Electron‐Transfer Reactions Triggered by Uncharged or Cationic Photosensitizer: Methodology for Generation of o‐Quinodimethane and Analysis of Back Electron‐Transfer Process

Abstract

Photoinduced electron transfer (PET) followed by back electron transfer (BET) reactions of 1,2-bis(a-styryl)benzenes 1,g enerates o-quinodimethane derivative 2 via the corresponding radical cation 2 · + .T his process serves as af acile methodt of orm o-quinodimethane intermediates. The intermediacy of 2 · + and 2 werec onfirmed by using various spectroscopicm ethods and trapping reactions with 3 O 2 and dienophiles. Kinetic analysisu sing nanosecond time-re-solved absorption showed that 2 · + was transformed to 2 via aB ET process, in which the decay of 2 · + and rise of 2 have almostt he same rate constants of k DECAY = 5.6 10 5 s À1 and k RISE = 5.9 10 5 s À1 ,r espectively.T he net BET rate constant evaluated by using Marcus theory is much faster than these experimental values, owing to the reduction based on diffusion.[a] Prof.form (M · + + +A · )a nd thes econdE Ti n( D + +M · + )f orms (D · + + +M). Ther esulting long distance betweenD · + andA · effectivelyp reventsf ormationo f[ D · + //A · ], thereforet he lifetime of D · + is substantially extended.I nt erms of analyzingt he BETp rocess, the most importanti nt hise fforti sB ET between( D · + + +A · ), whichi s formed in theP ET reactionss hown in Scheme 1b and1 c. As showni nS cheme1d, ther atec onstant (k BET )i sd efined fort he BETreactioni nt he encounterc omplex [D · + //A · ] EC that relatest o theassociation anddissociationof(D · + + A · ).Uncharged organic sensitizers, such as 9,10-dicyanoanthracene (DCA, Figure 1) and 1,4-dicyanonaphthalene (DCN), are conventional. Typical cationic sensitizers such as N-methylquinoliniumt etrafluoroborate (NMQ + BF 4 À ), [6] 2,4,6-triphenylpyrylium tetrafluoroborate( TPP + BF 4 À ), and N-methyl-9-mesitylacridinium perchlorate (Mes-Acr + ClO 4 À ,F ukuzumip hotocatalyst 7 ) can promote reactions of free D · + .R ecently,o rganometallic complexes,s uch as Ru(bpy) 3 2 + (Cl À ) 2 and fac-Ir(ppy) 3 ,h ave also been used in organic synthesis. [8] Upon excitation by visible light, these complexes undergo intersystem crossing to form long-lived triplet excited states.T he long lifetime induces ah igh possibility of collision with D, while the triplet state at alow energy level results in relatively low oxidation ability. g-Irradiationi naspecific solvent can also induce ET without any sensitizer and is often used forspectroscopic measurements.Kinetic analysiss uch as Stern-Volmer analysis is ap owerful tool to examine the synthetics tudy on aP ET reaction. When using an on-fluorescents ensitizer,l aser flash photolysis (LFP) provides informationa bout the formation and decay of D · + (Figure 2a). However, information on the recovery of Db yB ET,

Sensitizer-controlled photochemical reactivityviaupconversion of red light

“…In the present study, we targeted oxidative substrate activations from the uorescent singlet excited state of DCA ( 1* DCA), which has a very high oxidation potential of +1.99 V vs. SCE. 40 There have been many previous reports on photocatalytic applications of DCA [41][42][43][44][45][46][47][48][49] and related cyanoarenes, 40 though usually involving direct ultraviolet or blue excitation. Here, we targeted the formation of 1* DCA via upconversion of red light, to accomplish chemical transformations requiring high oxidative potentials, contrasting our recent studies, in which excitation of DCAc − led to thermodynamically challenging reductions under red irradiation.…”

Rates of Ring Opening of Radical Cation Intermediates Govern Differences in Thermoluminescence between 1‐ and 2‐Naphthyl‐Substituted Methylenecyclopropanes

“…TL in this case arises by decay of excited states in the form of fluorescence (FL) or phosphorescence (PH). A significant feature of this phenomenon is that it can readily provide insights into reactivities of radical ions, that play an important role not only in TL but also in photoredox reactions and organic electronic devices …”

“…TL in this case arises by decay of excited states in the form of fluorescence (FL) or phosphorescence (PH). A significant feature of this phenomenon is that it can readily provide insights into reactivities of radical ions, [3,4] that play an important role not only in TL but also in photoredox reactions [5,6] and organic electronic devices. [7][8][9] Previously, we reported that γ- [10,11] or X-ray [12] irradiation of methylcyclohexane (MCH) matrices (77 K) containing 2-aryl-1methylene-2-phenylcyclopropanes 1 (Scheme 1) followed by annealing leads to TL at around 130 K. For example, the 2,2diphenyl derivative 1 a displays vivid green TL.…”