Dynamics of Photoinduced Electron-Transfer Processes in Fullerene-Based Dyads: Effects of Varying the Donor Strength

Bridge effect on charge transfer and energy transfer in fullerene-chromophore dyads

Fullerene-chromophore dyads have attracted a great deal of research interest because these complexes can be potentially designed as nanoscale artificial photosynthetic centers, in which the chromophore and fullerene function as the electron donor and acceptor, respectively. The basic operation of this dyad-type artificial reaction center is photoinduced electron transfer from the donor to the acceptor. The fullerene and chromophore are usually covalently linked so that sufficient electronic coupling between these two moieties can facilitate the electron transfer. However, other deactivation pathways for the chromophore excited state, such as energy transfer to the fullerene, may reduce the quantum yield of the photoinduced electron transfer. Here, a series of C60-perylene dyads is exploited to interrogate the effect of the linkage on deactivation mechanisms of the chromophore excited state. For the C60-perylene dyads with a single or double bond bridge, we find that the decay of the singlet state of the chromophore is dominated by the electron transfer, and the corresponding time constant is determined to be 45 ps. On the other hand, for the dyad with a triple bond bridge, the singlet state of the chromophore is quickly quenched through energy transfer to fullerene, and the time constant is as short as 7.9 ps. Our finding suggests that the bond order of the bridge in the fullerene-chromophore dyads can be utilized to control the deactivation pathways of the excited state.

One-Photon Excitation Followed by a Three-Step Sequential Energy-Energy-Electron Transfer Leading to a Charge-Separated State in a Supramolecular Tetrad Featuring Benzothiazole-Boron-Dipyrromethene-Zinc Porphyrin-C60

A panchromatic triad, consisting of benzothiazole (BTZ) and BF₂ -chelated boron-dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using ¹ H NMR spectroscopy, ESI-MS, UV-visible, steady-state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ-BODIPY-ZnP, and dyads, BTZ-BODIPY and BODIPY-ZnP, along with the reference compounds BTZ-OMe, BODIPY-OMe, and ZnP-OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ-BODIPY-ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ-BODIPY-ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from ¹ BTZ^* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the ¹ BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two-step singlet-singlet energy transfer. Further, a supramolecular tetrad, BTZ-BODIPY-ZnP:ImC₆₀ , was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC₆₀ ), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from ¹ ZnP* to ImC₆₀ . Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, ¹ BTZ^* -BODIPY-ZnP:ImC₆₀ →BTZ-¹ BODIPY^* -ZnP:ImC₆₀ →BTZ-BODIPY-¹ ZnP^* :ImC₆₀ →BTZ-BODIPY-ZnP^.+ :ImC₆₀ ^.- in the supramolecular tetrad. The evaluated rate of energy transfer, k_EnT , was found to be 3-5×10¹⁰  s^-1 , which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC₆₀ . The rate constants for charge separation and recombination, k_CS and k_CR , respectively, calculated by monitoring the rise and decay of C₆₀ ^.- were found to be 5.5×10¹⁰ and 4.4×10⁸  s^-1 , respectively, for the BODIPY-ZnP:ImC₆₀ triad, and 3.1×10¹⁰ and 4.9×10⁸  s^-1 , respectively, for the BTZ-BODIPY-ZnP:ImC₆₀ tetrad. Initial excitation of the tetrad, promoting two-step energy transfer and a final electron-transfer event, has been successfully demonstrated in the present study.

Non-photoisomerizable butterfly shaped tetrasubstituted azobenzenes: synthesis and photophysical studies

The trans-azobenzene (AB) substituted derivatives with bulky carbazole (1), phenothiazine (2), and phenyl (3) have been synthesized to understand the effect of steric crowding around the –NN– linkage on the photoisomerization process.

Influence of hierarchization on electron transfers in structured MFI-type zeolites

H-ZSM-5 zeolite (Si/Al = 19.3) was hydrothermally synthesized. Alkaline and/or acid post-synthesis treatments were carried out to give rise to an interconnected mesoporous volume. The desilication treatment parameters have been tuned (temperature, organic base addition) to obtain a series of samples with increasing mesoporous volume and a constant number of acid sites. The physico-chemical properties of the resulting materials were fully characterized by many techniques (NMR, BET, PXRD, and pyridine thermal desorption followed by infrared spectroscopy). To assess the effect of post-treatments on sample reactivity, the charge separation processes between the zeolite framework and adsorbed trans-stilbene (t-St) molecule were investigated by UV-visible diffuse reflectance. The spectra obtained after t-St adsorption show clear differences depending on the applied post-treatments. It appears that the desilication treatments performed without acidic washing highly stabilize the radical cation resulting from the t-St spontaneous ionization. In contrast, by applying acidic washing after desilication, the ionization process becomes significantly weaker. The results show that the proportion of strong Lewis acid sites in the vicinity of Brønsted sites named Brønsted Strong Lewis Pairs (BSLP), are responsible for the amount of radical cations observed in the different samples. More precisely, it exists an optimal proportion of BSLP to achieve a high ionization rate. On the basis of the experimental results a mechanism for the formation of the t-St radical cation and the charge transfer complex (CTC) is proposed.

