Intramolecular Charge Transfer Absorption Research Articles

Design of Isoindigo-Based Small-Molecule Donors for Bulk Heterojunction Organic Solar Cell Applications in Combination with Nonfullerene Acceptors.

The development of small-molecule organic solar cells with the required efficiency depends on the information obtained from molecular-level studies. In this context, 39 small-molecule donors featuring isoindigo as an acceptor moiety have been meticulously crafted for potential applications in bulk heterojunction organic solar cells. These molecules follow the D2-A-D1-A-D2 and D2-A-π-D1-π-A-D2 framework. Similar molecules considered in the previous experimental study (molecules R1 ((3E,3″E)-6,6″-(benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione)) and R2 ((3E,3″E)-6,6″-(4,8-dimethoxybenzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(1,1'-dimethyl-[3,3'-biindolinylidene]-2,2'-dione))) have been chosen as reference molecules. Molecules with and without π-spacers have been considered to understand the impact of the length of the π-spacer on intramolecular charge-transfer transitions and absorption properties. A detailed investigation is carried out to establish the relationship between the structure and photovoltaic parameters using density functional theory and time-dependent density functional theory methods. The newly developed molecules exhibit better electronic, excited-state, and charge transport properties than the reference molecules. Additionally, model donor-acceptor interfaces are constructed by integrating the designed donor molecules with fullerene/nonfullerene acceptors. The electronic and excited-state properties of these interfaces are rigorously evaluated. Results elucidate that the donor comprising of isoindigo-bithiophene-pyrroloindacenodithiophene (IIG-T2-PIDT) emerges as a promising candidate for bulk heterojunction solar cells based on nonfullerene acceptors. This research provides systematic design strategies for the development of small-molecule donors for organic solar cells.

Bis(9-Boraphenanthrene) and Its Stable Biradical.

Selective and site-specific boron-doping of polycyclic aromatic hydrocarbon frameworks often give rise to redox and/or photophysical properties that are not easily accessible with the analogous all-carbon systems. Herein, we report ligand-mediated control of boraphenanthrene closed- and open-shell electronic states, which has led to the first structurally characterized examples of neutral bis(9-boraphenanthrene) (2-3) and its corresponding biradical (4). Notably, compounds 2 and 3 show intramolecular charge transfer absorption from the 9-boraphenanthrene units to p-quinodimethane, exhibiting dual (red-shifted) emission in solution due to excited state conjugation enhancement (ESCE). Moreover, while boron-centered monoradicals are ubiquitous, biradical 4 represents a rare type of open-shell singlet compound with 95% biradical character, among the highest of any reported boron-based polycyclic species with two radical sites.

Design and synthesis of novel D-A copolymers based on fused thienothiophene for efficient broad absorption electrochromism

Developing conjugated polymers with broad spectrum modulation is highly desirable in electrochromic field. Herein, three novel polymers based on fused thienothiophene derivative (tris(thienothiophene) (TTT) or indacenodithieno[3,2-b]thiophene (IDTT)) and two acceptors (5,8-bis(4-(2-butyloctyl)thiophen-2-yl)-dithieno[3′,2':3,4; 2″,3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (TBT) and 2,1,3-Benzothiadiazole (BT)) are synthesized via direct arylation polymerization (DArP). The neutral copolymers exhibit visibly broadband absorption due to the combination of the π-π* transition and intramolecular charge transfer absorption, which can be tuned by the feed ratio of the donor and acceptor monomers. As a result, the copolymer synthesized by IDTT, TBT and BT with a feed ratio of 3/2/1 presents a black color in the neutral state and high transparency in the oxidized state, which gives an average optical contrast of 34% in the visible band (400 nm–700nm), good stability and high coloration efficiency (370 cm2/C). Additionally, an electrochromic device with excellent electrochromic performance is fabricated based on the synthesized polymers. All of these confirm the synthesized polymers are attractive candidates for broad absorption electrochromism.

Nonfullerene acceptors with cyano-modified terminal groups for organic solar cells: Effect of substitution position on photovoltaic properties

Two kinds of nonfullerene acceptors based on cyano-modified 1,1-dicyanomethylene-3-indanone (IC) as end groups and the dithienothiophen[3,2-b]-pyrrolobenzothiadiazole (BTP) unit as a core are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption with the maximum absorption peak at 832 nm, and the absorption edge extends to 953 nm, corresponding to the narrow bandgap down to 1.30 eV. Comparing with the electron acceptor with cyano groups at mixed γ/δ position, the acceptor with cyano substituted at the β position shows more twisted structure and slightly higher energy level. When a wide bandgap polymer PM6 is adopted as the donor, the blend film with β-cyano substituted acceptor shows more suitable phase separation morphology and more balanced charge transport. The device based on the β-cyano substituted acceptor achieves a larger open-circuit voltage (VOC) of 0.88 V than the acceptor with cyano groups at mixed γ/δ position (VOC = 0.78 V), and the energy loss was calculated to be 0.45 eV, leading to a higher power conversion efficiency (PCE) of 12.41% comparing with the devices with the γ/δ-cyano substituted acceptor (PCE = 11.08%).

