Fluorescence competition with

Chemical reactions can be studied at the single-molecule level by measuring the fluorescence lifetime of an excited state that can undergo reaction in competition with fluorescence. Reactions involving electron transfer (section C3.2) are among the most accessible via such teclmiques, and are particularly attractive candidates for study as a means of testing relationships between charge-transfer optical spectra and electron-transfer rates. If the physical parameters that detennine the reaction probability, such as overlap between the donor and acceptor orbitals. 

Laser-induced fluorescence (LIF). Laser-induced fluorescence measurements have been applied to the atmosphere since the suggestion of Baardsen and Ter-hune in 1972 that this method should be feasible. Figure 11.43 shows the energy levels and transitions involved in LIF measurements. OH is excited from its ground X2n state into the first electronically excited A22 state. The v" = 0 to r = 0 transition is around 308 nm and the v" = 0 to v = 1 at 282 nm. Two schemes have been used excitation using 282 nm into v = 1 of the upper electronic state, or excitation using 308 nm into v = 0 of the upper state. Collisional quenching deactivates some of the v = 1 into u = 0 in competition with fluorescence, mainly in the (1,1) band of the electronic transition (that is, from v = 1 of the upper state into v" =1 of the lower state). Collisional deactivation of v = 0 then occurs in competition with fluorescence in the (0,0) band at 308 nm... 

The (1,4) substituted naphthalenophanes undergo [4 + 4] photocycloaddition when irradiated at X > 280 nm, in addition to fluorescence. This photoreaction is competitive with fluorescence, and requires a conformational change that can be suppressed at low temperature 93). The few reports of the lifetime or quantum yield of naphthalenophane fluorescence indicate the effects of photocycloaddition. For the anti-[2.2](1,4) isomer, kpu/ku = 0.021 in cyclohexane 93) the lifetime of syn-[3.3](l,4) naphthalenophane fluorescence was given as 15.3 ns107). Both values are low relative to the naphthalene solution excimer (kpu/kjj 0.2 xD 80 ns 71)), and this may be due in part to the photoreaction of the (1,4) naphthalenophanes. 

Formation of eaqfrom Excited States Corresponding to Internal Molecular Transition. Competition with Fluorescence... 

At temperatures above fm (Section IV.B.4), where the viscosity of the medium is relatively low, internal conversion, not contributing to trans - cis isomerization, plays no significant role since the sum of internal rotation, t - p, leading to trans- cis isomerization in competition with fluorescence. 

Double Activated Mechanism. For 4,4 -NDS in toluene (as an example of a solvent within a certain polarity range), a new double activated mechanism for trans - cis photoisomerization has been suggested [152,159], The two consecutive activated steps are (1) population of an excited state (A ), having strong CT character and essentially transoid geometry, in competition with fluorescence, and (2) rotation about the C=C double bond to a twisted excited state (probably p ). At high temperatures the cis form is formed via the sequence... 

A common form of coupling is spin-orbit coupling. As the strength of the spin-orbit coupling increases, with increasing mass of the atoms involved, so too does the predissociation rate (i.e. the rate at which product atoms are formed). For predissociation rates of the order 10 s there is strong competition with fluorescence the fluorescence yield drops to very low values (often < 1 per cent). In extreme cases the molecule dissociates directly ( 10 s), and no fluorescence is observed. 

The study of compounds such as 1 has provided much information about the design of photochemical electron transfer systems based on biological molecules. Specifically, the roles of the linkage and the thermodynamics in making electron transfer competitive with fluorescence, intersystem crossing, and internal conversion are now qualitatively understood. However, a dyad system cannot have a branch point at an intermediate state in the electron transfer path and therefore cannot be used to study the competition between forward and recombination electron transfer. Also, the dyad system fails to mimic the long lifetimes of hundreds of milliseconds characteristic of... 

Modern experimental measurements and the new computational techniques just discussed are now providing results that can rationalize issues such as the efficiency of 1C at a surface crossing, the competition with fluorescence when an excited state barrier is present, and the relationship between the molecular structure at the intersection and the structure of the photoproducts. Experiments on isolated molecules in cold-matrices or expanding-jets have revealed the presence of thermally activated fast radiationless decay channels. For example, Christensen et al. have proposed that (under isolated conditions in a cool-jet) trans — cis motion in all-tra 5-octa-1.3,5,7-tetraene (all-trow -OT) induces the opening of an efficient nonadiabatic radiationless deactivation channel on Si (2Ag). We now discuss this experiment and complementary theoretical results that illustrate the way in which theory and experiment can be used in concert. 

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