On the Fluorescence Properties and Nonradiative Transitions in Medium-Sized All-Trans Polyenes

Area of Science:

• Computational Chemistry
• Photochemistry
• Quantum Chemistry

Background:

• Linear polyenes are crucial organic molecules with tunable electronic properties.
• Understanding their excited states is key to controlling their photophysical behavior.

Purpose of the Study:

• To investigate the nonadiabatic photodynamics of all-trans linear polyenes (N=4-8).
• To elucidate the dependence of low-lying excited states on chain length.
• To map the crossing seam between the 1B and 2A singlet states.

Main Methods:

• State-Average Complete Active Space Self-Consistent Field (SA-CASSCF) method.
• Second-order perturbation theory variant (MS-CASPT2) for accurate energy calculations.
• Mapping of the 1B-2A crossing seam in a 2D coordinate space.

Main Results:

• Computed excitation energies and stationary points for singlet states.
• Determined the minimum of the 1B-2A crossing seam.
• Observed a substantial, monotonous increase in the seam's minimum energy with increasing N.

Conclusions:

• The energy increase of the crossing seam explains the fluorescence crossover.
• Smaller N exhibit S1(2A) fluorescence, while larger N show S2(1B) or dual fluorescence.
• Electronic structure calculations provide insights into polyene photophysical properties.