Structure and Dynamics of Photogenerated Triplet Radical Ion Pairs in DNA Hairpin Conjugates with Anthraquinone End Caps

Charge Transfer and Intersystem Crossing in Compact Naphthalenediimide-Phenothiazine Triads: Synthesis and Study of the Photophysical Property with Transient Optical and Electron Paramagnetic Resonance Spectroscopic Methods

In order to obtain a long-lived charge separation (CS) state in compact electron donor-acceptor molecular systems, we prepared a series of naphthalenediimide (NDI)-phenothiazine (PTZ) triads, with phenylene as the linker between the donor and acceptor. Conformation restriction is imposed to control the mutual orientation of the NDI and PTZ units by attaching methyl groups on the phenylene linker to tune the electronic coupling between the donor and the acceptor. Moreover, the PTZ moiety was oxidized to sulfoxide to tune the ordering of the CS state and the 3LE state (LE: locally excited state). UV-vis absorption spectra indicate electronic coupling between NDI with the phenylene linker as well as the PTZ units, manifested by the appearance of a charge-transfer (CT) absorption band, whereas this coupling is devoid in the triads with conformation restriction imposed. Fluorescence is strongly quenched in the triads compared to the reference compound, indicating electron transfer upon photoexcitation. Femtosecond transient absorption spectra indicate that the CS takes 0.8 ps, and then the 3LE state is formed by charge recombination in 83 ps. Nanosecond transient absorption (ns-TA) spectra show that the 3NDI state was observed in nonpolar solvents such as cyclohexane (triplet state lifetime: 95.7 μs), whereas the CS state was observed in more polar solvents. The CS state lifetimes are up to 1.2 μs (in toluene). Time-resolved electron paramagnetic resonance spectra of the triads in toluene consist of two types of signals: CS states (narrower signals, ∼10 mT) and 3LE states (broader signals, ∼50 to 200 mT). In the spectra of the triads containing PTZ, the CS state signals dominate, whereas for the triads containing oxidized PTZ, the 3NDI signals (zero-field splitting D ≈ 2000 MHz) prevail, both observations being in agreement with the ns-TA spectral studies. The electron spin polarization phase pattern of the 3NDI states of the triads indicates that the intersystem crossing (ISC) mechanism is spin-orbit charge-transfer ISC. Considering the 3CS state as ion pairs, the electron-exchange energy (J) is determined to be -39 to -59 MHz, and the electron spin dipolar interaction is 83-92 MHz.

An anthraquinone-based bismuth-iron metal-organic framework as an efficient photoanode in photoelectrochemical cells

Metal-organic frameworks (MOFs) are appealing candidate materials to design new photoelectrodes for use in solar energy conversion because of their modular nature and chemical versatility. However, to date there are few examples of MOFs that can be directly used as photoelectrodes, for which they must be able to afford charge separation upon light absorption, and promote the catalytic dissociation of water molecules, while maintaining structural integrity. Here, we have explored the use of the organic linker anthraquinone-2, 6-disulfonate (2, 6-AQDS) for the preparation of MOFs to be used as photoanodes. Thus, the reaction of 2, 6-AQDS with Bi(iii) or a combination of Bi(iii) and Fe(iii) resulted in two new MOFs, BiPF-10 and BiFePF-15, respectively. They display similar structural features, where the metal elements are disposed in inorganic-layer building units, which are pillared by the organic linkers by coordination bonds through the sulfonic acid groups. We show that the introduction of iron in the structure plays a crucial role for the practical use of the MOFs as a robust photoelectrode in a photoelectrochemical cell, producing as much as 1.23 mmol H2 cm-2 with the use of BiFePF-15 as photoanode. By means of time-resolved and electrochemical impedance spectroscopic studies we have been able to unravel the charge transfer mechanism, which involves the formation of a radical intermediate species, exhibiting a longer-lived lifetime by the presence of the iron-oxo clusters in BiFePF-15 to reduce the charge transfer resistance.

