Semi-Transparent Luminescent Solar Concentrators Based on Intramolecular Energy Transfer in Polyurethane Matrices.

Luminescent solar concentrators (LSCs) are spectral conversion devices offering interesting opportunities for the integration of photovoltaics into the built environment and portable systems. The Förster-resonance energy transfer (FRET) process can boost the optical response of LSCs by reducing energy losses typically associated to non-radiative processes occurring within the device under operation. In this work, a new class of FRET-based thin-film LSC devices is presented, in which the synthetic versatility of linear polyurethanes (PU) is exploited to control the photophysical properties and the device performance of the resulting LSCs. A series of luminescent linear PUs are synthesized in the presence of two novel bis-hydroxyl-functionalized luminophores of suitable optical properties, used as chain extenders during the step-growth polyaddition reaction for the formation of the linear macromolecular network. By synthetically tuning their composition, the obtained luminescent PUs can achieve a high energy transfer efficiency (≈90%) between the covalently linked luminophores. The corresponding LSC devices exhibit excellent photonic response, with external and internal photon efficiencies as high as ≈4% and ≈37%, respectively. Furthermore, their optimized power conversion efficiency combined with their enhanced average visible-light transmittance highlight their suitability for potential use as transparent solar energy devices.

Two-photon absorption properties of simple neutral Ir(III) complexes.

A series of neutral Ir(2-phenylpyridine)3 derivatives substituted on the para-position of the pyridyl ligands with a π-conjugated substituent possessing different donor abilities has been prepared. Their two-photon absorption properties have been determined using the Z-scan technique. Such simple iridium(III) neutral complexes, which are easy to synthesize, show good two-photon absorption activity, with relevant TPA cross sections (the best is 750 GM), giving rise to multifunctional chromophores, since they present also high second-order NLO properties.

Recent Investigations on the Use of Copper Complexes as Molecular Materials for Dye-Sensitized Solar Cells.

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a route for harnessing the sun's energy and converting it into electricity. Since then, an impressive amount of work has been devoted to improving the global photovoltaic efficiency of DSSCs, trying to optimize all components of the device. Up to now, the best efficiencies have usually been reached with ruthenium(II) photosensitizers, even if in the last few years many classes of organic compounds have shown record efficiencies. However, the future of DSSCs is stringently connected to the research and development of cheaper materials; in particular, the replacement of rare metals with abundant ones is an important topic in view of the long-term sustainability of DSSCs intended to replace the consolidated fossil-based technology. In this context, copper is a valid candidate, being both an alternative to ruthenium in the fabrication of photosensitizers and a material able to replace the common triiodide/iodide redox couple. Thus, recently, some research papers have confirmed the great potential of copper(I) coordination complexes as a cheap and convenient alternative to ruthenium dyes. Similarly, the use of copper compounds as electron transfer mediators for DSSCs can be an excellent way to solve the problems related to the more common I3-/I- redox couple. The goal of this mini-review is to report on the latest research devoted to the use of versatile copper complexes as photosensitizers and electron shuttles in DSSCs. The coverage, from 2022 up to now, illustrates the most recent studies on dye-sensitized solar cells based on copper complexes as molecular materials.

Iridium and Ruthenium Complexes Bearing Perylene Ligands.

The present review summarizes the work carried out mostly in the last decade on iridium and ruthenium complexes bearing various perylene ligands, of particular interest for bioimaging, photodynamic therapy, and solar energy conversion. In these complexes, the absorption spectra and the electrochemical properties are those of the perylene subunit plus those of the metal moiety. In contrast, the emissions are completely changed with respect to perylenes considered alone. Thus, fully organic perylenes are characterized by a strong fluorescence in the visible region, lifetimes of a few nanoseconds, and luminescence quantum yields approaching 100%, whereas perylene Ir and Ru complexes usually do not emit; however, in few cases, weak phosphorescent emissions, with lifetimes in the range of microseconds and relatively low quantum yields, are reported. This is due to a strong interaction between the perylene core and the heavy metal center, taking place after the excitation. Nevertheless, an important advantage deriving from the presence of the heavy metal center is represented by the ability to generate large amounts of singlet oxygen, which plays a key role in photodynamic therapy.

Luminescent 1,10-Phenanthroline ß-Diketonate Europium Complexes with Large Second-Order Nonlinear Optical Properties.

Substitution of the diglyme ligand of [Eu(hfa)3(diglyme)] (where hfa is hexafluoroacetylacetonate) with a simple 1,10-phenanthroline leads to a six-fold increase of the product µßEFISH, as measured by the Electric-Field-Induced Second Harmonic generation (EFISH) technique. Similarly, [Eu(tta)3(1,10-phenanthroline)] (where Htta is 2-thenoyltrifluoroacetone) is characterized by a large second-order NLO response. Both 1,10-phenanthroline europium complexes have great potential as multifunctional materials for photonics.

