Robert Władysław Góra

Solutions of 5’-methoxy-1’,3’,3’-trimethyl-6-nitrospiro[chromene-2,2’-indoline] in alcohols are intensely pink in the dark, become colorless under white light and recover their color thermally, with Arrhenius activation energies of 101.1± 0.5, 104.9± 1.6 and 105.3± 1.4 kJ mol−1 in ethanol, methanol and acidified ethanol, respectively. The measured process is therefore the thermal formation of the merocyanine (MC) from the photogenerated spiropyran (SP). The absorbance of the equilibrated ethanol solution, combined with the calculated oscillator strength, indicates that MC constitutes only a small fraction of the compound, of the order of a few per cent. With continuum solvation alone no cis-merocyanine intermediate (TCC) is found, whereas in a complex with five explicit ethanol molecules a fast SP⇌TCC pre-equilibrium precedes a considerably slower isomerization to the trans isomer (TTC). The reversible kinetic model shows that the measured rate constant reflects the position of the

Spiropyrans exhibit reversible photochromism that strongly depends on solvent polarity; yet a quantitative understanding of this relationship remains incomplete. Here we combine detailed spectroscopic and kinetic experiments with ab initio calculations to investigate 5’-methoxy-1',3',3'-trimethyl-6-nitrospiro[chromene-2,2'-indoline] (SP). Ultraviolet irradiation at 368 nm populates the S₃ (¹ππ*) state, initiating the ring-opening reaction to the merocyanine (MC) form, which is subsequently followed by thermal ring-closing. Theoretical calculations indicate that photoexcited SP undergoes nonradiative relaxation to the S₁ charge-transfer state, from which the system reaches the ground state via a ring-opened conical intersection. Experimental kinetics studies have revealed that increasing solvent polarity slows relaxation and raises the activation energy from 89.7 kJ mol⁻¹ in dioxane to 104.6 kJ mol⁻¹ in acetone. Calculations have reproduced this trend, showing that polar solvents preferentially stabilize MC over the transition state. The observed blue shift of the MC absorption band with increasing polarity confirms the presence of negative solvatochromism. This combined experimental-theoretical analysis provides a quantitative framework linking solvent polarity, photochromic efficiency, and the activation energy in a model nitrospiropyran system.

A series of five donor–π–acceptor stilbene dyes show efficient trans–cis photoisomerization and photoinduced birefringence. Their high photosensitivity and optical memory highlight potential for advanced photonic and optoelectronic applications.

Ab initio calculations elucidate plausible intramolecular photoinduced relaxation pathways involving ring puckering and an efficient intersystem crossing in A–U and A–T base pairs, potentially hindering electron-driven proton transfer detection.

Numerous biologically active compounds in medicine derive their therapeutic properties from sulfur-bearing five-membered aromatic rings. These compounds are inherently photoactive, and their transient accumulation beneath the skin of living organisms or in sunlit water may lead to photodegradation under sunlight. Among them, 2-aminothiazole is widely known as a molecular scaffold employed in therapeutics, but its photochemical properties remain unknown. Our UV-irradiation experiments and quantum-chemical calculations reveal that substituting a ring sulfur in the 2-aminoazoles family enables absorption in the UV-B range and initiates an ultrafast ring-opening photochemical process through C-S bond cleavage. The resulting reactive intermediate, with a lifetime of ~80 ps, can undergo a reversal to the initial structure or the addition of water molecules to its structure, potentially altering therapeutic agents. Consequently, modified pharmaceutics might not perform their intended functions, trigger harmful effects on living organisms and could become pollution for the biosphere.

He natural environment and living organisms that surround us are made up of chemical compounds, called chromophores, which can absorb photons coming from UV light from the Sun. The one-photon absorption process leads to ultrafast alteration in the electron density of chromophores, resulting in the population of short-lived excited states. These UV-induced electronic states can be responsible for performing high-energy chemical reactions, which cannot be observed in the chemistry of the ground state. Consequently, photochemical processes could damage the initial structure of a chromophore or allow molecules to undergo the chemical transformation to photoproducts. Therefore, understanding the photochemical and photophysical properties of essential chemical molecules for biology, medicine, renewable energy, and other fields is crucial to improve and find applications of light- sensitive systems in daily life. To scrutinize UV-induced chemistry, time-resolved spectroscopy is widely used as an experimental tool to investigate photochemical events. However, the experimental approach cannot provide detailed information about the molecular mechanisms of photochemical processes that occur in the excited states. Therefore, experimental methods in conjunction with computational photochemistry are used to elucidate the behaviour of UV-excited chemical molecules. Only the synergistic approach can comprehensively describe the photochemical picture of UV-induced molecules. This concise review contains a short introduction to the applications of computational chemistry in the studies of the photochemical properties of chromophores, and major radiationless deactivation pathways occurring in heteroaromatic compounds are briefly discussed. Furthermore, two very recent achievements of joint experimental and theoretical photochemistry studies are outlined, demonstrating how solvent or solute molecules can actively participate in photorelaxation channels of chromophores, allowing for an excited- state intermolecular electron transfer mechanism. The selected and discussed research results show that computational chemistry plays an invaluable role in answering questions about molecular mechanisms in the excited states and enables prediction of unexpected chemical processes

Dithiomalonates proved to be active nucleophiles in the stereoselective additions to chalcones, dienones, and en-ynones affording the desired Michael adducts with good to high yields and enantioselectivities. In contrast, the analogous dibenzyl malonate remained inactive. Bifunctional Cinchona-based squaramides secured the effective chirality transfer and the selectivity towards Michael adducts of various bisthiomalonates following the soft enolization approach. The thioester’s nature impacted the reactivity and stability of the reactants or products. While the reactions performed in solution led to the products, the required time along with byproducts formation such as thio-Michael adducts, limiting the applicability of reactive dithioesters. On the contrary, reactions performed under solvent-free, ball milling conditions furnished adducts up to six times faster, with subtly or no byproduct generation. Therefore, the mechanochemical approach revealed to be an effective tool for supporting the hardly effective reactions under standard solution conditions. Detailed KS-DFT studies supported the experimental observations shedding more light on the intricate active nucleophile formation, and different chemical reaction pathways, as well as indicating the crucial transitions state governing the observed stereoselectivities

UV-induced photolysis of aqueous guanine nucleosides produces 8-oxo-guanine and Fapy-guanine, which can induce various types of cellular malfunction.