Fluorescence and Phosphorescence of Flavylium Cation Analogues of Anthocyanins

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Abstract
Fluorescence spectra and lifetimes were determined for 16 synthetic flavylium cation analogues of anthocyanin plant pigments in dry acetonitrile acidified with trifluoroacetic acid (TFA). Phosphorescence was also observed from the lowest excited triplet state for all of the flavylium cations at 77 K in a rigid TFA-acidified isopropanol glass. The fluorescence quantum yields and lifetimes depend in a systematic manner on the nature and position of the substituents on the flavylium chromophore and three specific substitution patterns associated with significant decreases in the fluorescence quantum yield were identified. A 4′-bromo or 4′-iodo substituent in the B-ring of the flavylium cation produced a small but normal heavy-atom effect, reducing the fluorescence quantum yield and the phosphorescence lifetime relative to analogues without the halogen atom. In contrast, three flavylium cations with a 3′-bromo substituent exhibited an “inverse” heavy atom effect, i.e., an increase in the fluorescence quantum yield rather than a decrease, which was rationalized on the basis of the nodal properties of the natural transition orbitals (NTOs) involved in the S0→S1 radiative transition.
Funding Information
  • Conselho Nacional de Desenvolvimento Científico e Tecnológico (465454/2014-3, 444061/2018-5, 408181/2016-3, 407774/2021-1, CEX-APQ-00321-15, Finance Code 001, IST-ID/93/2018, PTDC/QUI-QFI/29527/2017, 2015/08539-1)
  • Fundação de Amparo à Pesquisa do Estado de Minas Gerais (465454/2014-3, 444061/2018-5, 408181/2016-3, 407774/2021-1, CEX-APQ-00321-15, Finance Code 001, IST-ID/93/2018, PTDC/QUI-QFI/29527/2017, 2015/08539-1)
  • Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (465454/2014-3, 444061/2018-5, 408181/2016-3, 407774/2021-1, CEX-APQ-00321-15, Finance Code 001, IST-ID/93/2018, PTDC/QUI-QFI/29527/2017, 2015/08539-1)
  • Fundação para a Ciência e a Tecnologia (465454/2014-3, 444061/2018-5, 408181/2016-3, 407774/2021-1, CEX-APQ-00321-15, Finance Code 001, IST-ID/93/2018, PTDC/QUI-QFI/29527/2017, 2015/08539-1)
  • Fundação de Amparo à Pesquisa do Estado de São Paulo (465454/2014-3, 444061/2018-5, 408181/2016-3, 407774/2021-1, CEX-APQ-00321-15, Finance Code 001, IST-ID/93/2018, PTDC/QUI-QFI/29527/2017, 2015/08539-1)

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