The driving forces for twisted or planar intramolecular charge transfer

Area of Science:

• Photochemistry
• Computational Chemistry
• Molecular Spectroscopy

Background:

• Donor-acceptor (D-A) chromophores exhibit excited-state structural transformations.
• These transformations can lead to twisted intramolecular charge transfer (TICT) or planar intramolecular charge transfer (PICT) states.
• The underlying driving forces for these geometric changes remain an active area of research.

Purpose of the Study:

• To elucidate the driving forces behind excited-state twisting or planarization in D-A chromophores.
• To determine the factors that dictate whether a chromophore adopts a TICT or PICT geometry.
• To establish a predictive framework for excited-state molecular behavior.

Main Methods:

• Fragment orbital interaction analysis was employed.
• Excited state energy decomposition analyses were performed.
• These computational methods were applied to several well-established TICT/PICT molecules.

Main Results:

• Three key driving forces were identified: energy gap, hole-electron interactions, and excited-state relaxation.
• The energy gap difference between twisted and planar geometries was found to be the dominant factor in most cases.
• Molecular structure significantly influences the contributions of these driving forces.

Conclusions:

• The energy gap between different excited-state geometries is crucial for determining chromophore behavior.
• Evaluating frontier orbital interactions between donor and acceptor units can predict excited-state planarization or twisting.
• This provides a valuable tool for designing chromophores with specific photophysical properties.