In order to further advance research on the functionalization of semiconductor surfaces and polymer materials, the axial complexes of (octakis(3,5-di-tert-butylphenoxy)phthalocyaninato)cobalt(ii) (CoPc) with nicotinic/isonicotinic (Nic/Isonic) acid in a 1 : 2 stoichiometry were obtained and characterized as highly stable, with stability constants in the range of similar to 104-105 M-1. The spatial and electronic structures were studied using UV-vis, 1H NMR, and ESR spectroscopy, mass spectrometry, and DFT calculations. The axial Nic/Isonic ligands retain reactive carboxyl groups for further covalent immobilization on semiconductor surfaces. The redox behavior of the obtained axial complexes and the dynamics of ultrafast processes in these complexes upon photoexcitation were studied, respectively, by cyclic voltammetry and femtosecond transient absorption spectroscopy. It showed the correspondence between the decay dynamics/excited state lifetimes of (Isonic)2CoPc and (Nic)2CoPc and the corresponding parameters of uncoordinated CoPc, which confirms the stability of CoPc with respect to axial modifications and provides a fundamental basis for the design of donor-acceptor systems.

The photophysical and photochemical properties of carborane-substituted zinc(II) phthalocyanine (ZnBPc) and its noncovalent complexes with albumin were studied to elucidate the reactivity triplet-state and photoinduced electron transfer pathways relevant to photodynamic therapy (PDT). The binding constant of the ZnBPc-BSA complex (1.5 & times; 106 M-1) indicates strong affinity facilitating its efficient delivery to malignant cells. Steady-state and time-resolved spectroscopy, including nanosecond transient absorption and femtosecond pump-probe techniques showed that ZnBPc in both free and albumin-bound forms efficiently populates a long-lived triplet state capable of generating singlet oxygen (Phi Delta = 0.54), comparable to unsubstituted ZnPc. Quenching the triplet state with molecular oxygen and ascorbic acid demonstrated that BSA complexation introduces steric hindrance decreasing bimolecular quenching rate constants relative to free ZnBPc. Photoinduced electron transfer from ascorbic acid to the triplet state yielded ZnBPc radical anions, characterized by absorption at 550-630 nm and second-order decay kinetics due to dark back electron transfer. In aqueous HSA-containing systems, femtosecond experiments provided direct evidence for ultrafast (0.5 ps) electron transfer from aromatic amino acid residues to ZnBPc, generating an excited singlet radical-ion pair 1[HSA(Tyr center dot+)-ZnBPc center dot-]. The decay kinetics of the radicalions pair occurs via back fast electron transfer (50 ps) limiting superoxide generation via this pathway. These results indicate that the albumin-ZnBPc complex maintains the high reactivity of the triplet state toward oxygen by modulating the electron transfer and energy transfer dynamics, which opens the potential for selective photochemical action in PDT applications involving superoxide and singlet oxygen.

Time-resolved femtosecond absorption spectroscopy data together with TD DFT calculations for CD6, a bis(aza-18crown-6)-containing dienone derivative of cyclohexanone, as well as its complex with EtNH3+ confirm that the latter undergoes photoinduced recoordination. This process takes hundreds of femtoseconds and involves N & centerdot;& centerdot;& centerdot;H & centerdot;& centerdot;& centerdot;N bond cleavage along with the formation of new O & centerdot;& centerdot;& centerdot;H & centerdot;& centerdot;& centerdot;N bonds. There is no change in the conformation of the complex upon photorecoordination.

