Charge Transfer Complexes in Spectroscopic and Photophysical Applications

Charge transfer complexes arise when an electron-rich donor molecule interacts with an electron-poor acceptor to form a transient association characterised by partial electron displacement. This interaction leads to new optical transitions, often in the ultraviolet–visible region, and underpins a wide range of applications in both analytical spectroscopy and photophysics. In spectroscopic analysis, charge transfer complexes serve as the basis for sensitive, selective assays in pharmaceutical quality control and environmental monitoring, exploiting shifts in absorption maxima and enhanced molar absorptivities. In photophysical contexts, these complexes enable tuning of electronic structures to control band gap energies, luminescence properties and interfacial charge separation processes, with applications in sensors, organic photovoltaics and light-emitting devices. The precise matching of donor–acceptor pairs, coupled with modern computational methods such as density functional theory, affords predictive design of complexes with bespoke optical and electronic features. Globally, this field contributes to greener analytical methods, high-throughput screening platforms and the development of next-generation photonic materials.

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Technical terms

Charge transfer complex (CTC): An association between an electron-rich donor and an electron-poor acceptor that exhibits new absorption bands due to partial electron transfer.

Donor and acceptor molecules: Species that respectively supply or withdraw electron density in a CTC, determining its optical and electronic characteristics.

Molar absorptivity (ε): A measure of how strongly a complex absorbs light at a particular wavelength, reflecting its formation constant.

Band gap energy: The energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital in a CTC, key to photophysical behaviour.

References

1. Charge Transfer Complex of Lorlatinib with Chloranilic Acid: Characterization and Application to the Development of a Novel 96-Microwell Spectrophotometric Assay with High Throughput. Molecules (2023).
2. Synthesis, spectroscopic characterization, DNA binding and DFT/PCM calculations of new Hydrogen-bonded charge transfer complex between 4-dimethylaminopyridine and Chloranilic acid. Results in Chemistry (2023).