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The possibility to control luminescence properties of purely organic emitting salts is receiving an ever-growing interest. A full understanding of the manifold effects of counterions on the photophysical properties of charged luminophors deserves thorough investigation. Here, we disclose the impact of different anions, i.e. triflate, iodide and nitrate, on the multiple emissions of a new class of ionic luminophors, based on 1,3-dimethyl-2-(p-tolyl)-1H-benzo[d]imidazol-3-ium, 1. The specific ion-pairing plays a key role in the emissive properties of solids, depending on both the electronegativity of the anion and its distance from the cation. Three main effects emerge from strong ion-pairing: (i) fluorescence quenching, (ii) phosphorescence enhancement and iii) efficient direct population of the triplet-state. As a result, a weak interaction (1-OTf) guarantees intense fluorescence (QY > 50%), while a strong pairing (1-NO3·0.5EtOH and 1-I·0.5CH2Cl2) promotes molecular room temperature phosphorescence (RTP) enhancement. The anion-controlling strategy also enables long-lasting RTP (288 ms) in 1-OTf and ultrabright phosphorescence (QY > 70%) in 1-NO3·0.5EtOH, under direct triplet-excitation at 350 nm.

A comprehensive investigation of the functional properties of heteroleptic donor-M-acceptor dithiolene complexes Bu4N[MII(L1)(L2)] is presented (M = Pd, Pt). The acceptor L1 consists of the chiral (R)-(+)α-methylbenzyldithiooxamidate ((R)-α-MBAdto), the donor L2 is 2-thioxo-1,3-dithiole-4,5-dithiolato (dmit) in 1 (Pd) and 2 (Pt), 1,2-dicarbomethoxyethylenedithiolate (ddmet) in 3 (Pd) and 4 (Pt), or [4',5':5,6][1,4]dithiino[2,3-b]quinoxaline-1',3'-dithiolato (quinoxdt) in 5 (Pd) and 6 (Pt). L1 is capable of undergoing proton exchange and promoting crystal formation in noncentrosymmetric space groups. L2 has different molecular structures while it maintains similar electron-donating capabilities. Thanks to the synergy of the ligands, 1-6 behave as H+ and Ag+ switchable linear chromophores. Moreover, the compounds exhibit a H+-switchable second-order NLO response in solution, which is maintained in the bulk for 1, 3, and 4 when they are embedded into a PMMA poled matrix. 5 and 6 show unique anti-Kasha H+ and Ag+ tunable colored emission originating from the quinoxdt ligand. A correlation between the electronic structure and properties is shown through density functional theory (DFT) and time-dependent DFT calculations.

The structures and properties, including second-order non-linear optical properties in solution and in polymethylmethacrylate (PMMA) films, of new nickel heteroleptic donor-acceptor dithiolene complexes are reported here ([Ni(Bn(2)pipdt)L1-4], 1-4). The acceptor dithione system (Bn(2)pipdt = 1,4-dibenzylpiperazine-2,3-dithione) is invariant, whereas the donor dithiolate moiety varies in 1 to 4 (i-mnt = 2,2-dicyanoethylene-1,1-dithiolate) (L1), sqdt = 1,2-dithiosquarate (L2), ddmedt = 1,2-dicarbomethoxyethylene dithiolate (L3), and dddmedt = 5,6-bis(methoxycarbonyl)-1,4-dithiine-2,3-bis(thiolate) (L4). Molecular structural characterization of 1, 3 and 4 points out that the metal center exhibits a square-planar geometry bound by the dithione ligand Bz(2)pipdt, and by three different dithiolate-type ligands. Complexes 1-4 are characterized in the visible region by a peak of moderately strong intensity, which undergoes negative solvatochromism. The molecular quadratic optical nonlinearities were determined in DMF solution by the electric field induced second harmonic generation technique, working with a 1907 nm incident wavelength. The mu beta(1907) values (-680 (1), -855 (2), -1080 (3); and -1180 (4)) (10(-48) esu) were measured, showing that 1-4 exhibit negative second-order polarizabilities whose values depend on the donor ligand. While 1-4 exhibit similar thermolytic behavior, when embedded in a PMMA poled matrix, only 3 maintains second-order NLO properties in the solid state. A good NLO response with a value of the three nonzero coefficients of the second-order susceptibility tensor chi(2)33, chi(2)31, and chi(2)15 = 2.20 +/- 0.44 pm V-1, 0.56 +/- 0.12 pm V-1 and 0.42 +/- 0.08 pm V-1, respectively, was found. According to computational studies, the mu beta(1907) sequence is mainly related to the donor capability of the dithiolate ligands, which affects the HOMO-LUMO energy gap (2.05 (1), 1.80 (2), 1.56 (3), and 1.36 (4) eV), and it can thus modulate the position of the low-energy absorption and related properties in the series.

In this work, we have prepared a series of A4 ZnII porphyrins, carrying in the β-pyrrolic-position one or two π-delocalized ethynylphenyl moieties with a −NO2 acceptor or a −NMe2 donor pendant, and measured their second-order NLO response in CHCl3 solution at 1907 nm via the electric-field-induced second harmonic generation (EFISH) technique. For some of these compounds, we have recorded an unexpected sign and/or absolute value of μβ1907. Since their sterically hindered A4 structure should ensure the lack of significant aggregation processes in solution, we explain such anomalous EFISH results by invoking a non-negligible contribution of the electronic cubic term γ(−2ω; ω, ω, 0) to γEFISH, as supported by a qualitative evaluation of the third-order response through the measure of the cubic hyperpolarizability (γTHG) and by computational evidence., If the polarity of β-substituted ZnII porphyrins is low, then the contribution of the third-order term γ0(−2ω; ω, ω, 0) to γEFISH cannot be neglected.