The charge separation mechanism in spinach photosystem II core complexes (PSII-CC) and D1/D2/cytb559 reaction centers (D1D2-RC) was investigated using broadband femtosecond absorption spectroscopy in combination with kinetic modeling. The population dynamics of electronic states was analyzed based on decomposing the spectral-temporal matrices ΔA(λ,t) into contributions from excited states of chlorophyll (Chl) and difference spectra of the oxidized electron donor P680 and the reduced pheophytin acceptor PheoD1 obtained by the steady-state photoaccumulation technique. The results are discussed within the models, suggesting that in D1D2-RC at 279 K/77 K: (i) the predominant mechanism of primary charge separation is formation of the ChlD1+PheoD1- ion-radical pair in 3.6/7.6 ps followed by the hole transfer to P680 in 30/108 ps; (ii) the excitation energy transfer from ChlZ molecules to central RC pigments occurs within 19/28 ps, overlapping in time with the electron-transfer reactions; (iii) the free energy level of ChlD1+PheoD1- is higher than that of the relaxed exciton state by ∼30 meV, but the states become equally energetic during the ChlD1+ → P680 hole transfer. The charge separation kinetics in D1D2-RC at 77 K was found to be significantly heterogeneous. In PSII-CC at 279 K, (i) the apparent time of energy transfer from the excited antenna to RC is ∼60 ps, while the effective time of the ChlD1+PheoD1- formation in a small fraction of complexes with directly excited RC is comparable to that in D1D2-RC (3.7 ps); (ii) unlike D1D2-RC, the primary charge separation occurs with a decrease in free energy between RC⁎ and ChlD1+PheoD1-.

The analysis of time-resolved S1–Sn absorption spectra in the 0–500 ps range, together with quantum-chemical calculations, uncovered a photorecoordination reaction for the following complexes of CD6 (a bis(aza-18-crown-6)-containing dienone (ketocyanine dye) with a central cyclohexanone fragment): CD6·(Mn+)2 (M = Ba2+, Sr2+, Ca2+, K+). This process takes place over hundreds of fs and involves an “axial-to-equatorial” conformational change, with the solvation shell undergoing rearrangement as well. The characteristic photorecoordination times were found to correlate with the stability constants of the complexes. The lifetimes for the fluorescent states of CD6 and its complexes, namely CD6·(Mn+)2 (M = Ba2+, Sr2+, Ca2+, K+), are different; ergo, there is no photoejection of crowned cations into the solution. The calculated conformational profiles in the ground and excited states indicate the presence of an energy barrier in this process. A general photorelaxation pathway is suggested for CD6·(Mn+)2 metal complexes (M = Ba2+, Sr2+, Ca2+, K+). The coordination of cations via the carbonyl moiety in the dye molecule promotes photorecoordination of metal cations in the cavities of the azacrown ether fragment. Photorecoordination times were found to correlate with the degree of conjugation between the lone pairs in the N atoms of the aza-18-crown-6 ether and the π subsystem in the dye molecules (established for the CD4–CD6 metal–dye complex series, where CD4 and CD5 are related dyes with central cyclobutanone and cyclopentanone fragments, respectively).

The Orange Carotenoid Protein (OCP) is a unique water-soluble photoactive protein that plays a critical role in regulating the balance between light harvesting and photoprotective responses in cyanobacteria. The challenge in understanding OCP´s photoactivation mechanism stems from the heterogeneity of the initial configurations of its embedded ketocarotenoid, which in the dark-adapted state can form up to two hydrogen bonds to critical amino acids in the protein’s C-terminal domain, and the extremely low quantum yield of primary photoproduct formation. While a series of experiments involving point mutations within these contacts helped us to identify these challenges, they did not resolve them. To overcome this, we shifted from classical mutagenesis to the translational introduction of non-canonical amino acid residues into the OCP structure. In this work, we demonstrate that replacing a single meta-hydrogen in tyrosine-201 with a halogen atom (chlorine, bromine, or iodine) leads to targeted modifications in the keto-carotenoid-protein matrix interaction network, both in the dark-adapted state and upon photoactivation. We found that such atomic substitutions allow us to effectively weaken key hydrogen bonds without disrupting protein folding, thereby increasing the yield of OCP photoactivation products. Such genetically encoded chemical modification of individual atoms and their systematic in situ variation in complex protein structures establishes a foundation for transforming OCP into a practical tool for optogenetics and other applications.

