Unraveling the influence of organic cations on tuning electronic structures and spin-splitting in two-dimensional layered organic-inorganic tin-iodine perovskites.

The solar cell and light-emitting device research community is currently focusing on investigating two-dimensional (2D) hybrid perovskite materials owing to their remarkable stability and intriguing optoelectronic characteristics, which hold significant promise for various applications. In general, the introduction of chirality in hybrid perovskites arises from symmetry breaking within their inorganic frameworks. Nevertheless, despite this understanding, the specific factors driving the observed increase in splitting remain obscure due to a lack of comprehensive investigations. Our research delves into the electronic properties of 2D layered hybrid perovskites, considering their behavior with and without spin-orbit coupling. We specifically focus on effect of Rashba splitting and the impact of electronic structure variation across a range of chiral perovskites by introducing chiral organic cations with differing degrees of π-conjugation, resulting in significant changes in spin-splitting magnitude. Systematic first principles investigations confirm that the distortion of the cage and d-spacing of chiral perovskites are crucial design parameters for achieving strong spin-splitting in 2D layered perovskites. Furthermore, our investigation reveals that these systems exhibit remarkable absorption capabilities in the visible light spectrum, as demonstrated by their computed optoelectronic characteristics. The chiral perovskites described in this study, which exhibit substantial spin-splitting, present a distinctive prototype with potential implications for spintronics and photovoltaics.

Organoboron Complexes as Thermally Activated Delayed Fluorescence (TADF) Materials for Organic Light-Emitting Diodes (OLEDs): A Computational Study.

We report on organoboron complexes characterized by very small energy gaps (ΔEST) between their singlet and triplet states, which allow for highly efficient harvesting of triplet excitons into singlet states for working as thermally activated delayed fluorescence (TADF) devices. Energy gaps ranging between 0.01 and 0.06 eV with dihedral angles of ca. 90° were registered. The spin-orbit couplings between the lowest excited S1 and T1 states yielded reversed intersystem crossing rate constants (KRISC) of an average of 105 s-1. This setup accomplished radiative decay rates of ca. 106 s-1, indicating highly potent electroluminescent devices, and hence, being suitable for application as organic light-emitting diodes.

Correction: Surface modification of graphene with functionalized carbenes and their applications in the sensing of toxic gases: a DFT study.

[This corrects the article DOI: 10.1039/D3RA02557H.].

Surface modification of graphene with functionalized carbenes and their applications in the sensing of toxic gases: a DFT study.

Graphene and other 2D materials have gained significant attention in the development of gas sensors. In this study, we employed Density Functional Theory (DFT) to investigate the adsorption properties of diazomethanes (1a-1g) with various functional groups (R = OH (a), OMe (b), OEt (c), OPr (d), CF3 (e), Ph (f)) on pristine graphene. Furthermore, we explored the adsorption behavior of activated carbenes (2a-2g) generated from the decomposition of diazomethanes on graphene, as well as the functionalized graphene derivatives (3a-3g) resulting from [2 + 1] cycloaddition reactions between (2a-2g) and graphene. The interaction between these functionalized derivatives (3a-3g) and toxic gases was also investigated. Our results revealed that carbenes exhibited a stronger affinity for graphene compared to diazomethanes. The adsorption energy of esters (3b, 3c, and 3d) on graphene decreased relative to compound 3a, while 3e exhibited increased adsorption energy due to the electron-withdrawing effect of fluorine atoms. Additionally, the adsorption energy of phenyl and nitrophenyl groups (3f and 3g) decreased due to their π-stacking interaction with graphene. Importantly, all functionalized derivatives (3a-3g) demonstrated favorable interactions with gases. Notably, the derivative 3a, acting as a hydrogen bonding donor, exhibited superior performance. Furthermore, modified graphene derivatives exhibited the highest adsorption energy with NO2 gas, highlighting their potential for selective NO2 sensing applications. These findings contribute to the understanding of gas-sensing mechanisms and the design of novel graphene-based sensor platforms.

Tuning anticancer properties and DNA-binding of Pt(ii) complexes via alteration of nitrogen softness/basicity of tridentate ligands.

