Design, synthesis, and photophysical studies of a porphyrin-fullerene dyad with parachute topology; charge recombination in the marcus inverted region

Inverted region in the reaction of the quinone reduction in the A1-site of photosystem I from cyanobacteria

Photosystem I from the menB strain of Synechocystis sp. PCC 6803 containing foreign quinones in the A1 sites was used for studying the primary steps of electron transfer by pump-probe femtosecond laser spectroscopy. The free energy gap (- ΔG) of electron transfer between the reduced primary acceptor A0 and the quinones bound in the A1 site varied from 0.12 eV for the low-potential 1,2-diamino-anthraquinone to 0.88 eV for the high-potential 2,3-dichloro-1,4-naphthoquinone, compared to 0.5 eV for the native phylloquinone. It was shown that the kinetics of charge separation between the special pair chlorophyll P700 and the primary acceptor A0 was not affected by quinone substitutions, whereas the rate of A0 → A1 electron transfer was sensitive to the redox-potential of quinones: the decrease of - ΔG by 400 meV compared to the native phylloquinone resulted in a ~ fivefold slowing of the reaction The presence of the asymmetric inverted region in the ΔG dependence of the reaction rate indicates that the electron transfer in photosystem I is controlled by nuclear tunneling and should be treated in terms of quantum electron-phonon interactions. A three-mode implementation of the multiphonon model, which includes modes around 240 cm-1 (large-scale protein vibrations), 930 cm-1 (out-of-plane bending of macrocycles and protein backbone vibrations), and 1600 cm-1 (double bonds vibrations) was applied to rationalize the observed dependence. The modes with a frequency of at least 1600 cm-1 make the predominant contribution to the reorganization energy, while the contribution of the "classical" low-frequency modes is only 4%.

Polymerization-Mediated Through-Space Charge Transfer: An Emerging Strategy for Light-Emitting Materials

Through-space charge transfer (TSCT) has attracted increasing attention owing to its great potential in designing efficient light-emitting molecules and polymers. Complementary to through-bond charge transfer and Förster resonance energy transfer, TSCT offers an alternative approach for the manipulation of molecular fluorescence. Recently, the synergy between TSCT and polymer systems through polymerization-mediated charge transfer has fostered the advancements of innovative light-emitting functional materials featuring thermally activated delayed fluorescence and/or aggregation-induced emission. This perspective highlights the significant progress in tailoring emission properties through structural engineering of donor and acceptor groups within polymeric systems, leveraging the TSCT mechanism. This strategy has transcended the limitations of traditional charge transfer systems with its tolerance to extended donor-acceptor distance, paving the way for novel applications beyond organic light-emitting diodes. The discussion concludes with a forward-looking analysis of potential future research trajectories in the field of polymerization-mediated charge transfer for developing next-generation light-emitting materials.

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).

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.

The effect of dark states on the intersystem crossing and thermally activated delayed fluorescence of naphthalimide-phenothiazine dyads

A series of 1,8-naphthalimide (NI)-phenothiazine (PTZ) electron donor-acceptor dyads were prepared to study the thermally activated delayed fluorescence (TADF) properties of the dyads, from a point of view of detection of the various transient species. The photophysical properties of the dyads were tuned by changing the electron-donating and the electron-withdrawing capability of the PTZ and NI moieties, respectively, by oxidation of the PTZ unit, or by using different aryl substituents attached to the NI unit. This tuning effect was manifested in the UV-vis absorption and fluorescence emission spectra, e.g., in the change of the charge transfer absorption bands. TADF was observed for the dyads containing the native PTZ unit, and the prompt and delayed fluorescence lifetimes changed with different aryl substituents on the imide part. In polar solvents, no TADF was observed. For the dyads with the PTZ unit oxidized, no TADF was observed as well. Femtosecond transient absorption spectra showed that the charge separation takes ca. 0.6 ps, and admixtures of locally excited (3LE) state and charge separated (1CS/3CS) states formed (in n-hexane). The subsequent charge recombination from the 1CS state takes ca. 7.92 ns. Upon oxidation of the PTZ unit, the beginning of charge separation is at 178 fs and formation of 3LE state takes 4.53 ns. Nanosecond transient absorption (ns-TA) spectra showed that both 3CS and 3LE states were observed for the dyads showing TADF, whereas only 3LE or 3CS states were observed for the systems lacking TADF. This is a rare but unambiguous experimental evidence that the spin-vibronic coupling of 3CS/3LE states is crucial for TADF. Without the mediating effect of the 3LE state, no TADF is resulted, even if the long-lived 3CS state is populated (lifetime τCS ≈ 140 ns). This experimental result confirms the 3CS → 1CS reverse intersystem crossing (rISC) is slow, without coupling with an approximate 3LE state. These studies are useful for an in-depth understanding of the photophysical mechanisms of the TADF emitters, as well as for molecular structure design of new electron donor-acceptor TADF emitters.

