Heavy Atom Effect on the Charge Recombination Dynamics of Photogenerated Geminate Ion Pairs

Abstract

The charge recombination dynamics of geminate ion pairs formed by electron transfer quenching of cyanoanthracene derivatives by bromo- and iodo-anisole in acetonitrile was investigated using various ultrafast spectroscopic techniques. Without a heavy atom, the only charge recombination pathway is that leading to the neutral ground state. With heavy atom substituted anisoles, charge recombination to the local triplet state of the excited precursor is observed. Time constants for triplet charge recombination ranging from 400 ps to less than 10 ps, depending on the heavy atom and on the energy gap between the ion pair and the triplet state, have been measured. This heavy atom effect was observed with ion pairs formed upon electron-transfer quenching with driving force going from −0.15 to −0.6 eV, suggesting that these intermediates are in fact exciplexes. A new scheme for producing free ions with a high yield using this effect and a secondary electron donor is also demonstrated

Investigations of bimolecular photoinduced electron transfer reactions in polar solvents using ultrafast spectroscopy

“…4 shows the transient grating spectra measured at various time delays after excitation of 9,10-dicyanoanthracene (DCA) by bromoanisole in ACN [67]. As explained in detail elsewhere [75], a transient grating spectrum is very similar to a transient absorption spectrum, the major difference being that, in the first case, the intensity is always positive.…”

“…Another insight in the nature of the CS product has been obtained by investigating the heavy atom effect on the CR dynamics of ion pairs [67]. As shown by the energy level scheme in Fig.…”

“…Several attempts from our group to observe the normal region for such CR processes proved to be unsuccessful [63,67]. For example, the weakly exergonic CR ( G CR = −0.65 eV) of the ion pair formed upon the highly exergonic quenching ( G CS = −2.17 eV) of tetracyanoanthracene (TCA) by N,N,N N -tetramethyl-p-phenylenediamine (TMPD) is complete after a few picoseconds [63]!…”

Ultrafast excited-state dynamics of strongly coupled porphyrin/core-substituted-naphthalenediimide dyads

“…We suggest that this very small T 1 population is generated upon triplet CR from the CSS. This process, which has been observed in geminate ion pairs with heavy atoms, 59 is possible when the CSS is located between the emitting state and its corresponding T 1 state and in the presence of heavy atoms that scramble the spin state of the CSS. All these conditions are probably fulfilled here with Zn and Br atoms and with a CSS lying very close in energy to the emitting state.…”

Photoinduced Electron Transfer Reactions: From the Elucidation of Old Problems in Bulk Solutions Towards the Exploration of Interfaces

“…[3,9,[12][13][14][15][16][17][18] According to the second hypothesis, highly exergonic CS should yield two ions at remote distance, and thus CR should be relatively slow. Our measurements using a donor/acceptor (D/A) pair with ∆G CS = -2.2 eV reveal that more than 90% of the ion pairs undergo CR in less than 1 ps.…”