The effect of thionation of the carbonyl group on the photophysics of compact spiro rhodamine-naphthalimide electron donor-acceptor dyads: intersystem crossing, charge separation, and electron spin dynamics

Herein, a spiro rhodamine (Rho)-thionated naphthalimide (NIS) electron donor-acceptor orthogonal dyad (Rho-NIS) was prepared to study the formation of a long-lived charge separation (CS) state via the electron spin control approach. The transient absorption (TA) spectra of Rho-NIS indicated that the intersystem crossing (ISC) occurs within 7-42 ps to produce the 3NIS state via the spin orbit coupling ISC (SOC-ISC). The energy order of 3CS (2.01 eV in n-hexane, HEX) and 3LE states (1.68 eV in HEX) depended on the solvent polarity. The 3NIS state having n-π* character and a lifetime of 0.38 μs was observed for Rho-NIS in toluene (TOL). Alternatively, in acetonitrile (ACN), the long-lived 3CS state (0.21 μs) with a high CS state quantum yield (ΦCS, 97%) was produced with the 3NIS state as the precursor and the CS took 134 ps. On the contrary, in the case of the reference Rho-naphthalimide (NI) Rho-NI dyad without thionation of its carbonyl group, a long-lived CS state (0.94 μs) with a high energy level (ECS = 2.12 eV) was generated even in HEX with a lower ΦCS (49%). In the presence of an acid, the Rho unit in the Rho-NIS adopted an open form (Rho-o) and the 3NIS state was produced within 24-47 ps with the 1Rho-o state as the precursor. Subsequently, slow intramolecular triplet-triplet energy transfer (TTET, 0.11-0.60 μs) produced the 3Rho-o state (9.4-13.6 μs). According to the time-resolved electron paramagnetic resonance (TREPR) spectra of NIS-NH2, the zero-field splitting (ZFS) parameter |D| and E of the triplet state were determined to be 6165 MHz and -1233 MHz, respectively, indicating that its triplet state has significant nπ* character, which was supported by its short triplet state lifetime (6.1 μs).

Direct observation of chirality-induced spin selectivity in electron donor-acceptor molecules

The role of chirality in determining the spin dynamics of photoinduced electron transfer in donor-acceptor molecules remains an open question. Although chirality-induced spin selectivity (CISS) has been demonstrated in molecules bound to substrates, experimental information about whether this process influences spin dynamics in the molecules themselves is lacking. Here we used time-resolved electron paramagnetic resonance spectroscopy to show that CISS strongly influences the spin dynamics of isolated covalent donor-chiral bridge-acceptor (D-Bχ-A) molecules in which selective photoexcitation of D is followed by two rapid, sequential electron-transfer events to yield D•+-Bχ-A•-. Exploiting this phenomenon affords the possibility of using chiral molecular building blocks to control electron spin states in quantum information applications.

Long-Lived Charge-Separated State in Naphthalimide-Phenothiazine Compact Electron Donor-Acceptor Dyads: Effect of Molecular Conformation Restriction and Solvent Polarity

To study the charge separation (CS) and long-lived CS state, we prepared a series of dyads based on naphthalimide (NI, electron acceptor) and phenothiazine (PTZ, electron donor), with an intervening phenyl linker attached on the N-position of both moieties. The purpose is to exploit the electron spin control effect to prolong the CS-state lifetime by formation of the 3CS state, instead of the ordinary 1CS state, the spin-correlated radical pair (SCRP), or the free ion pairs. The electronic coupling magnitude is tuned by conformational restriction exerted by the methyl groups on the phenyl linker. Differently from the previously reported NI-PTZ analogues containing long and flexible linkers, we observed a significant CS emission band centered at ca. 600 nm and thermally activated delayed fluorescence (TADF) with a lifetime of 13.8 ns (population ratio: 42%)/321.6 μs (56%). Nanosecond transient absorption spectroscopy indicates that in cyclohexane (CHX), only the 3NI* state was observed (lifetime τ = 274.7 μs), in acetonitrile (ACN), only the CS state was observed (τ = 1.4 μs), whereas in a solvent with intermediate polarity, such as toluene (TOL), both the 3NI* (shorter-lived) and the CS states were observed. Observation of the long-lived CS state in ACN, yet lack of TADF, confirms the spin-vibronic coupling theoretical model of TADF. Femtosecond transient absorption spectroscopy indicates that charge separation occurs in both nonpolar and polar solvents, with time constants ranging from less than 1 ps in ACN to ca. 60 ps in CHX. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of the 3NI* and CS states for the dyads upon photoexcitation. The electron spin-spin dipole interaction magnitude of the radical anion and cation of the CS state is intermediate between that of a typical SCRP and a 3CS state, suggesting that the long CS-state lifetime is partially due to the electron spin control effect.