A Novel Class of Cyclometalated Platinum(II) Complexes for Solution-Processable OLEDs.

Substitution of the chlorido ligand of cyclometalated [Pt (5-R-1,3-di(2-pyridyl) benzene)Cl] (R = methyl, mesityl, 2-thienyl, or 4-diphenylamino-phenyl) by 4-phenylthiazole-2-thiolate leads to related thiolato complexes, which were fully characterized. Their photophysical properties were determined in degassed dichloromethane solution. The emission color of the novel complexes can be easily tuned by the nature of the substituents on the terdentate ligand, as is the case for the parent chlorido complexes. Their luminescence Quantum Yield is high, with that of the compounds with the 2-thienyl or 4-diphenylamino-phenyl substituents being much higher than that of the related chloride complexes. The platinum complex with the cyclometalated 5-(2-thienyl)-1,3-di(2-pyridyl) benzene was used as the emitter for the fabrication of a yellow solution-processable OLED.

Introduction of a triphenylamine substituent on pyridyl rings as a springboard for a new appealing brightly luminescent 1,3-di-(2-pyridyl)benzene platinum(II) complex family.

The preparation and characterization of three new complexes, namely [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-4,6-difluoro-benzene)Cl] ([PtL1Cl]), [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-5-triphenylamine-benzene)Cl] ([PtL2Cl]), and [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-5-mesityl-benzene)Cl] ([PtL3Cl]), is reported. All of them are highly luminescent in dilute deaerated dichloromethane solution (Φlum = 0.88-0.90, in the yellow-green region; the λmax,em in nm for the monomers are: 562, 561 and 549 for [PtL1Cl], [PtL2Cl] and [PtL3Cl], respectively).[PtL1Cl] is the most appealing, being characterized by a very long lifetime (103.9 µs) and displaying intense NIR emission in concentrated deaerated solution (Φlum = 0.66) with essentially no "contamination" by visible light < 600 nm. This complex allows the fabrication of both yellow-green and deep red/NIR OLEDs; OLED emissions are in the yellow-green (CIE = 0.38, 0.56) and deep red/NIR (CIE = 0.65, 0,34) regions, for [PtL1Cl] 8 wt% (with 11% ph/e EQE) and pure [PtL1Cl] (with 4.3% ph/e EQE), respectively.

Recent Investigations on Thiocyanate-Free Ruthenium(II) 2,2'-Bipyridyl Complexes for Dye-Sensitized Solar Cells.

Three decades ago, dye-sensitized solar cells (DSSCs) emerged as a method for harnessing the energy of the sun and for converting it into electricity. Since then, a lot of work has been devoted to create better global photovoltaic efficiencies and long term stability. Among photosensitizers for DSSCs, thiocyanate-free ruthenium(II) complexes have gained increasing interest due to their better stability compared to conventional thiocyanate-based complexes, such as benchmark dyes N719 and Z907. In this mini-review, two classes of thiocyanate-free Ru(II) complexes are presented: (a) bis-bipyridyl compounds bearing an ancillary cyclometalating bidentate ligand; (b) bipyridyl compounds bearing non-cyclometalating ancillary ligands. The coverage, mainly from 2014 up to now, is not exhaustive, but illustrates the most recent design strategies and photovoltaic properties of these two families of ruthenium(II) dyes.

Exohedral Functionalization of Fullerene by Substituents Controlling of Molecular Organization for Spontaneous C60 Dimerization in Liquid Crystal Solutions and in a Bulk Controlled by a Potential.

A study was carried out on the possibility of orderly and spontaneous dimerization at room temperature of C60 cages in fullerene liquid crystal fullerene dyads (R-C60). For this purpose, dyads with a structural elements feature supporting π-stacking and Van der Waals interactions were tested, due to the presence of terthiophene donors linked through an α-position or dodecyloxy chains. In addition, this possibility was also tested and compared to dyads with shorter substituents and the pristine C60. Research has shown that only in dyads with the features of liquid crystals, π-dimerization of C60 units occurs, which was verified by electrochemical and spectroelectrochemical (ESR) measurements. Cyclic voltammetry and differential voltammetry studies reveal π-dimerization in liquid crystal dyad solution even without the possibility of previous polymerization (cathodic or anodic) under conditions in the absence of irradiation and without the availability of reaction initiators, and even with the use of preliminary homogenization. These dyads undergo six sequential, one-electron reductions of π-dimer (R-C60···C60-R), where two electrons are added successively to each of the two fullerene cages and first form two radical anion system (R-C60)•-(R-C60)•- without pairing with the characteristics of two doublets. Similarly, the second reductions of π-dimer occur at potentials that are close to the reduction potential for the conversion to a system of two triplet dianions (R-C60)2-(R-C60)2-. Electron paramagnetic resonance spectra indicate a significant interaction between C60 cages. Interestingly, the strength of intermolecular bonds is so significant that it can overcome Coulombic repulsion, even with such highly charged particles as dianions and trianions. Such behavior has been revealed and studied so far only in covalently bonded C60 dimers.

