Relaxation energy, charge-transfer

The genesis of the failure of TDDFT can be traced to the fact that TDDFT is a linear response theory. When an excitation moves charge from one area in a molecule to another, both ends of that molecule will geometrically relax. While charge transfer between molecules can be well approximated by ground-state density functional calculations of the total energies of the species involved, TDDFT must deduce the correct transitions by... 

Much of chemistry occurs in the condensed phase solution phase ET reactions have been a major focus for theory and experiment for the last 50 years. Experiments, and quantitative theories, have probed how reaction-free energy, solvent polarity, donor-acceptor distance, bridging stmctures, solvent relaxation, and vibronic coupling influence ET kinetics. Important connections have also been drawn between optical charge transfer transitions and thennal ET. 

A number of fluorescent dyes with internal charge transfer mechanism allow the molecule to twist (rotate) between the electron donor and electron acceptor moieties of the fluorescent dipole. In most cases, the twisted conformation is energetically preferred in the excited Si state, whereas the molecule prefers a planar or near-planar conformation in the ground state. For this reason, photoexcitation induces a twisting motion, whereas relaxation to the ground state returns the molecule to the planar conformation. Moreover, the Si — So energy gap is generally smaller in the twisted conformation, and relaxation from the twisted state causes either a... 

Excimer may relax (i) by emission of characteristic structureless band shifted to about 6000 cm-1 to the red of the normal fluorescence, (ii) dissociate nonradiatively into original molecules, (iii) form a photodimer. Those systems which give rise to photodimers may not decay by excimer emission. The binoing energy for excimer formation is provided by interaction between charge transfer (CT) state A+A- A-A and charge resonance state AA s A A. 

---