Intersystem crossing aromatic hydrocarbons

Thus we see that in molecules possessing ->- 77 excited states inter-combinational transitions (intersystem crossing, phosphorescence, and non-radiative triplet decay) should be efficient compared to the same processes in aromatic hydrocarbons. This conclusion is consistent with the high phosphorescence efficiencies and low fluorescence efficiencies exhibited by most carbonyl and heterocyclic compounds. 

Further proof of the importance of Franck-Condon factors is shown by the dramatically increased triplet-state lifetimes of aromatic hydrocarbons that have been deuterated. The effect of this deuteration is to decrease the rate of Ti A/W> S0 intersystem crossing, which is accompanied by a corresponding increase in triplet-state lifetime (Table 5.1). 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

However, the heavy atom effect can be small for some aromatic hydrocarbons if (i) the fluorescence quantum yield is large so that de-excitation by fluorescence emission dominates all other de-excitation processes (ii) the fluorescence quantum yield is very low so that the increase in efficiency of intersystem crossing is relatively small (iii) there is no triplet state energetically close to the fluorescing state (e.g. perylene)10 . 

Photoinduced electron transfer (PET Scheme 6.2) is a mild and versatile method to generate radical ion pairs in solution," exploiting the substantially enhanced oxidizing or reducing power of acceptors or donors upon photoexcitation. The excited state can be quenched by electron transfer (Eq. 7) before (aromatic hydrocarbons) or after intersystem crossing to the triplet state (ketones, quinones). The resulting radical ion pairs have limited lifetimes they readily undergo intersystem ... 

Several examples of heavy atom quenching of aromatic hydrocarbon states are known for example, carbon tetrabromide is an efficient quencher of the fluorescence of anthracene167 and carbon tetrachloride behaves similarly with p-terphenyl.188 Since quenching results in formation of the triplet state, it has been possible to use the heavy atom effect to measure intersystem crossing efficiencies ( ). Because of the elegance of this technique 169 and the importance of the results in photochemistry, we shall cover it in some detail. 

In aromatic hydrocarbons, the oxygen-quenching of singlet states does not involve energy transfer but is entirely due to enhanced intersystem crossing, which may proceed via a CT-complex state (Section 6.6.2). 

Fig. 5. The dependence of the nonradiative lifetime for intersystem crossing (from the first excited triplet to the ground state) on the electronic energy gap (O) CxHy ( ) CxDy. All data were obtained for aromatic hydrocarbons in solid solutions. The nonradiative lifetime (j3 = r r) was calculated by Siebrand30 from eq. (3-1) taking r, = 30 sec-1 for all the aromatic hydrocarbons. This figure is reproduced from Siebrand s paper.30...

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

Wilkinson has recently described a novel approach.113 It has long been known that solvents containing heavy atoms markedly quench the fluorescence of aromatic hydrocarbons, and it has been shown that this effect arises from an enhancement of the rate of intersystem crossing. Thus the ratio of phosphorescence to fluorescence for naphthalene irradiated at 77°K can be increased more than a hundredfold upon addition of some isopropyl iodide to the solvent.114 The same effect has been noted upon changing from hydrocarbon glasses to frozen krypton and xenon matrices.115 Wilkinson found that the decrease in fluorescence intensity from irradiated solutions of anthracene and some of its derivatives upon addition of bromobenzene is attended by an increase in T-T absorption intensity.116 The fluorescence quenching follows the Stern-Volmer law ... 

In the presence of heptakis(6-bromo-6-deoxy-(3-CD) ((3-CD7Br), Femia and Cline Love observed the room-temperature phosphorescence of phenanthrene and other polynuclear aromatic hydrocarbons in N2-purged N,N-dimethylfol-mamide (DMF)-water mixtures [24], On the other hand, Hamai and Monobe observed room-temperature phosphorescence of 2-chloronaphthalene from a deaerated solution containing a 1 1 complex of 6-iodo-6-deoxy-(3-CD ((3-CDI) and 2-chloronaphthalene [25], This result indicates that even only one iodine atom on the (3-CD rim can accelerate the intersystem crossing rate of 2-chloronaphthalene included in the CD cavity. The room-temperature phosphorescence of 6-bromo-2-naphthol and 3-bromoquinoline was also observed for the complexes with (3-CDI [26],... 

McGlynn Mid Boggus describe the phenomenon thus absorption in the charge transfer bMid is followed either by the converse emission or by intersystem crossing (according to Kasha [124]) to a dissociative level of the complex which yields the aromatic in its first excited triplet state. The aromatic hydrocarbon then phosphoresces. 

It should be pointed out that the above two types of results obtained for aromatic hydrocarbons and N-hetero-cyclics are consistent with the following conclusion the strongly radiative zf level is also one of the most favored zf levels in the intersystem crossing process. 

Triplet quantum yields (high rate of intersystem crossing (much faster than for typical aromatic hydrocarbons) and the inefficient fluorescence. These values were determined by photothermal and spectroscopic methods and agree with lower limits determined from singlet oxygen quantum yields [9,10]. (see below). A cyclodextrin complex also produces the triplet state efficiently in water [39]. 

In a later paper, Lewis and Saunders observed that triplet quenchers (cis-piperylene, oxygen) failed to affect the course of the direct photolysis of alkyl azides, from which it was concluded that the photolysis proceeded via a singlet azide and singlet nitrene. This was further supported by the observation that hexyl azide acted as an efficient quencher of aromatic hydrocarbon fluorescence, and that this singlet sensitization of hexyl azide led to the decomposition of the azide with an efficiency similar to that of direct photolysis Thus, although triplet sensitization leads to decomposition of alkyl azides, it appears that direct photolysis proceeds by way of an excited singlet azide without intersystem crossing to the triplet. 

Figure 5.5. Relationship between the energy gap AE(T, - Sg) and the logarithm of the rate constant kj of intersystem crossing in aromatic hydrocarbons (data from Birks. 1970).

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