Zwitterionic excited states

Fig. 21 (a) Rectification in the AR sense electron transfers occur first between electrode MD and the HOMO of the D-o-A molecule (1 ), and between the LUMO and MA (1), thus establishing the zwitterionic excited state D+-a-A , which then (2) relaxes back into the neutral state, (b) Rectification in the anti-AR sense if auto-ionization occurs first (1), forming the excited state D+-a-A by an interaction of the intense electric field and the molecule, followed by transfers to and from the electrodes (2) and (2 ), resulting in the electron passing from MD to MA. The molecular energy levels, which in reality must perforce shift dramatically during the electron transfer process, are drawn here for simplicity as if the molecule were still isolated in the gas phase ... 

In contrast to the absorption maxima of ANS, the fluorescence maxima are sensitive to solvent changes, but to a different extent in solvents of varying polarity. The fluorescence of ANS arises from two different excited-state molecules, the apolar locally excited state Si p and the zwitterionic excited state Si t, which emerges from Si p by intramolecular electron or charge transfer. The first emission, predominant in nonpolar solvents, varies only modestly with solvent polarity, as expected for a transition Si,np So between two states of similar charge separation. The second emission, observed in more polar solvents, is quite sensitive to solvent polarity, as anticipated for a transition Si,ct So between two states of very different charge separation [119, 120, 340]. 

Separate optimization of the relevant function spaces zwitterionic excited states of polyenes and the sudden polarization effect... 

V. HIGHLY POLARIZED OR ZWITTERIONIC EXCITED STATE OF TETRAPHENYLETHYLENE, STUDIED BY SPECTROSCOPIC TECHNIQUES... 

A picosecond optical calorimetric technique was employed to understand the singlet excited state of TPE. Picosecond optical calorimetry provides the energetics involved in the solvent polarity-dependent nature of tiie decay of a polar or zwitterionic excited state. It was found that the dipole moment of the singlet excited TPE is around 6.3D and energy of excited TPE decreases with increase in solvent polarity. These results on the TPE singlet excited state dipole moment, decay rate, and decrease in the energy as the solvent polarity increases are cited as evidence in favor of the polar or zwitterionic character associated with the twisted TPE (p ) excited state [63,64]. 

The same technique, picosecond optical calorimetry, was employed to study the kinetics of p (twisted zwitterionic singlet excited state) decay in various TPE derivatives [64]. The results obtained indicate that two excited states are involved 1.) the vertically excited fluorescent state and 2.) the twisted zwitterionic excited state (p ). Furthermore, they indicated that these two states are in equilibrium in nonpolar solvents. 

Aryl substituted cyclobutenes undergo cycloreversion to arylalkynes but can also give addition products in hydroxylic solvents. These reactions are singlet state processes. The aryl derivatives of cyclobutene do not open to 1,3-butadienes, and based on substituent effects, the excited states appear to have zwitterionic character. These results suggest that the aryl substituent favors the formation of a zwitterionic excited state. 

The rate of energy transfer from the benzotriazole chromophore to the hydroperoxy groups is controlled by the lifetime of the excited state, as long as it is higher than 1.5 ev approximately. Details of decay mechanisms of the excited states will be published later. Here we will note that the principal feature of the deactivation mechanism involves an intramolecular proton transfer process which may occur before vibrational equilibration of the vertical excited state is completed. The fluorescence has a blue (X-max = 405 nm) and a red (X.max = 585 nm) component, with the blue component only being present at room temperature in dilute solution, and at low temperatures in polar matrices. The red component is present in emission at room temperature from polycrystalline powders and at low temperatures in hydrocarbon matrices. It may be postulated that the blue component arises from a vibra-tionally excited 0-protonated species, while the red component arises from a proton transferred zwitterionic excited state. Phosphorescence is detected from the model compound (II) in polar matrices at 77K. Table II gives some excited state lifetime data on the copolymer and model systems. 

Bonacic-Koutecky V (1978) Sudden polarization in zwitterionic excited states of organic intermediates in photochemical reactions. On a possible mechanism for bicyclo[3.1.0]hex-2-ene formation. J Am Chem Soc 100 396... 

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