Hydrogen-bonding dynamics in photoinduced electron transfer in a ferrocene-quinone linked dyad with a rigid amide spacer

A newly designed ferrocene-quinone dyad with an amide space (Fc-Q) is employed to examine formation of the hydrogen bonding in the one-electron reduced form (Q*-) and the dynamics in the photoinduced electron-transfer reaction from the ferrocene to the quinone moiety. Photoexcitation of the Q moiety in Fc-Q in deaerated PhCN with 388 nm results in intramolecular electron transfer from Fc to the singlet excited state of Q to produce Fc+-Q*- without changing the conformation (<1 ps), followed by hydrogen bond formation with the amide proton of the spacer (tau = approximately 5 ps). The resulting radical ion pair decays via a back electron transfer to the ground state at a longer time scale with a rate constant of 2.6 x 108 s-1.     

Novel zinc phthalocyanine-benzoquinone rigid dyad and its photoinduced electron transfer properties

While preparing the first structurally rigid zinc phthalocyanine-benzoquinone (ZnPc-BQ) dyad as a model for photoinduced charge separation mimicking natural photosynthesis, a convenient method is developed for in situ generation of a benzoquinone chromophore in the dyad using an iso-butyryl mask. The dyad has no rotamers and possesses a fixed distance between ZnPc and BQ moieties (center-to-center and edge-to-edge distances are 9.40 and 2.14 A, respectively). The dyad displays unusual electronic perturbation in the ground state, resulting from the interactions between Pc and BQ, and exhibits photoinduced electron transfer with a lifetime of 40 ps of the charged separated states. The steady-state fluorescence and electrochemical behavior of the dyad are evaluated. This study opens a route to subsequent dyads, triads, and complex architectures of electron donor-acceptor arrays with rigid structures and long charge separation states.     

Metal ion-promoted intramolecular electron transfer in a ferrocene-naphthoquinone linked dyad. Continuous change in driving force and reorganization energy with metal ion concentration

Thermal intramolecular electron transfer from the ferrocene (Fc) to naphthoquinone (NQ) moiety occurs efficiently by the addition of metal triflates (M(n)()(+): Sc(OTf)(3), Y(OTf)(3), Eu(OTf)(3)) to an acetonitrile solution of a ferrocene-naphthoquinone (Fc-NQ) linked dyad with a flexible methylene and an amide spacer, although no electron transfer takes place in the absence of M(n)()(+). The resulting semiquinone radical anion (NQ(*)(-)) is stabilized by the strong binding of M(n)()(+) with one carbonyl oxygen of NQ(*)(-)( )()as well as hydrogen bonding between the amide proton and the other carbonyl oxygen of NQ(*)(-). The high stability of the Fc(+)()-NQ(*)(-)/M(n)()(+)() complex allows us to determine the driving force of electron transfer by the conventional electrochemical method. The one-electron reduction potential of the NQ moiety of Fc-NQ is shifted to a positive direction with increasing concentration of M(n)()(+), obeying the Nernst equation, whereas the one-electron oxidation potential of the Fc moiety remains the same. The driving force dependence of the observed rate constant (k(ET)) of M(n)()(+)-promoted intramolecular electron transfer is well evaluated in light of the Marcus theory of electron transfer. The driving force of electron transfer increases with increasing concentration of M(n)()(+) [M(n)()(+)], whereas the reorganization energy of electron transfer decreases with increasing [M(n)()(+)] from a large value which results from the strong binding between NQ(*)(-) and M(n)()(+).     

Photoinduced intramolecular electron transfer and triplet energy transfer in a steroid-linked norbornadiene-carbazole dyad

Bichromophoric compound 3 beta-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17 beta-yl-[2-(N-carbazolyl)acetate] (NBD-S-CZ) was synthesized and its photochemistry was examined by fluorescence quenching, flash photolysis, and chemically induced dynamic nuclear polarization (CIDNP) methods. Fluorescence quenching measurements show that intramolecular electron transfer from the singlet excited state of the carbazole to the norbornadiene group in NBD-S-CZ occurs with an efficiency (Phi SET) of about 14 % and rate constant (kSET) of about 1.6 x 10(7) s-1. Phosphorescence and flash photolysis studies reveal that intramolecular triplet energy transfer and electron transfer from the triplet carbazole to the norbornadiene group proceed with an efficiency (TET + TT) of about 52 % and rate constant (kTET + kTT) of about 3.3 x 10(5) s-1. Upon selective excitation of the carbazole chromophore, nuclear polarization is detected for protons of the norbornadiene group (emission) and its quadricyclane isomer (enhanced absorption); this suggests that the isomerization of the norbornadiene group to the quadricyclane proceeds by a radical-ion pair recombination mechanism in addition to intramolecular triplet sensitization. The long-distance intramolecular triplet energy transfer and electron transfers starting both from the singlet and triplet excited states are proposed to proceed by a through-bond mechanism.     

