Treatment of P(X)(X′)Cl with KC₈ gave the crystalline diphosphine [P(X)X′]₂ (1) which dissociated reversibly into the phosphinyl radical ˙P(X)X′ (2), a plausible intermediate in the reaction of 1 with [Cr(CO)₆], [Co(NO)(CO)₃] or P₄, yielding [Cr{P(X)X′}₂(CO)₃] (3), [Co{P(X)X′}(CO)₃] (4), or 1,4-P₄[P(X)X′]₂ (5); the P(X)X′ substituent is pyramidal at P in 5 but planar in 4 [X = N(SiMe₃)₂, X′ = NPrⁱ ₂].

Two novel sensitizers, 2a and 2b, with D/A–π–D–π–A architectures were designed and synthesized based on a phenothiazine framework connected via ethylene with 3,5-bis(trifluoromethyl)phenyl (A) and p-methoxyphenyl (D), and with cyanoacrylic acid as an anchoring group, acting as an acceptor (A). A comparative analysis was performed to investigate the effect of the electronic character of end-capped D and A groups on the photophysical properties and photovoltaic performance of 2a and 2b. The optical, thermal, electrochemical and photovoltaic properties of the dyes were thoroughly investigated and confronted with DFT and TD-DFT calculations. Compound 2b of D–π–D–π–A type showed better photovoltaic parameters, which verify the general trends, although the parameters for compound 2a of A–π–D–π–A type do not differ significantly from those for 2b. The metal-free 2a and 2b-based DSSC devices exhibited higher power conversion efficiencies of 6.11% (2a) and 5.03% (2b) compared to 3.56% for the N719-based device.

New D-π-D-π-A low-molecular-weight compounds, based on a phenothiazine scaffold linked via an acetylene unit with various donor moiety and cyanoacrylic acid anchoring groups, respectively, were successfully synthesized. The prepared phenothiazine dyes were entirely characterized using nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The compounds were designed to study the relationship between end-capping donor groups’ structure on their optoelectronic and thermal properties as well as the dye-sensitized solar cells’ performance. The effect of π-conjugation enlargement by incorporation of different heterocyclic substituents possessing various electron–donor affinities was systematically experimentally and theoretically examined. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were implemented to determine the electronic properties of the novel molecules.

Treatment of PCl[N(SiMe₃)₂](NPrⁱ ₂) (1) with potassium–graphite in thf afforded the colourless, crystalline diphosphine {P[N(SiMe₃)₂](NPrⁱ ₂)}₂ (2) in good yield. Sublimation of 2in vacuo yielded the yellow phosphinyl radical P[N(SiMe₃)₂](NPrⁱ ₂) (3), which upon cooling reverted to 2; the latter in C₆D₆ at 298 K was a mixture of rac and meso diastereoisomers. The yellow, crystalline phosphenium salt {P[N(SiMe₃)₂](NPrⁱ ₂)}[AlCl₄] (4) was obtained from 1 and ½Al₂Cl₆ in CH₂Cl₂. By single-crystal X-ray diffraction (XRD) the structures of the known compound 1 and of 2 and 4 were determined. The structure of the radical 3, formed by the thermal homolytic dissociation of the diphosphine 2, was determined in the gas phase by electron diffraction (GED), utilising data from UMP2/6-31+G* ab initio calculations. The model of the molecule in the GED structure analysis was described by a set of internal coordinates and an initial set of Cartesian coordinates from ab initio calculations, facilitating the structure analysis. The experimental data were found to be consistent with the presence of a single conformer of the radical in the gas phase. The computed standard homolytic dissociation enthalpy of the P–P bond in the corresponding diphosphine 2, corrected for BSSE, 54 kJ mol⁻¹, is substantially reduced compared to the dissociation enthalpy of tetramethyldiphosphine by the reorganisation energies of the fragments that form upon dissociation. The intrinsic energy content of the P–P bond in the diphosphine 2 was estimated to be 286 kJ mol⁻¹, in agreement with the results of previous work on a series of crowded diphosphines.