Nonpolymer Organic Solar Cells: Microscopic Phonon Control to Suppress Nonradiative Voltage Loss via Charge-Separated State

Recent remarkable developments on nonfullerene solar cells have reached a photoelectric conversion efficiency (PCE) of 18% by tuning the band energy levels in small molecular acceptors. In this regard, understanding the impact of small donor molecules on nonpolymer solar cells is essential. Here, we systematically investigated mechanisms of solar cell performance using diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP) conjugates of C4-DPP-H₂BP and C4-DPP-ZnBP, where C4 represents the butyl group substituted at the DPP unit as small p-type molecules, while an acceptor of [6,6]-phenyl-C₆₁-buthylic acid methyl ester is employed. We clarified the microscopic origins of the photocarrier caused by phonon-assisted one-dimensional (1D) electron-hole dissociations at the donor-acceptor interface. Using a time-resolved electron paramagnetic resonance, we have characterized controlled charge-recombination by manipulating disorders in π-π donor stacking. This ensures carrier transport through stacking molecular conformations to suppress nonradiative voltage loss capturing specific interfacial radical pairs separated by 1.8 nm in bulk-heterojunction solar cells. We show that, while disordered lattice motions by the π-π stackings via zinc ligation are essential to enhance the entropy for charge dissociations at the interface, too much ordered crystallinity causes the backscattering phonon to reduce the open-circuit voltage by geminate charge-recombination.

A long-lived charge-separated state of spiro compact electron donor-acceptor dyads based on rhodamine and naphthalenediimide chromophores

Spiro rhodamine (Rho)-naphthalenediimide (NDI) electron donor-acceptor orthogonal dyads were prepared to generate a long-lived charge separation (CS) state based on the electron spin control approach, i.e. to form the ³CS state, not the ¹CS state, to prolong the CS state lifetime by the electron spin forbidden feature of the charge recombination process of ³CS → S₀. The electron donor Rho (lactam form) is attached via three σ bonds, including two C-C and one N-N bonds (Rho-NDI), or an intervening phenylene, to the electron acceptor NDI (Rho-Ph-NDI and Rho-PhMe-NDI). Transient absorption (TA) spectra show that fast intersystem crossing (ISC) (<120 fs) occurred to generate an upper triplet state localized on the NDI moiety (³NDI*), and then to form the CS state. For Rho-NDI in both non-polar and polar solvents, a long-lived ³CS state (lifetime τ = 0.13 μs) and charge separation quantum yield (Φ _CS) up to 25% were observed, whereas for Rho-Ph-NDI (τ _T = 1.1 μs) and Rho-PhMe-NDI (τ _T = 2.0 μs), a low-lying ³NDI* state was formed by charge recombination (CR) in n-hexane (HEX). In toluene (TOL), however, CS states were observed for Rho-Ph-NDI (0.37 μs) and Rho-PhMe-NDI (0.63 μs). With electron paramagnetic resonance (EPR) spectra, weak electronic coupling between the Rho and NDI moieties for Rho-NDI was proved. Time-resolved EPR (TREPR) spectra detected two transient species including NDI-localized triplets (formed via SOC-ISC) and a ³CS state. The CS state of Rho-NDI features the largest dipolar interaction (|D| = 184 MHz) compared to Rho-Ph-NDI (|D| = 39 MHz) and Rho-PhMe-NDI (|D| = 41 MHz) due to the smallest distance between Rho and NDI moieties. For Rho-NDI, the time-dependent e,a → a,e phase change of the CS state TREPR spectrum indicates that the long-lived CS state is based on the electron spin control effect.

A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.

Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science

More than a half century ago, the NMR spectra of diamagnetic products resulting from radical pair reactions were observed to have strongly enhanced absorptive and emissive resonances. At the same time, photogenerated radical pairs were discovered to exhibit unusual electron paramagnetic resonance spectra that also had such resonances. These non-Boltzmann, spin-polarized spectra were observed in both chemical systems as well as in photosynthetic reaction center proteins following photodriven charge separation. Subsequent studies of these phenomena led to a variety of chemical electron donor-acceptor model systems that provided a broad understanding of the spin dynamics responsible for these spectra. When the distance between the two radicals is restricted, these observations result from the formation of spin-correlated radical pairs (SCRPs) in which the spin-spin exchange and dipolar interactions between the two unpaired spins play an important role in the spin dynamics. Early on, it was recognized that SCRPs photogenerated by ultrafast electron transfer are entangled spin pairs created in a well-defined spin state. These SCRPs can serve as spin qubit pairs (SQPs), whose spin dynamics can be manipulated to study a wide variety of quantum phenomena intrinsic to the field of quantum information science. This Perspective highlights the role of SCRPs as SQPs, gives examples of possible quantum manipulations using SQPs, and provides some thoughts on future directions.

