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Proton transfer-driven intersystem crossing in apigenin.

Sharan Babu1, Anshuman Bera1, Sivaranjana Reddy Vennapusa1

  • 1School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram, Maruthamala PO, Vithura, Thiruvananthapuram, Kerala, 695551, India. siva@iisertvm.ac.in.

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Area of Science:

  • Photochemistry
  • Quantum Chemistry
  • Molecular Dynamics

Background:

  • Apigenin, a naturally occurring flavonoid, exhibits complex excited-state dynamics.
  • Understanding these dynamics is crucial for applications in photomedicine and materials science.

Purpose of the Study:

  • To theoretically investigate the energetics and dynamics of excited-state events in apigenin originating from the S1 state.
  • To elucidate the interplay between excited-state intramolecular proton transfer (ESIPT) and intersystem crossing (ISC).

Main Methods:

  • Theoretical calculations
  • Quantum chemical simulations
  • Potential energy surface mapping
  • Spin-orbit coupling analysis

Main Results:

  • The dominant excited-state event is barrierless ESIPT, converting the normal tautomer to the proton-transferred tautomer.
  • Intersystem crossing (ISC) competes with ESIPT, particularly at the Franck-Condon region and along the proton-transfer coordinate.
  • The T4 triplet state is an energetically feasible receiver due to small energy gaps and spin-orbit coupling.
  • Higher-lying triplet states (T3, T2) play a role in ISC beyond the Franck-Condon point.
  • Ultrafast internal conversion within the triplet manifold was observed.

Conclusions:

  • Proton transfer significantly promotes triplet formation in apigenin.
  • The interplay of ESIPT and ISC pathways dictates the excited-state fate of apigenin.
  • Theoretical insights provide a foundation for controlling apigenin's photophysical properties.