Absorption and emission in oligo-phenylene vinylene nanoaggregates: The role of disorder and structural defects

Abstract

The impact of exciton-phonon coupling and defect states on the photophysical properties of p-distyrylbenzene nanoaggregates is studied numerically. Molecular packing within aggregates is based on the known crystal structures of poly-p-phenylene vinylene (Type I) and the five phenyl group oligomer (Type II). Calculations of absorption and emission are conducted using a reduced basis set consisting of all one- and two-particle vibronic states. The calculated spectra are very similar for both aggregate types, the only substantial difference being the polarization directions for the J-band and 0-0 emission line. Under the noninteracting domains approximation the calculated nanoaggregate absorption spectrum is in excellent agreement with experiment, assuming an exciton coherence length of approximately 20 Å. In the calculated emission spectrum the 0-0 emission is uniquely polarized compared with the rest of the vibronic progression, also in agreement with experiment. The 0-0 emission intensity in defect-free Type I and II aggregates is linearly proportional to the total number of molecules, becoming superradiant beyond a certain size threshold. The 0-0 emission is highly sensitive to stacking faults and dislocations. These defects account for the measured Stokes shift, but quench the 0-0 emission (and superradiance) while only slightly affecting the rest of the vibronic progression. Adding orientational point defects to an aggregate with stacking faults and/or dislocations enhances the 0-0 oscillator strength, bringing the 0-0 emission intensity into good agreement with experiment.