Electronic and Inertial Effects of Methylation on Excited-State Hydrogen Transfer

Area of Science:

• Physical Chemistry
• Photochemistry
• Quantum Dynamics

Background:

• Excited-state intramolecular hydrogen transfer (ESIHT) is a fundamental ultrafast reaction crucial for photoprotective molecules and functional chromophores.
• The symmetric malonaldehyde (MA) prototype exhibits competing decay channels, including C═C torsional motion, complicating its photodynamics.
• Understanding substituent effects is key to controlling ESIHT and designing novel photoactive materials.

Purpose of the Study:

• To investigate the influence of methylation on the ultrafast dynamics of ESIHT in acetylacetone (AcAc) compared to malonaldehyde (MA).
• To elucidate the roles of electronic and inertial effects in shaping the excited-state decay pathways initiated on the S₂(ππ*) state.
• To compare the dynamics on the S₁(nπ*) state following intersystem crossing.

Main Methods:

• Utilized XMS-CASPT2 nonadiabatic dynamics simulations on the singlet electronic manifold.
• Analyzed wavepacket evolution, bond-length alternation, and torsional motion.
• Compared dynamics of malonaldehyde (MA) and acetylacetone (AcAc) to discern substituent effects.

Main Results:

• In AcAc, methylation destabilizes the S₁ state, reducing the S₂/S₁ energy gap and leading to S₂/S₁ decay preceding hydrogen transfer.
• AcAc exhibits two distinct decay channels on S₁: an early ballistic rise (~75 fs) and a slower torsional motion, attributed to inertial mismatch.
• Malonaldehyde (MA) lacks the ballistic channel observed in AcAc, showing simpler decay dynamics.

Conclusions:

• Methylation in AcAc significantly alters ESIHT pathways, introducing unique ultrafast dynamics driven by electronic and inertial effects.
• The observed dynamics in AcAc align with experimental time-resolved photoelectron spectroscopy, validating the ultrafast S₂ lifetime.
• Propose X-ray spectroscopy to probe site-specific motion during H-transfer, offering deeper insights into these rapid photochemical processes.