Identification of Photoexcited Singlet Quinones and Their Ultrafast Electron-TransfervsIntersystem-Crossing Rates

Abstract

Photoexcitation of chloranil (CA) produces initially the excited singlet state 1 CA*, as demonstrated for the first time by time-resolved spectroscopy on the femtosecond/picosecond time scale. Electron transfer from aromatic donors (D) to singlet chloranil leads to short-lived (ca. 5 ps) singlet radical-ion pairs, 1[D•+, CA •-]. This ultrafast quenching process competes with intersystem crossing (k ISC ≈ 1011 s-1) to generate the triplet excited state, 3 CA*. The follow-up electron transfer from D to 3 CA* yields triplet radical-ion pairs, which are distinguished from their singlet analogues by their long (nanosecond) lifetimes. The competition between electron transfer and intersystem crossing on the early picosecond time scale also pertains to a wide variety of other photoexcited quinones related to chloranil. Electron transfer to singlet quinone as established here adds a new dimension to the generally accepted mechanisms which proceed from the triplet state, and the inclusion of reactions on both the triplet and the singlet manifolds provides a complete picture of photoinduced electron transfer to various quinone acceptors.

Mechanistic Studies of the Photoinduced Quinone Trimethyl Lock Decaging Process

“…23,24 The present system, however, is significantly perturbed, electronically by the sulfur substituent and geometrically by the bulky trimethyl lock system. The long lifetime of the transient from the flash photolysis studies, and the fact that it is quenched efficiently by oxygen, suggest that the transient is a triplet.…”

“…We have been unable to observe luminescence from 1a, either in fluid media at room temperature or at 77K in a frozen matrix, as is typical of quinones. [23][24][25][26][27][28][29][30] Figure 3. Absorbance spectrum of 1a in methanol.…”

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

“…On the other hand, the one-electron reduction potential of 1 DDQ* is also estimated to be 3.8 V versus SCE from the E red value of 3 DDQ* (3.18 V vs. SCE) [14] and the singlet-triplet energy gap of p-chloranil (0.64 eV). [17] The E ox value of PhNO 2 is significant smaller than that of E red of 1 DDQ*. Thus, electron transfer from PhNO 2 to 1 DDQ* is energetically feasible to generate the radical ion pair.…”

“…The one-electron oxidation potentials of PhCN, PhNO 2 , and PhCF 3 were not reported. However, the ionization potentials of PhCN (9.72 eV), PhNO 2 (9.93 eV), and PhCF 3 (9.70 eV) are significantly higher than the value of benzene (9.23 eV).[16] The singlet excited state of DDQ ( 1 DDQ*) is expected to have a much stronger oxidizing ability than that of 3 DDQ because the singlet-triplet energy gap in the excited state of a p-benzoquinone derivative (p-chloranil) is known to be 0.64 eV.[17] However, the singlet excited state ( 1 DDQ*) is spontaneously relaxed to 3 DDQ* with a lifetime of a few picoseconds, [17] which is too short to be able to oxidize PhCN at low concentrations. If a neat aromatic substrate is used as a solvent, 1 DDQ* would be able to oxidize the substrate.…”

A Mechanistic Study of Photoinduced Electron Transfer from Triplet Erythrosin to Various Quinones Using Time Resolved Absorption and ESR-CIDEP Measurements

“…The triplet state of bromanil, 3 BN, shows absorption in the 440-550 nm range and BN − has an absorption maximum at 450 nm [34]. The lifetime of bromanil triplet 3 BN is given as 2.3 μs [26]. In Figure 5b is the decay profile for the Ery 2− /BN at 470 nm, the decay curve is fitted with the tri-exponential decay having contributions from 3 BN, BN − and Ery − caused by their different time profiles following the first order decay kinetics.…”