We report here the formation of a long-lived charge-separated state of a self-assembled donor-acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) to the zinc center of a subphthalocyanine-triphenylamine-zinc porphyrin (SubPc-TPA-ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0 x 10(4) M(-1), suggesting a moderately stable complex formation. The geometric and electronic structures of the covalently linked SubPc-TPA-ZnP triad and self-assembled SubPc-TPA-ZnP:C(60)Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc-TPA-ZnP(*+):C(60)Im(*-)). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C(60)Im with a lifetime of 1.1 ns. Interestingly, slow charge recombination (1.6 x 10(5) s(-1)) and the long lifetime of the charge-separated state (6.6 micros) were obtained in toluene by utilizing the nanosecond transient measurements.
Based on X-ray analyses the molecular structures of the electron donor-acceptor C2. 21-and [3.3]paracyclophanes 1,2,4,6,10, and 11 are discussed in terms of steric and electronic effects. The with in correlation to the variation of the strength of the electron Elektron-Donor-Acceptor-Verbindungen, 46'). In the preceding paper') the syntheses of the 4,5,7,8-tetracyano[2.2]paracyclophanes 1 -4, of the 5,6,8,9-tetracyano[3.3]paracyclophanes 5 -7 and of the 6,7,9,10-tetracyano[4.4]paracyclophanes 8 and 9 have been described. In this series of compounds the tetracyanobenzene (TCNB) unit as the common electron acceptor is facing various electron donors of different ionisation potential and with graduated donor-acceptor distances. In the present paper we deal with the X-ray structure analyses of some typical representatives of this series and with investigations concerning charge-transfer (CT) absorptions which we try to correlate with the molecular structures. Molecular Structures of TetracyanoparacyclophanesX-Ray Structure Analysis2': Crystal and data collection parameters for 1, 2, 4, 6, 10, and 11 are listed in Table 1. Intensity data were collected by using graphite-monochromated Mo-K, radiation and applying 0 / 2 0 scan technique. The structures of 1, 4, 6, 10, and 11 were solved by direct methods (MULTAN) and were refined by full-matrix least-squares technique using anisotropic temperature factors for non-hydrogen atoms and isotropic temperature factors for hydrogen atoms. In the case of 2 the solution of the structure was not possible by direct method. Based on a Patterson synthesis and considering the structures of 1 and of pseudoyem-tetramethoxy-[2.2]paracyclophane3) a molecular model was constructed and introduced into the elementary cell. By variation of the x and z components for the centre of the molecule and by variation of the three Euler angles the location and orientation of the molecule of 2 was refined using anisotropic temperature factors for non-hydrogen and isotropic temperature factors for hydrogen atoms. The R values are listed in Table 1 Figure 1 A shows the molecular structure of 1 in a top-view perpendicular to the planes of the aromatic rings. The bond lengths given in this figure do not deviate significantly from normal values with the exception of the central bonds in the bridges which, obviously due to transanular 7c-K repulsion, are considerably elongated. There is only a slight deviation from an ecliptic arrangement of the two rings, the axes through the bridge-head atoms of which form an angle of 4" with each other. In Figure 1 B a side-view of 1 is presented which shows the boat-type deformation of the aromatic rings which is similar to that of C2.2lparacyclo-phane itself4'. The deviation from planarity is somewhat stronger for the acceptor ring whereas the bridge bonds to the bridge-head atoms deviate from the corresponding tnangle planes stronger on the unsubstituted side. The exocyclic bonds on the ring atoms deviate from the sp2 plane of these atoms into the direction towards the...
Silicon-phthalocyanine-cored fullerodendrimers with up to eight fullerene substituents (SiPc-n C(60); n=2, 4, and 8) have been synthesized. Photophysical properties of newly synthesized SiPc-n C(60) have been investigated by time-resolved fluorescence and transient absorption analysis with pulsed laser light. Laser photolysis measurements suggest the occurrence of a charge-separation process from (1)SiPc* to the C(60) subunits. The nanosecond transient absorption spectra in the near-IR region indicate that the lifetimes of the formed radical ion pairs are prolonged on the order of SiPc-8 C(60)>SiPc-4 C(60)>SiPc-2 C(60), which may be related to the electron migration among the C(60) subunits. The usefulness of SiPc-n C(60) as light-harvesting systems, evaluated as a ratio of the rates of charge recombination to those of charge separation, increases markedly with the dendrimer generation.
