Locating Cytosine Conical Intersections by Laser Experiments and Ab Initio Calculations.

Area of Science:

• Photochemistry
• Quantum Chemistry
• Molecular Spectroscopy

Background:

• Cytosine (Cyt) is a fundamental DNA/RNA base.
• Understanding excited-state decay mechanisms is crucial for photochemistry and photobiology.
• Substitution effects on molecular excited states are key to tuning reactivity.

Purpose of the Study:

• To elucidate the S0 → S1 excited decay mechanism of cytosine and its derivatives.
• To investigate how chemical substitution impacts excited-state decay pathways and energetics.
• To correlate experimental spectroscopic data with theoretical calculations.

Main Methods:

• Jet-cooled spectroscopy, including nanosecond resonant two-photon ionization (R2PI) and picosecond lifetime measurements.
• Computational chemistry using CASPT2//CASSCF methods.
• Synthesis and study of eight cytosine derivatives.

Main Results:

• Cytosine and several derivatives exhibit rapid internal conversion at low vibrational energies (250–1200 cm⁻¹ above 0₀⁰).
• The decay mechanism is confirmed to proceed via a "C5-C6 twist" conical intersection.
• Substituents stabilizing charge shifts increase the barrier to this decay pathway.
• Clamped derivatives (TMCyt, 1M-TMCyt) show distinct decay along an N3 out-of-plane coordinate up to higher energies (+3500 and +4500 cm⁻¹).

Conclusions:

• The "C5-C6 twist" conical intersection is the primary decay channel for excited cytosine at low vibrational energies.
• Substitution patterns can effectively tune the excited-state decay barriers and mechanisms.
• Spectroscopic and computational methods provide a powerful combination for understanding molecular excited-state dynamics.