The Z-isomer of N,N’-diammoniopropyl derivative of di(3-pyridyl)ethylene was synthesized. The structure and stability of complexes between this non-planar weak acceptor (A, (Z)-2) and a planar strong donor, the E-isomer of bis(18-crown-6)stilbene (D, (E)-1), were studied using X-ray diffraction, 1H NMR spectroscopy, and optical spectroscopy, including 1H NMR and spectrofluorimetric titrations. In MeCN, the components form a very stable pseudocyclic bimolecular complex (logKD·A = 8.48) due to homoditopic coordination of the ammonium groups of the acceptor to the crown moieties of the donor through numerous hydrogen bonds. Intrasupramolecular photo-driven electron transfer (ET) in the isomeric complexes of (E)-1 with (E)- and (Z)-2 was studied using steady-state absorption and fluorescence spectroscopy with time-resolved pulse absorption spectroscopy. It was found that back ET is approximately two times faster in complex (E)-1·(Z)-2 than in closely related (E)-1·(E)-2. Meanwhile, it is ~67 times slower in complex (E)-1·(E)-2 than in the isomeric complex based on N,N’-diammoniopropyl derivative of (E)-di(4-pyridyl)ethylene. Quantum chemical (DFT, TD-DFT) calculations suggest the actual photorelaxation pathway for the complexes under study.

(Octakis-(3,6-di-tert-butyl-9H-carbazole)phthalocyaninato)cobalt(II) (CoPc(Carb)8) was synthesized as a donor platform for coordination-driven self-assembly with 1-N-methyl-2-(pyridin-4-yl)pyrrolidino[3',4':1,2][60]fullerene (PyC60) and 4-(10-phenylanthracen-9-yl)pyridine. The formation of 1:1 coordination complexes (dyads) was confirmed by spectroscopic techniques (ultraviolet-visible (UV-vis), fluorescence, IR, MALDI-TOF), chemical thermodynamics and kinetics, and density functional theory (DFT) calculations. A model study with pyridine revealed the formation of both 1:1 and 1:2 complexes. DFT calculations were performed to determine the bonding energies and frontier orbital energies of CoPc(Carb)8 and its dyads. Similar highest occupied molecular orbital (HOMO) energies of CoPc(Carb)8 and its dyads result from their comparable electronic structures, while the variation in lowest unoccupied molecular orbital (LUMO) energies arises from differences in orbital composition, with contributions from the phthalocyanine core's nonmetallic atoms and the cobalt 3dyz orbital. To evaluate the influence of carbazole substituents, analogous dyads based on (octakis(3,5-di-tert-butylphenoxy)phthalocyaninato)cobalt(II) (CoPc(3,5-tBuPhO)8) were investigated. The combination of quantum chemical calculations and the ultrafast femtosecond transient absorption spectroscopy method has allowed to discover and study the charge-separated states such as PyC60•- -CoPc(Carb)8•+ and PyC60•- -CoPc(3,5-tBuPhO)8•+, with lifetimes of 90.7 and >500 ps, respectively, in toluene. These findings enhance the understanding of cobalt(II) phthalocyanine-based donor-acceptor systems and inform the design of materials for photovoltaic applications.

An analysis of the results of time-resolved transient S1→Sn absorption spectroscopy studies and quantum chemical calculations of the dye (2E,5E)-2,5-bis[4-(1,4,7,10,13-pentaoxa-16-azacyclooctadecane-16-yl)benzylidene]cyclopentanone (1) and its metal complexes confirm the generality of the phenomenon of photoinduced recoordination of metal cations in the complexes of bis(aza-18-crown-6)-containing derivatives of the dibenzylidenecyclobutanone (dibenzylidenecyclopentanone) series. The results obtained confirmed the existence of the first stage of photoinduced recoordination of metal cations in the 1 • (Mn+)2 (M = Ba2+, Ca2+, K+) complexes which completes within a few hundreds of femtoseconds. The process involves cleavage of the N—M bond followed by displacement of the metal cation from its equilibrium position in the azacrown ether cavity and by transformation of the “axial” conformation of the complex to “equatorial” one. It was demonstrated that the barrier photoinduced recoordination of the cation in the 1 • (Ba2+)2 complex is accompanied by the change of the type of the solvation shell of the crowned Ba2+ cation in the following order: (2+1)MeCN, (3+1)MeCN, 4MeCN. In a low-temperature butyronitrile glassy matrix at 77 K, the photoinduced recoordination is completely suppressed. The Mg2+, Li+, and Na+ cations can form not only the inclusion complexes, but also the 1: 3 complexes ((Mn+) • 1 • (Mn+)2) of moderate stability through additional coordination to the carbonyl group of the dye. The radii, rS, of solvates of different-stoichiometry complexes containing the same metal cation determined from the data of anisotropy decay kinetics of S1→Sn absorption are about 9.2 Å for 1 • (Mg2+)2 and nearly 11.5 Å for Mg2+ • 1 • (Mg2+)2.

