Inter-system crossing

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

Scheme 30 represents the energy diagram for the photorearrangement shown in Scheme 29. Quenching of the triplet state of the sensitizer by the cis allyl phosphate, c/s-1, generates the triplet state, T , of the 1,2-biradical 2. The 1,2-biradical is trapped by the phosphorus atom to afford the triplet state, TP, of the spirophosphoranyl 1,3-biradical 3. Then, inter-system crossing generates the...

All this indicates that ISC (inter-system-crossing) plays an important role in the reaction, since the occurrence of channels (2b), (2c), and (2d), which account for about 2/3 of the overall reaction yield, can only be rationalized assuming that ISC between triplet and singlet PESs is occurring very efficiently. Theoretical work on both triplet and singlet PESs of C2H4O, with inclusion on nonadiabatic couplings, is desirable and would... 

Figure 1 Jablonskii diagram illustrating the energy levels and photophysical processes for an aromatic chromophore (ISC = inter-system crossing IC = internal conversion).

A close correspondence is found in the photoluminescence spectrum in solution and in solid films [71], Using the inter-system crossing to populate the Ti... 

As shown in Fig. 5, even if the MRCI-derived upward correction of 3.4 kcal/mol to the energy of A2-lb is included, the energy difference between 1 A2-lb and 3b is estimated to be only 5 kcal/mol. Therefore, at equilibrium, a small amount of singlet lb should be present at ambient temperatures. Inter-system crossing of singlet lb to triplet lb should then lead to the irreversible conversion of 3b to the triplet ground state of lb. As already mentioned, the reversion of 3b to triplet lb has, in fact, been observed in inert solvents.38... 

Fig. 9. Jablonsky Diagram for energy conversion pathways of an excited molecule. While fluorescence occurs between states of the same spin, an ISC (inter system crossing) leads to spin inversion and a delay in emission (phosphorescence halftimes from 1CT4 s to minutes or even hours)...

Heavy atom enhancement of intersystem crossing has been used to determine the mechanism of acridine photoreduction in ethanol.115 It was found that addition of sodium iodide decreased the fluorescence intensity and the rate of disappearance of acridine to the same extent, confirming that the singlet state is responsible for photoreduction. From the increase in triplet state absorption upon addition of iodide it was found that Of for acridine was 0.76. Thus the short singlet lifetime (0.8 nsec) of acridine is due to rapid inter-system crossing to unreactive triplet states. 

Absorption by the chromophore ligand is commonly (6) followed by inter-system crossing (ISC) to the triplet state of the ligand. This state can decay either by emission (ligand phosphorescence) or by... 

Radiative transitions Non-radiative decay Inter-System Crossing Energy Transfer... 

The temperature sensitivity arises due to disposition of T2 state with respect to S, state. If T2 is considerably above S, transfer to T, is less probable because of unfavourable Franck-Condon factor. As a consequence, fluorescence is the easiest way for deactivation and fluorescence yield is nearly unity. No dependence on temperature is expected. On the other hand, if T2 is sufficiently below S so that the density of state is high at the crossing point, fluorescence quantum yield should be less than unity as triplet transfer is fecilitated. Again no temperature dependence is observed. But if T2 is nearly at the same energy as S, a barrier to inter-system crossing is expected and fluorescence yield will show temperature dependence. 

It is further observed that an increase in reaction efficiency corresponds to decrease in fluorescence efficiency. The last column gives the sum of the two processes which may be taken as the contribution by the singlet excited state. The values vary within small limits. The remainder of the excited molecules must either degrade to the triplet state by inter-system crossing or to the ground state by internal conversion. The scheme of reaction is suggested as follows ... 

Phosphorescence In phosphorescence the electron falls from the excited singlet state by inter-system crossing to the lowest triplet state (Tt lifetime 10 6-l() s), where it initially remains because a transition to the singlet state is symmetry-forbidden, and subsequently returns to the ground state (Fig. 5.2). The length of time between excitation and emission is therefore significantly greater than in fluorescence. 

Scheme 11.5 Photophysics of sensitised lanthanide sensors sens-Ln such as 11.32 and 11.33. Key k = rate constant, em = emission, eT = electron transfer, q = quenching, isc = inter-system crossing to triplet state, (B)ET = (back) energy transfer (reproduced with permission from Reference 24).

The rates km cover the km o accounting for the excited state decay of chromophore m (by radiative as well as non-radiative transitions) and the SC ) originated by inter-system crossing to triplet states (ISC rate). The simple km do not include the effect of excited state wave function delocalization and a possible decay out of exciton states [45], Therefore, we shortly demonstrate the computation of the photon emission part of the km including such a delocalization effect (determination of excitonic augment rates). It will be important for the mixed quantum classical simulations discussed in the following (for more details see also [11]). 

Figure 2.23 Schematic illustrating the dye sensitization of a semiconductor electrode via electron transfer straight lines indicate radiative transitions, curved lines electron transfer, and wavy lines non-radiative (nr) transitions. Photoexcitation into the Si state of the dye may result in charge injection into the conduction band of the semiconductor or fluorescence and inter-system crossing, from where charge injection may occur from the triplet state or phosphorescence...

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