The development of building integrated photovoltaic (BIPV) technology and its implementation in construction of the building envelop provide aesthetical, economical and technical solutions toward the zero-energy building. In this perspective dye-sensitized solar cells (DSSCs), which can be obtained in transparent form and with tunable different colors, offer not only an alternative to the traditional silicon solar cells to be applied in particular to decorative effects on windows and glass integrated facades, but also to indoor structures (and furnishings) in order to recapture the energy spent for the inner lighting, thanks to their peculiar ability of operating in diffuse light condition. In this context, porphyrin-based molecules have an immense potential as light harvesting component of dye-sensitized nanocrystalline TiO2 solar cells, reaching now efficiencies up to about 13%. However the multistep synthesis of the best performing porphyrin dyes, showing a meso substitution pattern, is characterized by very low overall yields compromising their possible applicative development for instance in large photovoltaic (PV) glass modules in competition with the actual commercial PV glass modules based on CuInGaSe2 or CdTe thin voltaic films. In this review the renewed interest in the role of the β-substituted ZnII porphyrins for PV application, less studied than the meso substituted ones, is highlighted. Indeed they can rely on a more accessible synthetic procedure since their tetraaryl porphyrinic core can be easily obtained by a one pot reaction between pyrrole and the appropriate aryl aldehyde. Moreover, their remarkable light harvesting properties in the visible range as well as their peculiar steric hindrance, which strongly opposes to the charge recombination process at the photoanode/dye/electrolyte interface, make this kind of cost-effective porphyrinic dyes more promising for application in new PV glass modules based on DSSC technology, to be applied BIPV.

Three perfluorinated Zn

Dye-Sensitized Solar Cells (DSSCs) are a highly promising alternative to conventional photovoltaic silicon-based devices, due to the potential low cost and the interesting conversion efficiencies. A key-role is played by the dye, and porphyrin sensitizers have drawn great interest because of their excellent light harvesting properties mimicking photosynthesis. Indeed, porphyrins are characterized by strong electronic absorption bands in the visible region up to the near infrared and by long-lived π* singlet excited states. Moreover, the presence of four meso and eight β-pyrrolic positions allows a fine tuning of their photoelectrochemical properties through structural modification. Trans-A2BC push–pull ZnII porphyrins, characterized by a strong and directional electron excitation process along the push–pull system, have been extensively investigated. On the other hand, A4 β-pyrrolic substituted tetraaryl ZnII porphyrins, which incorporate a tetraaryl porphyrinic core as a starting material, have received lower attention, even if they are synthetically more attractive and show several advantages such as a more sterically hindered architecture and enhanced solubility in most common organic solvents. The present contribution intends to review the most prominent A4 β-substituted ZnII porphyrins reported in the literature so far for application in DSSCs, focusing on the strategies employed to enhance the light harvesting capability of the dye and on a comparison with meso-substituted analogs.

Three perfluorinated ZnII porphyrins were evaluated as n-type sensitizers in photoelectrosynthetic cells for HBr and water splitting. All the dyes are featured by the presence of pentafluorophenyl electron-withdrawing groups to increase the ground-state oxidation potential and differ for the nature and position of the ?-conjugate linker between the core and anchoring group tasked to bind the metal oxide, in order to assess the best way of coupling with the semiconductor. A phenyl-triazole moiety was used to link the carboxylic anchoring group onto the meso position, while an ethynyl-phenyl linker was chosen to bridge carboxylic and cyanoacrylic groups onto the ?-pyrrolic position. A combination of electrochemical, computational, and spectroscopic investigations confirmed the strong electron-withdrawing effect of the perfluorinated porphyrin core, which assures all the investigated dyes of the high oxidation potential required to the coupling with water oxidation catalysts (WOC). Such an electron-poor core, however, affects the charge separation character of the dyes, as demonstrated by the spatial distribution of the excited states, leading to a nonquantitative charge injection, although tilting of the molecules on the semiconductor surface could bring the porphyrin ring closer to the semiconductor, offering additional charge-transfer pathways. Indeed, all the dyes demonstrated successful in the splitting of both aqueous HBr and water, with the best results found for the SnO2/TiO2 photoanode sensitized with the ?-substituted porphyrin equipped with a cyanoacrylic terminal group, achieving 0.4 and 0.1 mA/cm2 photoanodic currents in HBr and water under visible light, respectively. The faradaic yield for oxygen evolution in the presence of an IrIV catalyst was over 95%, and the photoanode operation was stable for more than 1000 s. Thus, the perfluorinated porphyrins with a cyanoacrylic anchoring group at the ?-position should be considered for further development to improve the charge-transfer character.

