Complex Photophysical Behaviors Affect Single Conjugated Molecule Optical Anisotropy Measurements

Polarization modulation depth (M) measurements have been widely used as a technique to report on transition dipole alignment and thus molecular conformation of single conjugated polymers and aggregates. Such extrapolation of conformation from these measurements, however, is complicated by the fact that photophysical processes (which themselves are coupled to conformation) may influence M values. Here, we show that the presence of intensity-dependent partial photoluminescence quenching can suppress M values, with this suppression more prominent in molecules with highly aligned transition dipoles. We show that these findings on M values are a direct consequence of behaviors of fluorescence intensity maxima and minima as a function of excitation polarization in poly­[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] single molecules, as supported by a simulation that reproduces the experimental results. Our findings show that interpreting M values of molecules with complex photophysics should be done with caution