Tuning the HOMO energy levels of organic dyes for dye-sensitized solar cells based on Br-/Br3- electrolytes

A series of novel metal-free organic dyes TC301-TC310 with relatively high HOMO levels were synthesized and applied in dye-sensitized solar cells (DSCs) based on electrolytes that contain Br(-)/Br(3)(-) and I(-)/I(3)(-). The effects of additive Li(+) ions and the HOMO levels of the dyes have an important influence on properties of the dyes and performance of DSCs. The addition of Li(+) ions in electrolytes can broaden the absorption spectra of the dyes on TiO(2) films and shift both the LUMO levels of the dyes and the conduction band of TiO(2), thus leading to the increase of J(sc) and the decrease of V(oc). Upon using Br(-)/Br(3)(-) instead of I(-)/I(3)(-), a large increase of V(oc) is attributed to the enlarged energy difference between the redox potentials of electrolyte and the Fermi level of TiO(2), as well as the suppressed electron recombination. Incident photon to current efficiency (IPCE) action spectra, electrochemical impedance spectra, and nanosecond laser transient absorption reveal that both the electron collection yields and the dye regeneration yields (Φ(r)) depend on the potential difference (the driving forces) between the oxidized dyes and the Br(-)/Br(3)(-) redox couple. For the dyes for which the HOMO levels are more positive than the redox potential of Br(-)/Br(3)(-) sufficient driving forces lead to the longer effective electron-diffusion lengths and almost the same efficient dye regenerations, whereas for the dyes for which the HOMO levels are similar to the redox potential of Br(-)/Br(3)(-), insufficient driving forces lead to shorter effective electron-diffusion lengths and inefficient dye regenerations.     

Phenoxazine dyes for dye-sensitized solar cells: relationship between molecular structure and electron lifetime

A series of metal-free organic dyes with a core phenoxazine chromophore have been synthesized and tested as sensitizers in dye-sensitized solar cells. Overall conversion efficiencies of 6.03-7.40% were reached under standard AM 1.5G illumination at a light intensity of 100 mW cm(-2) . A clear trend in electron lifetime could be seen; a dye with a furan-conjugated linker showed a shorter lifetime relative to dyes with the acceptor group directly attached to the phenoxazine. The addition of an extra donor unit, which bore insulating alkoxyl chains, in the 7-position of the phenoxazine could increase the lifetime even further and, together with additives in the electrolyte to raise the conduction band, an open circuit voltage of 800 mV could be achieved. From photoelectron spectroscopy and X-ray absorption spectroscopy of the dyes adsorbed on TiO(2) particles, it can be concluded that the excitation is mainly of cyano character (i.e., on average, the dye molecules are standing on, and pointing out, from the surface of TiO(2) particles).     

Change of Boron Substitution Improves the Lasing Performance of Bodipy Dyes: A Mechanistic Rationalisation

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen (¹O₂). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the ¹O₂ generation capacity of these dyes as well as their rate of reaction with ¹O₂, thereby enhancing their lifetimes even under lasing conditions.     

Theoretical Insight into the Origin of Large Stokes Shift and Photophysical Properties of Anilido‐Pyridine Boron Difluoride Dyes

The geometric and electronic structures and photophysical properties of anilido‐pyridine boron difluoride dyes 1 – 4 , a series of scarce 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time‐dependent DFT (TD‐DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index—the charge‐transfer distance. It is found that PBE0 provides satisfactory overall results. An in‐depth insight into Huang–Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2 , while 3 shows a relatively blue‐shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1 . Finally, compound 4 , which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm^?1), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light‐emitting diodes. The theoretical results could complement and assist in the development of BODIPY‐based dyes with both large Stokes shift and high quantum efficiency.     

Boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors--en route to a new class of highly emissive fluorophores for the red spectral range

The X-ray crystallographic, optical spectroscopic, and electrochemical properties of a newly synthesized class of boron-diindomethene (BDI) dyes and their tetrahydrobicyclo precursors (bc-BDP) are presented. The BDI chromophore was designed to show intensive absorption and strong fluorescence in an applicationary advantageous spectral range. Its modular architecture permits fusion of a second subunit, for example, a receptor moiety to the dye's core to yield directly linked yet perpendicularly prearranged composite systems. The synthesis was developed to allow facile tuning of the chromophore platform and to thus adjust its redox properties. X-ray analysis revealed a pronounced planarity of the chromophore in the case of the BDIs, which led to a remarkable close packing in the crystal of the simplest derivative. On the other hand, deviation from planarity was found for the diester-substituted bc-BDP benzocrown that exhibits a "butterfly"-like conformation in the crystal. Both families of dyes show charge- or electron-transfer-type fluorescence-quenching characteristics in polar solvents when equipped with a strong donor in the meso-position of the core. These processes can be utilized for signaling purposes if an appropriate receptor is introduced. Further modification of the chromophore can invoke such a guest-responsive intramolecular quenching process, also for receptor groups of low electron density, for example, benzocrowns. In addition to the design of various prototype molecules, a promising fluoroionophore for Na+ was obtained that absorbs and emits in the 650 nm region and shows a strong fluorescence enhancement upon analyte binding. Furthermore, investigation of the remarkable solvatokinetic fluorescence properties of the "butterfly"-like bc-BDP derivatives suggested that a second intrinsic nonradiative deactivation channel can play a role in the photophysics of boron-dipyrromethene dyes.     

