Excitation/detection energy controlled anisotropy dynamics in asymmetrically cyano substituted tri-podal molecules
In the present work, the photophysical properties of two series of asymmetrical tri-podal molecules is studied, in order to determine the dependence of energy localization/delocalization phenomena on excitation and detection wavelength, by means of steady state, femtosecond time resolved fluorescence and anisotropy spectroscopy. The molecules bear tri-phenylamine as an electron donating core and acetylenic or olefinic π-conjugated bridge. At the periphery, they are substituted by none, one, two or three -CN groups used as electron acceptors. Thus, the compounds with only one or two -CN groups are asymmetrically substituted. As a comparison, the photophysics of their dipolar and quadrupolar analogues is also presented. The steady state absorption spectra of the asymmetrical tri-podal compounds exhibit a broadening and a low energy shoulder due to the splitting of the excited states. The fluorescence spectra are more red-shifted in the tri-podal molecules with a single -CN group, providing a first evidence for its mostly dipolar nature. Time resolved anisotropy measurements by using different excitation and detection wavelengths provide clear evidence that the asymmetrical tri-podal molecules with one or two -CN groups behave like octupolar molecules upon high-energy excitation (the initial anisotropy is found 0.1-0.15), while upon low-energy excitation they reveal a behavior expected for linear dipolar or V-shaped quadrupolar molecules (the initial anisotropy is very close to 0.4 and 0.17 respectively). The symmetrical tri-podal compounds with none or three cyano groups, exhibit anisotropy depolarization time of 2.5 ps attributed to energy hopping. The amplitude of this energy hopping component is wavelength dependent and increases as the excitation is shifted towards the long wavelength edge.