Inter/Intrachain Interactions Behind the Formation of Charge Transfer States in Polyspirobifluorene: A Case Study for Complex Excited-State Dynamics in Different Polarity Index Solvents

摘要

In this work, we demonstrate the complex excited-state nature of the conjugated polymer, polyspirobifluorene (PSBF), using steady-state and time-resolved spectroscopy techniques to understand the origin of excited charge transfer state (CT) formation and their contribution to the total photoluminescence (PL). The measurements were compared in two solvents with different polarity, for example, methyl cyclohexane (MCH) and 2-methyltetrahydrofuran (2-MeTHF), which allow us to reveal solvent quality and temperature dependent CT state formation arising from inter/intrachain interaction phenomena. The inter/intrachain interactions are explained by means of spatial conformational changes of the polymer chain configuration, such as coiling and collapse of the backbone with concomitant side chain reorganization. It has been found that the PL emission at room temperature (RT) demonstrates a mixed state configuration containing contributions from (1)(pi, pi*) excited states along with the CT states contribution, with the spectra arising from a mixture of the two emissive species. However, with decreasing temperatures to ca. 145 K (prior to the freezing point) in 2-MeTHF, the two emissive species become separated, with the emission from the CT state showing a red-shift with decreasing temperature. At 145 K, we observe the formation of an unstructured, wholly new emission band, which is strongly red-shifted relatively to the 1(pi, pi*) excited-state and shows classic Gaussian line shape. This emission is attributed to the formation of inter/intrachain CT states. In the case of frozen solutions (similar to 90 K), the spectra dramatically blue-shifts and loses all contribution from the inter/intrachain species, and emission then arises completely from the pure intrachain CT excitonic state. The behavior of the polymer is strongly dependent on both solvent quality and temperature effects on the excited state geometry relaxation by means of the local solvent-solute interactions that stabilize the CT states, due to solvation of the new charge distribution, and also changes on the transition states via manipulating energy barriers.