We investigated the impact of singly occupied molecular orbital (SOMO) energy on the n-doping efficiency of benzimidazole derivatives. By designing and synthesizing a series of new air-stable benzimidazole-based dopants with different SOMO energy levels, we demonstrated that an increase of the dopant SOMO energy by only similar to 0.3 eV enhances the electrical conductivity of a benchmark electron-transporting naphthalenediimide-bithiophene polymer by more than 1 order of magnitude. By combining electrical, X-ray diffraction, and electron paramagnetic resonance measurements with density functional theory calculations and analytical transport simulations, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and crystallinity of the doped polymer as a function of the dopant SOMO energy. Our findings strongly indicate that charge and energy transport are dominated by the (relative) position of the SOMO level, whereas morphological differences appear to play a lesser role. These results set molecular-design guidelines for next-generation n-type dopants. Funding Agencies|Swedish Foundation for Strategic Research, VINNOVA [2015-04859]; Swedish Research CouncilSwedish Research Council [2016-03979]; AForsk [18-313]; Advanced Functional Materials Center at Linkoping University [2009-00971]; Regione LombardiaRegione Lombardia; Fondazione CariploFondazione Cariplo; SmartMatLab Centre project [2014-42639194]; Finnish Cultural FoundationFinnish Cultural Foundation; Universita degli Studi di Milano (Piano Sostegno alla Ricerca 2018 LINEA 2 Azione A-Giovani Ricercatori)

A series of new N-arylated ferrocenepyrazole structures, carrying different donor or acceptor substituents in the para position of the aryl ring, has been synthesized by the Chan-Lam cross-coupling reaction. The nonplanar geometric molecular structure of some of these chromophores together with their crystal packing was determined by X-ray diffraction, and the HOMO and LUMO energy levels were evaluated by electrochemical and optical measurements and by density functional theoretical (DFT) calculations. By the investigation of solvent effects and time-dependent DFT (TD-DFT) calculations, the intense electronic absorption band around 270-310 nm was confirmed to be an internal-chargetransfer (ICT) band, showing a significant red shift by increasing the electron withdrawing properties of the substituent on the para position of the aryl ring. TD-DFT calculations and electric field induced second harmonic generation (EFISH) measurements of the quadratic hyperpolarizability have shown that also the second-order nonlinear optical (NLO) response of these new ICT chromophores can be tuned by changing the nature of the substituent. Both theoretical ?? and ? and experimental EFISH ??1907 and ?1907 values are significant, with a quite satisfactory correlation of the general trend of theoretical ?? and EFISH ??1907 values. The highest value of EFISH ??1907 (410 × 10-48 esu) was measured for the chromophore carrying the strong electron withdrawing NO2 group.

Complex [Pt(iPr

Complex [Pt(iPr(2)pipdt)(Quinoxdt)] (iPr(2)pipdt=1,4-diisopropyl-piperazine-2,3-dithione; Quinoxdt=[1,4]dithiino[2,3-b]quinoxaline-2,3-dithiolate) exhibits a remarkable green emission at 570nm (room temperature), which is above the lowest excited state. The complex is characterized by negative solvatochromism as well as a high second-order polarizability. Addition of Ag-I ions induces 1)hypsochromic shift of the lowest frequencies and 2)reversible quenching of luminescence. The corresponding Ni and Pd complexes have also been prepared and investigated to assist interpretation of optical properties within the triad. Computational studies based on DFT and time-dependent DFT highlight the electronic properties of [Pt(iPr(2)pipdt)(Quinoxdt)]. The preferential site of interaction between the Pt complex and incoming Ag-I is evidenced by the shape of the Fukui functions, pointing to the thiolic sulfur and platinum atoms as the most reactive sites towards a soft cation. Calculated optical properties are in agreement with experimental findings. This study sheds light on the structure-property relationship for this class of compounds.

Porphyrins are characterized by strong light-harvesting abilities and chemical, photochemical and thermal stability. Since their energy levels can also be easily tuned by chemical modification, this kind of molecules are a good candidate to be implement in photovoltaic devices. Push-pull porphyrins featured by a strong internal charge transfer, have reached interesting photon-to-current conversion efficiencies in dye-sensitized solar cells (DSSCs),1 which best perform in diffuse light conditions2 and can be considered as a potential solution for the development of photovoltaic glass, both for the outdoor and the indoor, to be applied in the making of future zero-energy buildings (ZEBs). However push-pull porphyrins are obtained by multistep synthesis with low overall yields, on the contrary ?-functionalized porphyrins offer the possibility of a straightforward synthesis with higher yields, and therefore more adapted for a large scale production. 3 Here we will present our recent results in the study of ?-substituted porphyrins as dyes in DSSC prototype devices, where the components will be realized by means of different thin-film deposition techniques, such as screen and ink-jet printing. Finally, we will also present our preliminary results in the implementation of photoactive porphyrin thin layer in hetero-junction photovoltaic cells realized through physical vapor deposition. References 1. Urbani, M. et al. Meso-Substituted Porphyrins for Dye-Sensitized Solar Cells. Chem. Rev. 114, 12330-12396 (2014). 2. Freitag, M. et al. Dye-sensitized solar cells for efficient power generation under ambient lighting. Nat. Photonics 11, 372-378 (2017). 3. Di Carlo, G., Orbelli Biroli, A., Tessore, F., Caramori, S. & Pizzotti, M. ?-Substituted ZnIIporphyrins as dyes for DSSC: A possible approach to photovoltaic windows. Coord. Chem. Rev. 358, 153-177 (2018).