Nine tridentate Schiff base ligands of the type (N^N^O) were synthesized from reactions of primary amines {2-picolylamine (Py), N-phenyl-1,2-diaminobenzene (PhN), and N-phenyl-1,2-diaminoethane(EtN)} and salicylaldehyde derivatives {3-ethoxy (OEt), 4-diethylamine (NEt2) and 4-hydroxy (OH)}. Complexes with the general formula Pt(N^N^O)Cl were synthesized by reacting K2PtCl4 with the ligands in DMSO/ethanol mixtures. The ligands and their complexes were characterized by NMR spectroscopy, mass spectrometry and elemental analysis. The DNA-binding behaviours of the platinum(ii) complexes were investigated by two techniques, indicating good binding affinities and a two-stage binding process for seven complexes: intercalation followed by switching to a covalent binding mode over time. The other two complexes covalently bond to ct-DNA without intercalation. Theoretical calculations were used to shed light on the electronic and steric factors that lead to the difference in DNA-binding behavior. The reactions of some platinum complexes with guanine were investigated experimentally and theoretically. The binding of the complexes with bovine serum albumin (BSA) indicated a static interaction with higher binding affinities for the ethoxy-containing complexes. The half maximal inhibitory concentration (IC50) values against MCF-7 and HepG2 cell lines suggest that platinum complexes with tridentate ligands of N-phenyl-o-phenylenediamine or pyridyl with 3-ethoxysalicylimine are good chemotherapeutic candidates. Pt-Py-OEt and Pt-PhN-OEt have IC50 values against MCF-7 of 13.27 and 10.97 µM, respectively, compared to 18.36 µM for cisplatin, while they have IC50 values against HepG2 of 6.99 and 10.15 µM, respectively, compared to 19.73 µM for cisplatin. The cell cycle interference behaviour with HepG2 of selected complexes is similar to that of cisplatin, suggesting apoptotic cell death. The current work highlights the impact of the tridentate ligand on the biological properties of platinum complexes.

Tuning the anticancer properties of Pt(ii) complexes via structurally flexible N-(2-picolyl)salicylimine ligands.

Three tridentate Schiff base ligands were synthesized from the reactions between 2-picolylamine and salicylaldehyde derivatives (3-ethoxy (OEt), 4-diethylamino (NEt2) and 4-hydroxy (OH)). Complexes with the general formula Pt(N^N^O)Cl were obtained from reactions between the ligands and K2PtCl4. The ligands and their complexes were characterized by NMR spectroscopy, mass spectrometry and elemental analysis. Further confirmation of the structure of Pt-OEt was achieved by single-crystal X-ray diffraction. The DMSO/chlorido exchange process at Pt-OEt was investigated by monitoring the change in conductivity, revealing very slow dissociation in DMSO. Moreover, solvent/chlorido exchange for Pt-OEt and Pt-NEt2 were investigated by NMR spectroscopy in DMSO and DMSO/D2O; Pt-NEt2 forms an adduct with DMSO while Pt-OEt forms adducts with DMSO and water. The DNA-binding behaviour of the platinum(ii) complexes was investigated by two techniques. Pt-NEt2 has the best apparent binding constant. The intercalation mode of interaction with ct-DNA was suggested by molecular docking studies and the increase in the relative viscosity of ct-DNA with increasing concentrations of the platinum(ii) complexes. However, the gradual decrease in the relative viscosity over time at constant concentration of platinum(ii) complexes indicated a shift from intercalation to a covalent binding mode. Anticancer activities of the ligands and their platinum(ii) complexes were examined against two cell lines. The platinum(ii) complexes exhibit superior cytotoxicity to that of their ligands. Among the platinum(ii) complexes, Pt-OEt possesses the best IC50 against both cell lines, its cytotoxicity being comparable to that observed for cisplatin. Cell cycle arrest in the HepG2 cell line upon treatment with Pt-OEt and Pt-NEt2 was investigated and compared to that of cisplatin; the change in the cell accumulation patterns supports the presumption of an apoptotic cell death pathway. The optimized structures of the B-DNA trimer adducts with the platinum complexes showed hydrogen-bonding interactions between the ligands and nucleobases, affecting the inter-strand hydrogen bonding within the DNA, and highlighting the strong ability of the complexes to induce conformational changes in the DNA, leading to the activation of apoptotic cell death. In summary, the current study demonstrates promising new anticancer platinum(ii) complexes with highly flexible tridentate ligands; the functional groups on the ligands are important in tuning their DNA binding/anticancer properties.

Hydroaminoalkylation of alkenes using transition metals complexes grafted on silica SBA15 as catalysts.