An yttrium ion-selective fluorescence sensor based on metal ion-controlled photoinduced electron transfer in zinc porphyrin-quinone dyad

A highly Y3+-selective fluorescence sensor has been developed using zinc porphyrin-CONH-quinone dyad (ZnP-CONH-Q). The selective binding of the Q moiety of ZnP-CONH-Q with Y3+ retards electron transfer from the singlet excited state (1ZnP*) to Q, leading to a remarkable enhancement of the fluorescence intensity.     

Intra- vs intermolecular photoinduced electron transfer reactions of a macrocyclic donor-acceptor dyad

The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraazacyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2]2+ (Chart 1). By contrast, Stern-Volmer analysis of intermolecular quenching of [ZnL2]2+ by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted.     

Ultrafast photoinduced intramolecular charge separation and recombination processes in the oligothiophene-substituted benzene dyads with an amide spacer

Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.     

Solvent mediated intramolecular photoinduced electron transfer in a fluorene-perylene bisimide derivative

Photoinduced electron transfer from fluorene to perylene bisimide has been studied for 2,7-bis(N-(1-hexylheptyl)-3,4:9,10-perylene-bisimide-N'-yl))-9,9-didodecylfluorene (PFP) in 11 different organic solvents. The intramolecular charge-separated state in PFP is almost isoenergetic with the locally excited state of the perylene bisimide. As a consequence of the small change in free energy for charge separation, the electron transfer rate strongly depends on subtle changes in the medium. The rate constant k(CS) for the electron transfer from fluorene to perylene bisimide moiety in the excited state varies over more than 2 orders of magnitude ( approximately 10(8)-10(10) s(-1)) with the solvent but does not show the familiar increase with polarity. The widely differing rate constants can be successfully explained by considering (1) the contribution of the polarization energy of the dipole moment in the transition state and by (2) the classical Marcus-Jortner model and assuming a spherical cavity for the charge-separated state. Using the first model, we show that lnk(CS) should vary linearly with Deltaf [Deltaf = (epsilon(r) - 1)/(2epsilon(r) + 1) - (n(2) - 1)/(2n(2) + 1), where epsilon(r) and n represent the static dielectric constant and the refractive index of the solvent, respectively], in accordance with experimental results. The second model, where the reorganization energy scales linearly with Deltaf, provides quantitative agreement with experimental rate constants within a factor of 2.     

Binaphthalene with substituted tetrathiafulvalene and trichloroquinone: a new example of metal ion-promoted electron transfer

A new binaphthalene (R)-1 with two substituted TTF and trichloroquinone units is reported. Both absorption and ESR spectral studies show that electron transfer occurs between TTF and trichloroquinone units of (R)-1 in the presence of metal ions (Pb²⁺, Sc³⁺, Zn²⁺, and Ca²⁺). We also propose a possible mechanism for this electron transfer process. But, the CD spectral change of (R)-1 in the presence of metal ions is rather small.     

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.     

芘-对二氰基乙烯基苯的光诱导分子内电子转移及其盐效应的研究

本工作设计并合成了一系列以不同链长相连的芘-对二氰基乙烯基苯(Py-DCVB)化合物, 利用紫外可见吸收光谱、荧光光谱、核磁共振谱研究了Py-DCVB的光诱导分子内电子转移激基复合物的形成与基态构象的关系, 并且利用激光闪光光解开展了盐效应的研究, 瞬态吸收光谱的结果证实了Py-DCVB分子内光诱导电子转移反应经历激基复合物的中间过程。     

Temperature dependence of the rate of photoinduced intramolecular electron transfer in a Zn-porphyrin-quinone system

The U-shaped dependence of the rate constant of photoinduced intramolecular electron transfer vs. temperature observed for zinc-porphyrin-bridge-quinone donor-acceptor system in a 2-methyltetrahydrofuran solution is fitted using the stochastic multichannel model. The mechanism of this behavior is revealed. It is found that the drop in the rate upon the decline in temperature over the range of 320 to 125 K is related to two factors: the dynamic effect of the solvent (the increased relaxation time of solvent) and the low activation barrier. The substantial growth of the rate constant with the temperature drop in the range of 125 to 80 K is due to suppression of the dynamic effect of the solvent and a simultaneous lowering of the activation barrier due to the increased exergonicity of the reaction.     

Control of photoinduced electron transfer in zinc phthalocyanine-perylenediimide dyad and triad by the magnesium ion

Photoexcitation of a zinc phthalocyanine-perylenediimide (ZnPc-PDI) dyad and a bis(zinc phthalocyanine)-perylenediimide [(ZnPc) 2-PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg (2+) ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc (*+)-PDI (*-)/Mg (2+) and (ZnPc) 2 (*+)-PDI (*-)/Mg (2+)) in which PDI (*-) forms a complex with Mg (2+). Formation of the CS states in the presence of Mg (2+) was confirmed by appearance of the absorption bands due to ZnPc (*+) and PDI (*-)/Mg (2+) complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( E red) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg (2+) to PDI (*-), whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg (2+) to PDI (*-) was confirmed by the ESR spectrum, which is different from that of PDI (*-) without Mg (2+). The energy of the CS state (ZnPc (*+)-PDI (*-)/Mg (2+)) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc- (3)PDI*: 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg (2+) instead of the triplet excited state of the PDI moiety.     

