Philippe P. Lainé

Theoretical procedure for optimizing dye-sensitized solar cells: from electronic structure to photovoltaic efficiency.

A step-by-step theoretical protocol based on density functional theory (DFT) and time-dependent DFT at both the molecular and periodic levels is proposed for the design of dye-sensitized solar cell (DSSC) devices including dyes and electrolyte additives. This computational tool is tested with a fused polycyclic pyridinium derivative as a novel dye prototype. First, the UV-vis spectrum of this dye alone is computed, and then the electronic structure of the system with the dye adsorbed on an oxide semiconductor surface is evaluated. The influence of the electrolyte part of the DSSC is investigated by explicitly taking into account the electrolyte molecules co-adsorbed with the dye on the surface. We find that tert-butylpyridine (TBP) reduces the electron injection by a factor of 2, while lithium ion increases this injection by a factor of 2.4. Our stepwise protocol is successfully validated by experimental measurements, which establish that TBP divides the electronic injection by 1.6 whereas Li(+) multiplies this injection by 1.8. This procedure should be useful for molecular engineering in the field of DSSCs, not only as a complement to experimental approaches but also for improving them in terms of time and resource consumption.

Modeling Dye-Sensitized Solar Cells: From Theory to Experiment

Density functional theory (DFT) and time-dependent DFT are useful computational approaches frequently used in the dye-sensitized solar cell (DSSC) community in order to analyze experimental results and to clarify the elementary processes involved in the working principles of these devices. Indeed, despite these significant contributions, these methods can provide insights that go well beyond a purely descriptive aim, especially when suitable computational approaches and methodologies for interpreting and validating the computational outcomes are developed. In the present contribution, the possibility of using recently developed computational approaches to design and interpret the macroscopic behavior of DSSCs is exemplified by the study of the performances of three new TiO2-based DSSCs making use of organic dyes, all belonging to the expanded pyridinium family.

Long-range coupling in a mixed-valence diruthenium complexes containing bis-terpyridine ligands of various lengths as bridges

Dinuclear ruthenium(II) complexes have been prepared which contain back-to-back bis-terpyridine ligands of various lengths (Ru ⋯ Ru distances between 7 and 20 A) as bridges; significant electronic coupling is observed in mixed-valence states, even for the system with the longest separation.

Expanded Pyridiniums: Bis-cyclization of Branched Pyridiniums into Their Fused Polycyclic and Positively Charged Derivatives—Assessing the Impact of Pericondensation on Structural, Electrochemical, Electronic, and Photophysical Features

This study evaluates the impact of the extension of the π-conjugated system of pyridiniums on their various properties. The molecular scaffold of aryl-substituted expanded pyridiniums (referred to as branched species) can be photochemically bis-cyclized into the corresponding fused polycyclic derivatives (referred to as pericondensed species). The representative 1,2,4,6-tetraphenylpyridinium (1(H)) and 1,2,3,5,6-pentaphenyl-4-(p-tolyl)pyridinium (2(Me)) tetra- and hexa-branched pyridiniums are herein compared with their corresponding pericondensed derivatives, the fully fused 9-phenylbenzo[1,2]quinolizino[3,4,5,6-def]phenanthridinium (1(H)f) and the hitherto unknown hemifused 9-methyl-1,2,3-triphenylbenzo[h]phenanthro[9,10,1-def]isoquinolinium (2(Me)f). Combined solid-state X-ray crystallography and solution NMR experiments showed that stacking interactions are barely efficient when the pericondensed pyridiniums are not appropriately substituted. The electrochemical study revealed that the first reduction process of all the expanded pyridiniums occurs at around -1 V vs. SCE, which indicates that the lowest unoccupied molecular orbital (LUMO) remains essentially localized on the pyridinium core regardless of pericondensation. In contrast, the electronic and photophysical properties are significantly affected on going from branched to pericondensed pyridiniums. Typically, the number of absorption bands increases with extended activity towards the visible region (down to ca. 450 nm in MeCN), whereas emission quantum yields are increased by three orders of magnitude (at ca. 0.25 on average). A relationship is established between the observed differential impact of the pericondensation and the importance of the localized LUMO on the properties considered: predominant for the first reduction process compared with secondary for the optical and photophysical properties.

