Biradical excited states

V. Bonacic-Koutecky and J. Michl, Charge-transfer-biradical excited states Relation to anomalous fluorescence, Negative Sl-Tl splitting in twisted aminoborane, J. Am. Chem. Soc. 107, 1765 (1985). 

There are large families of chemical compounds between those of DMABN and ethylene that are either polar or nonpolar in S0, but after photoexcitation the twisted charge-separated biradical excited states develop via adiabatic pathways (Section III). This has been displayed by experimental2 and theoretical14 studies. 

The enrichment of the concentration of the polar solvent component in the cage and, therefore, the relative amount of the red shift of the fluorescence band is a function of viscosity, since the diffusion-controlled reaction time must be smaller than the excited-state lifetime. This lifetime limitation of the red shift is even more severe if the higher value of the excited-state dipole moment is not a property of the initial Franck-Condon state but of the final state of an adiabatic reaction. Nevertheless, the additional red shift has been observed for the fluorescence of TICT biradical excited states due to their nanosecond lifetime together with a quenching effect of the total fluorescence since the A to 50 transition is weak (symmetry forbidden) (Fig. 2.25). 

In contrast to Varma and co-workers96,97 we wish to point out that exciplex formation can, but is not forced to, occur with internal twisting. In appropriate solutions, of course, it may be connected with the TICT reaction mechanism, but the formation of the twisted biradical excited state plays the governing role. 

On the other hand, since biradical excited states are in principle easy polarizable,39 105 they will have something in common with the charge-separated states of a broken dative bond. In order to find out the essentials about the excited states of the dative bonds, especially about the geometries corresponding to minima from which emission might take place, we will proceed in four steps. 

He invoked the intermediacy of a weakly bound peroxide biradical resulting from trapping of the biradical excited state of the sensitizer by oxygen. This was able to transfer oxygen to the substrate. In the same years, Lewis had identified the metastable state as a triplet, or, by adopting a term more often used by organic chemistry practitioners, a biradical (see Chap. 3). Thus, no wonder that such an intermediate may enter in radical reactions, and in fact a number of photochemical reactions of aldehydes and ketones had been explained exactly via such biradical... 

The energies of this Cl and of the other ones calculated in this work are listed in Table III. The calculated CASSCF values of the energies of the two lowest electronically states are 9.0 eV (5i, vertical) and 10.3 eV ( 2, vertical) [99]. They are considerably higher than the expenmental ones, as noted for this method by other workers [65]. In all cases, the computed conical intersections lie at much lower energies than the excited state, and are easily accessible upon excitation to Si. In the case of the H/allyl Cl, the validity confirmation process recovered the CHDN and 1,3-CHDN anchors. An attempt to approach the third anchor [BCE(I)] resulted instead in a biradical, shown in Figure 43. The bhadical may be regarded as a resonance hybrid of two allyl-type biradicals. 

The parameterization of MNDO/AM1/PM3 is performed by adjusting the constants involved in the different methods so that the results of HF calculations fit experimental data as closely as possible. This is in a sense wrong. We know that the HF method cannot give the correct result, even in the limit of an infinite basis set and without approximations. The HF results lack electron correlation, as will be discussed in Chapter 4, but the experimental data of course include such effects. This may be viewed as an advantage, the electron correlation effects are implicitly taken into account in the parameterization, and we need not perform complicated calculations to improve deficiencies in fhe HF procedure. However, it becomes problematic when the HF wave function cannot describe the system even qualitatively correctly, as for example with biradicals and excited states. Additional flexibility can be introduced in the trial wave function by adding more Slater determinants, for example by means of a Cl procedure (see Chapter 4 for details). But electron cori elation is then taken into account twice, once in the parameterization at the HF level, and once explicitly by the Cl calculation. 

Also in this case calculation results fit the experimental data (Fig. 7) [99H(50)1115]. In fact, the singlet excited state can evolve, giving the Dewar thiophene (and then isomeric thiophenes) or the corresponding excited triplet state. This triplet state cannot be converted into the biradical intermediate because this intermediate shows a higher energy than the triplet state, thus preventing the formation of the cyclopropenyl derivatives. 

The dioxygen molecule exists in two forms a triplet or ground state in which it is a stable biradical and a singlet or excited state in which it is not a radical. Reactions of carotenoids with singlet oxygen have already been presented in this chapter and we now focus on the reactions of carotenoids and oxygen in the ground or triplet state. 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

A direct observation of a triplet state of substituted trimethylene biradical has recently been claimed.84) This is in good agreement with the prediction 104,105) that the lowest excited state of cyclopropane will have an energy minimum at a geometry in which one C—C bond is broken. 

All of the elements of stereo- and regioselectivity and reactivity that theory must explain are found in the above reactions. The triplet excited states of the aryl carbonyl compounds demonstrate regioselectivity that has been previously explained on the basis of the relative stabilities of the two possible biradical intermediates, 1 and 2. 65>66> The selectivity... 

The final type of reaction that will be discussed is the highly interesting cross photocycloaddition of cyclic a, (3- unsaturated ketones with olefins. Examples were given in Eqs. 28—31. A general mechanism 94), to which there may be exceptions to be discussed later, would involve a triplet state of the enone and the reactions steps given in Eqs. 32, 33, and 35, complex formation, biradical formation, and product formation. An earlier idea that two different excited triplet states were reacting has been discounted. 100,141,142) The inefficiency of the reaction is attributed to an alternate decay of complex 77,78,ioo,i42)( an(j the excited state has a n-n configuration. 100,142)... 

Finally, we should indicate that we have not ruled out the possibility that there is a contribution to initiating photopolymerization in maleimide/vinyl ether systems from an exciplex type complex between an excited state maleimide and ground state vinyl ether. A biradical formed from such a complex might initiate free radical polymerization in lieu of cyclization to form a 2 + 2 adduct. However, we note that at present we have no evidence for such a reactive exciplex. 

Arnold, B.R. Bucher, G. Netto-Ferreira, J.C. Platz, M.S. Scaiano, J.C. Biradicals, Radicals in Excited States, Carbenes and Related Species. In Landolt-Bomstein, Radical Reaction Rates in Liquids, W. Marlienssen, Ed. SpringenBerlin, 1998 Vol. New Series II/18E2, Chapter 14, pp 141-349. 

For chemical systems of interest, photolysis produces intermediates, such as radicals or biradicals, whose energetics relative to the reactants are unknown. The energetics of the intermediate can be established by comparison of the acoustic wave generated by the non-radiative decay to create the intermediate, producing thermal energy , with that of a reference or calibration compound whose excited-state decay converts the entire photon energy into heat, / (ref). The ratio of acoustic wave amplitudes, a, represents the fraction of the photon energy that is converted into heat. 

Free-radical chemistry is important in organic photochemistry as, generally speaking, all organic photochemical reactions of in, ) or 3(n,tt ) excited states and all reactions of 3(tt,Jt ) excited states result in the formation of either a pair of radicals or a biradical. We shall consider... 

The first step of the reaction involves the (n, it ) excited state of the carbonyl compound reacting with the ground-state alkene. For aromatic ketones, rapid intersystem crossing from the excited singlet state to the excited triplet state occurs, forming initially a 1,4-biradical and then the oxetane ... 

If the alkene can exist as cis and trans isomers then we need to be aware of the stereospecificity of the reaction. If the reaction involves an excited triplet state then the biradical formed will be able to undergo bond rotation in the lifetime of the excited state. The reaction is, therefore, nonstereospecific, forming a mixture of oxetane isomers from either alkene isomer ... 

---