Nature of the “sudden polarization” effect and its role in photochemistry

Abstract

The "sudden polarization" effect is described within the framework of the four-state model of asymmetrical diradicals. Its applicability for photochemical predictions is illustrated by the results obtained from the ob initio minimal-basis-set calculations followed by a very limited CI. The analysis of the reasons for the large charge separation in the low-lying singlet excited states and their particular polarities has been performed with the PPP full C1 approach. The connection between an alternant-symmetry braking within the r-electron model and the origin of the "sudden polarization" effect for the hydrocarbon intermediates is demonstrated. A possible extension of similar phenomena to the excited states of molecular system with pronounced polar character is discussed.

The Ultrafast Photoisomerizations of Rhodopsin and Bathorhodopsin Are Modulated by Bond Length Alternation and HOOP Driven Electronic Effects

“…(Notice that near the conical intersection this charge translocation reaches values above 0.9. This is a manifestation of the "sudden polarization effect" 73,74 that, consistently with early results, occurs when the system is approaching the conical intersection and that has a lifetime limited to tens of femtoseconds. See also Figure 3B for Rh.)…”

Dark State of the Thiele Hydrocarbon: Efficient Solvatochromic Emission from a Nonpolar Centrosymmetric Singlet Diradicaloid

“…These pieces of evidence, applied to TTH , suggest that after relaxation to the SE state via planarization of the p QDM core (Figure a) and fast internal conversion to the lower-lying DE state, the long exocyclic CC bond length afforded in the ionic DE state along with its proximity to the SE state assist the intramolecular torsion, ultimately leading to a charge-separated twisted excited state whose stabilization will be preferential in polar solvents (Figure b). Such twisted, charge-separated (zwitterionic) excited states are well known in olefins and justified by sudden polarization, − a mechanism explaining, in this case, the observed solvent-dependent Stokes shift of TTH. Thus, the extreme elongation of the exocyclic CC bonds in the excited DE state of TTH , rather unusual for other more extended p QDM diradicaloids, makes it comparable to short olefins.…”

“…To assess the role of twisting around such CC bonds, CASSCF/NEVPT2 calculations were carried out on a smaller p QDM derivative, the fully fluorinated tetracyano-quinodimethane F4TCNQ (Figure S18), featuring a similar low-lying DE state and documented luminescence properties (maximum value of PLQY = 0.04 in cyclohexane) . Twisting around one exocyclic CC bond of F4TCNQ generates asymmetric excited conformers featuring non-negligible dipole moments as a result of sudden polarization, − leading to a solvent-sensitive dipolar structure with red-shifted emission driven by the mixing of the DE and SE ionic states. Calculations show that such twisted excited states may be preferentially stabilized in polar solvents (Figure S18).…”

Tuning Stilbene Photochemistry by Fluorination: State Reordering Leads to Sudden Polarization near the Franck–Condon Region

“…The sudden polarization of alkenes in the 90°-twisted molecular geometry, a manifestation of the pseudo-Jahn–Teller effect, was theoretically predicted in 1970s and thoroughly discussed − because of its crucial role in photochemical reactions . An experimental verification of the effect came decades later, with the progress in ultrafast spectroscopy and owing to suitable probes, such as stilbene. − …”