The antipodal introduction of two bromine atoms on the 2,12 β-pyrrolic position of 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrin was successfully achieved by a light-induced reaction of the substrate with excess NBS. Complexation with Ni(II) of the major regioisomer led to good quality crystals, suitable for X-ray structure determination with unprecedented probability levels. The regiospecific character of the synthetic procedure and the exactness of the bromine atom position assignment were thus confirmed, suggesting an unexpected electrophilic aromatic substitution pathway rather than a free-radical halogenation process. A QTAIM topological analysis on the DFT-optimized wave function of the monosubstituted free-base porphyrin intermediate carrying a bromine atom in C2 β-pyrrolic position confirmed the largest negative charge for the C12 carbon atom in antipodal position, in agreement with the proposed electrophilic aromatic substitution mechanism.

Both linear- and nonlinear-optical properties of Bu4N[Pt(L1)(L2)] (1; L1 = [4′,5′:5,6][1,4]dithiino[2,3-b]quinoxaline-1′,3′dithiolato; L2 = (R)-α-MBAdto dithiooxamidate, where (R)-α-MBA = (R)-(+)-α-methylbenzyl) upon HCl addition at room temperature change dramatically: the color turns from deep blue to green; the luminescence switches from deep red to green; the nonlinear-optical response (first hyperpolarizability) increases by a factor of 12. Thus, 1 behaves as a unique multiresponsive optical switch whose properties can be followed by the naked eye.

A series of 5,15 push-pull meso-diarylzinc(II) porphyrinates, carrying one or two -COOH or -COOCH3 acceptor groups and a -OCH3 or a -N(CH3)2 donor group, show in N,N-dimethylformamide and CHCl3 solutions a negative and solvent-dependent second-order nonlinear-optical (NLO) response measured by the electric-field-induced second-harmonic generation (EFISH) technique, different from the structurally related zinc(II) porphyrinate carrying a -N(CH3)2 donor group and a -NO2 acceptor group, where a still solvent-dependent but positive EFISH second-order response was previously reported. Moreover, when a -N(CH3)2 donor group and a -COOH acceptor group are part of a sterically hindered 2,12 push-pull beta-pyrrolic-substituted tetraarylzinc(II) porphyrinate, the EFISH response is positive and solvent-independent. In order to rationalize these rather intriguing series of observations, EFISH measurements have been integrated by electronic absorption and IR spectroscopic investigations and by density functional theory (DFT) and coupled-perturbed DFT theoretical and 1H pulsed-gradient spin-echo NMR investigations, which prompt that the significant concentration effects and the strong influence of the solvent nature on the NLO response are originated by a complex whole of different aggregation processes induced by the -COOH group.

The combination of ?-substituted Zn2+ porphyrin dyes and copper-based electrolytes represents a sustainable route for economic and environmentally friendly dye-sensitized solar cells. Remarkably, a new copper electrolyte, [Cu(2-mesityl-1,10-phenanthroline) 2]+/2+, exceeds the performance reached by Co2+/3+ and I-/I3- reference electrolytes.

Transient optical studies, at the nanosecond and femtosecond time scales, have been carried out on three structurally different porphyrin dyes to probe their charge transfer dynamics relevant to spectral sensitization and dye-sensitized solar cell (DSSC) operation. Two of them, showing a ? substitution pattern, were previously reported: ZnB bearing an ethynylphenyl cyanoacrylic acceptor group and ZnBT bearing an additional ?-conjugated dithienylethylene (DTE) chromophore in the acceptor pendant. The new porphyrin ZnMT, showing a donor-acceptor system in 5,15-meso position and the DTE unit as well, is characterized by a smaller HOMO-LUMO energy gap and by a more significant push-pull character which guarantees more intense and red-shifted absorption bands compared to ?-substituted dyes. Besides, the introduction of the DTE unit promotes a remarkable gain in spectral sensitization both in ? and meso positions. We have demonstrated that the population of the absorption manifold resulting from the presence of the additional DTE chromophore leads to charge separation on a time scale of ca. 200 fs, irrespective of either the meso or ? substitution position, resulting in the generation of a long-lived charge-separated state surviving for several microseconds after injection. Thus, dyes equipped with the DTE bridge display the ability to convert photons in the 450-550 nm range, where more common push-pull designs do not show optimal absorption. Further, we have confirmed that ? substitution may represent an effective strategy to combine extended spectral sensitization and optimal charge transfer rates with a synthetic and up-scalable feasibility.

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-beta-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO2 interface in PSSCs. Although the meso push-pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I-/I-3(-)-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO2 interface. We have found that charge injection yields into TiO2 are quite similar for both dyes and that the regeneration efficiencies by I-, are also comparable and in the range of 75-85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO2 layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO2. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of pi-staking aggregation of the dye, promotes a superior shielding of the TiO2 surface against charge recombination involving oxidized species of the electrolyte.