Interaction of Photogenerated Spin Qubit Pairs with a Third Electron Spin in DNA Hairpins

The designing of tunable molecular systems that can host spin qubits is a promising strategy for advancing the development of quantum information science (QIS) applications. Photogenerated radical pairs are good spin qubit pair (SQP) candidates because they can be initialized in a pure quantum state that exhibits relatively long coherence times. DNA is a well-studied molecular system that allows for control of energetics and spatial specificity through careful design and thus serves as a tunable scaffold on which to control multispin interactions. Here, we examine a series of DNA hairpins that use naphthalenediimide (NDI) as the hairpin linker. Photoexcitation of the NDI leads to subnanosecond oxidation of guanine (G) within the duplex or a stilbenediether (Sd) end-cap to give NDI^•--G^•+ or NDI^•--Sd^•+ SQPs, respectively. A 2,2,6,6-tetramethylpiperdinyl-1-oxyl (TEMPO) stable radical is covalently attached to the hairpin at varying distances from the SQP spins. While TEMPO has a minimal effect on the SQP formation and decay dynamics, EPR spectroscopy indicates that there are significant spin-spin dipolar interactions between the SQP and TEMPO. We also demonstrate the ability to implement more complex spin manipulations of the NDI^•--Sd^•+-TEMPO system using pulse-EPR techniques, which is important for developing DNA hairpins for QIS applications.

The effect of one-atom substitution on the photophysical properties and electron spin polarization: Intersystem crossing of compact orthogonal perylene/phenoxazine electron donor/acceptor dyad

A perylene (Pery)-phenoxazine (PXZ) compact orthogonal electron donor/acceptor dyad was prepared to study the relationship between the molecular structures and the spin-orbit charge transfer intersystem crossing (SOCT-ISC), as well as the electron spin selectivity of the ISC process. The geometry of Pery-PXZ (80.0°) is different from the previously reported perylene-phenothiazine dyad (Pery-PTZ, 91.5°), although there is only one atom variation for the two dyads. Pery-PXZ shows a high singlet oxygen quantum yield (84%). Femtosecond transient absorption spectra indicate that the charge separation (CS, faster than 120 fs) is faster than the Pery-PTZ analog (CS, 250 fs) and charge recombination (CR, i.e., SOCT-ISC, 5.98 ns) of Pery-PXZ is slower than the Pery-PTZ analog (CR, 0.9 ns). The intrinsic triplet state lifetime of Pery-PXZ is 242 µs vs the lifetime of 181 µs for the Pery-PTZ analog. Moreover, the triplet state lifetime of Pery-PXZ in the solid polymer matrix is extended to 4.45 ms, which indicates that the triplet state of Pery-PXZ in fluid solution is deactivated not only by the triplet-triplet annihilation effect but also by other factors such as vibration coupled relaxation. Interestingly, with pulsed laser excited time-resolved electron paramagnetic resonance spectroscopy, the electron spin polarization (ESP) pattern of the triplet state of the current dyad is opposite to that of Pery-PTZ. These results demonstrated the rich electron spin chemistry of the ISC of compact electron donor/acceptor dyads, e.g., the ESP is dependent on not only the molecular geometry but also the structure of the electron donor (or acceptor).

Excitation wavelength dependent ICT character and ISC efficiency in a photocleavage agent of 1-aminoanthraquinone

Anthraquinone derivatives have been widely used as photocleavage agents and dyes. Here, photoinduced excited state intramolecular charge transfer (ICT) dynamics of 1-aminoanthraquinone in ethanol are studied by femtosecond transient absorption (fs-TA) spectroscopy and quantum chemical (QC) calculations. Four decay associated difference spectra and corresponding lifetime components were obtained by singular value decomposition and global fitting analysis from the fs-TA spectra. The QC calculations prove that the S₁ state has obvious ICT character. Planar ICT (PICT) and twisted ICT (TICT) reaction coordinates are observed. On the PICT coordinate, vibrational relaxation (VR) and intersystem crossing (ISC) processes are observed. The ISC efficiency is dependent on excitation wavelength, which elucidates that T'₂ state participates in the ISC process on a higher-level than the S'₁ state. On the TICT coordinate, the TICT process is dependent on excitation wavelength, which elucidates a direct experimental evidence of an energy barrier in excited state TICT potential energy.

