Steady-State and Ultrafast Fluorescence Depolarisation in Rigid-Rod Conjugated Polymers

Abstract

Polarised spectroscopic techniques were used to investigate the underlying physics of steady-state and ultrafast fluorescence depolarisation in conjugated polymers. Depolarisation is due to fluorescence anisotropy: the angular difference between the absorption and emission transition dipole moments of a molecule. Polarised spectroscopy results from a polymer with a flexible backbone, poly (9,9-di(ethylhexyl)fluorene) were compared with those from two rigid backbone polymers: methyl-substituted ladder-type poly (para-phenylene) and the newly synthesised naphthylene ladder-type polymer (2,6-NLP). This revealed that there is an intrinsic anisotropy directly associated with the molecular backbone. This work is the first reported on 2,6-NLP. Fluorescence anisotropy was shown to be dependent upon the conjugation length; the transition dipole moments show larger angles for short lengths, tending to a minimum as the length increases. For rigid-rod polymers, this behaviour is replicated at each vibronic position. In the flexible polymer, planarisation of the backbone elongates the excited state over more conjugated bonds, changing the angle between the transition dipole moments, whereas in rigid-rod polymers, such elongation can only be electronic. Linear dichroism results obtained for all the polymers has shown the angle between the absorption transition dipole moment and the molecular backbone is large and that the emission transition dipole moment is aligned with the backbone. “Off-chain” to “on-chain” transition dipole moments arise from transitions from localised to delocalised states suggesting that the excited state in conjugated polymers is delocalised. Time-dependent measurements show that the main fluorescence depolarisation mechanism occurs in under 5 ps for both flexible and rigid polymers. The ultrafast timescale and the similarity of the two systems requires the process to be electronic in origin and not linked to a physical deformation. This work proposes that ultrafast fluorescence depolarisation is a result of the delocalisation of the electronic state as the conjugation length extends over more of the polymer