Reaching Optimal Light-Induced Intramolecular Spin Alignment within Photomagnetic Molecular Device Prototypes

Ground-state (GS) and excited-state (ES) properties of novel photomagnetic molecular devices (PMMDs) are investigated by means of density functional theory. These organic PMMDs undergo a ferromagnetic alignment of their intramolecular spins in the lowest ES. They are comprised of: 1) an anthracene unit (An) as both the photosensitizer (P) and a transient spin carrier (SC) in the triplet ES ((3)An*); 2) imino-nitroxyl (IN) or oxoverdazyl (OV) stable radical(s) as the dangling SC(s); and 3) bridge(s) (B) connecting peripheral SC(s) to the An core at positions 9 and 10. Improving the efficiency of the PMMDs involves strengthening the ES intramolecular exchange coupling (J(ES)) between transient and persistent SCs, hence the choice of 2-pyrimidinyl (pm) as B elements to replace the original p-phenylene (ph). Dissymmetry of the pm connectors leads to [SC-B-P-B-SC] regio-isomers int. and ext., depending on whether the pyrimidinic nitrogen atoms point towards the An core or the peripheral SCs, respectively. For the int. regio-isomers we show that the photoinduced spin alignment is significantly improved because the J(ES)/k(B) value is increased by a factor of more than two compared with the ph-based analogue (J(ES)/k(B)>+400 K). Most importantly, we show that the optimal J(ES)/k(B) value ( approximately +600 K) could be reached in the event of an unexpected saddle-shaped structural distortion of the lowest ES. Accounting for this intriguing behavior requires dissection of the combined effects of 1) borderline intramolecular steric hindrance about key An-pm linkages, which translates into the flatness of the potential energy surface; 2) spin density disruption due to the presence of radicals; and 3) possibly intervening photochemistry, with An acting as a light-triggered electron donor while pm, IN, and OV behave as electron acceptors. Finally, potentialities attached to the [(SC)-pm-An-pm](int) pattern are disclosed.

Theoretical insights into branched and fused expanded pyridiniums by the means of density functional theory.

With the aim of getting insights into the peculiar electronic, structural, and photophysical properties of four expanded pyridinium systems of potential use as electron acceptors in supramolecular architectures, their electronic and geometrical structures, at both the ground and the excited states, were investigated by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT). Solvent effects were included by the means of a polarizable continuum model (PCM) at both the ground and the excited states. In particular, the computed photophysical behaviors (absorption and emission) of the fused architectures were compared to those of the respective branched precursors in order to clarify the origin(s) of (i) the extension of their electronic absorption toward the visible region and (ii) the increase of their luminescence quantum yields and red-shifted emission wavelengths experimentally observed. The theoretical insights gained allow for a clear-cut explanation of the different behavior of these systems of interest as electron acceptors and luminophores for more complex supramolecular architectures and opens the route for a joint experimental and theoretical design of new pyridinium-based acceptors.

Molecular Dyads of Ruthenium(II)– or Osmium(II)–Bis(terpyridine) Chromophores and Expanded Pyridinium Acceptors: Equilibration between MLCT and Charge-Separated Excited States

The synthesis, characterization, redox behavior, and photophysical properties (both at room temperature in fluid solution and at 77 K in rigid matrix) of a series of four new molecular dyads (2-5) containing Ru(II)- or Os(II)-bis(terpyridine) subunits as chromophores and various expanded pyridinium subunits as electron acceptors are reported, along with the reference properties of a formerly reported dyad, 1. The molecular dyads 2-4 have been designed to have their (potentially emissive) triplet metal-to-ligand charge-transfer (MLCT) and charge-separated (CS) states close in energy, so that excited-state equilibration between these levels can take place. Such a situation is not shared by limit cases 1 and 5. For dyad 1, forward photoinduced electron transfer (time constant, 7 ps) and subsequent charge recombination (time constant, 45 ps) are evidenced, while for dyad 5, photoinduced electron transfer is thermodynamically forbidden so that MLCT decays are the only active deactivation processes. As regards 2-4, CS states are formed from MLCT states with time constants of a few dozens of picoseconds. However, for these latter species, such experimental time constants are not due to photoinduced charge separation but are related to the excited-state equilibration times. Comparative analysis of time constants for charge recombination from the CS states based on proper thermodynamic and kinetic models highlighted that, in spite of their apparently affiliated structures, dyads 1-4 do not constitute a homologous series of compounds as far as intercomponent electron transfer processes are concerned.