The gas-phase hydroaminoalkylation reaction of propene catalyzed by group 4 (M = Ti, Zr and Hf) metal amido complexes [(≡Si-O-)(M(-NMe2)3] was investigated by using PBE0-D3/SVP//TZVP level of theory. The geometrical analysis traced the formation of the metallaaziridines and the azametallacyclopentanes as key intermediates in these reactions. The metallaaziridines were simulated through the activation of α-C-H bonds of the amido groups; while the azametallacyclopentanes were configured by slotting the propene double bond onto the M - C bonds of the metallaaziridines. The latter reaction was considered the rate-determining step. Thermochemical calculations showed that the order of catalytic activity is: Ti ≥ Zr > Hf; while the preference of the azametallacyclopentanes is: Hf > Zr ≥ Ti.

Synthesis, Characterization, DFT and Photocatalytic Studies of a New Pyrazine Cadmium(II) Tetrakis(4-methoxy-phenyl)-porphyrin Compound.

This study describes the synthesis, theoretical investigations, and photocatalytic degradational properties of a new (pyrazine)(meso-tetrakis(4-tert-methoxyphenyl)-porphyrinato)-cadmium (II) ([Cd(TMPP)-Pyz]) complex (1). The new penta-coordinated CdII porphyrin complex (1) was characterized by various spectroscopic techniques, including FT-IR, NMR, UV-visible absorption, fluorescence emission, and singlet oxygen, while its molecular structure was studied using single crystal X-ray diffraction. The UV-Vis spectroscopic study highlighted the redshift of the absorption bands after the insertion of the Cd(II) metal ion into the TMPP ring. The co-coordination of the pyrazine axial ligand enhanced this effect. A fluorescence emission spectroscopic study showed a significant blueshift in the Q bands, accompanied by a decrease in the fluorescence emission intensity and quantum yields of Φf = 0.084, Φf = 0.06 and Φf = 0.03 for H2-TMPP free-base porphyrin, [Cd(TMPP)] and [Cd(TMPP)(Pyz)] (1) respectively. Singlet oxygen revealed that the H2-TMPP porphyrin produced the most efficient singlet oxygen quantum yield of (ΦΔ = 0.73) compared to [CdTMPP] (ΦΔ = 0.57) and [Cd(TMPP)(Pyz)] (1) (ΦΔ = 0.13). In the crystal lattice, the [Cd(TMPP)Pyz] was stabilized through non-covalent intermolecular interactions (NCI), such as the hydrogen bonds C-H···N and C-H···Cg. Additionally, crystal explorer software was then utilized to measure the quantitative analysis of the intermolecular interactions in the unit cell of the crystal structure and established that the C-H···π interaction dominated. The Natural bond orbital (NBO) analysis revealed that each molecule is stabilized by hyperconjugation and charge delocalization. As a photocatalyst, the coordination complex 1 showed excellent photocatalytic activity toward the degradation of Levafix Blue CA reactive dye (i.e., dye photo-degradation of 80%).

Zn2+-Schiff's Base Complex as an "On-Off-On" Molecular Switch and a Fluorescence Probe for Cu2+ and Ag+ Ions.

The present study presents a thorough theoretical analysis of the electronic structure and conformational preference of Schiff's base ligand N,N-bis(2-hydroxybenzilidene)-2,4,6-trimethyl benzene-1,3-diamine (H2L) and its metal complexes with Zn2+, Cu2+ and Ag+ ions. This study aims to investigate the behavior of H2L and the binuclear Zn2+ complex (1) as fluorescent probes for the detection of metal ions (Zn2+, Cu2+ and Ag+) using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The six conformers of the H2L ligand were optimized using the B3LYP/6-311 + + G** level of theory, while the L-2-metal complexes were optimized by applying the B3LYP functional with the LANL2DZ/6-311 + + G** mixed basis set. The gas-phase and solvated Enol-cis isomer (E-cis) was found to be the most stable species. The absorption spectra of the E-cis isomer and its metal complexes were simulated using B3LYP, CAM-B3LYP, M06-2X and ωB97X functionals with a 6-311 + + G** basis set for C, O, N and H atoms and a LANL2DZ basis set for the metal ions (Zn2+, Cu2+ and Ag+). The computational results of the B3LYP functional were in excellent agreement with the experimental results. Hence, it was adopted for performing the emission calculations. The results indicated that metal complex (1) can act as a fluorescent chemosensor for the detection of Ag+ and Cu2+ ions through the mechanism of intermolecular charge transfer (ICT) and as a molecular switch "On-Off-On" via the replacement of Cu2+ by Ag+ ions, as proved experimentally.

