Electronic and structural elements that regulate the excited-state dynamics in purine nucleobase derivatives

Area of Science:

• Photochemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Purine derivatives are fundamental components of nucleic acids.
• Understanding their excited-state dynamics is crucial for photobiology and photochemistry.

Purpose of the Study:

• To investigate the excited-state dynamics of purine free base and 9-methylpurine.
• To elucidate the relaxation pathways and the role of solvent and structure.

Main Methods:

• Femtosecond broadband transient absorption spectroscopy.
• Ab initio static and surface-hopping dynamics simulations.
• Experimental and theoretical investigations.

Main Results:

• Excitation leads to ultrafast S2(ππ*) to (1)nπ* state conversion.
• The (1)nπ* state acts as a doorway to efficient triplet manifold population.
• Intersystem crossing rate increases significantly in nonpolar solvents, indicating a solvent-dependent energy barrier.
• C6 functionalization inhibits (1)nπ* access and promotes relaxation to the ground state.

Conclusions:

• Accessibility of nπ* states and potential energy surface topology near conical intersections govern purine excited-state dynamics.
• C6 functionalization is key in regulating radiative and nonradiative relaxation pathways.
• Purine chromophore itself is not responsible for ultrafast internal conversion in adenine and guanine monomers.