Excitonic coupling directly revealed on single chains of polyfluorene by combined force spectroscopy and fluorescence microscopy

Molecular aggregates were discovered in 1930’s, yet, the forces and excitonic coupling energy associated with the aggregate formation have not been detected so far. We directly measure such force and energy on single chains of the conjugated polymer polyfluorene using atomic force and fluorescence microscopes. The polyfluorene chain is attached on either side to a substrate and an AFM tip, respectively, and mechanically stretched under intense laser irradiation. The force – extension curves show force peaks that are attributed to gradual unfolding of the chain. Upon the irradiation, neighboring conjugated segments interact via excitonic coupling when in contact and experience an attractive force which is detected by the AFM. Analysis of the force curves provides excitonic coupling energy which is of same order as theoretically calculated values for a face-to-face fluorene dimer, and in agreement with the energy obtained from single-chain fluorescence spectra. Apart from contributing an essential piece of knowledge in the field of molecular photophysics, the work demonstrates on molecular scale a novel energy conversion mechanism from light to mechanical energy which could be potentially used, e.g., as a driving mechanism for molecular motors.

Planar hexaphyrin-like macrocycles turning into bis-BODIPYs with box-shaped structures exhibiting excitonic coupling

This present study reveals the synthesis and excited-state dynamics of carbazole embedded hexaphyrin-like macrocycles and their bis-BODIPY complexes. Time-resolved spectroscopy was performed to elucidate the excitonic coupling in conjunction with electronic structure calculations.