Long-lived charge-separated state produced by photoinduced electron transfer in a zinc imidazoporphyrin-C(60) dyad

[reaction: see text] Photoexcitation of a zinc imidazoporphyrin-fullerene dyad with a short linkage results in formation of the charge-separated state by photoinduced electron transfer. The charge-separated state has a lifetime of 310 micros in benzonitrile at 278 K, which is the longest lifetime in solution ever reported for electron donor-acceptor-linked dyads.     

The role of hydrogen bonding in excited state intramolecular charge transfer

Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.     

Mechanism and energetics of intramolecular hydrogen transfer in amide and peptide radicals and cation-radicals

Intramolecular hydrogen transfer in five model amide and peptide radicals and cation-radicals was investigated by combined B3LYP-MP2 calculations. Hypervalent ammonium radicals produced by electron capture in protonated peptides undergo competitive elimination of ammonia, H-atom loss, and H-atom migration to neighboring amide carbonyls. The calculated transition state energies for H-atom migration are slightly but uniformly lower than those for H-atom loss. Transition state theory calculations with inclusion of quantum tunneling effects predict k(H migration)/k(H loss) branching ratios that increase with the ring size of the cyclic transition state for the migration. Intramolecular hydrogen-atom migration in amide and peptide radicals can be described by the proton-coupled electron transfer mechanism. The migrating hydrogen atom shows a negligible spin density and substantial positive charge that are typical of a proton migration. Electron transfer occurs through a pi-orbital system and proceeds in the same (clockwise) or opposite (counterclockwise) direction as the proton motion, depending on the electronic properties of the chain connecting the ammonium group and the amide bond.     

Site-selective photoinduced electron transfer from alcoholic solvents to the chromophore facilitated by hydrogen bonding: a new fluorescence quenching mechanism

Solute-solvent intermolecular photoinduced electron transfer (ET) reaction was proposed to account for the drastic fluorescence quenching behaviors of oxazine 750 (OX750) chromophore in protic alcoholic solvents. According to our theoretical calculations for the hydrogen-bonded OX750-(alcohol)(n) complexes using the time-dependent density functional theory (TDDFT) method, we demonstrated that the ET reaction takes place from the alcoholic solvents to the chromophore and the intermolecular ET passing through the site-specific intermolecular hydrogen bonds exhibits an unambiguous site selectivity. In our motivated experiments of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD), it could be noted that the ultrafast ET reaction takes place as fast as 200 fs. This ultrafast intermolecular photoinduced ET is much faster than the diffusive solvation process, and even significantly faster than the intramolecular vibrational redistribution (IVR) process of the OX750 chromophore. Therefore, the ultrafast intermolecular ET should be coupled with the hydrogen-bonding dynamics occurring in the sub-picosecond time domain. We theoretically demonstrated for the first time that the selected hydrogen bonds are transiently strengthened in the excited states for facilitating the ultrafast solute-solvent intermolecular ET reaction.     

Dependence of amide vibrations on hydrogen bonding

The effect of hydrogen bonding on the amide group vibrational spectra has traditionally been rationalized by invoking a resonance model where hydrogen bonding impacts the amide functional group by stabilizing its [(-)O-C=NH (+)] structure over the [O=C-NH] structure. However, Triggs and Valentini's UV-Raman study of solvation and hydrogen bonding effects on epsilon-caprolactum, N, N-dimethylacetamide (DMA), and N-methylacetamide (NMA) ( Triggs, N. E.; Valentini, J. J. J. Phys. Chem. 1992, 96, 6922-6931) casts doubt on the validity of this model by demonstrating that, contrary to the resonance model prediction, carbonyl hydrogen bonding does not impact the AmII' frequency of DMA. In this study, we utilize density functional theory (DFT) calculations to examine the impact of hydrogen bonding on the C=O and N-H functional groups of NMA, which is typically used as a simple model of the peptide bond. Our calculations indicate that, as expected, the hydrogen bonding frequency dependence of the AmI vibration predominantly derives from the C=O group, whereas the hydrogen bonding frequency dependence of the AmII vibration primarily derives from N-H hydrogen bonding. In contrast, the hydrogen bonding dependence of the conformation-sensitive AmIII band derives equally from both C=O and N-H groups and thus, is equally responsive to hydrogen bonding at the C=O or N-H site. Our work shows that a clear understanding of the normal mode composition of the amide vibrations is crucial for an accurate interpretation of the hydrogen bonding dependence of amide vibrational frequencies.     

Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene

Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium triflate [Sc(OTf)(3)] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)(3). The rate constants of the Sc(OTf)(3)-promoted photoinduced electron-transfer reactions (k(et)) of acridine to afford the complex between acridine radical anion and Sc(OTf)(3) remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)(3). In contrast to the case of acridine, the k(et) value of the Sc(OTf)(3)-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)(3) to exhibit first-order dependence on [Sc(OTf)(3)] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k(et) on [Sc(OTf)(3)] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)(3). The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)(3) as compared to those in the absence of Sc(OTf)(3) have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)(3)-promoted photoinduced electron transfer from hexamethylbenzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O(2)-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)(3). The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.