A Symmetry-Broken Charge-Separated State in the Marcus Inverted Region

We report a long-lived charge-separated state in a chromophoric pair (DC-PDI₂ ) that uniquely integrates the advantages of fundamental processes of photosynthetic reaction centers: i) Symmetry-breaking charge-separation (SB-CS) and ii) Marcus-inverted-region dependence. The near-orthogonal bichromophoric DC-PDI₂ manifests an ultrafast evolution of the SB-CS state with a time constant of τ S B - C S ${{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ =0.35±0.02 ps and a slow charge recombination (CR) kinetics with τ C R ${{\tau }_{{\rm C}{\rm R}}}$ =4.09±0.01 ns in ACN. The rate constant of CR of DC-PDI₂ is 11 686 times slower than SB-CS in ACN, as the CR of the PDI radical ion-pair occurs in the deep inverted region of the Marcus parabola ( - Δ G C R ${{-{\rm \Delta }G}_{{\rm C}{\rm R}}}$ >λ). In contrast, an analogous benzyloxy (BnO)-substituted DC-BPDI₂ showcases a ≈10-fold accelerated CR kinetics with τ C R / τ S B - C S ${{\tau }_{{\rm C}{\rm R}}/{\tau }_{{\rm S}{\rm B}-{\rm C}{\rm S}}}$ lowering to ≈1536 in ACN, by virtue of a decreased CR driving force. The present investigation demonstrates a control of molecular engineering to tune the energetics and kinetics of the SB-CS material, which is essential for next-generation optoelectronic devices.

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.

Naphthalimide-phenothiazine dyads: effect of conformational flexibility and matching of the energy of the charge-transfer state and the localized triplet excited state on the thermally activated delayed fluorescence

In order to investigate the joint influence of the conformation flexibility and the matching of the energies of the charge-transfer (CT) and the localized triplet excited (³LE) states on the thermally activated delayed fluorescence (TADF) in electron donor–acceptor molecules, a series of compact electron donor–acceptor dyads and a triad were prepared, with naphthalimide (NI) as electron acceptor and phenothiazine (PTZ) as electron donor. The NI and PTZ moieties are either directly connected at the 3-position of NI and the N-position of the PTZ moiety via a C–N single bond, or they are linked through a phenyl group. The tuning of the energy order of the CT and LE states is achieved by oxidation of the PTZ unit into the corresponding sulfoxide, whereas conformation restriction is imposed by introducing ortho-methyl substituents on the phenyl linker, so that the coupling magnitude between the CT and the ³LE states can be controlled. The singlet oxygen quantum yield (Φ_Δ) of NI-PTZ is moderate in n-hexane (HEX, Φ_Δ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. Triplet state was observed in the nanosecond transient absorption spectra with lifetimes in the 4–48 μs range. Computational investigations show that the orthogonal electron donor–acceptor molecular structure is beneficial for TADF. These calculations indicate small energetic difference between the ³LE and ³CT states, which are helpful for interpreting the ns-TA spectra and the origins of TADF in NI-PTZ, which is ultimately due to the small energetic difference between the ³LE and ³CT states. Conversely, NI-PTZ-O, which has a higher CT state and bears a much more stabilized ³LE state, does not show TADF.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Making Nitronaphthalene Fluoresce

Nitroaromatic compounds are inherently nonfluorescent, and the subpicosecond lifetimes of the singlet excited states of many small nitrated polycyclic aromatic hydrocarbons, such as nitronaphthalenes, render them unfeasible for photosensitizers and photo-oxidants, despite their immensely beneficial reduction potentials. This article reports up to a 7000-fold increase in the singlet-excited-state lifetime of 1-nitronaphthalene upon attaching an amine or an N-amide to the ring lacking the nitro group. Varying the charge-transfer (CT) character of the excited states and the medium polarity balances the decay rates along the radiative and the two nonradiative pathways and can make these nitronaphthalene derivatives fluoresce. The strong electron-donating amine suppresses intersystem crossing (ISC) but accommodates CT pathways of nonradiate deactivation. Conversely, the N-amide does not induce a pronounced CT character but slows down ISC enough to achieve relatively long lifetimes of the singlet excited state. These paradigms are key for the pursuit of electron-deficient (n-type) organic conjugates with promising optical characteristics.