Synthesis and Photoinduced Electron-Transfer Reactions in a La2 @Ih -C80 -Phenoxazine Conjugate

A newly designed electron donor-acceptor conjugate consisting of an endohedral dimetallofullerene (La₂ @I_h -C₈₀ ) and phenoxazine (POZ) was successfully synthesized under Prato conditions. Our results document that the 1,3-dipolar cycloaddition took place across the [5,6] junction to afford exclusively the corresponding [5,6] cycloadduct. The structure of the conjugate was characterized by means of NMR spectroscopy, absorption spectroscopy, and electrochemical studies. Computational calculations suggest that the electron density of the highest occupied molecular orbital (HOMO) is distributed on the POZ moiety, whereas that of the lowest unoccupied molecular orbital (LUMO) is located at the endohedral La atoms, leading to efficient separation of the HOMO and LUMO in the conjugate. Time-resolved absorption spectroscopic investigations and spectroelectrochemical measurements corroborate the formation of the energetically low-lying (La₂ @I_h -C₈₀ )^.- -(POZ)^.+ radical-ion-pair state by means of ultrafast through-space electron transfer.

Matrix-assisted laser desorption/ionisation quadrupole ion trap time-of-flight mass spectrometry of novel shape-persistent macrocycles

RATIONALE

Shape-persistent macrocycles (SPMs) represent innovative molecular building blocks for the development of highly organised supramolecular architectures with application in nanotechnology, chemistry, catalysis and optoelectronics. Systematic mass spectrometric characterisation of SPMs and their collision-activated decay is not available to date.

METHODS

Characterisation of alkoxy-decorated SPMs was performed by matrix-assisted laser desorption/ionisation quadrupole ion trap time-of-flight mass spectrometry (MALDI-QIT-TOFMS) and collision-induced dissociation (CID).

RESULTS

Laser excitation of SPMs leads to the formation of stable cation radicals which show characteristic fragmentation patterns. All the product ions formed were identified. Photoelectrons generated during the MALDI process and full-ring conjugation were found to be fundamental for the formation of molecular cation radicals and their stabilisation, respectively. Formation of supramolecular aggregates of SPMs by π-π stacking was proven. SPMs are suitable motifs for the preparation of novel fullerene-based donor-acceptor systems.

CONCLUSIONS

Alkoxy-decorated SPMs represent promising electron-donating building blocks that can be exploited in electronics and optoelectronics for the development of robust and highly efficient laser-activated supramolecular switches.

The effect of structural changes on charge transfer states in a light-harvesting carotenoid-diaryl-porphyrin-C60 molecular triad

We present a detailed study of charge transfer (CT) excited states for a large number of configurations in a light-harvesting Carotenoid-diaryl-Porphyrin-C60 (CPC60) molecular triad. The chain-like molecular triad undergoes photoinduced charge transfer process exhibiting a large excited state dipole moment, making it suitable for application to molecular-scale opto-electronic devices. An important consideration is that the structural flexibility of the CPC60 triad impacts its dynamics in solvents. Since experimentally measured dipole moments for the triad of ∼110 D and ∼160 D strongly indicate a range in structural variability in the excited state, studying the effect of structural changes on the CT excited state energetics furthers the understanding of its charge transfer states. We have calculated the variation in the lowest CT excited state energies by performing a scan of possible variation in the structure of the triad. Some of these configurations were generated by incrementally scanning a 360° torsional (dihedral) twist at the C60-porhyrin linkage and the porphyrin-carotenoid linkage. Additionally, five different CPC60 conformations were studied to determine the effect of pi-conjugation and particle-hole Coulombic attraction on the CT excitation energies. Our calculations show that configurational changes in the triad induces a variation of ∼0.6 eV in CT excited state energies in the gas-phase. The corresponding calculated excited state dipoles show a range of 47 D-188 D. The absorption spectra and density of states of these structures show little variation except for the structures where the porphyrin and aryl conjugation is changed.