The photochemistry of lipoic acid: photoionization and observation of a triplet excited state of a disulfide.

Under short-wavelength UV irradiation, lipoic acid (LipSS) and its reduced form, dihydrolipoic acid (DHLA), undergo photoionization processes through a bi- or monophotonic pathway. After ionization, the LipSS radical cation (LipSS*+) and radical anion (LipSS*-) are generated. LipSS*- can be converted to equimolar amounts of LipSS and DHLA through second-order decay. Triplet acetone can be quenched by LipSS and DHLA with a rate close to the diffusion-controlled limit. The mechanism was further confirmed by continuous irradiation experiments. When LipSS is directly irradiated with UVA light, the first excited triplet state of LipSS is observed, with a lifetime tau=75 ns. Characteristic reactions include triplet energy transfer to oxygen and beta-carotene and addition to isoprene. The lifetime of triplet LipSS is also shortened by addition of water and methanol.     

trans-cis Photoisomerization of the styrylpyridine Ligand in [Re(CO)3(2,2'-bipyridine)(t-4-styrylpyridine)]+: role of the metal-to-ligand charge-transfer excited states

The trans-cis isomerization of the styrylpyridine carbon-carbon double bond induced by visible light irradiation in fac-[Re(CO)(3)(bpy)(stpy)](+) (bpy = 2,2'-bipyridine; stpy = t-4-styrylpyridine) has been investigated by means of quantum-chemical methods. The structures of the various cis and trans conformers of [Re(CO)(3)(bpy)(stpy)](+) have been optimized at the density functional theory (DFT) level. Three rotational conformers for the most stable trans isomer lie within 2.3 kJ mol(-1) each other. The energy difference between the cis and trans isomers is 27.0 kJ mol(-1). The electronic spectroscopy of the most stable conformers has been investigated by time-dependent DFT (TD-DFT) and complete active space self-consistent field/CAS second order perturbation theory (CASSCF/CASPT2) calculations. The lowest absorption bands are dominated by metal-to-ligand charge-transfer (MLCT, d(Re)-->pi*(bpy)) transitions calculated at about 25,000 cm(-1) and by a strong intraligand (1)IL (pi(stpy)-->pi*(stpy)) transition in the near UV region. On the basis of CASSCF potential energy curves (PECs) calculated as a function of the torsion angle of the C=C bond of the styrylpyridine ligand, it is shown that the role of the low-lying MLCT states is important in the photoisomerization mechanism. In contrast to the free organic ligand, in which the singlet mechanism is operational via the (1)IL (S(1)) and electronic ground (S(0)) states, coordination to the rhenium steers the isomerization to the triplet PEC corresponding to the (3)IL state. From the (3)IL(t) (t = trans) the system evolves to the perpendicular intermediate (3)IL(p) (p = perpendicular) following a 90 degrees rotation around the styrylpyridine C=C bond. The metal center acts as a photosensitizer because of the presence of photoactive MLCT states under visible irradiation. The position of the crossing between the (3)IL and electronic ground state PEC determines the quantum yield of the isomerization process.     

The Origin of the Halogen Effect on the Phthalocyanine Green Pigments

The structure and the electronic and optical properties of halogenated copper‐phthalocyanine (nα,mβ(Hal)‐CuPc) molecules are investigated, according to the variation in the substituted halogen‐atom species (Hal=Cl or Br) at the α and β positions of isoindole ring with different numbers (n and m=0, 4, 8, or 16). Our results show that the halogen effect mainly results from a structural deformation rather than caused by electronic effects. A nonplanar deformation of the phthalocyanine chromophore of the nα,mβ(Hal)‐CuPc molecule causes a significant change only in the HOMO and HOMO‐1 levels, rather than in the LUMO levels, which leads to the appearance of a green color arising from the large red‐shifts of the Soret and Q bands. The present result may serve as an important reference point for designing novel halogen‐free green pigments, in accordance with the environmental regulations for the restriction of hazardous substances (RoHS) in electronic and electrical devices.     