Dimeric Corrole Analogs of Chlorophyll Special Pairs

Chlorophyll special pairs in photosynthetic reaction centers function as both exciton acceptors and primary electron donors. Although the macrocyclic natural pigments contain Mg(II), the central metal in most synthetic analogs is Zn(II). Here we report that insertion of either Al(III) or Ga(III) into an imidazole-substituted corrole affords an exceptionally robust photoactive dimer. Notably, attractive electronic interactions between dimer subunits are relatively strong, as documented by signature changes in NMR and electronic absorption spectra, as well as by cyclic voltammetry, where two well-separated reversible redox couples were observed. EPR spectra of one-electron oxidized dimers closely mimic those of native special pairs, and strong through-space interactions between corrole subunits inferred from spectroscopic and electrochemical data are further supported by crystal structure analyses (3 Å interplanar distances, 5 Å lateral shifts, and 6 Å metal to metal distances).

A charge transfer state induced by strong exciton coupling in a cofacial μ-oxo-bridged porphyrin heterodimer

Photosensitizers with high energy, long lasting charge-transfer states are important components in systems designed for solar energy conversion by multistep electron transfer. Here, we show that in a push-pull type, μ-oxo-bridged porphyrin heterodimer composed of octaethylporphyrinatoaluminum(iii) and octaethylporphyrinatophosphorus(v), the strong excitonic coupling between the porphyrins and the different electron withdrawing abilities of Al(iii) and P(v) promote the formation of a high energy CT state. Using, an array of optical and magnetic resonance spectroscopic methods along with theoretical calculations, we demonstrate photodynamics of the heterodimer that involves the initial formation of a singlet CT which relaxes to a triplet CT state with a lifetime of ∼130 ps. The high-energy triplet CT state (3CT = 1.68 eV) lasts for nearly 105 μs prior to relaxing to the ground state.

Switching Competition between Electron and Energy Transfers in Porphyrin-Fullerene Dyads

Porphyrin-fullerene dyads were intensively studied as molecular donor-acceptor systems providing efficient photoinduced charge separation (CS). A practical advantage of the dyads is the possibility to tune its CS process by the porphyrin periphery modification, which allows one to optimize the dyad for particular applications. However, this tuning process is typically composed of a series of trial stages involving the development of complex synthetic schemes. To address the issue, we synthesized a series of dyads with properties switching between electron and energy transfer in both polar (benzonitrile) and nonpolar (toluene) media and developed a computation procedure with sufficient reliability by which we can predict the CS properties of the dyad in different media and design new dyads. The dyads photochemistry was established by conducting ultrafast transient absorption studies in toluene, anisole, and benzonitrile. The most crucial step in computational modeling was to establish a procedure for correction of the electronic-state energies obtained by DFT so that the effects of the electron correlation and the long-range interactions are properly incorporated. We also carried out standard electrochemical measurements and show that our computation approach predicts better thermodynamics of the dyads in different solvents.

Long-Lived Triplet Excited State Accessed with Spin-Orbit Charge Transfer Intersystem Crossing in Red Light-Absorbing Phenoxazine-Styryl BODIPY Electron Donor/Acceptor Dyads

Orthogonal phenoxazine-styryl BODIPY compact electron donor/acceptor dyads were prepared as heavy atom-free triplet photosensitizers (PSs) with strong red light absorption (ϵ=1.33×105  M-1  cm-1 at 630 nm), whereas the previously reported triplet photosensitizers based on the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, a long-lived triplet state (τT =333 μs) was observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect (HAE) is much shorter (τT =1.8 μs). Long triplet state lifetime is beneficial to enhance electron or energy transfer, the primary photophysical processes in the application of triplet PSs. Our approach is based on SOCT-ISC, without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible light-harvesting chromophore, which shows red-light absorption. Femtosecond transient absorption spectra indicated the charge separation (109 ps) and SOCT-ISC (charge recombination, CR; 2.3 ns) for BDP-1. ISC efficiency of BDP-1 was determined as ΦT =25 % (in toluene). The dyad BDP-3 was used as triplet PS for triplet-triplet annihilation upconversion (upconversion quantum yield ΦUC =1.5 %; anti-Stokes shift is 5900 cm-1 ).