Three novel dyes based on ZnII porphyrinates combined, in β-pyrrolic position, with the π unit dithienylethylene (DTE) have been synthesized and investigated for application in DSSCs. The panchromatic effect due to elongation of the π-delocalized system through a bridge between the porphyrinic ring and the DTE unit such as the 4-ethynylstyryl (1), ethynyl (2), and ethenyl (3) bonds have been investigated by computational, electrochemical, and photoelectrochemical methods. For all three dyes the π conjugated substituents in the β position produced the expected panchromatic effect with broadened electronic absorption spectra over a wide range of wavelengths and IPCE spectra featuring a broad plateau in the region 430-650 nm. In addition both DFT computational and electrochemical data have shown a smaller HOMO-LUMO energy gap for dye 3, when compared to dye 2 suggesting a slightly more facile conjugation between the porphyrinic core and the DTE unit through the ethenylic bond. Conversely the photoelectrochemical investigation showed improved DSSC performances from 3 to 1. These results have been rationalized by an in-depth DFT computational study of dyes 2 and 3 interacting with a cluster of 82 TiO2 units. The small energetic overlap between the LUMO and the TiO2 conduction band characterizing the more structurally distorted dye 3 would suggest low quantum yield of electron injection, while dye 2 shows a greater interaction between the LUMO of the dye and the semiconductor. Consequently the increased linearity and planarity of the structure of dye 1 seems to be the origin of its best performance in DSSC. Therefore it appears that the nature of the bridge between the DTE unit and the porphyrinic ring is quite relevant for the efficiency of these dyes for DSSC, due to distortion from the planarity and linearity of the structure of the dye and the consequent changes on the dye π conjugation. © 2014 American Chemical Society.

Tricyanofuran (TCF) derivatives attached to both anthracene and pyrene moieties were synthesised and characterised by optical, electrochemical and computational techniques. Both compounds exhibited similar absorption profile as well as electrochemical behaviour, however the pyrene derivative showed 20-fold higher non-linear optical activity measured by the EFISH technique. This huge difference has been assigned to (i) a lower molar absorption and (ii) a higher torsion angle for the anthracene derivative, confirmed by both experimental X-ray diffraction and DFT calculations. Furthermore, we note that the μβ1.907 value of −1700 × 10−48 esu recorded for the pyrene derivative in CHCl3/pyridine is remarkable for a NLO chromophore lacking a classical push–pull structure.

A novel square-planar diimine dithiolate non-linear optical (NLO) chromophore, the complex [Ni(o-phen)(bdt)] (1), is reported in this paper (o-phen = dianion of 1,2-phenylendiamina; bdt = 1,2-benzenedithiolate). This compound has been fully characterized by single-crystal X-ray diffraction, UV-vis-NIR spectroscopy, cyclic voltammetry, DFT, and TD-DFT calculations. 1 crystallizes in the P-21 space group and shows an almost planar molecule. The cyclic voltammetry measurements present an irreversible anodic peak at 0.87 V and reversible and quasi-reversible reduction waves at -0.35 and -1.13 V, respectively. A medium-intense (epsilon = 1.91 x 10(4) mol(-1) dm(3) cm(-1)) absorption with a maximum at 728 nm appears in the UV-vis-NIR spectrum. The molecular second order NLO properties of 1 have been measured by the electric field induced second-harmonic generation technique in DMF solution, giving values of -1000 x 10(-48) esu for mu beta(1.907) and 356 x 10(-48) esu for mu beta(0). Furthermore, remarkable solid-state responses have been measured for polymeric films of 1. Indeed, this complex has been embedded into poly(methyl methacrylate) poled films showing high NLO response (d(33) = 1.90 +/- 0.38 pm/V), higher than those previously reported for similar push pull metal complexes. Moreover, I represents the first example of a diimine dithiolate chromophore incorporated into a NLO-active film.

The introduction of alkoxy chains in the molecular architecture of meso push-pull porphyrins is of paramount importance aiming at high performing dye-sensitized solar cells (DSSCs) based on these specific sensitizers. Recently, we have demonstrated that the same approach is fruitful even if it is applied to tetraarylporphyrins with an acceptor/anchoring substituent in the [small beta]-pyrrolic position. In particular, among the ortho-ortho, the ortho-para and the ortho-functionalization of the aryl rings with an octyloxy chain, we identified the latter as the most performing in the series, showing a good balance between the dye loading and the reduction of [small pi]-[small pi] aggregation. Herein, focusing our attention on the mono-ortho-functionalized molecular structure, we have investigated the effect of the alkoxy chain length and nature on the reduction of dye-to-dye aggregation as well as on the enhancement of light harvesting capabilities, finding an almost linear relationship between the device photon conversion efficiency (PCE) and the alkoxy chain length both in the presence and in the absence of a co-disaggregating agent.

Two novel green beta-substituted Zn-II-porphyrins, G1 and G2, based on a 4D-pi-1A type substitution pattern have been synthesized. Their enhanced push-pull character, by reduction of H-L energy gaps, promotes broadening and red-shifting of absorption bands. The effective synthetic pathway and the remarkable spectroscopic properties make G2 ideal for BIPV application.

Push-pull Zn(II)-porphyrinates have recently shown attracting performances as light harvesting systems in dye-sensitized solar cells (DSSCs). To fully exploit their intrinsically high efficiency it is important to finely tune their HOMO and LUMO levels, which can be achieved by proper choice of the push and pull substituents. Of course such target-oriented molecular design requires the availability of reliable relationships between molecular structure and electronic properties; therefore we have carried out a combined electrochemical, spectroscopic and computational investigation on a wide, systematic range of Zn(II)-porphyrinates 5,15 meso substituted with phenylethynyl linkers, including a first symmetric series carrying on the opposite terminals the same substituent (-N(CH3)2,-OCH3,-COOCH3,-COOH,-NO2); and a second push-pull one, with the terminal positions carrying one donor and one acceptor group belonging to the series above. Moreover, two suitably modified porphyrins allowed evaluation of the effects of (i) the presence or absence of the phenyl group in the linker between the porphyrin core and the acceptor group, and (ii) the effect of perfluorination on the same phenyl group. A rationalization scheme is proposed encompassing the whole porphyrin set, affording inter alia interesting clues on the different localization of the redox centres and effective conjugation between the porphyrin core and the side chains as a function of the molecular design.