Photoinduced electron transfer in Os(terpyridine)-biphenylene-(bi)pyridinium assemblies.

A series of linearly arranged donor-spacer-acceptor (D-S-A) systems 1-3, has been prepared and characterized. These dyads combine an Os(II)bis(terpyridine) unit as the photoactivable electron donor (D), a biphenylene (2) or phenylene-xylylene (3) fragment as the spacer (S), and a N-aryl-2,6-diphenylpyridinium electrophore (with aryl = 4-pyridyl or 4-pyridylium in 1 or 2/3, respectively) as the acceptor (A). Their absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution) have been studied. The electronic structure and spectroscopic properties of a representative compound of the series (i.e., 2) have also been investigated at the theoretical level, performing Density Functional Theory (DFT)-based calculations. Time-dependent transient absorption spectra of 1-3 have also been recorded at room temperature. The results indicate that efficient photoinduced oxidative electron transfer takes place in the D-S-A systems at room temperature in fluid solution, for which rate constants (in the range 4 × 10(8)-2 × 10(10) s(-1)) depend on the driving force of the process and the spacer nature. In all the D-S-A systems, charge recombination is faster than photoinduced charge separation, in spite of the relatively large energy of the D(+)-S-A(-) charge-separated states (between 1.47 and 1.78 eV for the various species), which would suggest that the charge recombination occurs in the Marcus inverted region. Considerations based on superexchange mechanism suggest that the reason for the fast charge recombination is the presence of a virtual D-S(+)-A(-) state at low energy--because of the involvement of the easily oxidizable biphenylene spacer--which is beneficial for charge recombination via superexchange but unsuitable for photoinduced charge separation. To further support the above statement, we prepared a fourth D-S-A species, 4, analogous to 2 but with a (hardly oxidizable) single phenylene fragment serving as the spacer. For such a species, charge recombination (about 3 × 10(10) s(-1)) is slower than photoinduced charge separation (about 1 × 10(11) s(-1)), thereby confirming our suggestions.

Intramolecular Spin Alignment within Mono‐Oxidized and Photoexcited Anthracene‐Based π Radicals as Prototypical Photomagnetic Molecular Devices: Relationships Between Electrochemical, Photophysical, and Photochemical Control Pathways

AnOV is a pi-conjugated radical built from an anthracene (An) unit linked by a p-phenylene to an oxoverdazyl (OV) moiety. The mono-oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV(+) was demonstrated by combining spectroelectrochemistry, electron-spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations. The intramolecular spin alignment (ISA) within AnOV(+) results from the ferromagnetic coupling (J(electrochem)>0) of the two unpaired electrons located on the oxidized electron donor (An(+)) and on the pendant OV radical. The spin-density distribution pattern of AnOV(+) is akin to that of AnOV when photopromoted (AnOV*) to its high-spin (HS) lowest excited quartet (S=3/2) state. This high-spin state results from the ferromagnetic coupling (J(photophys)>0) of the triplet locally excited state of An ((3)An*) with the doublet ground state of OV. As a shared salient feature, AnOV(+) and AnOV* (HS) show a spin delocalization within the domain of activated An in either An(+) or (3)An* (nexus states) forms. The present study essentially contributes to establish and clarify relationships between electrochemical, photophysical, and photochemical pathways to achieve ISA processes within AnOV. In particular, we discuss the impact of the spin polarization of the unpaired electron of OV on electronic features of the An electron-donating subunit. Close analysis of this polarizing interplay allows one to derive a novel functional paradigm to manipulate electron spins at the intramolecular level with light and under an external magnetic field. Indeed, two original functional elements are identified: light-triggered donors of spin-polarized electrons and spin-selective electron acceptors, which are of potential interest for molecular spintronics.

Photophysical properties of osmium( II ) complexes with the novel 4′- p -phenylterpyridine-triarylpyridinium ligand

A new family of electron acceptor ligands complexed with osmium(II) and associated with appropriate electron donor subunits within heteroleptic compounds, results in multicomponent assemblies of potential interest for both synthetic chemistry and supramolecular photochemistry.