Dyes in Liquid Crystals: Experimental and Computational Studies of a Guest–Host System Based on a Combined DFT and MD Approach

Practical applications of guest–host liquid crystal systems are critically dependent on the alignment of the guest species within the liquid crystal host. UV/Vis absorption spectroscopy shows that the 1,5‐dihydroxy‐2,6‐bis‐(4‐propylphenyl)‐9,10‐anthraquinone dye aligns within the E7 nematic host, giving an experimental dichroic ratio of 9.40 and dye order parameter of 0.74. This alignment was modelled by using a combination of density functional theory (DFT) and molecular dynamics (MD) computational approaches that do not require the input of experimental data. Time‐dependent DFT calculations show that the electronic transition dipole moment is highly aligned with the long molecular axis of the dye. Fully atomistic MD simulations show that the long axis of the dye is less highly aligned within the E7 host, indicating that this contribution limits the overall dye alignment and, thereby, the potential practical applications of this particular system. Importantly, this study demonstrates an experimental and combined DFT and MD computational approach that may be applied generally to guest–host systems, providing a potential route to their rational design.     

Substituent Effect on the π Linkers in Triphenylamine Dyes for Sensitized Solar Cells: A DFT/TDDFT Study

A series of metal‐free organic donor–π bridge–acceptor dyes are studied computationally using density functional theory (DFT) and time‐dependent DFT (TDDFT) approaches to explore their potential performances in dye‐sensitized solar cells (DSSCs). Taking triphenylamine (TPA) and cyanoacrylic acid moieties as donor and acceptor units, respectively, the effects of different substituents of the π linkers in the TPA‐based dyes on the energy conversion efficiency of the DSSCs are theoretically evaluated through optimized geometries, charge distributions, electronic structures, simulated absorption spectra, and free energies of injection. The results show that the molecular orbital energy levels and electron‐injection driving forces of the TPA dyes can be tuned by the introduction of substituents with different electron‐withdrawing or ‐donating abilities. The electron‐withdrawing substituent always lowers the energies of both frontier orbitals, while the electron‐donating one heightens them simultaneously. The efficiency trend of these TPA derivatives as sensitizers in DSSCs is also predicted by analyzing the light‐harvesting efficiencies and the free energies of injection. The following substituents are shown to increase the efficiency of the dye: OMe, OEt, OHe, and OH.     

Studies of 2‐Azaazulenium Derivatives: Unsymmetrical Trimethine Cyanine Dyes Bearing a 2‐Azaazulenium Moiety as One of the Terminal Groups

We report here the synthesis of a series of symmetrical and unsymmetrical trimethine cyanine dyes derived from 2‐azaazulene, combined spectral and quantum‐chemical investigations of their molecular geometry and electron structure, as well as the nature of the lowest electron transitions. Based on the analysis of both calculations and experimental data obtained from absorption and ¹³C NMR spectra, it was concluded that the 2‐azaazulene residue can be treated as a weakly basic terminal group; its donor properties are provided with the participation of the HOMO?1, in contrast to the typical Brooker’s terminal residues with their donor HOMOs. The new classification of the terminal groups of cyanine dyes, and hence the classification of types of unsymmetrical cyanines, is proposed. It is shown that the nature of the higher electron transitions (delocalized or local) in the cyanine dyes depends on their type. In the unsymmetrical trimethine cyanine of the mixed type, negative deviations are observed in their absorption spectra.     

Theoretical study of metal‐free organic dyes based on different configurations for efficient dye‐sensitized solar cells

Considering different solar dyes configuration, four novel metal‐free organic dyes based on phenoxazine as electron donor, thiophene and cyanovinylene linkers as the ‐conjugation bridge and cyanoacrylic acid as electron acceptor were designed to optimize open circuit voltage and short circuit current parameters and theoretically inspected. Density functional theory and time‐dependent density functional theory calculations were used to study frontier molecular orbital energy states of the dyes and their optical absorption spectra. The results indicated that D2‐4 dyes can be suitable candidates as sensitizers for application in dye sensitized solar cells and among these three dyes, D3 showed a broader and more bathochromically shifted absorption band compared to the others. The dye also showed the highest molar extinction coefficient. This work suggests optimizing the configuration of metal‐free organic dyes based on simple D‐ ‐A configuration containing alkyl chain as substitution, starburst conformation, and symmetric double D‐ ‐A chains would produce good photovoltaic properties.     