This short review outlines the main results obtained by our research group over the last 15 years in the field of porphyrins and metal porphyrins for second order nonlinear optics (NLO). This overview aims to provide a general framework of the key factors which affect the second order NLO response of porphyrin chromophores. The pivotal role of the porphyrin ring as π-conjugated linker, the nature of the metal center, the substitution pattern which features the geometrical arrangement of donor and acceptor substituents in the different classes of porphyrin NLO-phores, as well as the aggregation phenomena and the role of solvents are addressed in detail.

Excimer formation in a new class of terthiophene-based fluorophores covalently bonded to a cyclotetrasiloxane scaffold has been demonstrated and the photophysical process ruling it has been investigated in detail and modeled theoretically. In contrast to the conventional systems in which long-living fluorophores such as pyrene are linked in the same molecule, an excimer is formed only when two terthio-phene-based branches nano-organized on the same cyclotetrasiloxane scaffold are close enough together when excitation takes place. In such a case, excimer formation is extremely efficient, and the new bound excited states are quite stable. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

The synthesis and characterization of novel fluorinated Er3+ complexes emitting at the telecommunications wavelengths are presented. These new knowledge-based materials have been designed and optimized on the basis of an extensive photophysical investigation on the processes controlling the overall quantum yield, namely (i) the energy transfer process from the light harvesting conjugated antenna to the emitting ion and (ii) the nonradiative quenching of the lanthanide emission. The study of their photophysical properties has been used for the assessment of their application as active materials in a new generation of low-cost, optical communication amplifiers, based on solution-processed planar waveguides pumped by light emitting diodes (LEDs). Suitable ligands possessing a high-absorption cross-section (where commercially available LEDs operate), good energy level matching those of the ion, and high solubility in fluorinated polymers have been developed. © 2009 American Chemical Society.

We report time-dependent (TD) and coupled-perturbed (CP) DFT/HF investigations, on the linear and non-linear optical properties of M(II)-complexes of [5-[(4′-dimethylaminophenyl)-ethynyl]-15-[(4″-nitrophenyl)ethynyl]-10,20-diphenylporphyrin], with M = Zn, Cu, Ni. The TD-DFT absorption spectrum is in good agreement with the experimental one, allowing us to identify the transitions mainly responsible of the NLO response. CP-DFT/CP-HF calculations provide similar hyperpolarizability values for the three complexes, suggesting an irrelevant role of the metal center. A sharp hyperpolarizability increase is calculated at an incident laser wavelength of 1.064 μm compared to 1.907 μm, which seems to be responsible for the large differences between Zn, Cu and Ni complexes reported experimentally.

A series of sodium arylcyclotetrasiloxanolates properly functionalized in the para position of the aromatic ring, like [4-RC6H4Si(O)ONa]4 and the corresponding (trimethylsiloxy)cyclotetrasiloxanes [4-RC6H4Si(O)OSiMe3]4 (R = Cl, Br, CH CH2, CH2Cl), were prepared as scaffolds for anchoring second order NLO active or fluorescent organic chromophores. Such cyclic molecules can be regarded as 2D models reproducing supramolecular organizations as monolayers. The crystalline structures of some of the compounds synthesized in this work were characterized by single crystal X-ray diffraction analysis at low temperature. All the compounds have phenyl rings in an all-cis arrangement with respect to the tetrasiloxane cycle which, in the trimethylsiloxy derivatives, is quite planar. In these latter compounds the phenyl rings are almost parallel one to the other, although not orthogonal to the ring.

The synthesis, characterization, nonlinear optical properties and DFT calculations for a new Ni-diiminedithiolate complex [Ni(Cl4-phen)(mi-5edt)] (Cl4-phen = 3,4,7,8-tetrachloro-1,10-phenanthroline; mi-5edt = 1-(N-methylindol-5-yl)-ethene-1,2-dithiolate), are reported. The UV–Vis spectrum shows a solvatochromic absorption at 700 nm in DMF (e = 5520 mol−1 dm3 cm−1) due to a charge-transfer (CT) highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition, typical of push–pull complexes. The CT character of this electronic transition is confirmed by DFT calculations with the HOMO mainly centered on the mi-5edt moiety and the LUMO on the Cl4-phen ligand. By comparison with the unsubstituted analogue compound, computational studies confirm the role of the chlorination enhancing the optical properties of this complex. The second order nonlinear optical properties were measured by EFISH technique (at 1.907 μm), giving values of −2030 and −810 × 10−48 esu for μβ1.907 and μβ0, respectively. These values are among the highest reported so far for the class of d8 square-planar push–pull compounds.

Silicon nanocrystals functionalized with tetraphenylporphyrin Zn(ii) chromophores at the periphery perform as light harvesting antennae: excitation of the porphyrin units in the visible spectral region yields sensitized emission of the silicon nanocrystal core in the near infrared with a long lifetime (λmax= 905 nm,τ= 130 μs). This result demonstrates that this hybrid material has a potential application as a luminescent probe for bioimaging.

