Helically chiral N,N,O,O‐boron chelated dipyrromethenes showed solution‐phase circularly polarized luminescence (CPL) in the red region of the visible spectrum (λ em(max) from 621 to 663 nm). The parent dipyrromethene is desymmetrised through O chelation of boron by the 3,5‐ortho‐phenolic substituents, inducing a helical chirality in the fluorophore. The combination of high luminescence dissymmetry factors (|g lum| up to 4.7 ×10−3) and fluorescence quantum yields (Φ F up to 0.73) gave exceptionally efficient circularly polarized red emission from these simple small organic fluorophores, enabling future application in CPL‐based bioimaging.
A series of redox tunable polyoxometalate–bodipy conjugates display variable charge transfer dynamics occuring down to 54 ps.
A series of photosensitizers for NiO-based dye-sensitized solar cells is presented. Three model compounds containing a triphenylamine donor appended to a boron dipyrromethene (bodipy) chromophore have been successfully prepared and characterised using emission spectroscopy, electrochemistry and spectroelectrochemistry, to ultimately direct the design of dyes with more complex structures. Carboxylic acid anchoring groups and thiophene spacers were appended to the model compounds to provide five dyes which were adsorbed onto NiO and integrated into dye-sensitized solar cells. Solar cells incorporating the simple Bodipy-CO₂H dye were surprisingly promising relative to the more complex dye 4. Cell performances were improved with dyes which had increased electronic communication between the donor and acceptor, achieved by incorporating a less hindered bodipy moiety. Further increases in performances were obtained from dyes which contained a thiophene spacer. Thus, the best performance was obtained for 7 which generated a very promising photocurrent density of 5.87 mA cm(-2) and an IPCE of 53%. Spectroelectrochemistry combined with time-resolved transient absorption spectroscopy were used to determine the identity and lifetime of excited state species. Short-lived (ps) transients were recorded for 4, 5 and 7 which are consistent with previous studies. Despite a longer lived (25 ns) charge-separated state for 6/NiO, there was no increase in the photocurrent generated by the corresponding solar cell.
Dye-sensitized solar cells are photoelectrochemical devices, which are of great interest due to their ease of fabrication and attractive design.
Understanding what influences the formation and lifetime of charge-separated states is key to developing photoelectrochemical devices. This paper describes the use of time-resolved infrared absorption spectroscopy (TRIR) to determine the structure and lifetime of the intermediates formed on photoexcitation of two organic donor-π-acceptor dyes adsorbed to the surface of NiO. The donor and π-linker of both dyes is triphenylamine and thiophene but the acceptors differ, maleonitrile (1) and bodipy (2). Despite their structural similarities, dye 1 outperforms 2 significantly in devices. Strong transient bands in the fingerprint region (1 and 2) and nitrile region (2300-2000 cm) for 1 enabled us to monitor the structure of the excited states in solution or adsorbed on NiO (in the absence and presence of electrolyte) and the corresponding kinetics, which are on a ps-ns timescale. The results are consistent with rapid (<1 ps) charge-transfer from NiO to the excited dye (1) to give exclusively the charge-separated state on the timescale of our measurements. Conversely, the TRIR experiments revealed that multiple species are present shortly after excitation of the bodipy chromophore in 2, which is electronically decoupled from the thiophene linker. In solution, excitation first populates the bodipy singlet excited state, followed by charge transfer from the triphenylamine to the bodipy. The presence and short lifetime (τ ≈ 30 ps) of the charge-transfer excited state when 2 is adsorbed on NiO (2|NiO) suggests that charge separation is slower and/or less efficient in 2|NiO than in 1|NiO. This is consistent with the difference in performance between the two dyes in dye-sensitized solar cells and photoelectrochemical water splitting devices. Compared to n-type materials such as TiO, less is understood regarding electron transfer between dyes and p-type metal oxides such as NiO, but it is evident that fast charge-recombination presents a limit to the performance of photocathodes. This is also a major challenge to photocatalytic systems based on a "Z-scheme", where the catalysis takes place on a µs-s timescale.
Dye-sensitized photocathodes (p-DSSCs) were first proposed as a way to increase the efficiency of dye-sensitized solar cells over a decade ago.1 Such photocathodes would replace the platinised counter electrode of the Grätzel cell, and by harvesting lower energy photons than the photoanode increase spectral coverage as well as increasing photovoltage. In this way, the p-DSSC could help drive advances from the current DSSC record of 13%, 2 towards (or beyond) the 25% attained by crystalline Si: 3 the resulting tandem DSSCs would have a maximum theoretical efficiency of 43%, vs. 33% for a single junction device. But to date, no tandem DSSC has exceeded the power conversion efficiency of a state-of-the-art, single-junction TiO 2 n-DSSC, because they are limited by the poor performance of the series connected p-DSSC. The only p-DSSC power conversion efficiencies 41% have been achieved using redox mediators whose negative redox potentials would severely reduce the photovoltage produced by the TiO 2 side of any tandem cell. 4 With the more typical n-DSSC I 3 À /I À redox couple, the p-DSSC record efficiency is just 0.61%. 5The poor performance of p-DSSCs results from the small energy difference between the valence band (VB) of the p-type semiconductor (usually NiO) and redox mediator, rapid back transfer of photogenerated holes from NiO to the electrolyte, and slow hole diffusion through NiO.6 Consequently, open circuit voltages (V OC ) are low -the maximum obtained with I 3 À /I À is 350 mV using special high crystallinity NiO and a dye with a highly extended conjugated system. 7 More typically, V OC is around 100 mV, and rarely exceeds 150 mV. At the same time, fill factors are low (ca. 30%) and photocurrents ( J SC ) are moderate (up to 8.2 mA cm À2 ). 8 While the improvements achieved through sensitizer design have been impressive, 4,6,8 the necessary step change in p-DSSC performance will require exploration of new strategies, and materials. One simple modification employed in n-DSSCs, but little investigated in p-DSSCs, 9a is the use of coadsorbents such as cholic acid derivatives and alkyl phosphonic acids. 9b-e These can prevent dye aggregation, and passivate the surface helping to suppress recombination, reduce dark current and increase stability and efficiency. We expect that in p-DSSCs co-adsorbents can play a similar role, while potentially also providing a remote electron acceptor, isolated from the NiO VB that can relay electrons to the mediator and impede recombination. This strategy may have advantages over incorporating the acceptor into the dye -it facilitates a rapid, combinatorial approach to new cells and should reduce communication between electron and hole. Owing to their fast electrochemistry, tunable potentials and stability to redox cycling, 10 polyoxometalates (POMs) appear a good but as yet untested choice of electron acceptor coadsorbent for p-DSSCs. Ultrafast electron transfer (ET) has been observed between POMs and dyes on TiO 2 , 11 and in n-DSSCs, POMs can enhance performance by incre...
The optical and electrochemical properties of a series of polyoxometalate (POM) oxoclusters decorated with two bodipy (boron‐dipyrromethene) light‐harvesting units were examined. Evaluated here in this polyanionic donor‐acceptor system is the effect of the solvent and associated counterions on the intramolecular photoinduced electron transfer. The results show that both solvents and counterions have a major impact upon the energy of the charge‐transfer state by modifying the solvation shell around the POMs. This modification leads to a significantly shorter charge separation time in the case of smaller counterion and slower charge recombination in a less polar solvent. These results were rationalized in terms of Marcus theory and show that solvent and counterion both affect the driving force for photoinduced electron transfer and the reorganization energy. This was corroborated with theoretical investigations combining DFT and molecular dynamics simulations.
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