N—>tt* states

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

In addition, it can be shown that second-order vibronic perturbation will make possible some intersystem crossing to the 3B3u(n, tt ) state. However, this second-order perturbation should be much less important than the first-order spin-orbit perturbation.(19) This will produce the unequal population of the spin states shown in Figure 6.1. In the absence of sir the ratios of population densities n are given by the following equations ... 

Zeng, J., Craw, J. S., Hush, N. S. and Reimers, J. R. Medium effects on molecular and ionic electronic spectra. Application to the lowest l(n,Tt ) state of dilute pyridine in water, Chem.Phys.Lett., 206 (1993), 323-328... 

Both the Si and Ti excited states arise from the promotion of an electron from the n molecular orbital to the Jt molecular orbital. They are referred to as (njji ) and 3(n,tt ) states, respectively. The S2 and T2 states arise from the promotion of an electron from the n molecular orbital to the Jt molecular orbital and are referred to as 1(n,nx ) and 3(jt,Jt ) states, respectively. The state diagram for methanal is shown in Figure 1.11. With regard to the different spin states in molecules, the following ideas are important ... 

There are two types of electronic transition commonly responsible for photochemically induced reactions in organic molecules. The first of these is the n—>ir transition in which an electron in a nonbonding atomic orbital is excited to an antibonding ir orbital, the excited state being referred to as n, it. This occurs in nitrogen-, oxygen-, and sulfur-containing molecules, and the nature of the n, 77 state of the carbonyl function has been the subject of considerable study.5,6 Excitation to the n, tt state in aldehydes and ketones occurs at approximately 290 nm. 

Figure 2.8. Energy level diagram illustrating the vibronic coupling between the 41 level of the S n.Tt ) state and the S2(n,4s) Rydberg state, which induces intensity into the Si <— S0 transition.

Although esters do not have low-lying n,tt states, their carbonyl groups make them excellent electron acceptors. Thus with a good electron acceptor... 

Figure 17-1. Potential energy profiles of the lowest tt, tt state (squares), the lowest n, tt state (diamonds), the lowest tt, ct state (triangles), and the S0 state (circles) of 9H-adenine, as a function of the N9-H stretch reaction coordinate. Geometries optimized at the CASSCF level and energy profiles obtained with TDDFT (adapted from Ref. [57])...

The estimated vertical excitations in water are presented in Table 17-1. From qualitative arguments, an increase of the energies on the (n, tt ) states can be expected because of the stabilization of the lone pairs by hydrogen bonding. In fact, the estimated blue shifts in solution are 0.6 eV and 0.8 eV for the nN, tt and nQ, tt states, respectively. For the (tt, tt ) state, the calculations predict a blue shift of approximately 0.2 eV in water solution with respect to the gas phase excitation maximum this is in contrast to the experimental red shift of 0.1 eV, and the failure to reproduce the experimental trend is probably due to the approximations of the... 

Retinol. The extensively investigated retinol molecule offers a relatively simple photophysical system due to the absence of a low-lying (n, tt ) state. Two main patterns characterize the fluorescence of all-trans retinol at 295°K (1) the break-... 

The short-chain-length molecules (n = 2-4) are nonfluorescent because the - -(n, TT ) state is the lowest. The fact that the long-chain molecules exhibit a wavelength- and solvent-independent emission, both at 298°K and 77°K, is attributed to a state reversal, making the - -Ag the lowest. In the intermediate (n =... 

The involvement of upper excited states following irradiation of the phthalocyanine complexes Rh(Pc)(MeOH)X (where X = Cl, Br, or I) has been investigated. All compounds have the same action spectrum for photoinduced H-abstraction and the emission at 420 nm is attributed to relaxation of an upper tt, tt ) excited state. At high photonic fluxes it has been shown that biphotonic photochemistry involves the n, tt state. A study of the photoaquation of [RhCNH,), ] in fully and partially hydrated zeolite Y has been reported. Reaction within the cavities occurs with a quantum yield that is only 15—20% of that in aqueous solution this is attributed to the decrease in mobility of the water in the zeolite and to the exclusion of water from the ligand-exchange site by the zeolite lattice. ... 

Later, Falk [66] used TET to explain the photoisomerization of pyrrole pigments. Markov [67] applied the theory to photoketonization of dicarbonylic compounds. Hamanoue [68] showed that TET predictions were consistent with the relative reactivity of (n, n ) and (n, Tt ) states of an-thraquinones. Following a suggestion by Isaacs [69], Okamoto [70] claimed that the increase in the KIE of the H abstraction of methanol by benzo-phenone was evidence for a nuclear tunnelling mechanism, but this claim is not entirely consistent with the predictions of Isaacs. Shizuka [71] proposed TET could be used to explain excited state proton transfers, but this may only be the case for intramolecular proton transfers or reactions in apolar solvents, otherwise the system effective reduced mass will be too high [72]. 

Recently, studies were carried out to explain the exo/endo selectivity of the Paternb-Btichi reaction [30], These studies were carried out mostly with achiral or racemic substrates. Excited monocyclic aromatic aldehydes 33 react in their n, TT state with cyclic enol ether derivatives like 2,3-dihydrofuran 34 (Scheme 8) [31], In these cases, the sterically disfavored endo isomer 35a was obtained as major product. This result was explained by the fate of the triplet biradical intermediate G. In order to favor cyclization to the oxetanes 35a,b, the radical /J-orbitals have to approach in a perpendicular fashion to increase the spin-orbit coupling needed for the triplet to singlet intersystem crossing [32]. The sterically most favored arrangement of this intermediate is depicted as G. The encumbering Ar substituent is orientated upside and anti to the trihydrofuranyl moiety. Cyclization from this conformation yields the major isomer 35a. 

The nature of the excited state of a carbonyl group in the Norrish type II scission is highly controversial. Wagner and Hammond [645, 646] and Dalton and Turro [176] maintain that both excited 1 (n, 7r ) and 3(n, tt ) states take part in the type II process with aliphatic ketones. 

Cyclobutanone was considered in both the singlet fundamental and the first excited (n — tt ) state [16]. The geometry was fully optimized, at the RHF and HPHF levels, respectively, using a dummy atom at the center of the molecule X), and the minimal basis set [7s,3p/2s,lp] [28]. 

Generally, the calculated spectra reproduce the general qualitative features of the experimental spectra quite well. The visible bands of magnesium and free-base porphine are predicted to arise from weakly-allowed transitions to a doubly-degenerate (n, tt ) state and a pair of (tt, n ) states, respectively, a picture basically similar to the 4-orbital model. In the Soret spectral region, however, the present studies reveal that this band is composed of a number of intense tt tt transitions, a result which represents a fundamental departure from the basic 4-orbital model [13]. The results further suggest that resolution of the Soret spectral region into individual electronic transitions would be difficult experimentally, and perhaps is best achieved currently with a computational approach. 

We discussed how the n,TT states of carbonyls, especially the triplets, have considerable radical character. In addition, oxygen-centered radicals are very reactive because of the high strengths of O-H and O-C bonds. Hydrogen atom abstraction should be expected, and indeed it is a common reaction. A typical reaction involves a carbonyl n,TT state undergoing a bimolecular hydrogen atom abstraction, and that is the basis for the common process called photoreduction. 

Pyridine shows no luminescence. Kasha and co-workers have explained this by observing that the n n state of pyridine is lower in energy than the n n state, although the extinction coefficient for absorption is extremely small. Pyridine radiationlessly crosses to the n -> tt state rapidly and this state does not emit. Propose a mechanism for the rapid, radiationless relaxation of an n excitation in pyridine. 

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