This review concerns the main results obtained in the last few years by our research group at the Dipartimento di Chimica Inorganica Metallorganica e Analitica of the Universita degli Studi di Milano, in the field of non-linear optics. After a brief introduction on the basic concepts of non-linear optics (NLO), we report our investigations on the second-order NLO properties of one-dimensional (1D) and 2D coordination and organometallic compounds and hybrid inorganic–organic crystalline materials. Our research has allowed to define some general rules for the design of new appealing NLO compounds and to isolate and identify some new interesting photonic materials.

The axial coordination of push–pull E-stilbazoles carrying electron donor or electron withdrawing groups with tetraphenylporphyrinates of Zn(II), Ru(II) and Os(II) does not produce any significant increase in second order NLO response, the exception being the E-stilbazole carrying the electron withdrawing CF3 group. The lack of an increase in the second order NLO response due to the polarising action on the E-stilbazole push–pull system, by the metal centres acting as Lewis acids, is attributed to the axial π backbonding of the metal atom to the E-stilbazole, such backbonding producing a screening effect that opposes σ donation. This effect becomes dominant when the E-stilbazole carries a strong electron withdrawing group. Thus, it appears that the metals in porphyrin complexes may behave in a dual way when they interact axially with soft π delocalised ligands, such as E-stilbazoles where σ acceptor or π donor properties prevail according to the nature of the E-stilbazole itself. Infrared, electronic spectra and dipole moments suggest both axial and equatorial dissipation of the electronic density located on the metal, via π backbonding processes which influence the Lewis acid behaviour of the metal centres. A particular crystal packing, with boxes formed by four porphyrinic moieties and with large empty channels within the boxes, appears to be characteristic of axially substituted tetraphenylporphyrinates, resulting in materials able to absorb gases and solvents in their void spaces.

This work describes the synthesis and the electronic characterisation by electronic absorption spectroscopy, cyclic voltammetry and dipole moments of diphenyl porphyrins and their ZnII complexes substituted at the meso position by a pseudo-linear π-delocalised substituent carrying an electron-donor or an electron-withdrawing group. The second-order NLO response was investigated by the EFISH technique working with a nonresonant incident wavelength of 1.907 μm. This work confirms the ambivalent role of the polarisable porphyrin ring, which, already in the ground state, acts as a donor or acceptor depending on the nature (acceptor or donor) of the substituent in the meso position, as was pointed out in our previous work on tetraphenyl porphyrins substituted at the β-pyrrolic position. The second-order NLO response has been discussed by comparing the nature of the substituent (electron donor or acceptor) and the kind of substitution (β pyrrolic or meso). This comparison evidenced a significant increase of the electron-donor properties of NBu2 or NMe2 when they are connected to the meso position or to the β-pyrrolic position, probably because of an auxiliary donor effect of the electron-rich porphyrin ring. When the substituent is the electron-withdrawing NO2 group, the substituent position (meso or β pyrrolic) is influential, with the existance of a significant increase in the second order NLO response when substitution occurs at the meso position. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Coordination of 4'-(C6H4-p-X)-2,2':6',2''-terpyridines [X = NO2, NBu2, (E)-CH=CH-C6H4-p-NBu2, (E,E)-(CH=CH)2-C6H4-p-NMe2] to Zn(II), Ru(III), and Ir(III) metal centers induces a significant enhancement of the absolute value of the second-order nonlinear optical (NLO) response of the terpyridine, measured by means of both electric field induced second harmonic generation and solvatochromic methods. By varying the nature of the metal center, the enhanced second-order NLO response shifts from positive to negative. Such a shift is controlled by electronic charge-transfer transitions, such as metal-to-ligand or ligand-to-metal transitions, in addition to the intraligand charge transfer. The enhancement generated by coordination is also controlled by the chelation effect and by fine-tuning of the ancillary ligands.

In this work, we report an ab initio investigation of second-order nonlinear optical (NLO) properties and absorption electronic spectra of push−pull transition metal chromophores of the formula [M(CO)5L] (M = Cr, W; L = pyridine (Py), 4-formyl-pyridine (PyCHO), pyrazine (Pyz), trans-1,2-bis(4-pyridyl)ethylene (BPE)). Pyz and BPE are considered either with one nitrogen atom free or interacting with the strong acceptor BF3. All of the molecular properties have been calculated using two different and methodologically independent approaches: the time dependent and coupled perturbed density functional theories (TDDFT and CPDFT) and the sum-over-states (SOS) approach, where the excited states are obtained via the single configuration interaction (SCI) ab initio method. DFT results are in acceptable agreement with the experimental energy values of electronic transitions (with the exception of chromophores with the large π-delocalization, like BPE); SCI calculations overestimate excitation energies and produce a...

Density functional theory (DFT) investigations of title organometallic compounds led to identification of electronic features which determine the magnitude and the orientation of the dipole moment. The analysis of our theoretical results shows that the presence of a strong acidic BF3 group, or more generally, the substitution of ligands in the M(CO)(x)ClyL complexes can affect the orientation of the dipole moments, out of the direction of the first charge transfer (CT) excitation and, as a consequence, the magnitude of the vector part of the second order hyperpolarizabilities. (C) 2003 Elsevier Science B.V. All rights reserved.

We report the synthesis, electronic properties, and second-order nonlinear optical (NLO) response of asymmetric heteronuclear push−pull bimetallic complexes with pyrazine (pyz) and trans-1,2-bis(4-...