Recent Advances in Dye-Sensitized Solar Cells.

Dye-sensitized solar cells (DSSCs) are an effective alternative for delivering clean energy from the sun compared to the most widely deployed technologies based upon semiconductor photovoltaics [...].

Novel Ferrocene-Appended ß-Ketoimines and Related BF2 Derivatives with Significant Aggregation-Induced Emission and Second-Order Nonlinear Optical Properties.

A series of new ß-ketoimines containing a ferrocene moiety and their BF2 complexes have been synthesized and structurally characterized. The solvatochromism of the ß-ketoimines was studied, putting in evidence a redshift with increasing solvent polarity. This positive solvatochromism can be attributed to a more polarized excited state compared with the ground state, due to intramolecular charge transfer (ICT) transitions. The ß-ketoimines exhibited weak emission, attributable to the excited-state intramolecular proton transfer (ESIPT) phenomenon. This ESIPT effect is suppressed upon restriction of the keto-enamine tautomerism, induced upon addition of BF3 ⋅OEt2 , which afforded the related BF2 complexes, characterized by an enhancement of the fluorescence through the ICT effect. Both the ß-ketoimines and BF2 complexes exhibited significant aggregation-induced emission behavior in mixtures of CH3 CN/H2 O, due to restriction of intramolecular rotation in the aggregated state. The frontier molecular orbital levels, ground- and excited-state dipole moments (µg and µe ), and the origin of electronic absorption spectra were studied by time-dependent DFT calculations. The second-order nonlinear optical (NLO) properties were determined by the electric-field-induced second-harmonic generation technique. The µß1907 values of the ß-ketoimines increased upon the formation of the related BF2 complexes, mainly due to an enhancement of the ground-state dipole moment. The results presented here reveal that some of these novel compounds are excellent multifunctional candidates for NLO and luminescence applications.

Copper Complexes as Alternative Redox Mediators in Dye-Sensitized Solar Cells.

Thirty years ago, dye-sensitized solar cells (DSSCs) emerged as a method for harnessing the sun's energy and converting it into electricity. Since then, a lot of work has been dedicated to improving their global photovoltaic efficiency and their eco-sustainability. Recently, various articles showed the great potential of copper complexes as a convenient and cheap alternative to the traditional ruthenium dyes. In addition, copper complexes demonstrate that they can act as redox mediators for DSSCs, thus being an answer to the problems related to the I3-/I- redox couple. The aim of this review is to report on the most recent impact made by copper complexes as alternative redox mediators. The coverage, mainly from 2016 up to now, is not exhaustive, but allows us to understand the great role played by copper complexes in the design of eco-sustainable DSSCs.

NLO-active Y-shaped ferrocene conjugated imidazole chromophores as precursors for SHG polymeric films.

New Y-shaped ferrocene conjugated imidazole chromophores were prepared and fully characterized. The Y-shaped structure was confirmed by the single crystal X-ray diffraction technique. The chromophores show interesting second-order nonlinear optical (NLO) properties in solution, as determined by the Electric-Field Induced Second Harmonic generation (EFISH) technique. Remarkably, the trifluoro substituted compound 3 is characterized by a high µßEFISH value and has good potential as a molecular building block for composite films with Second Harmonic Generation (SHG) properties. For all compounds, the dipole moments and frontier orbital energies were calculated by the Density Functional Theoretical method.

Improving the efficiency of copper-dye-sensitized solar cells by manipulating the electrolyte solution.

The use of a copper(i) dye, bearing a 2,9-dimesityl-1,10-phenanthroline and a 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid, was investigated in DSSCs with various electrolyte solutions based on two different redox mediators, namely the common I-/I3- couple and an interesting copper electron shuttle. The experimental results provide evidence of the importance of the redox mediator concentration and the crucial role of additives such as 4-tert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide in the performance of sustainable "full-copper" DSSCs, consolidating the way to DSSCs with Earth-abundant components.

A Highly Luminescent Tetrahydrocurcumin IrIII Complex with Remarkable Photoactivated Anticancer Activity.

Curcumin has chemopreventative properties against a variety of tumours, but has poor bioavailability. Here, two new bis-cyclometallated iridium(III) complexes have been prepared, featuring the natural product curcumin (CUR) or its reduced form, tetrahydrocurcumin (THC), as bidentate, anionic O O-binding ligands. The iridium THC complex is highly luminescent in deoxygenated solution and efficiently generates singlet oxygen under aerated conditions, whereas in the CUR analogue, other non-radiative decay pathways are competitive. The complexes are rapidly taken up by a variety of human tumour cell lines from solutions of micromolar concentration. They show negligible cytotoxicity in the absence of irradiation. When briefly irradiated with visible light, Ir-THC becomes highly phototoxic, inducing rapid apoptosis within 2 h. The results show the high potential of such complexes as sensitizers in photodynamic therapy (PDT).