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Q(y) band. Absorption changes of FRL-PSII were monitored at 278 K in the 400-800 nm spectral range on a timescale of 0.1-500 ps upon selective excitation at 740 nm of four chlorophyll (Chl) f molecules in the light harvesting antenna, or of one Chl d molecule at the Chl(D1) position in the reaction center (RC) upon pumping at 710 nm. Numerical analysis of absorption changes and assessment of the energy levels of the presumed ion-radical states made it possible to identify P(D1)(+)Chl(D1)(-) as the predominant primary charge-separated radical pair, the formation of which upon selective excitation of Chl d has an apparent time of similar to 1.6 ps. Electron transfer to the secondary acceptor pheophytin Pheo(D1) has an apparent time of similar to 7 ps with a variety of excitation wavelengths. The energy redistribution between Chl a and Chl f in the antenna occurs within 1 ps, whereas the energy migration from Chl f to the RC occurs mostly with lifetimes of 60 and 400 ps. Potentiometric analysis suggests that in canonical PSII, P(D1)(+)Chl(D1)(-) can be partially formed from the excited (P(D1)Chl(D1))* state.

The operation of bacteriorhodopsin (BR) from the archaeon Halobacterium salinarum is based on the photochromic reaction of isomerization of the chromophore group (the retinal protonated Schiff base, RPSB) from the all-trans to the 13-cis form. The ultrafast dynamics of the reverse 13-cis → all-trans photoreaction was studied using femtosecond transient absorption spectroscopy in comparison with the forward photoreaction. The forward photoreaction was initiated by photoexcitation of BR by pulse I (540 nm). The reverse photoreaction was initiated by photoexcitation of the product K590 at an early stage of its formation (5 ps) by pulse II (660 nm). The conversion of the excited K590 to the ground state proceeds at times of 0.19, 1.1, and 16 ps with the relative contributions of ~20/60/20, respectively. All these decay channels lead to the formation of the initial state of BR as a product with a quantum yield of ~1. This state is preceded by vibrationally excited intermediates, the relaxation of which occurs in the 16 ps time range. Likely, the heterogeneity of the excited state of K590 is determined by the heterogeneity of its chromophore center. The forward photoreaction includes two components—0.52 and 3.5 ps, with the relative contributions of 91/9, respectively. The reverse photoreaction initiated from K590 proceeds more efficiently in the conical intersection (CI) region but on the whole at a lower rate compared to the forward photoreaction, due to significant heterogeneity of the potential energy surface.

Objective: Rhodopsin from the eubacterium Exiguobacterium sibiricum (ESR) performs the lightdependent proton pumping function. The operation of ESR is based on the ultrafast photochemical reaction of isomerization of the retinal chromophore, which triggers dark processes closed in a photocycle. Many parameters of the photocycle are determined by the presence of a hydrogen bond between the primary counterion Asp85 and the chromophore. ESR in detergent micelles pumps protons most efficiently at pH > 9, when such a bond is most probable. Methods: In the present study, the photochemical reaction of ESR at pH 9.5 was investigated by femtosecond laser absorption spectroscopy. Results and Discussion: It was shown that photoisomerization of the chromophore group occurs in 0.51 ps, with the contribution from the reactive excited state being ca. 80