A series of five organic donor-π-bridge-acceptor (D-π-A) sensitizers is investigated within the context of their photoinduced charge-transfer properties. Thereby, the focus is set on the impact of structural modifications of the molecular architecture on the π-systems of the dyes. In particular, two different modes of systematic extension of the sensitizers' π-systems, namely, (i) within the electron donating site and (ii) within the π-bridge, are investigated by means of steady-state and time-resolved spectroscopic methods. The photophysical studies of the molecules in solution and as deposited on Al 2 O 3 or TiO 2 films reveal that different effects on the charge-transfer characteristics evolve dependent where − within the molecular structure − the modification of the π-system is performed. Hence, π-extension of the donor sites, for instance, leads to a strong red shift of the absorption features and a variation of light-harvesting properties. Modifying the π-bridges results in a spatial decoupling of the HOMO and LUMO orbitals, which goes along with changes of the electronic coupling to TiO 2 . Furthermore, solution studies show that the electronic structure of the dyes governs their singlet excited-state features. As shown, the results obtained from these studies then allow important predictions about the deactivation of the excited states of these molecules adsorbed on TiO 2 . Finally, quantum chemical methods − among others, time-dependent density functional theory calculations − provide conclusive insight into the relationship between the electronic structure of the dyes and its impact on the photoinduced charge-transfer characteristics. ■ INTRODUCTIONThe advantages of metal-free organic sensitizers, for the application in dye-sensitized solar cells (DSSCs), place them more and more into the focus of sensitizer research and development. Superior to their ruthenium-containing alternatives 1 are their extinction coefficients and the nearly unlimited possibilities of varying parameters such as the overall π-conjugation, the distance between the donor and the acceptor, and the nature of the anchoring groups by wellestablished synthetic methods.2 This allows for fine-tuning the spectral and electronic features as well as for a control over charge-transfer kinetics and electronic coupling to TiO 2 . Understanding the influence of structural variations to the sensitizer, such as the length of π-conjugation or anchoring/ acceptor group choice on the DSSC device performance, is crucial for advancing this field and technology. 3,4 Critical factors that influenced performance properties are (i) the excited-state redox potentials, which should be properly aligned with the conduction band of TiO 2 , (ii) the light-harvesting features of the sensitizer, (iii) the conjugation across the donor and anchoring groups, and (iv) the electronic coupling between the lowest unoccupied molecular orbital (LUMO) and the conduction band of the TiO 2 .5 It has been shown that a sensitive interplay between all of these fact...
Photoinduced electron-transfer processes of the newly synthesized rodlike covalent donor−acceptor molecules consisting of electron-donating ferrocenes (Fc) with electron-accepting perylenediimides (PDIs) with core-substituted cyano and pyrrolidine groups, forming Fc-PDI(CN)2 dyad, Fc2-PDI(CN)2 triad, and Fc-PDI(Py)2 dyad, have been investigated in benzonitrile. The geometric and electronic structures of the dyads and triad were probed by ab initio B3LYP/6-311G methods. The distribution of the highest occupied molecular orbitals (HOMOs) was on the ferrocene entities, while the distribution of the lowest unoccupied molecular orbitals (LUMOs) was on the PDI entities. Free-energy calculations verify that the light-induced processes from excited states of PDIs are exothermic. The excited state photochemical events are monitored by femtosecond and nanosecond transient absorption techniques. In benzonitrile, the quenching pathway involves fast and efficient charge separation from the ferrocenes to the excited PDIs. The finding that the lifetime of the Fc2 +-[PDI(CN)2]•− triad (59 ps) was found to be longer than that of the Fc+-[PDI(CN)2]•− dyad (25 ps) in benzonitrile reflects the effect of the second ferrocene entity in stabilizing the radical ion pairs in the triad. In addition, photoinduced electron transfer in the Fc-PDI(Py)2 dyad occurs via the drastic quenching of singlet state of PDI(Py)2, resulting in the enhancement of triplet state of PDI(Py)2 due to charge recombination of the radical-ion pairs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.