Towards efficient sustainable full-copper dye-sensitized solar cells.

Two new heteroleptic copper(i) sensitizers bearing 6,6'-dimethyl-2,2'-bipyridine-4,4'-dibenzoic acid, to anchor the dye on the titania surface, and a π-delocalized 2-(R-phenyl)-1H-phenanthro[9,10-d]imidazole (R = NPh2 or O-hexyl) ancillary ligand were prepared and well characterized. Their performance as dyes in DSSCs is quite similar to that of the related complex bearing 2,9-dimesityl-1,10-phenanthroline as an ancillary ligand, when using the common I-/I3- redox couple or homoleptic copper complexes as electron shuttles. The experimental results along with theoretical calculations confirm the great potential of full-copper DSSCs.

Novel cyclometallated 5-π-delocalized donor-1,3-di(2-pyridyl)benzene platinum(ii) complexes with good second-order nonlinear optical properties.

Five new platinum(ii) complexes bearing a cyclometallated 5-π-delocalized donor-1,3-di(2-pyridyl)benzene were prepared and fully characterized. Their second-order nonlinear optical (NLO) properties were determined by the Electric-Field Induced Second Harmonic generation (EFISH) technique, working in DMF solution with an incident wavelength of 1907 nm, whereas the dipole moments were determined by Density Functional Theory (DFT) calculations. Remarkably, the platinum(ii) complex with a cyclometallated 5-guaiazulene-CH[double bond, length as m-dash]CH-1,3-di(2-pyridyl)benzene appears as a very good candidate for application in photonics, being characterized by the largest second-order NLO response. Besides, it appeared that the nature of substituents on thiophene, chosen as π-delocalized moiety in position 5 of the 1,3-di(2-pyridyl)benzene, strongly affects the NLO properties.

An investigation on the second-order nonlinear optical response of cationic bipyridine or phenanthroline iridium(iii) complexes bearing cyclometallated 2-phenylpyridines with a triphenylamine substituent.

The synthesis and characterisation of six new cationic iridium(iii) complexes bearing either 4,4'-di-tert-butyl-2,2'-bipyridine (dtBubpy) or 5-NO2-1,10-phenanthroline along with two cyclometallated 2-phenylpyridine derivative ligands, decorated with triphenylamine groups either meta or para to the Ir-CC^N bond or para to the Ir-NC^N bond, are reported. The second-order nonlinear optical (NLO) properties of all the compounds have been determined by the electric field induced second harmonic generation technique and show that the µßEFISH value can be tuned by the nature of the iridium coordination sphere. The dipole moment, µ, necessary to evaluate the quadratic hyperpolarizability ßEFISH, was theoretically determined. The linear optical properties of the complexes are also presented and rationalised by quantum-chemical calculations. One of the prepared iridium compounds was incorporated into a polystyrene film, affording the first example of a second-order NLO active polymeric film based on a cationic organometallic complex.

Photochromic DTE-Substituted-1,3-di(2-pyridyl)benzene Platinum(II) Complexes: Photomodulation of Luminescence and Second-Order Nonlinear Optical Properties.

We disclose a new family of photochromic cyclometalated platinum(II) complexes (PtDTE1 and PtDTE2), where a dithienylethene (DTE) unit is connected at the para-position of the central phenyl ring of (N^C^N) cyclometalated ligand, through two different linkages. Their syntheses are presented along with the X-ray characterizations of both the open and closed isomers of PtDTE1. The investigation of their photophysical properties is made, including absorption, photochromism, emission, and second-order nonlinear properties. We report a quantitative photoisomerization for both PtDTE1 and PtDTE2, irrespective of the nature of the connecting mode between the DTE unit and the platinum(II) moiety. The efficient photochromism allows a significant NLO photomodulation, both in solution and in thin films. In addition, we show that the photoluminescence of the PtDTE1 and PtDTE2 can be controlled by the open/closed isomerization of the DTE unit.

Intriguing C-HCu interactions in bis-(phenanthroline)Cu(i) redox mediators for dye-sensitized solar cells.

We have synthesized and characterized a series of bis-(phenanthroline)Cu(i) complexes of interest as redox mediators for dye-sensitized solar cells. This study led to the discovery of intriguing anagostic interactions between the hydrogen atom and the copper center as evidenced by X-ray diffraction studies on a single crystal. Remarkably, an anagostic interaction was found between a H atom of a methyl group and a copper site.