New Carbazole-Based Sensitizers for p-Type DSSCs: Impact of the Position of Acceptor Units on Device Performance

We report the design, synthesis, and characterization of two new carbazole-based organic dyes PC1–2 as potential sensitizers for NiO-based p-type dye-sensitized solar cells (p-DSSCs). The D-A-π-A’ configured PC1 dye comprises a thienyl unit as a π-spacer and a malononitrile as an end-capping acceptor unit, whereas in PC2, the cyanovinylene group serves as an acceptor unit and a thienyl group acts as a donor unit in a D–A–D configuration. These molecules achieved excellent solubility due to their long-branched alkyl chains. The current work encompasses their structural, photophysical, thermal, electrochemical, theoretical, and photoelectrochemical studies, establishing structure–property relationships. PC1–2 exhibit λabs and λemi in the range of 389–404 and 448–515 nm, respectively, with a band gap in the range of 2.88–2.92 eV. Electrochemical studies confirm the feasibility of electron injection, regeneration, and recombination. The introduction of an additional electron-withdrawing group (cyanovinylene group) on the dye PC1 skeleton endows it with a higher dye loading capacity, high hole injection, and a strengthened intramolecular charge transfer (ICT) effect, resulting in a redshifted ICT absorption with a higher molar extinction coefficient. Among the two new dyes, the device based on PC1 achieved the highest power conversion efficiency (PCE) of 0.027% with a short-circuit current density (JSC) of 1.29 mA·cm–2, open-circuit voltage (VOC) of 67 mV, and fill factor (FF) of 31%, whereas the device with dye PC2 performed less efficiently (PCE: 0.018%, JSC: 0.92 mA·cm–2, VOC: 68 mV, and FF: 30%). Conclusively, the study provides insights into the intricacies involved in the structural modification of carbazole-based p-type dyads for the development of highly efficient DSSCs.

Synthesis and photovoltaic performance of nitrogen-bridged star-shaped fused-ring electron acceptors

<p indent="0mm">A new nitrogen-bridged star-shaped fused-ring electron acceptor (BTDTP-C8C9-6Cl) with benzo[1,2-<italic>b</italic>:3,4-<italic>b</italic>′-5,6-<italic>b</italic>′′]tri(4-<italic>H</italic>-dithieno[3,2-<italic>b</italic>:2′,3′-<italic>d</italic>]pyrrole) as a fused ring skeleton and 5,6-dichloro-3-(dicyano methylene)indone as end groups has been designed and synthesized. The electron acceptor shows an strong intramolecular charge transfer absorption in the range of 500~<sc>800 nm</sc> with the molar extinction coefficient of 3.2×10⁵ M⁻¹ cm⁻¹ and the optical bandgap of <sc>1.56 eV.</sc> Two wide bandgap polymer donors, PM6 and D18, which have complementary absorption spectra with BTDTP-C8C9-6Cl, are blended with BTDTP-C8C9-6Cl to fabricate organic solar cell devices, respectively. Compared with the blend film based on PM6 and BTDTP-C8C9-6Cl, the blend film based on D18 and BTDTP-C8C9-6Cl shows stronger crystallinity and more suitable fibrous interpenetrating network phase separation morphology. The investigation of the charge transport and recombination behaviors reveals that the device based on D18:BTDTP-C8C9-6Cl exhibits more balanced charge transport, weakened recombination, and effective exciton dissociation and charge collection. Finally, the organic solar cell device based on D18:BTDTP-C8C9-6Cl achieves power conversion efficiencies of 12.00%.

Nitrogen-bridged star-shaped fused-ring electron acceptors for organic solar cells

Two kinds of nitrogen-bridged star-shaped fused-ring electron acceptors based on benzo[1,2-b:3,4-b’-5,6-b’’]tri(4-H-dithieno[3,2-b:2’,3’-d]pyrrole) (BTDTP) core with linear and branched alkyl side chains on bridging nitrogen atoms are developed. Both electron acceptors exhibit strong intramolecular charge transfer absorption bands at 500–800 nm (molar extinction coefficient of 3.0 × 105 M −1 cm−1) with narrow optical bandgaps around 1.58 eV, but reveal different molecular packing and morphology. Compared to acceptor with linear side chains, the acceptor bearing branched side chains exhibits ordered face-on orientation with tight π-π stacking, as well as more favorable phase separation morphology and balanced charge transport when blended with polymer donor PM6. Organic solar cell device based on acceptors bearing branched side chains achieves a power conversion efficiency (PCE) of 9.95% with a fill factor of 74.98% and a short-circuit current density of 14.90 mA cm−2, which is much superior to the device using acceptors bearing linear side chains (a PCE of 6.74%).