MALDI-Fourier transform mass spectrometric and theoretical studies of donor-acceptor and donor-bridge-acceptor fullerenes

Fourier-transform ion cyclotron resonance (FTICR) mass spectrometric studies have been performed on donor-acceptor and donor-bridge acceptor fullerene-based systems. Matrix-assisted laser desorption/ionization (MALDI) was used for ion production; both the positive and negative ion modes were utilized. In addition, collision-induced dissociation (CID) experiments were carried out to study the movement of the charge (electron or hole) upon fragmentation. The experiments are complemented by ab initio theoretical calculations yielding both molecular orbital energies and electron density distributions. It was found that the theoretical electron density map predicted the experimentally observed fragmentation correctly in every case. Both the calculations and the MS experiments may be useful in studying these and related donor-acceptor systems in view of their use for charge separation and eventually, solar energy production.

Incorporation and electron transfer of anthracene in pores of ZSM-5 zeolites. Effect of Brønsted acid site density

The sorption course of anthracene (ACENE-3) into Brønsted-acidic medium pore MFI zeolites was monitored by in situ EPR and diffuse reflectance UV-visible absorption over one year. Weighed amounts of solid ACENE-3 were merely exposed to H(n)ZSM-5 (H(n)(AlO(2))(n)(SiO(2))(96-n)), with the following Brønsted acid site (BAS) densities, n = 0.0, 0.17, 0.57, 0.95, 2.0, 3.4, 6.6, dehydrated at 623 K under argon. The weighed amounts correspond to 1 ACENE-3 per zeolite unit cell. ACENE-3 is found to be incorporated as intact molecules in purely siliceous MFI (silicalite-1). Monte Carlo simulations indicate that ACENE-3 lies in the intersection of straight and zigzag channels. In contrast, the presence of BASs on the inner surface of channels induces spontaneous ionization of ACENE-3 (ionization potential = 7.44 eV). The charge separation as ACENE-3*(+)@H(n)ZSM-5*(-) is caused by the strong Coulombic field gradient of Si-O(-)(H(+))-Al BAS in the absence of any Lewis acid site. The rate and yield of ionization are found to increase dramatically with BAS density increase. The stabilization of ACENE-3*(+)@H(n)ZSM-5*(-) is explained by the tight fit between the rod-shape ACENE-3 and the channel dimensions and especially by the compartmentalization of ejected electrons as AlO(4)H*(-) centers away from the initial site of ionization. The final charge recombination occurs after more than one year and leads to ACENE-3 occluded in the straight channel in close proximity to BAS without any protonation of ACENE-3 (pK(a) = -13.5).

Generation of long-lived radical ions through enhanced photoinduced electron transfer processes between [60]fullerene and phenothiazine derivatives

Photoinduced electron transfer processes between fullerenes (C60) and four phenothiazine derivatives (PTZs) in the absence and presence of hexylviologen dication (HV2+) have been studied by the transient absorption method in the visible and near-IR regions. Electron-transfer takes place from PTZs to the triplet states of fullerenes (3C60*) giving the radical anion of fullerenes (C60.-) and the radical cations of PTZs (PTZ.+). The rate constants and efficiencies of electron transfer are quite high, because of the high electron-donor abilities of PTZs as elucidated by their low oxidation potentials. On addition of HV2+ to the C60 and PTZ systems, the electron-mediating process occurs from C60.- to HV2+, yielding the viologen radical cation (HV.+). In the presence of a sacrificial donor, HV.+ persisted for a long time.

Photoinduced Electron Transfer in α-Cyclodextrin-Based Supramolecular Dyads: A Free-Energy-Dependence Study

Photoinduced electron transfer (PET) between alpha-cyclodextrin-appended pyrene (PYCD) and a few acceptor molecules was studied in aqueous solutions. The pyrene moiety in PYCD is located above the narrower rim of the alpha-CD and is fully exposed to water. The acceptors are monocyclic organic molecules and, upon dissolution in water in the presence of PYCD, a fraction of the donor-acceptor systems is present as supramolecular dyads and the remaining fraction as free molecules. Free-energy-dependence studies showed that electron transfer in the supramolecular dyads follows the Marcus equation. The donor-acceptor coupling and the reorganization energy were determined from fits of the data to the Marcus equation. The electronic coupling was found to be similar to those reported for hydrogen-bonded systems. It appears that the actual lambdaout values are somewhat lower than values calculated with the continuum model. The experimental design has also allowed, for the first time, a visual demonstration of the inverted region on the basis of the raw fluorescence lifetime data.