Long-Lived Charge-Transfer State Induced by Spin-Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in a Compact Spiro Electron Donor/Acceptor Dyad

We prepared conceptually novel, fully rigid, spiro compact electron donor (Rhodamine B, lactam form, RB)/acceptor (naphthalimide; NI) orthogonal dyad to attain the long‐lived triplet charge‐transfer (³CT) state, based on the electron spin control using spin‐orbit charge transfer intersystem crossing (SOCT‐ISC). Transient absorption (TA) spectra indicate the first charge separation (CS) takes place within 2.5 ps, subsequent SOCT‐ISC takes 8 ns to produce the ³NI* state. Then the slow secondary CS (125 ns) gives the long‐lived ³CT state (0.94 μs in deaerated n‐hexane) with high energy level (ca. 2.12 eV). The cascade photophysical processes of the dyad upon photoexcitation are summarized as ¹NI*→¹CT→³NI*→³CT. With time‐resolved electron paramagnetic resonance (TREPR) spectra, an EEEAAA electron‐spin polarization pattern was observed for the naphthalimide‐localized triplet state. Our spiro compact dyad structure and the electron spin‐control approach is different to previous methods for which invoking transition‐metal coordination or chromophores with intrinsic ISC ability is mandatory.

Selectively Addressable Photogenerated Spin Qubit Pairs in DNA Hairpins

Photoinduced electron transfer can produce radical pairs having two quantum entangled electron spins that can act as spin qubits in quantum information applications. Manipulation of these spin qubits requires selective addressing of each spin using microwave pulses. In this work, photogenerated spin qubit pairs are prepared within chromophore-modified DNA hairpins with varying spin qubit distances, and are probed using transient EPR spectroscopy. By performing pulse-EPR measurements on the shortest hairpin, selective addressing of each spin qubit comprising the pair is demonstrated. Furthermore, these spin qubit pairs have coherence times of more than 4 μs, which provides a comfortable time window for performing complex spin manipulations for quantum information applications. The applicability of these DNA-based photogenerated two-qubit systems is discussed in the context of quantum gate operations, specifically the controlled-NOT gate.

Photo-induced spin switching in a modified anthraquinone modulated by DNA binding

An anthraquinone modified with a nitroxide radical and able to intercalate into DNA has been synthesized to obtain a molecule the spin state of which can be manipulated by visible light and DNA binding. The doublet ground state of the molecule can be photo-switched to either a strongly coupled spin state (quartet + doublet), when isolated, or to an uncoupled spin state (triplet and doublet), when bound to DNA. The different spin state that is obtained upon photoexcitation depends on the intercalation of the quinonic core into double-stranded DNA which changes the conformation of the molecule, thereby altering the exchange interaction between the excited state localized on the quinonic core and the nitroxide radical. The spin state of the system has been investigated using both continuous-wave and time-resolved EPR spectroscopy.

Ultrafast investigation of photoinduced charge transfer in aminoanthraquinone pharmaceutical product

We investigated the mechanism of intramolecular charge transfer and the following radiationless dynamics of the excited states of 1-aminoanthraquinone using steady state and time-resolved absorption spectroscopy combined with quantum chemical calculations. Following photoexcitation with 460 nm, conformational relaxation via twisting of the amino group, charge transfer and the intersystem crossing (ISC) processes have been established to be the major relaxation pathways responsible for the ultrafast nonradiative of the excited S₁ state. Intramolecular proton transfer, which could be induced by intramolecular hydrogen bonding is inspected and excluded. Time-dependent density functional theory (TDDFT) calculations reveal the change of the dipole moments of the S₀ and S₁ states along the twisted coordinate of the amino group, indicating the mechanism of twisted intra-molecular charge transfer (TICT). The timescale of TICT is measured to be 5 ps due to the conformational relaxation and a barrier on the S₁ potential surface. The ISC from the S₁ state to the triplet manifold is a main deactivation pathway with the decay time of 28 ps. Our results observed here have yield a physically intuitive and complete picture of the photoinduced charge transfer and radiationless dynamics in anthraquinone pharmaceutial products.