Synthesis, Structure, and Significant Energy Gap Modulation of Symmetrical Silafluorene-Cored Tetracyanobutadiene and Tetracyanoquinodimethane Derivatives.

A series of symmetrical tetracyanobutadiene and tetracyanoquinodimethane derivatives with a D-A-D'-A-D structural configuration and silafluorene core (D') were designed and readily synthesized via a [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction. We found that the photophysical properties and HOMO and LUMO energy levels and gaps of the silafluorene derivatives could be profoundly remolded through CA-RE reactions and modulated by varying the peripheral donor units from phenyl, m-dimethoxyphenyl, and N,N-dimethylaniline to triphenylamine groups. After CA-RE reactions, the HOMO-LUMO gaps of 1a-1j are in the range of 1.75-2.78 eV, with significant decreases of 0.52-1.46 eV compared to those of the parent silafluorene compounds 2a-2j. The intriguing crystal structures of 1f and 1j were analyzed and elucidated to show their unique potential porosity. The stability, electrochemical, and computational studies were systematically performed to unveil the reshaped electron-donating and -withdrawing nature in one molecular system. 1h-1j with peripherally strong amino donors exhibit an intense and broad intramolecular charge transfer absorption band in the near-infrared region from 550 to 900 nm. The molecular design and synthesis reported here broaden the types and fields of D-A molecular systems for potential applications in organic optoelectronic devices.

Binuclear Rhodium(I) Complex of a Dimethylvinylene-Bridged Distorted Hexaphyrin(2.1.2.1.2.1).

A highly distorted binuclear rhodium(I) complex, 2Rh, was successfully synthesized from hexaphyrin(2.1.2.1.2.1) containing dimethylvinylene-bridges between dipyrrin units. IR spectroscopy, 1H NMR spectroscopy, and X-ray crystallography revealed that the complex 2Rh consists of two rhodium(I) ions coordinated to two dipyrrin units. Rh complexation induced a transformation from a trans-/cis-/trans- to trans-/cis-/cis-conformation on the dimethylvinylene-bridges. This is the first example of rhodium(I)-ion-induced cis-/trans-isomerization in the porphyrin derivatives. Theoretical calculations of 2Rh predicted the presence of intramolecular charge-transfer absorption due to the distorted molecular structure.

Insight into the impact of the substituent modification on the photovoltaic performance of ferrocenyl chalcones based DSSCs

The design, synthesis, spectroscopic, computational and solar cell performance studies of two new ferrocenyl chalcones (Fc-O1 and Fc-S2) were synthesized by the Claisen-Schmidt condensation reaction. In the synthesized molecules, the enone moiety acts as an efficient π-bridge that links the donor ferrocene derivative with methyl-furan (Fc-O1) and bithiophene (Fc-S2) substituents which permits good viability in fine-tuning the intramolecular charge transfer (ICT) and optical absorption spectrum. The chemical structure of the compounds have been elucidated by proton and carbon NMR as well as FTIR spectroscopies. The comparison study between the structures of these ferrocenyl chalcones were done by XRD and computational investigations. The substitution of bithiophene derivative on the Fc-S2 revealed the improvement of the structure planarity as compared to slightly twisted structure of Fc-O1. Fc-O1 and Fc-S2 exhibited large maximum absorption wavelength within the UV–Vis region (~485 ​nm), suggesting their potential as dye sensitizer in DSSC application. The compounds also possessed suitable energy levels of HOMO and LUMO for utilization as sensitizer in DSSC to ensure the process of electron injection and dye regeneration. A good ICT due to its planarity, large absorption maximum in visible range and low-lying HOMO energy level of Fc-S2 give rise to the higher conversion efficiency in DSSC study as in comparison to Fc-O1.

Heavy-Atom-Free Bay-Substituted Perylene Diimide Donor-Acceptor Photosensitizers.

