2015 is the International Year of Light and Light-Based Technologies, and this special issue highlights achievements in photochemistry and photophysics research from across the globe. The research topics presented range from the sustainable production of medicines to the advancement of modern technology.

Titanium dioxide is a versatile inexpensive heterogeneous photocatalyst. Apart from many applications in environmental chemistry, numerous applications in organic synthesis have been reported. Selective oxidation and reduction of organic compounds and C–C bond formation reactions have been carried out.

2-Aryl-1,3-dimethylbenzimidazolines (DMBIHs) act as effective electron and hydrogen donors for photoinduced electron-transfer reduction of various organic substrates in which photoexcitation of the substrates as well as photosensitization utilizing substituted pyrenes or transition metal complexes as sensitizers are carried out.

Visible light excitation of 2-naphthyl- or 2-hydroxynaphthyl-substituted 1,3-dimethylbenzimidazolines (DMBIH) promoted the reduction reaction of some organohalides, involving the cleavage of aromatic as well as aliphatic carbon–halogen bonds. Addition of certain bases, such as 1,8-diazabicyclo[5.4.0]undec-7-ene, significantly enhanced the reducing ability of hydroxynaphthyl-substituted DMBIH.

Sunlight-induced bromination of benzylic compounds was conducted in a capillary microreactor without an artificial light source or additional temperature control. These reactions can be considered to be eco-friendly.

An efficient, one-flow Arndt–Eistert synthesis was demonstrated. A sequence of acid chloride formation–nucleophilic acyl substitution–Wolff rearrangement–nucleophilic addition was performed in a microflow system without isolating any intermediates, which included a potentially explosive compound. The microflow system was made from simple, inexpensive, and readily available instruments and tubes.

Photodecarboxylative benzylations of N-methoxyphthalimide were successfully realized and gave the corresponding benzylated hydroxyphthalimidines in moderate to good yields of 52–73 %. The photobenzylation was furthermore investigated in an advanced continuous-flow photoreactor, which enabled rapid optimization of the reaction conditions and achieved good yields after short residence times.

The photoinduced decarboxylation of carboxylate ions bearing electron-withdrawing groups using biphenyl and 1,4-dicyanonaphthalene leads to the efficient generation of carbanions under mild conditions. In particular, the cyanomethyl anion formed using this photochemical method can be added to benzaldehydes to give the corresponding adducts.

Unprecedented molecular architectures are accessed via intramolecular cycloadditions of photogenerated azaxylylenes. The approach is well-aligned with diversity-oriented synthesis, whereby photoprecursors are synthesised in a modular fashion allowing for up to four diversity inputs, with the complexity of the primary photoproducts being further enhanced using high-yielding post-photochemical steps.

The photochemical rearrangement of spirooxindole epoxides was performed catalytically with a chiral bifunctional xanthone. The substrates are non-covalently bound via hydrogen bonds to the photochemically active catalyst, which presumably acts as a triplet sensitizer. Upon irradiation with λ 366 nm, the 3-acylindolin-2-ones were formed in high yields (85–98 %) and with enantioselectivities up to 33 % ee.

The effect of solvent polarity and temperature, as well as excitation wavelength, was examined for the diastereoselectivities of the Paternó-Büchi reaction of chiral cyanobenzoate with 1-(1-naphthyl)-1-phenylethene to reveal the different nature of the excited charge-transfer complex (ECT) and the conventional exciplex (EX). The exciplex ensemble was critically modulated by increased donor–acceptor interactions.

The rate constant of intersystem crossing (ISC), k_ISC ~3 × 10⁷ s^–1, for cyclopentane-1,3-diyl diradicals was unequivocally determined by the simultaneous observation of the decay process of the triplet diradical (λ_obs = 320 nm) and the growth process of the corresponding singlet diradical (λ_obs = 560 nm). The two spin states were directly observed using a long-lived singlet 2,2-dimethoxy-1,3-diphenylcyclopentane-1,3-diyl diradical.

Intramolecular triplet sensitization of a simple alkene results in the formation of 1,2-biradical. Laser flash photolysis was used to detect the biradical directly; the biradical has a lifetime of a few microseconds. Density functional theory calculations reveal that two radical centres are localized on the adjacent carbon atoms.

By coprecipitating a sodium birnessite-like phase with and without phosphate (1.5 %), we are able to engineer two very similar but distinct materials – one that is nanocrystalline and the other that is amorphous.

Colorimetric detection of nitroaromatics using an organic photochromic compound is demonstrated.

The new red-emitting phosphor, La₂Zr₃(MoO₄)₉:Eu³⁺, exhibits broad absorption bands due to admixed Zr⁴⁺, Mo⁶⁺, and Eu³⁺ charge transfer (CT) bands. The influence of the various means of excitation on the emission is investigated. A bi-sigmoidal temperature-quenching behaviour was observed and discussed in relation to the CT bands.

Efficient luminophores with quantum yields of 48 % were obtained for Tb(diclo)₃phen and Tb(diclo)₃bipy complexes, while analogous Eu³⁺ complexes remained inefficient with quantum yields below 13 %.

Vertical and lateral carrier mobilities of a polymer donor–PC₆₁BM system were measured with the space charge limited current (SCLC) and field-effect transistor methods. Some blend films annealed at high temperatures exhibited an abrupt increase in the field-effect hole mobility, which was interpreted by an antiplasticization effect.

Boron subphthalocyanines (BsubPcs) have shown potential as functional materials within organic photovoltaics. In this paper, we revisit an understudied class of BsubPcs with an aryl axial substituent. We have described their relevant properties for application in organic photovoltaics, and incorporated them directly into photovoltaic cells. The resulting cells demonstrate the high potential of this class of BsubPcs within this area.

Donor–acceptor (D-A) copolymers of thieno[3,4-b]pyrazine (TP) and fluorene are reported in order to illustrate the ability to tune the light absorption properties of TP-based materials via basic structure–function relationships.

Narrow dispersity polymers with tethered fullerenes were synthesised and fabricated into organic photovoltaic devices, achieving up to 0.4 % efficiency.

Materials for organic polymer solar cells should have maximized light absorption across the solar spectrum. An approach is described to synthesize donor–acceptor conjugated polymers that are optimized for light harvesting properties. The best candidate polymer is characterized using a range of photophysical techniques and its performance in solar cell devices evaluated.

Tin oxide (SnO₂) has been widely explored as electron-transport material in solar cells owing to its excellent mobility and transparency. This work reports on the application of SnO₂ and SnO₂/Sn nanoparticles in perovskite solar cells (PSCs). The study demonstrates a new approach to improve the performance of PSCs through material engineering.