From truxenes to heterotruxenes: playing with heteroatoms and the symmetry of molecules

As a result of the modification of truxene, we can change the electronic structure or create multidimensional materials. Thus, the use of truxenes is very wide.

Influence of the heteroatom introduction on the physicochemical properties of 5-heterotruxenes containing nitrogen, oxygen and sulfur atom

5-heterotruxenes are a class of heterocyclic aromatic compounds derived from the parent heptacyclic hydrocarbon, namely truxene. Currently, few substances belonging to this group are known. These include derivatives of 5-oxatruxene, 5-azatruxene, and 5-thiatruxene, along with its sulfone and sulfoxide. The introduction of one heteroatom, not only enables selective monofunctionalization of the 5-heterotruxene system but also allows adjustment of physico-chemical properties depending on the needs. Two investigated compounds, namely 5-oxatruxene and 5-azatruxene, exhibit fluorescence enhancement by symmetry breaking (FESB) phenomena, manifested by a fourfold increase in the fluorescence quantum yield. Therefore, derivatives of 5-heterotruxenes may contribute to the development of new stable optoelectronic substances as well as other functional materials. Nevertheless, in the beginning, it is crucial to investigate their spectral, thermal, and electrochemical properties to learn more about the advantages and limitations of these aromatic systems. The following article also presents a new simplified and universal synthetic methodology, without use of anhydrous conditions or organometallic substances, giving easy access to 5-azatruxene and structural-related heteroaromatic systems. The discussed heteroatom influence is not limited to the truxene core but also helps to understand the physicochemical properties of other, currently top-rated high-symmetric heteroaromatic systems such as circulenes, sumanenes and their analogues.

Ferrocene Derivatives Functionalized with Donor/Acceptor (Hetero)Aromatic Substituents: Tuning of Redox Properties

A series of functionalized ferrocene derivatives carrying electron-donor and electron-withdrawing (hetero)aromatic substituents has been designed as potential alternative electrolyte redox couples for dye-sensitized solar cells (DSSC). The compounds have been synthesized and fully characterized in their optical and electrochemical properties. A general synthetic approach that implies the use of a microwave assisted Suzuki coupling has been developed to access a significative number of compounds. The presence of different electron-rich and electron-poor substituents provided fine tuning of optical properties and energy levels. HOMO and LUMO energy values showed that the substitution of one or two cyclopentadienyl rings of ferrocene can be successfully exploited to increase the maximum attainable voltage from a standard DSSC device using TiO2 as a semiconductor, opening the way to highly efficient, non-toxic, and cheap redox shuttles to be employed in solar energy technologies.

Recent Advances of Cobalt(II/III) Redox Couples for Dye-Sensitized Solar Cell Applications

In recent years dye-sensitized solar cells (DSSCs) have emerged as one of the alternatives for the global energy crisis. DSSCs have achieved a certified efficiency of >11% by using the I(-) /I3 (-) redox couple. In order to commercialize the technology almost all components of the device have to be improved. Among the various components of DSSCs, the redox couple that regenerates the oxidized sensitizer plays a crucial role in achieving high efficiency and durability of the cell. However, the I(-) /I3 (-) redox couple has certain limitations such as the absorption of triiodide up to 430 nm and the volatile nature of iodine, which also corrodes the silver-based current collectors. These limitations are obstructing the commercialization of this technology. For this reason, one has to identify alternative redox couples. In this regard, the Co(II/III) redox couple is found to be the best alternative to the existing I(-) /I3 (-) redox couple. Recently, DSSC test cell efficiency has risen up to 13% by using the cobalt redox couple. This review emphasizes the recent development of Co(II/III) redox couples for DSSC applications.

Very fast singlet and triplet energy transfers in a tri-chromophoric porphyrin dyad aided by the truxene platform

A trichromophoric dyad composed of an octa-β-alkyl-palladium(II)porphyrin (donor) and two tri-meso-aryl-zinc(II)porphyrins (acceptors) held by a truxene spacer exhibits very fast rates for triplet energy transfers at 77 (kET(T1) = 1.63 × 108 s-1) and 298 K (kET(T1) = 3.44 × 108 s-1), whereas the corresponding singlet energy transfer rates, kET(S1) = 3.9 × 1010 s-1 (77 K) and kET(S1) = 6.0 × 1010 s-1 (298 K), are also considered fast. The interpretation for these results is that the energy transfer processes proceed via a through bond Dexter mechanism (i.e. double electron exchange) supported by comparison with literature data and evidence for a moderate MO coupling between the donor and acceptor chromophores in the frontier MOs.

