered side chains to disordered side chains, i.e., the equilibrium constants between the two side-chain structures, were evaluated from fluorescence decay analysis. The equilibrium constants were found to be consistently smaller for 2 than for 1 in the temperature range examined. The stronger excimer emission observed with 2 is thus suggested due to the larger content of disordered side chains in 2 despite the better controlled chromophore orientation in its ordered side chains. This is consistent with the lower activation energy of excimer formation estimated for 2. The molar ellipticities of the ordered and disordered side chains were evaluated from the observed CD and the equilibrium constants. The possible ordered side-chain structures of 1 and 2 were explored by molecular mechanics calculations as well as theoretical CD calculations based on the low-energy conformations obtained thereby.