Abstract
The concept of aggregation-induced emission represents a means to rationalise photoluminescence of usually nonfluorescent excimers in solid-state materials. In this publication, we study the photophysical properties of selected diaminodicyanoquinone (DADQ) derivatives in the solid state using a combined approach of experiment and theory. DADQs are a class of high-dipole organic chromophores promising for applications in non-linear optics and light-harvesting devices. Among the compounds investigated, we find both aggregation-induced emission and aggregation-caused quenching effects rationalised by calculated energy transfer rates. Analysis of fluorescence spectra and lifetime measurements provide the interesting result that (at least) two emissive species seem to contribute to the photophysical properties of DADQs. The main emission peak is notably broadened in the long-wavelength limit and exhibits a blue-shifted shoulder. We employ high-level quantum-chemical methods to validate a molecular approach to a solid-state problem and show that the complex emission features of DADQs can be attributed to a combination of H-type aggregates, monomers, and crystal structure defects.
Highlights
In contrast to aggregation-caused quenching (ACQ), solid-state materials may experience quantum yields (QYs) enhancement due to close molecular packing
Due to an interplay of aggregation-induced emission (AIE) and ACQ effects, a reverse trend between fluorescence QYs in solution and in the solid state is observed with respect to the substituents. We show that this can be related to the degree of p–pstacking within the crystals which is supported by energy transfer rates computed according to the ideas of Radhakrishnan and co-workers.[33]
That depending on the size of the defect cavity, the monomer may show small amounts of fluorescence at various emission energies beyond the main emission peak. Using both experimental and computational approaches, we were able to explain the emergence of aggregation-induced emission in an organic crystal system
Summary
In contrast to ACQ, solid-state materials may experience QY enhancement due to close molecular packing. We subsequently apply a combination of energy transfer rate calculations, periodic quantumchemical methods, and the DFT/MRCI approach to examine the electronic and optical properties of DADQs in the solid state and investigate what may be the origin of the two emitting species. Please note that this analysis is based on the assumption that the optical properties of DADQs in the solid state can be reduced to the response of a monomer embedded in a crystal environment.
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