Truxene: a promising scaffold for future materials

Truxene (10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene), which is a heptacyclic polyarene structure, has attracted a great deal of interest due to its exceptional solubility, high thermal stability and ease with which it may be modified.

Judicious design of indoline chromophores for high-efficiency iodine-free dye-sensitized solar cells

Indoline photosensitizers exhibit impressive short-circuit photocurrent but generally low molar extinction coefficient and rapid charge recombination, which limits their application in thin-film dye-sensitizerd solar cells (DSCs). Here, we incorporate a new dithieno[3,2-b:2',3'-d]pyrrole (DTP) segment (i.e., dihexyloxy-triphenylamine (DHO-TPA) substituted DTP) as the conjugated π-linker to construct a series of high molar absorption coefficient indoline dyes (XW69, XW70, and XW71) for DSCs employing a cobalt(II/III) redox electrolyte. Interestingly, this DTP linker is demonstrated as an efficient building block, not only slowing down the kinetics of charge recombination of titania electrons with tris(1,10-phenanthroline)cobalt(III) ions but also making a great contribution to the light absorption properties in comparison with the dihexylaniline substituted DTP. With respect to the dihexyloxy-triphenylamine dye (XW68), these new indoline dyes exhibit stronger light-harvesting and thus better power conversion efficiency of DSCs made from thin titania films. Benefitting from the bulky rigidity of the donor and π-conjugation unit, the XW70 dye displays a promising conversion efficiency as high as 8.78%, with a short-circuit current density (J(SC)) of 13.3 mA cm(-2), open-circuit voltage (V(OC)) of 943 mV, and fill factor (FF) of 0.70 under AM 1.5 illumination (100 mW cm(-2)). Furthermore, the effect of light irradiation on these dyes adsorbed on nanocrystalline TiO2 films was investigated, proving the photostability of these indoline chromophores. Our work has valued the feasibility of judicious design of indoline chromophores to obtain organic photosensitizers for high-efficiency iodine-free DSCs made from thin titania films.

Origin of the temperature dependence of the rate of singlet energy transfer in a three-component truxene-bridged dyads

We report a truxene-based dyad built upon one donor (tri-meso-phenylzinc(II)porphyrin) and two acceptors (octa-β-alkylporphyrin free base) in which the donor exhibits free rotation around a Ctruxene-Cmeso single bond at 298 K in fluid solution but not at 77 K in a glass matrix, whereas the acceptors have very limited motion as they are blocked by β-methyl groups. This case is interesting because all the structural and spectroscopic parameters affecting the rate for singlet energy transfer according to a Förster Resonance Energy Transfer are only weakly temperature dependent, leaving only the Dexter mechanism explaining the larger variation in rate of energy transfers with the temperature hence providing a circumstantial evidence for a dual mechanism (Föster and Dexter) in truxene-based dyads (or polyads) in the S1 excited states.

Joint electrical, photophysical, and photovoltaic studies on truxene dye-sensitized solar cells: impact of arylamine electron donors

The judicious design of electron donors is one of the viable tactics to improve the efficiency of organic dyes for dye-sensitized solar cells (DSCs) employing outer-sphere redox couples. Herein, a hexahexyltruxene-substituted 4-(hexyloxy)-N-phenylaniline (HT-HPA) segment is constructed and employed as the electron donor in two organic push-pull dyes (M28 and M29) with high molar absorption coefficient values. Relative to its congener (C241) possessing the dihexyloxy-substituted triphenylamine electron donor, M29 exhibits red-shifted absorption as well as enhanced maximum molar absorption coefficient values. A thorough comparison with M29 and C241 demonstrates that the HT-HPA segment adequately insulates the TiO2 surface from the electrolyte, which prevents back-recombination and prolongs electron lifetime in the semiconductor. The diminishment of charge recombination not only enables attainment of strikingly high photovoltages (approaching 1 V), but also overcompensates the disadvantageous impact of lower dye-load amounts. As a result, the dye transformation from C241 to M29 brings forth an efficiency improvement from 7.3 % to 8.5 % at the 100 mW cm(-2) simulated AM1.5 conditions. Our work should shed light on the future design of more powerful push-pull organic photosensitizers for iodine-free DSCs.