Single- and co-sensitization of triphenylamine-based and asymmetrical squaraine dyes on the anatase (001) surface for DSSC applications: Periodic DFT calculations.

Dye aggregation causes poor performance of dye-sensitized solar cell (DSSC) applications through faster charge recombination of the photosensitizer with electrolyte. Triphenylamine (TBA)-based dyes feature a higher molar absorption coefficient and broadened wavelength but cannot absorb sunlight in the near-infrared (NIR) region. In contrast, the squaraine (SQ) photosensitizer, which is also called an NIR photosensitizer, has a maximum wavelength in the NIR region with high intensity. However, SQ dye suffers from dye aggregation due to its planar structure. The use of a co-sensitizer is one well-tested way to increase the power conversion efficiency (η) of solar cells by reducing dye aggregation and charge recombination. Using density functional theory (DFT) and time-dependent DFT (TDDFT), this work explains from a theoretical perspective the higher η values of the TZC1 and TZC2 dyes compared to that of asymmetric the SQ sensitizer (YR6) as free dyes. The electronic properties, reorganization energies, absorption and emission spectra, ICT parameters, and photovoltage parameters of the TZC1, TZC2, and YR6 dyes were computed using the M06/6-31G(d,p) level of theory in the gas phase and CH2Cl2 solvent (CPCM method). Additionally, the mono- and co-adsorption processes of TZC-based sensitizers with YR6 on the anatase (001) surface were investigated using periodic DFT calculations with the PBE + U/PAW method and the dispersion correction of the Grimme method D3. The results reveal that the use of the co-sensitized led to significant stabilization of the formed complexes by at least 1.21 eV, the panchromatic effect on the absorption spectra, and an increase in the light-harvesting ability in the NIR region, which improves the performance of DSSCs.

DNA-Binding Capabilities and Anticancer Activities of Ruthenium(II) Cymene Complexes with (Poly)cyclic Aromatic Diamine Ligands.

Ruthenium(II) arene complexes of the general formula [RuCl(η6-p-cymene)(diamine)]PF6 (diamine = 1,2-diaminobenzene (1), 2,3-diaminonaphthalene (2), 9,10-diaminophenanthrene (3), 2,3-diaminophenazine (4), and 1,2-diaminoanthraquinone (5) were synthesized. Chloro/aqua exchange was evaluated experimentally for complexes 1 and 2. The exchange process was investigated theoretically for all complexes, revealing relatively fast exchange with no significant influence from the polycyclic aromatic diamines. The calf thymus DNA (CT-DNA) binding of the complexes increased dramatically upon extending the aromatic component of the diamines, as evaluated by changes in absorption spectra upon titration with different concentrations of CT-DNA. An intercalation binding mode was established for the complexes using the increase in the relative viscosity of the CT-DNA following addition of complexes 1 and 2. Theoretical studies showed strong preference for replacement of water by guanine for all the complexes, and relatively strong Ru-Nguanine bonds. The plane of the aromatic systems can assume angles that support non-classical interactions with the DNA and covalent binding, leading to higher binding affinities. The ruthenium arenes illustrated in this study have promising anticancer activities, with the half maximal inhibitory concentration (IC50) values comparable to or better than cisplatin against three cell lines.

A new phenothiazine-based selective visual and fluorescent sensor for cyanide.

A new donor-π-acceptor derived from phenothiazine, namely 2-(2-((10-hexyl-10H-phenothiazin-3-yl)methylene)-3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (PTZON) was synthesized and fully characterized, and its potential as a fluorescent sensor for cyanide anion was investigated. The PTZON showed a visible absorption band at 564 nm corresponds to an intramolecular charge transfer (ICT) and an emission band at 589 nm in CH3CN/H2O. The results of cyanide anion titration revealed ratiometric changes in both absorption and fluorescence spectra as a result of the nucleophilic addition of cyanide anion via Michael addition. The optical studies, FT-IR spectra, NMR, high-resolution mass, and DFT calculations confirmed the sensing mechanism. The selectivity of PTZON as a cyanide anion fluorescent sensor was proved in mixed solvent solutions, and the sensitivity was as low as 0.011 µM, which is far lower than the value allowed by the United States Environmental Protection Agency for drinking water (1.9 µM). Also, the detection limit of PTZON was assessed to be 3.39 µM by the spectrophotometric method. The binding stoichiometry between PTZON and cyanide anion was found to be 1:1 as evidenced by mass spectra. TLC silica-coated plates test strips demonstrated the fluorescent detection of cyanide anion.

