Steric hindrance in the transfer of singlet electronic energy

Abstract

Within the frame of the scheme shown in IV, this conclusion is of course only valid if theinterconversions shown occur in the excited state. It is also perhaps interesting to note that the observed interconversion between 1 and 2 may also be rationalized through the initial excited state generation of 7 and 8 depicted in Scheme IV followed by electronic deactivation of these species and subsequent thermal, symmetry-controlled, closure to 2 and 1, respectively. Nonetheless, we tend to favor the mechanistic interpretation of Scheme IV owing to the obvious inability of the "thermal" process to readily accommodate the substantial photostationary predominance of the cis isomer irrespective of N substituent.

Singlet energy transfer to azoalkanes

“…In Figures 2-5 a Calculated from the Forster equation26 setting = 5 A. 6 k_d was calculated from eq 7 setting = 5 Á. D at 20°was obtained from Z) = 2&773?…”

“…Consider an energy transfer process that requires a close encounter, say Renc ~ . The rate constant for formation of (D*, A) pairs is equal to the familiar second-order diffusion constant fcd -4wNDa/l000 (6) where D is the sum of the diffusion coefficients of the donor (Z>D) and of the acceptor (Da)• The probability that energy transfer will occur during an encounter will equal kt(a)j(kt(a) + k~d), where kt(a) is the firstorder rate constant for energy transfer of (D*, A) pairs where fcq is the second-order constant associated with reaction 1.…”

Organic photochemistry. XXVI. Role of steric factors in the quenching of alkylbenzene fluorescence by dienes

“…This relatively small sensitivity to steric factors, in a quenching act which has been viewed as involving intimate contact between partners, seems contradictory and it is made even more interesting in the light of similar observations for short-range exothermic singlet-singlet4 and triplet-triplet3 energy transfer.16 With respect (16) Steric inhibition has been reported for singlet interactions between substituted cyclohexanones and electron deficient olefins6 (a process which appears to have a very specific directional stereoelectronic requirement) and singlet-singlet energy transfer between benzene and aliphatic ketones. 6 The fact that the latter study was made in the gas phase may be of some significance. 4 Froehlich, Morrison / Quenching of Alkylbenzene Fluorescence to the latter report we have done a preliminary study of phosphorescence quenching of several alkylbenzenes using an isopentane medium at 77°K;17 the data are presented in Table IV.…”

“…Quenching of Straight Chain Aromatic Hydrocarbons by cA-Piperylene in Hexane Number of carbon atoms in the chain 6. Average of three determinations.…”

Exciplex photophysics. 7. Steric effects in exciplex photophysics

“…24 Likewise, in the quenching of benzene by acetone and di-tert-butylketone, the ratio of k, between these two ketones was only 1.6 in cyclohexane, whereas it was 4.1 in the gas phase. 25 Since k D m is a linear function of the quencher diffusion coefficient, very accurate diffusion coefficients are clearly required to judge small changes in k,. These do not appear to be available for the quenchers in question.…”