Decelerating Charge Recombination Using Fluorinated Porphyrins in N,N-Bis(3,4,5-trimethoxyphenyl)aniline-Aluminum(III) Porphyrin-Fullerene Reaction Center Models

In supramolecular reaction center models, the lifetime of the charge-separated state depends on many factors. However, little attention has been paid to the redox potential of the species that lie between the donor and acceptor in the final charge separated state. Here, we report on a series of self-assembled aluminum porphyrin-based triads that provide a unique opportunity to study the influence of the porphyrin redox potential independently of other factors. The triads, BTMPA-Im→AlPorF_n-Ph-C₆₀ (n = 0, 3, 5), were constructed by linking the fullerene (C₆₀) and bis(3,4,5-trimethoxyphenyl)aniline (BTMPA) to the aluminum(III) porphyrin. The porphyrin (AlPor, AlPorF₃, or AlPorF₅) redox potentials are tuned by the substitution of phenyl (Ph), 3,4,5-trifluorophenyl (PhF₃), or 2,3,4,5,6-pentafluorophenyl (PhF₅) groups in its meso positions. The C₆₀ and BTMPA units are bound axially to opposite faces of the porphyrin plane via covalent and coordination bonds, respectively. Excitation of all of the triads results in sequential electron transfer that generates the identical final charge separated state, BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•-, which lies energetically 1.50 eV above the ground state. Despite the fact that the radical pair is identical in all of the triads, remarkably, the lifetime of the BTMPA^•+-Im→AlPorF_n-Ph-C₆₀^•- radical pair was found to be very different in each of them, that is, 1240, 740, and 56 ns for BTMPA-Im→AlPorF₅-Ph-C₆₀, BTMPA-Im→AlPorF₃-Ph-C₆₀, and BTMPA-Im→AlPor-Ph-C₆₀, respectively. These results clearly suggest that the charge recombination is an activated process that depends on the midpoint potential of the central aluminum(III) porphyrin (AlPorF_n).

Sc3N@Ih-C80 based donor–acceptor conjugate: role of thiophene spacer in promoting ultrafast excited state charge separation

Photoinduced charge separation and dark charge recombination occurring within picoseconds is observed in newly synthesized triphenylamine–thiophene-Sc₃N@I_h-C₈₀ and triphenylamine–thiophene-C₆₀ conjugates.

Theoretical analysis of the inverted region in photoinduced proton-coupled electron transfer

Photoinduced proton-coupled electron transfer (PCET) plays a key role in a wide range of energy conversion processes, and understanding how to design systems to control the PCET rate constant is a significant challenge. Herein a theoretical formulation of PCET is utilized to identify the conditions under which photoinduced PCET may exhibit inverted region behavior. In the inverted region, the rate constant decreases as the driving force increases even though the reaction becomes more thermodynamically favorable. Photoinduced PCET will exhibit inverted region behavior when the following criteria are satisfied: (1) the overlap integrals corresponding to the ground reactant and the excited product proton vibrational wavefunctions become negligible for a low enough product vibronic state and (2) the reaction free energies associated with the lower excited product proton vibrational wavefunctions contributing significantly to the rate constant are negative with magnitudes greater than the reorganization energy. These criteria are typically not satisfied by harmonic or Morse potentials but are satisfied by more realistic asymmetric double well potentials because the proton vibrational states above the barrier correspond to more delocalized proton vibrational wavefunctions with nodal structures leading to destructive interference effects. Thus, this theoretical analysis predicts that inverted region behavior could be observed for systems with asymmetric double well potentials characteristic of hydrogen-bonded systems and that the hydrogen/deuterium kinetic isotope effect will approach unity and could even become inverse in this region due to the oscillatory nature of the highly excited vibrational wavefunctions. These insights may help guide the design of more effective energy conversion devices.