Colloidal quantum dots of CsPbBr3 perovskites have been synthesized. The average size and polydispersity of nanocrystals were determined to be 8.3 nm and 16

Primary excitation energy transfer and charge separation in photosystem I (PSI) from the extremophile desert green alga Chlorella ohadii grown in low light were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850 nm and in the time range from 50 fs to 500 ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band and compared with similar processes in PSI from the cyanobacterium Synechocystis sp. PCC 6803. When PSI from C. ohadii was excited at 660 nm, the processes of energy redistribution in the light-harvesting antenna complex were observed within a time interval of up to 25 ps, while formation of the stable radical ion pair P700+A1− was kinetically heterogeneous with characteristic times of 25 and 120 ps. When PSI was excited into the red edge of the Qy band at 715 nm, primary charge separation reactions occurred within the time range of 7 ps in half of the complexes. In the remaining complexes, formation of the radical ion pair P700+A1− was limited by the energy transfer and occurred with a characteristic time of 70 ps. Similar photochemical reactions in PSI from Synechocystis 6803 were significantly faster: upon excitation at 680 nm, formation of the primary radical ion pairs occurred with a time of 3 ps in 30

The absorption dynamics of chlorophyll a in a symmetric tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP was measured by the broadband femtosecond laser pump-probe spectroscopy within the range from 400 to 780 nm with a time resolution of 20 f-200 ps. When BoWSCP was excited in the region of the Soret band at a wavelength of 430 nm, a nonradiative intramolecular conversion S3 → S1 was observed with a characteristic time of 83 ± 9 fs. When the complex was excited in the region of the Qy band at a wavelength of 670 nm, a relaxation transition between two excitonic states of the chlorophyll dimer was observed with a time of 105 ± 10 fs. Absorption spectra of excited singlet states S1 and S3 of chlorophyll a were obtained. It has been demonstrated that the delocalization of the excited state between exciton-coupled Chl molecules in the BoWSCP tetramer changes in time and depends on the excitation energy. Upon excitation of BoWSCP, an ultrafast photochemical reaction is observed in the Soret region, apparently due to the reduction of tryptophan in the vicinity of chlorophyll.

The paper reports on the absorption dynamics of chlorophyll a in a symmetric tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP. It was measured by a broadband femtosecond laser pump-probe spectroscopy within the range from 400 to 750 nm and with a time resolution of 20 fs-200 ps. When BoWSCP was excited in the region of the Soret band at a wavelength of 430 nm, nonradiative intramolecular conversion S 3 →S 1 was observed with a characteristic time of 83 ± 9 fs. When the complex was excited in the region of the Q y band at 670 nm, relaxation transition between two excitonic states of the chlorophyll dimer was observed in the range of 105 ± 10 fs. Absorption spectra of the excited singlet states S 1 and S 3 of chlorophyll a were obtained. The delocalization of the excited state between exciton-coupled Chl molecules in BoWSCP tetramer changed in time and depended on the excitation energy. When BoWSCP is excited in the Soret band region, an ultrafast photochemical reaction is observed. This could result from the reduction of tryptophan in the vicinity of chlorophyll.

The alloys of Zn0.34Cd0.66S QDs show a high photocatalytic activity when mediating the hydrogen production from water with a reaction rate of 233 ± 9 µmol h−1 g−1. The reaction is carried out in the presence of ascorbic acid as sacrificial electron donor. The alloy Zn0.34Cd0.66S/ZnS QDs with a ZnS shell showed a yield about seven times lower for the same process compared to the ZnCdS/ZnS alloy. The dynamics of the exciton reactions in the initial stages of the reaction mediated by these QDs was studied by femtosecond laser spectroscopy. Femtosecond transient absorption spectra is reported in a detailed and comprehensive way describing dynamics of the absorption peak A1 near the bleach band of edge exciton B1. This A1 absorption peak is attributed to the Stark shift due to exciton–exciton interactions due to the trapping of a hole or an electron on the QDs surface. In shell-less ZnCdSQDs, the A1 peak presented two kinetic components: the damping of the A1 peak amplitude due to the relaxation of “hot” excitons and the concomitant growth of the A1 peak at later delay times. We suggest that this was due to due to the carrier capture charge by surface traps leading to electric field redistribution in the QDs. The relaxation/decay of “hot” electrons in alloys of ZnCdS and ZnCdS/ZnS QDs occur with characteristic times of τZnCdS = 153 fs for ZnCdS and τZnCdS/ZnS = 193 fs.

