EXCITED STATES OF DNA AND ITS COMPONENTS AT ROOM TEMPERATURE—IV. SPECTRAL, POLARIZATION AND QUANTUM YIELD STUDIES OF EMISSIONS FROM CpC AND POLY rC

Abstract— Corrected normalized emission spectra are reported for poly rC, CpC and CMP in neutral aqueous solution at room temperature. The emissions from CpC and poly rC show progressive and extensive depolarization across the emission band compared with CMP. Self-consistent analysis of the polarization and spectral data allows the resolution of red-shifted component emissions from the overall spectra and the evaluation of component polarizations. By comparison with experimental reference spectra three long wavelength components are identified in the poly rC emission and assigned as excimer fluorescence, monomer phosphorescence and excimer phosphorescence, whereas for CpC only two components are observed. The quantum yields are interpreted quantitatively in terms of the known stacking behavior of CpC and poly rC. with excimer emissions originating from the stacked fraction. Quantitative differences between CpC and poly rC are due to their different hypochromism and degree of stacking. Differences of polarization of excimer fluorescence are a consequence of different stacking geometries, and the directional properties of the transition moment suggest the excimer fluorescence is largely charge-resonance in nature. The behavior of the CpC/poly rC system is compared with ApA/ poly rA.