Chiral Bifunctional Metalloporphyrin Catalysts for Kinetic Resolution of Epoxides with Carbon Dioxide

Chiral binaphthyl-strapped Zn(II) porphyrins with triazolium halide units were synthesized as bifunctional catalysts for kinetic resolution of epoxides with CO₂. Several catalysts were screened by changing the linker length and nucleophilic counteranions, and the optimized catalyst accelerated the enantioselective reaction at ambient temperature to produce optically active cyclic carbonates and epoxides.

Chiroptical and catalytic properties of doubly binaphthyl-strapped chiral porphyrins

Doubly (R)-binaphthyl-strapped porphyrins with methylene chains were synthesized.

Excited state dynamics and conformations of a Cu(ii)-phthalocyanine-perylenebisimide dyad

We investigate the excited state dynamics and the conformations of a new molecular donor-bridge-acceptor system, a Cu(ii)-phthalocyanine (CuPc) covalently linked via a flexible aliphatic spacer to a perylenebisimide (PBI). We performed time-resolved polarization anisotropy and pump-probe measurements in combination with molecular dynamics simulations. Our data suggest the existence of three conformations of the dyad: two more extended, metastable conformations with centre-of-mass distances >1 nm between the PBI and CuPc units of the dyad, and a highly stable folded structure, in which the PBI and CuPc units are stacked on top of each other with a centre-of-mass distance of 0.4 nm. In the extended conformations the dyad shows emission predominantly from the PBI unit with a very weak contribution from the CuPc unit. In contrast, for the folded conformation the PBI emission of the dyad is strongly quenched due to fast energy transfer from the PBI to the CuPc unit (3 ps) and subsequent intersystem-crossing (300 fs) from the first excited singlet state of CuPc unit into its triplet state. Finally, the CuPc triplet state is deactivated non-radiatively with a time constant of 25 ns.

Fullerenes – how 25 years of charge transfer chemistry have shaped our understanding of (interfacial) interactions

Over 25 years research in charge transfer chemistry are highlighted in terms of interfacial interactions between fullerenes and porphyrins in electron donor–acceptor systems.

Synthesis and Properties of Axially Symmetrical Rigid Visible Light-Harvesting Systems Containing [60]Fullerene and Perylenebisimide

Two visible light-harvesting perylenebisimide (PDI)-[60]fullerene (C₆₀) systems, dyad P1 with one C₆₀ unit and triad P2 with two C₆₀ units, have been synthesized. Both systems are axially symmetrical with a rigid biphenyl linker, ensuring a relatively fixed spatial distance between the donor and acceptor, preventing through-space interaction, and enhancing energy transfer. Steady-state and transient spectroscopy, electrochemistry, as well as theoretical calculations have been used to investigate the electrochemical and photophysical properties of the two systems. Steady-state and time-resolved spectroscopy demonstrate that the excited state is featured by an efficient intramolecular energy transfer from PDI to C₆₀. Then, the high efficient intrinsic intersystem crossing of C₆₀ eventually leads to the production of the triplet C₆₀. The extensive visible light absorption of PDI in the range of 400-650 nm and the final localization of the excited energy at the triplet C₆₀ make these compounds ideal singlet oxygen inducers. Further investigation shows that the photooxidation capability for both compounds is significantly enhanced with respect to either PDI or C₆₀ and even better than that of the commonly used triplet photosensitizer methylene blue (MB). The double C₆₀ moieties in P2 display a better result, and the photooxidation efficiency of P2 increases 1.3- and 1.4-fold compared to that of P1 and MB, respectively. The combination of a light-harvesting unit with an intersystem crossing unit results in a highly efficient photooxidation system, which opens up a new way to triplet photosensitizer design.