A series of di- and monosubstituted cis-platinum(II) silanolate complexes, Pt(OSiR3)2(dppe) (R=Et, 1; R=Me, 2) and Pt(OSiR3)Cl(dppe) (R=Et, 3; R=iPr, 4) where dppe is 1,2-bis(diphenylphosphino)ethane, have been isolated and characterised spectroscopically. Complex 1 does not react with CO and H2 under anhydrous conditions, but the complexes Pt{C(O)OCH3)}2(dppe) (6) and Pt(CO3)(dppe) (7) have been isolated bubbling CO in methanol and CO2 in moist benzene solutions of 1, respectively. The behaviour of 1 towards water or methanol is discussed on the basis of 1H, and 31P{1H} NMR spectroscopic data. The new complex Pt{S2C(OSiEt3)2}(dppe) (8) has been isolated by reaction of 1 with CS2 in benzene solution. This reactivity would suggest a high sensitivity towards water, but not towards H2 or CO, of the bonding of slightly oxidised platinum particles with silanol groups of silica surface.

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO2 interface in PSSCs. Although the meso push-pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I(-)/I3(-)-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO2 interface. We have found that charge injection yields into TiO2 are quite similar for both dyes and that the regeneration efficiencies by I(-), are also comparable and in the range of 75-85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO2 layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO2. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO2 surface against charge recombination involving oxidized species of the electrolyte.

A possible new process of activation of the OOH group in the mechanism of ethylene epoxidation catalysed by Pt(II) diphosphine complexes has been investigated by ab initio MO-LCAO calculations. The electronic and geometric features of YOOH species (Y = H, CH3, t-But, CF3, CH3CO, (PH3)2Pt(CF3), (PH3)3PtCl) have been evaluated and compared. Coordination of the OOH group to platinum induces an inversion of the polarity of the O-O bond when compared to any organic hydroperoxide; parallelly it favours the isomerisation of the OOH group from a hydroperoxo to an oxywater-like structure. This latter effect could be an important factor in favouring the reaction of the platinum coordinated OOH group with ethylene to form ethylene oxide. In fact, calculations on the energetics of the interaction between H2O2 and C2H4 have shown that isomerisation of HOOH to H2OO oxywater structure is the rate determining step for the epoxidation process.

The characterization of the nature of surface species is a primary requirement for a better insight into surface organometallic chemistry and therefore heterogeneous catalytic processes. To date, the structures of organometallic species formed on the surface of inorganic supports (e.g. mainly oxides such as SiO 2 , Al 2 O 3 , TiO 2 , MgO) have been investigated spectroscopically. However, the use of solvent extraction with or without surface reactions as a tool for a more precise characterization of these surface species is becoming a new approach. Extraction tests under various conditions constitute a first probe because the eventual extracted species can be easily characterized by classical methods. Moreover, reactions of the unextracted surface organometallic species with specific reagents may lead to products which can be extracted and characterized as well by classical methods. This approach can give an indirect evidence of the nature of the unextracted organometallic surface species. The nature of surface species can be further substantiated by the synthesis of well defined molecular organometallic models and by the investigation of their reactivity.

The mixed-ligand dithiolene complex [Pt(Bz(2)pipdt)(dcbdt)] (1) bearing the two ligands Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C(2)S(2)PtS(2)C(2) dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C(2)S(2) units exhibit a significant difference in Bz(2)pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz(2)pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (μβ(0) = -1296 × 10(-48) esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes.

Ab initio Hartree Fock calculations have been carried out on the complex L 2 Pt(η 2 -O 2 ) and its derivative L 2 Pt-(OOH)X and L 2 Pt(OOH)(XY) obtained by reaction with HX (HX=H 2 O, HCl, HOOCH, NH 3 ) and HXYH (HXYH=oxalic acid, o-catechol, hydrazine, hydroxylamine, ethylendiamine, o-phenylendiamine). In addition also the reaction L 2 Pt(OOH)X+HX→H 2 O 2 +L 2 PtX 2 leading to the formation of hydrogen peroxide has been studied. The theoretical investigation gave, for all considered species, optimum complex geometry, charge distribution and reaction energies being evaluated by means of a correlation energy density functional

In this work, the second harmonic generation (SHG) and the nonlinear optical (NLO) coefficients d33 of PMMA films, containing 4 wt % NLO active macrocyclic chromophores based on a cyclotetrasiloxane scaffold and of their monomeric structurally homologous NLO active chromophores, were obtained by an electrical poling process and measured at room temperature during a period of many months, after switching off of the orienting electric field. The composite films based on the macrocyclic NLO chromophores have shown much higher d33 values and stability in the time if compared to films based on the monomeric structurally homologous NLO chromophores. Such higher stability in the time is related to a higher decay time t2 and to a lower rotational diffusional coefficient DR, as a consequence of the increased size of the macrocyclic NLO chromophores. We have observed that also the shape of the NLO chromophores may play a role. A good stability in the time of the d33 values is maintained if the PMMA films, containing the macrocyclic NLO chromophores, are heated up to about 50 °C.

In this work we report some examples of fluorinated lanthanide complexes, in which the geometry of the coordination sphere is controlled by C–F···π system strong interactions involving the fluorinated ligands. Moreover we have also shown that a perfluorinated ligand, such as tris(pentafluorophenyl)phosphine oxide may have a role in increasing the pumping of energy, in the Er3+ complex with totally fluorinated ligands, from the π system of the perfluorinated acetylacetone ligand, acting as antenna, to the Er3+ emitting ion.