Raising the barrier for photoinduced DNA charge injection with a cyclohexyl artificial base pair

The effects of an artificial cyclohexyl base pair on the quantum yields of fluorescence and dynamics of charge separation and charge recombination have been investigated for several synthetic DNA hairpins. The hairpins possess stilbenedicarboxamide, perylenediimide, or naphthalenediimide linkers and base-paired stems. In the absence of the artificial base pair hole injection into both adenine and guanine purine bases is exergonic and irreversible, except in the case of stilbene with adenine for which it is slightly endergonic and reversible. Insertion of the artificial base pair renders hole injection endergonic or isoergonic except in the case of the powerful naphthalene acceptor for which it remains exergonic. Both hole injection and charge recombination are slower for the naphthalene acceptor in the presence of the artificial base pair than in its absence. The effect of an artificial base pair on charge separation and charge recombination in hairpins possessing stilbene and naphthalene acceptor linkers and a stilbenediether donor capping group has also been investigated. In the case of the stilbene acceptor–stilbene donor capped hairpins photoinduced charge separation across six base pairs is efficient in the absence of the artificial base pair but does not occur in its presence. In the case of the naphthalene acceptor–stilbene donor capped hairpins the artificial base pair slows but does not stop charge separation and charge recombination, leading to the formation of long-lived charge separated states.

Electronic Interactions of Michler's Ketone with DNA Bases in Synthetic Hairpins

The mechanism and dynamics of photoinduced electron transfer in two families of DNA hairpins possessing Michler's ketone linkers have been investigated by means of steady state and time-resolved transient absorption and emission spectroscopies. The excited state behavior of the diol linker employed in hairpin synthesis is similar to that of Michler's ketone in methanol solution. Hairpins possessing only a Michler's ketone linker undergo fast singlet state charge separation and charge recombination with an adjacent purine base, attributed to well-stacked ground state conformations, and intersystem crossing to the triplet state, attributed to poorly stacked ground state conformations. The failure of the triplet to undergo electron transfer reactions on the 7 ns time scale of our measurements is attributed to the low triplet energy and reduction potential of the twisted triplet state. Hairpins possessing both a Michler's ketone linker and a perylenediimide base surrogate separated by four base pairs undergo photoinduced hole transport from the diimide to Michler's ketone upon excitation of the diimide. The efficiency of hole transport is dependent upon the sequence of the intervening purine bases.

Driving force dependence of charge separation and recombination processes in dyads of nucleotides and strongly electron-donating oligothiophenes

Charge transfer in DNA has attracted great attention of scientists because of its importance in biological processes. However, our knowledge on excess-electron transfer in DNA still remains limited in comparison to numerous studies of hole transfer in DNA. To clarify the dynamics of excess-electron transfer in DNA by photochemical techniques, new electron-donating photosensitizers should be developed. Herein, a terthiophene and two 3,4-ethylenedioxythiophene oligomers were used as photosensitizers in dyads including natural nucleobases as electron acceptors. The charge separation and recombination processes in the dyads were investigated by femtosecond laser flash photolysis, and the driving force dependence of these rate constants was discussed on the basis of the Marcus theory. From this study, the conformation effect on charge recombination process was found. We expect that 3,4-ethylenedioxythiophene oligomers are useful in investigation of excess-electron-transfer dynamics in DNA.

Conformational control of benzophenone-sensitized charge transfer in dinucleotides

Charge transfer in DNA cannot be understood without addressing the complex conformational flexibility, which occurs on a wide range of timescales. In order to reduce this complexity four dinucleotide models 1X consisting of benzophenone linked by a phosphodiester to one of the natural nucleosides X = A, G, T, C were studied in water and methanol. The theoretical work focuses on the dynamics and electronic structure of 1G. Predominant conformations in the two solvents were obtained by molecular dynamics simulations. 1G in MeOH adopts mainly an open geometry with a distance of 12–16 Å between the two aromatic parts. In H2O the two parts of 1G form primarily a stacked conformation yielding a distance of 5–6 Å. The low-lying excited states were investigated by electronic structure theory in a QM/MM environment for representative snapshots of the trajectories. Photo-induced intramolecular charge transfer in the S1 state occurs exclusively in the stacked conformation. Ultrafast transient absorption spectroscopy with 1X reveals fast charge transfer from S1 in both solvents with varying yields. Significant charge transfer from the T1 state is only found for the nucleobases with the lowest oxidation potential: in H2O, charge transfer occurs with 3.2 × 10(9) s(-1) for 1A and 6.0 × 10(9) s(-1) for 1G. The reorganization energy remains nearly unchanged going from MeOH to the more polar H2O. The electronic coupling is rather low even for the stacked conformation with H(AB) = 3 meV and explains the moderate charge transfer rates. The solvent controls the conformational distribution and therefore gates the charge transfer due to differences in distance and stacking.