Guest and solvent modulated photo-driven charge separation and triplet generation in a perylene bisimide cyclophane

Cofacial positioning of two perylene bisimide (PBI) chromophores at a distance of 6.5 Å in a cyclophane structure prohibits the otherwise common excimer formation and directs photoexcited singlet state relaxation towards intramolecular symmetry-breaking charge separation (τCS = 161 ± 4 ps) in polar CH2Cl2, which is thermodynamically favored with a Gibbs free energy of ΔGCS = -0.32 eV. The charges then recombine slowly in τCR = 8.90 ± 0.06 ns to form the PBI triplet excited state, which can be used subsequently to generate singlet oxygen in 27% quantum yield. This sequence of events is eliminated by dissolving the PBI cyclophane in non-polar toluene, where only excited singlet state decay occurs. In contrast, complexation of electron-rich aromatic hydrocarbons by the host PBI cyclophane followed by photoexcitation of PBI results in ultrafast electron transfer (<10 ps) from the guest to the PBI in CH2Cl2. The rate constants for charge separation and recombination increase as the guest molecules become easier to oxidize, demonstrating that charge separation occurs close to the peak of the Marcus curve and the recombination lies far into the Marcus inverted region.

Studying the intersystem crossing rate and triplet quantum yield of meso-substituted porphyrins by means of pulse train fluorescence technique

Excited state dynamics, particularly intersystem crossing, of a set of meso-substituted porphyrins bearing different electron–donor and acceptor groups was studied by pulse train fluorescence technique. Triplet quantum yield was found to be critically dependent on the nature of meso-substituents in the porphyrin system. Porphyrins with meso methoxyphenyl groups were found to show high triplet quantum yields ([Formula: see text] between 0.70 and 0.81). Moreover, the quantity of methoxyphenyl groups and the substitution pattern directly influence [Formula: see text]. Alternatively, porphyrins attached to nitrophenyl group possess low triplet quantum yield values (~0.3). The observed structure-properties relationships suggest new ways for tuning the optical properties of porphyrins via chemical modification.

Electrochemical Oxidations ofp-Doped Semiconducting Single-Walled Carbon Nanotubes

Two oxidation peaks at 0.99, 1.48 V versus Fc/Fc+appear in the cyclic voltammograms of a series of defect-site functionalized SWNTs in methylene chloride solution in the presence of ferrocenes. These two peaks are demonstrated to be the electrochemical responses to the independent oxidation of v1and v2valence bands ofp-doped semiconducting SWNTs.

Charge separation and charge recombination photophysical studies in a series of perylene–C60linear and cyclic dyads

A new donor–acceptor doubly bridged perylenediimide–fullerene dyad (PDI–C₆₀,DB-3), where the perylenediimide (PDI) acts as a donor, has been synthesized and studied by time-resolved absorption spectroscopy.

Ultrafast charge separation and charge stabilization in axially linked ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ reaction center mimics

Sequential electron transfer leading to charge stabilization in newly synthesized vertically aligned ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ supramolecular triads is reported.

In vivo studies of nanostructure-based photosensitizers for photodynamic cancer therapy

Animal models, particularly rodents, are major translational models for evaluating novel anticancer therapeutics. In this review, different types of nanostructure-based photosensitizers that have advanced into the in vivo evaluation stage for the photodynamic therapy (PDT) of cancer are described. This article focuses on the in vivo efficacies of the nanostructures as delivery agents and as energy transducers for photosensitizers in animal models. These materials are useful in overcoming solubility issues, lack of tumor specificity, and access to tumors deep in healthy tissue. At the end of this article, the opportunities made possible by these multiplexed nanostructure-based systems are summarized, as well as the considerable challenges associated with obtaining regulatory approval for such materials. The following questions are also addressed: (1) Is there a pressing demand for more nanoparticle materials? (2) What is the prognosis for regulatory approval of nanoparticles to be used in the clinic?

Enhancing molecular recognition in electron donor-acceptor hybrids via cooperativity

Herein, we report the synthesis of guanidinium bis-porphyrin tweezers 1 and fullerene carboxylate 3, their assembly into a novel supramolecular 1@3 electron donor-acceptor hybrid, and its characterization. In solution, the binding constant affording 1@3 is exceptionally high. 1@3, which features a highly confined topography, builds up from a combination of guanidinium-carboxylate hydrogen bonding and π-π stacking/charge-transfer motifs. The latter is governed by interactions between the electron-donating porphyrin and the electron-accepting fullerene. Importantly, positive cooperativity between the applied binding motifs is corroborated by a number of experimental techniques, such as NMR, absorption, fluorescence, etc. In addition, transient absorption experiments shed light onto electron-transfer processes taking place in the ground state and upon photoexcitation. In fact, porphyrin excitation powers an electron transfer to the fullerene yielding charge separated state lifetimes in the nanosecond regime.