Effect of vinylene and 1,4-phenylene spacers on efficiency of the ground-state intramolecular charge-transfer in enlarged 4-dimethylamino-1-methylpyridinium cations

¹⁵N NMR chemical shifts of the exo- and endocyclic nitrogen atoms show how efficient is the ground-state intramolecular charge transfer between these sites in 4-dimethylamino-1-methylpyridinium cation (increased contribution of the quinoid resonance form results in a shielding and deshielding effect of their NMR signals, respectively). As it was anticipated, insertion of vinylene and/or 1,4-phenylene spacers to the cation considerably hinders the ground-state charge transfer. This hypothesis is further supported by an analysis of the C–NMe₂ bond lengths (X-ray data show that spacers elongate this bond). The selected valence angles in the compounds studied are also linearly dependent on δ(¹⁵N_endo) and δ(¹⁵N_exo) values. Although the correlation coefficient for the δ(¹⁵N_endo) versus δ(¹⁵N_exo) dependence is equal to 0.983, decrease of the net charge on one nitrogen atom is not compensated entirely by its increase on another nitrogen atom. This shows that exocyclic nitrogen atom is not the only acceptor of the positive charge in the molecule. The natural population analysis shows that the positive charge is transferred not only to the exocyclic N but also to, e.g., 1- and N-methyl C as well as to C3 and C5 atoms in pyridine ring. Ground-state charge transfer through the p-phenylene moiety was found to be less effective than through the trans-vinylene bridge. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Fine Tuning the Performance of DSSCs by Variation of the π‐Spacers in Organic Dyes that Contain a 2,7‐Diaminofluorene Donor

Organic dyes that contain a 2,7‐diaminofluorene‐based donor, a cyanoacrylic‐acid acceptor, and various aromatic conjugation segments, which are composed of benzene, fluorene, carbazole, and thiophene units, as a π‐bridge have been synthesized and characterized by optical, electrochemical, and theoretical investigations. The trends in the absorption and electrochemical properties of these dyes are in accordance with the electron‐donating ability of the conjugating segment. Consequently, the dyes that contained a 2,7‐carbazole unit in the π‐spacer exhibited red‐shifted absorption and lower oxidation potentials than their corresponding fluorene‐ and phenylene‐bridged dyes. However, the enhanced power‐conversion efficiency that was exhibited by the fluorene‐bridged dyes in the DSSCs was attributed to their broader and intense absorption. Despite the longer‐wavelength absorption and reasonable optical density, carbazole‐bridged dyes exhibited lower power‐conversion efficiencies, which were ascribed to the poor alignment of the LUMO level in these dyes, thereby leading to the inhibition of electron injection into the TiO₂ conduction band.     

In-depth probe of researching interfacial charge transfer process for organic solar cells: A promising bisadduct fullerene derivatives acceptor

The charge transfer (CT) mechanism at the donor/acceptor (D/A) interface plays an irreplaceable role in the photoelectric conversion of efficient bulk-heterojunction (BHJ) organic solar cells (OSCs), which affects the resulting competition between charge separation and charge recombination. Extensive CT studies have preferred monoadduct fullerene derivatives ( M60 , M70 ) due to their unique spherical geometry with fewer factors to consider. However, the effect of carbon cage size, substituent group properties and the number of CT properties have not been much discussed. Here, sulfur-containing bisadducts ( B60 , B70 ) were selected to explore whether they are also suitable for CT research like classical monoadducts. Using density functional theory and time-dependent density functional theory, interface stacking configuration, key parameters relevant to CT states, charge separation, and recombination rates were determined to confirm the characteristics of B60 and B70 as a good acceptor applied to interfacial research. This work points to the CT mechanism along the route of DA → D*A → D⁺A⁻ through a theoretical analysis and also provides candidates for the theoretical interface photoelectric process in BHJ OSCs: bisadduct fullerene derivatives as good acceptor materials.     

Cyanation: providing a three-in-one advantage for the design of n-type organic field-effect transistors

The theoretical work presented here demonstrates that, when substitution takes place at appropriate positions, cyanation could be a useful tool for reducing the internal reorganization energy of molecules. A molecular-orbital-based explanation is given for this fundamentally important phenomenon. Some of the cyanated pentacene derivatives (nCN-PENT-n) not only have internal reorganization energies for electron transfer (lambda(-)) smaller than that of pentacene, but the lambda(-) values are even of the same magnitude as the internal reorganization energy for hole transfer (lambda(+)) of pentacene, a small value that few organic compounds have surpassed. In addition, cyanation raises the electron affinity of the parent compound and may afford good electronic couplings between neighboring molecules, because of its ability in promoting pi-stacking. For the design of high performance n-Type Organic field-effect transistors, high electron affinities, large intermolecular electronic couplings, and small reorganization energies are necessary. Cyanation may help in all three aspects. Two cyanated trialkylsilylethynyl pentacene derivatives with known pi-stacking structures are predicted to provide reasonably small internal reorganization energies, large electronic couplings, and high electron affinities. They have the potential to outperform N-fluoroalkylated dicyanoperylene-3,4:9,10-bis(dicarboximides) (PDI-FCN(2)) in terms of electron mobility.