The structure and binding mode of citrate in the stabilization of gold nanoparticles.

Elucidating the binding mode of carboxylate-containing ligands to gold nanoparticles (AuNPs) is crucial to understand their stabilizing role. A detailed picture of the three-dimensional structure and coordination modes of citrate, acetate, succinate and glutarate to AuNPs is obtained by 13C and 23Na solid-state NMR in combination with computational modelling and electron microscopy. The binding between the carboxylates and the AuNP surface is found to occur in three different modes. These three modes are simultaneously present at low citrate to gold ratios, while a monocarboxylate monodentate (1κO1) mode is favoured at high citrate:gold ratios. The surface AuNP atoms are found to be predominantly in the zero oxidation state after citrate coordination, although trace amounts of Auδ+ are observed. 23Na NMR experiments show that Na+ ions are present near the gold surface, indicating that carboxylate binding occurs as a 2e- L-type interaction for each oxygen atom involved. This approach has broad potential to probe the binding of a variety of ligands to metal nanoparticles.

Vibrational Fingerprints of Low-Lying Pt(n)P(2n) (n = 1-5) Cluster Structures from Global Optimization Based on Density Functional Theory Potential Energy Surfaces.

Vibrational fingerprints of small Pt(n)P(2n) (n = 1-5) clusters were computed from their low-lying structures located from a global exploration of their DFT potential energy surfaces with the GSAM code. Five DFT methods were assessed from the CCSD(T) wavenumbers of PtP2 species and CCSD relative energies of Pt2P4 structures. The eight first Pt(n)P(2n) isomers found are reported. The vibrational computations reveal (i) the absence of clear signatures made by overtone or combination bands due to very weak mechanical and electrical anharmonicities and (ii) some significant and recurrent vibrational fingerprints in correlation with the different PP bonding situations in the Pt(n)P(2n) structures.

Non-precious bimetallic catalysts for selective dehydrogenation of an organic chemical hydride system.

Methylcyclohexane (MCH)-toluene (TOL) chemical hydride cycles as hydrogen carrier systems are successful with the selective dehydrogenation of MCH to TOL, which has been achieved only using precious Pt-based catalysts. Herein, we report improved selectivity using non-precious metal nickel-based bimetallic catalysts, where the second metal occupies the unselective step sites.

A highly selective copper-indium bimetallic electrocatalyst for the electrochemical reduction of aqueous CO2 to CO.

The challenge in the electrochemical reduction of aqueous carbon dioxide is in designing a highly selective, energy-efficient, and non-precious-metal electrocatalyst that minimizes the competitive reduction of proton to form hydrogen during aqueous CO2 conversion. A non-noble metal electrocatalyst based on a copper-indium (Cu-In) alloy that selectively converts CO2 to CO with a low overpotential is reported. The electrochemical deposition of In on rough Cu surfaces led to Cu-In alloy surfaces. DFT calculations showed that the In preferentially located on the edge sites rather than on the corner or flat sites and that the d-electron nature of Cu remained almost intact, but adsorption properties of neighboring Cu was perturbed by the presence of In. This preparation of non-noble metal alloy electrodes for the reduction of CO2 provides guidelines for further improving electrocatalysis.

Well-defined silica supported aluminum hydride: another step towards the utopian single site dream?