Incorporation of Anthracene into Zeolites: Confinement Effect on the Recombination Rate of Photoinduced Radical Cation-Electron Pair

FT-Raman spectrometry in combination with diffuse reflectance UV/Vis absorption (DRUVv) and fluorescence emission indicate that complete anthracene (ANT) sorption as intact molecules takes place over 6 months in the medium pores of non-Brønsted acidic M(n)ZSM-5 zeolites (n=0.0, 3.4, 6.6; M=Na+, K+, Rb+, Cs+) with 1 ANT per unit cell loading. The combined effect of confinement and electrostatic field induced by bulky cations (Rb+, Cs+) leads to specific changes in the occluded ANT Raman spectra after very long organization periods (one year). The laser photolysis (266 nm, 355 nm) of ANT@M(n)ZSM-5 equilibrated samples generates long-lived charge separated species in aluminum rich zeolites (n=3.4, 6.6). The very long-lived radical pairs are characterized by conventional DRUVv and CW-EPR spectroscopy. The direct charge recombination rates of ANT.+-electron pairs are dispersive, extending over a broad range of timescales. The kinetic constant values are found to increase dramatically with the aluminum content and increase markedly with M+ according to the following order Na+ < K+ < Rb+ < Cs+. The small reorganization energy (lambda) of ZSM-5 zeolite pores coupled with large negative free energy changes (-DeltaG degrees ) between the ground state ANT oxidation potential and Fermi level of aluminum rich M(n)ZSM-5 explain the observed trends of the ANT.+@M(n)ZSM-5.- charge recombination rates.

Solid Film versus Solution-Phase Charge-Recombination Dynamics of exTTF–Bridge–C60 Dyads

The charge-recombination dynamics of two exTTF-C60 dyads (exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene), observed after photoinduced charge separation, are compared in solution and in the solid state. The dyads differ only in the degree of conjugation of the bridge between the donor (exTTF) and the acceptor (C60) moieties. In solution, photoexcitation of the nonconjugated dyad C60-BN-exTTF (1) (BN = 1,1'-binaphthyl) shows slower charge-recombination dynamics compared with the conjugated dyad C60-TVB-exTTF (2) (TVB = bisthienylvinylenebenzene) (lifetimes of 24 and 0.6 micros, respectively), consistent with the expected stronger electronic coupling in the conjugated dyad. However, in solid films, the dynamics are remarkably different, with dyad 2 showing slower recombination dynamics than 1. For dyad 1, recombination dynamics for the solid films are observed to be tenfold faster than in solution, with this acceleration attributed to enhanced electronic coupling between the geminate radical pair in the solid film. In contrast, for dyad 2, the recombination dynamics in the solid film exhibit a lifetime of 7 micros, tenfold slower than that observed for this dyad in solution. These slow recombination dynamics are assigned to the dissociation of the initially formed geminate radical pair to free carriers. Subsequent trapping of the free carriers at film defects results in the observed slow recombination dynamics. It is thus apparent that consideration of solution-phase recombination data is of only limited value in predicting the solid-film behaviour. These results are discussed with reference to the development of organic solar cells based upon molecular donor-acceptor structures.

Derivatized Fullerenes Bearing Multiple Electron Donors: Synthesis and Charge Transfer Properties

A series of methano-C60 adducts bearing up to six electron donating N,N-dimethylaniline units (denoted as D compounds), along with their analogues without the dimethylamino groups as references (R compounds), were synthesized. The redox properties of the D compounds in solutions were evaluated spectroscopically in reference to the R compounds. According to UV/vis absorption results, there are obviously ground-state intramolecular charge-transfer complexes in the D series, and the charge-transfer effects apparently become saturated with only two donor units in the molecule. The photoinduced intramolecular electron-transfer properties of the D compounds were investigated via fluorescence measurements. The emission from intramolecular exciplexes can be found only in the D molecule with two electron donor units. Throughout the D series, the fluorescence properties are highly sensitive to the solvent polarity, with the emission completely quenched for all of the molecules in a polar solvent like methylene chloride. Mechanistic implications of the results are discussed.

Photoinduced Electron Transfer Competitive with Energy Transfer of the Excited Triplet State of [60]Fullerene to Ferrocene Derivatives Revealed by Combination of Transient Absorption and Thermal Lens Measurements

The quenching processes of the exited triplet state of fullerene (3C60) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although 3C60 was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (phi(et)) for the generation of the radical anion of C60 (C60*-) via 3C60 were quite low even in polar solvents; nevertheless, the free-energy changes (deltaG(et)) of electron transfer from Fc to 3C60 are sufficiently negative. In benzonitrile (BN), the phi(et) value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from 3C60 to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of 3C60. Therefore, energy transfer from 3C60 to ferrocene decreases the electron-transfer process from ferrocene to 3C60. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (phi(et) = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (phi(et) = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.