Photosystem I (PSI) of the cyanobacterium Acaryochloris marina is capable of performing an efficient photoelectrochemical conversion of far-red light due to its unique suite of cofactors. Chlorophyll d (Chl-d) has been long known as the major antenna pigment in the PSI from A. marina, while the exact cofactor composition of the reaction centre (RC) was established only recently by cryo-electron microscopy. The RC consists of four Chl-d molecules, and, surprisingly, two molecules of pheophytin a (Pheo-a), which provide a unique opportunity to resolve, spectrally and kinetically, the primary electron transfer reactions. Femtosecond transient absorption spectroscopy was here employed to observe absorption changes in the 400-860 nm spectral window occurring in the 0.1-500 ps timescale upon unselective antenna excitation and selective excitation of the Chl-d special pair P740 in the RC. A numerical decomposition of the absorption changes, including principal component analysis, allowed the identification of P740(+)Chld2(-) as the primary charge separated state and P740(+)Pheoa3(-) as the successive, secondary, radical pair. A remarkable feature of the electron transfer reaction between Chld2 and Pheoa3 is the fast, kinetically unresolved, equilibrium with an estimated ratio of 1:3. The energy level of the stabilised ion-radical state P740(+)Pheoa3(-) was determined to be ~60 meV below that of the RC excited state. In this regard, the energetics and the structural implications of the presence of Pheo-a in the electron transfer chain of PSI from A. marina are discussed, also in comparison with those of the most diffused Chl-a binding RC.

Transient absorption spectroscopy, kinetics of absorption anisotropy decay for bis-aza-18-crown-6-containing di-benzylidenecyclobutanone dye and its metal complexes, and quantum chemical calculations support the presence of the first stage in the photorecoordination mechanism for the complexes with the Ba2+, Ca2+, and K+ cations. The depolarization parameters, including the radii of solvates, are deduced from the experimental data. During the barrierless photorecoordination of the complex with two Ba2+ cations, the solvent shell of the crowned Ba2+ cation undergoes the following changes: (2 + 1)MeCN, (3 + 1)MeCN, 4 MeCN.

Photosystem I from the menB strain of Synechocystis sp. PCC 6803 containing foreign quinones in the A 1 sites was used for studying the primary steps of electron transfer by pump-probe femtosecond laser spectroscopy. The free energy gap (− ΔG) of electron transfer between the reduced primary acceptor A 0 and the quinones bound in the A 1 site varied from 0.12 eV for the low-potential 1,2-diamino-anthraquinone to 0.88 eV for the high-potential 2,3-dichloro-1,4-naphthoquinone, compared to 0.5 eV for the native phylloquinone. It was shown that the kinetics of charge separation between the special pair chlorophyll P 700 and the primary acceptor A 0 was not affected by quinone substitutions, whereas the rate of A 0 → A 1 electron transfer was sensitive to the redox-potential of quinones: the decrease of − ΔG by 400 meV compared to the native phylloquinone resulted in a ~ fivefold slowing of the reaction The presence of the asymmetric inverted region in the ΔG dependence of the reaction rate indicates that the electron transfer in photosystem I is controlled by nuclear tunneling and should be treated in terms of quantum electron–phonon interactions. A three-mode implementation of the multiphonon model, which includes modes around 240 cm −1 (large-scale protein vibrations), 930 cm −1 (out-of-plane bending of macrocycles and protein backbone vibrations), and 1600 cm −1 (double bonds vibrations) was applied to rationalize the observed dependence. The modes with a frequency of at least 1600 cm −1 make the predominant contribution to the reorganization energy, while the contribution of the “classical” low-frequency modes is only 4%.