Reaction of triisobutylaluminum with SBA15700 at room temperature occurs by two parallel pathways involving either silanol or siloxane bridges. It leads to the formation of a well-defined bipodal [([triple bond, length as m-dash]SiO)2Al-CH2CH(CH3)2] 1a, silicon isobutyl [[triple bond, length as m-dash]Si-CH2CH(CH3)2] 1b and a silicon hydride [[triple bond, length as m-dash]Si-H] 1c. Their structural identity was characterized by FT-IR and advanced solid-state NMR spectroscopies (1H, 13C, 29Si, 27Al and 2D multiple quantum), elemental and gas phase analysis, and DFT calculations. The reaction involves the formation of a highly reactive monopodal intermediate: [[triple bond, length as m-dash]SiO-Al-[CH2CH(CH3)2]2], with evolution of isobutane. This intermediate undergoes two parallel routes: transfer of either one isobutyl fragment or of one hydride to an adjacent silicon atom. Both processes occur by opening of a strained siloxane bridge, [triple bond, length as m-dash]Si-O-Si[triple bond, length as m-dash] but with two different mechanisms, showing that the reality of "single site" catalyst may be an utopia: DFT calculations indicate that isobutyl transfer occurs via a simple metathesis between the Al-isobutyl and O-Si bonds, while hydride transfer occurs via a two steps mechanism, the first one is a ß-H elimination to Al with elimination of isobutene, whereas the second is a metathesis step between the formed Al-H bond and a O-Si bond. Thermal treatment of 1a (at 250 °C) under high vacuum (10-5 mbar) generates Al-H through a ß-H elimination of isobutyl fragment. These supported well-defined Al-H which are highly stable with time, are tetra, penta and octa coordinated as demonstrated by IR and 27Al-1H J-HMQC NMR spectroscopy. All these observations indicate that surfaces atoms around the site of grafting play a considerable role in the reactivity of a single site system.

Mechanistic insights into the reductive dehydroxylation pathway for the biosynthesis of isoprenoids promoted by the IspH enzyme.

Here, we report an integrated quantum mechanics/molecular mechanics (QM/MM) study of the bio-organometallic reaction pathway of the 2H+/2e- reduction of (E)-4-hydroxy-3-methylbut-2-enyl pyrophosphate (HMBPP) into the so called universal terpenoid precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), promoted by the IspH enzyme. Our results support the viability of the bio-organometallic pathway through rotation of the OH group of HMBPP away from the [Fe4S4] cluster at the core of the catalytic site, to become engaged in a H-bond with Glu126. This rotation is synchronous with π-coordination of the C2[double bond, length as m-dash]C3 double bond of HMBPP to the apical Fe atom of the [Fe4S4] cluster. Dehydroxylation of HMBPP is triggered by a proton transfer from Glu126 to the OH group of HMBPP. The reaction pathway is completed by competitive proton transfer from the terminal phosphate group to the C2 or C4 atom of HMBPP.

CO dissociation on magnetic Fe(n) clusters.

This work theoretically investigates the CO dissociation on Fen nanoparticles, for n in the range of 1-65, focusing on size dependence in the context of the initial step of the Fischer-Tropsch reaction. CO adsorbs molecularly through its C-end on a triangular facet of the nanoparticle. Dissociation becomes easier when the cluster size increases. Then, the C atom is bonded to a square facet that is generated as a result of the adsorption if it does not yet exist in the bare cluster, while the O atom is adsorbed on a triangular facet. In the most stable situation, the two adsorbed atoms remain close together, both having in common one shared first-neighbor iron atom. There is a partial spin quenching of the neighboring Fe atoms, which become more positively charged than the other Fe atoms. The shared surface iron atom resembles a metal-cation from a complex. Despite the small size of the iron cluster considered, fluctuations due to specific configurations do not influence properties for n > 25 and global trends seem significant.

Modeling localized photoinduced electrons in rutile-TiO2 using periodic DFT+U methodology.

We propose a theoretical model for photocatalytic processes on titanium dioxide, described by its most stable phase and surface, rutile-TiO(2)(110). The excitation induced by light promotes electrons from the valence band to the conduction band. In this context, one important requirement is having a correct value of the magnitude of the electronic gap. The use of GGA+U or LDA+U functional with an appropriate U value allows this. The U correction has little consequence on the adsorption strength itself on the TiO(2)(110) surface. For the ground state, it only yields a slight increase of the interaction strength of some test molecules; the surface basicity is somewhat enhanced. This is interpreted by the shift of TiO(2) vacant levels. Photoexcitation is taken into account by imposing two unpaired electrons per cell of the same spin. The size of the cell therefore determines the number of excitations per surface area; the larger the cell, the smaller the electron-hole surface concentration and the smaller the energy for electronic excitation. For the excited state, careful attention must be focused on the localization of the excited electron and of the hole which are crucial for the determination of the lowest electronic states and for the surface reactivity. We found that the excited electron is localized on a pentacoordinated surface titanium atom while the hole is shared by two surface oxygen atoms not too far from it. The electronic levels associated to the reduced titanium atoms are low in energy; the projected density of states is superposed onto the valence band.