Perylene diimide (PDI) dyes are extensively investigated because of their favorable photophysical characteristics for a wide range of organic material applications. Fine-tuning of the optoelectronic properties is readily achieved by functionalization of the electron-deficient PDI scaffold. Here, we present four new donor-acceptor type dyads, wherein the electron donor units - benzo[1,2-b : 4,5-b']dithiophene, 9,9-dimethyl-9,10-dihydroacridine, dithieno[3,2-b : 2',3'-d]pyrrole, and triphenylamine-are attached to the bay-positions of the PDI acceptor. Intersystem crossing occurs for these systems upon photoexcitation, without the aid of heavy atoms, resulting in singlet oxygen quantum yields up to 80 % in toluene solution. Furthermore, this feature is retained when the system is directly irradiated with energy corresponding to the intramolecular charge-transfer absorption band (at 639 nm). Geometrical optimization and (time-dependent) density functional theory calculations afford more insights into the requirements for intersystem crossing such as spin-orbit coupling, dihedral angles, the involvement of charge-transfer states, and energy level alignment.

Synthesis and Properties of the Doubly Oxonium-Embedded Picenes as Electron-Deficient Polycyclic Aromatic Hydrocarbons

The doubly oxonium-embedded picenes have been synthesized for the first time. They have a rigid planar electron-deficient core and are stabilized by the pyrylium aromaticity. The doping positions of oxygen atoms influenced their electrochemical and optoelectronic properties because of the different charge distributions in the skeletons. The donor-acceptor-donor-type molecules baring electron-donating triphenylamine moieties exhibited intramolecular charge-transfer absorption at the visible regions, indicating the large impact of the substituents on their electronic properties.

Synthesis and Characterization of Metallo-Supramolecular Polymers Based on Benzodipyrrolidone.

A simple route to the preparation of benzodipyrrolidone (BDP) based monomeric building blocks containing 2,2′:6′,2″-terpyridines is reported from a common precursor 4′-(4-pinacolatoboronphenyl)-2,2′:6′,2″-terpyridine via Suzuki coupling reaction. Self-assembly polymerization with ruthenium (II) ions under mild conditions yielded a series of novel metallo-supramolecular polymers with weak donor-acceptor (D-A) structures based on benzodipyrrolidone. The structure of the bridge connected BDP with terpyridine have a significant impact on the wavelength and intensity of the intramolecular charge transfer (ICT) absorption peak. The resulting metallo-polymers exhibited strong double absorption bands around 315 nm and 510 nm involved in π-π* transitions and ICT or metal to ligand charge transfer (MLCT) absorption bands. The forming of D-A structure and coordination with ruthenium (II) ions is favorable to narrow the energy gap and the energy gaps of the resulting metallo-supramolecular polymers are 2.01 and 1.62 eV, respectively.

DFT and TD-DFT study on quadratic NLO response and optoelectronic activity in novel Y-shaped imidazole-based push-pull chromophores.

A theoretical analysis of a series of imidazole-based Y-shaped chromophores, D1-D8, is performed in order to investigate their non(linear) optical, fluorescence, and charge-transport properties. The calculations have been carried out employing DFT and TD-DFT methods at CAM-B3LYP and M06-2X levels of theory. FMO analysis reveals that in ground state, the highest occupied molecular orbital is localized on the 4,5-dimethylanilino donor moiety and imidazole core while the lowest unoccupied molecular orbital spreads on π-linker and nitro acceptor moieties. Vertical absorption and fluorescence transitions are characterized as intramolecular charge transfer and maximum absorption and fluorescence wavelengths show that by changing the π-bridge to the imidazole C2, we can tune fluorescence color from cyan to orange. Calculated (hyper)polarizabilities show that elongation of π-linker by polarizable subunits, such as double bonds or heteroaromatic rings, increases significantly the nonlinear response and shifts the charge-transfer band bathochromically. Calculated reorganization energies indicate that the studied compounds are hole-transporting materials rather than electron-transporters. Interestingly, D7 and D8, with higher hyperpolarizabilities, are predicted to be potent candidates for NLO-devices while D5 and D8 molecules are expected to be promising candidates for luminescent materials and good hole-transport materials for organic light-emitting diodes.

Single-Component Organic Solar Cells Based on Intramolecular Charge Transfer Photoabsorption.

Conjugated donor–acceptor molecules with intramolecular charge transfer absorption are employed for single-component organic solar cells. Among the five types of donor–acceptor molecules, the strong push–pull structure of DTDCPB resulted in solar cells with high JSC, an internal quantum efficiency exceeding 20%, and high VOC exceeding 1 V with little photon energy loss around 0.7 eV. The exciton binding energy (EBE), which is a key factor in enhancing the photocurrent in the single-component device, was determined by quantum chemical calculation. The relationship between the photoexcited state and the device performance suggests that the strong internal charge transfer is effective for reducing the EBE. Furthermore, molecular packing in the film is shown to influence photogeneration in the film bulk.

“Small in size but mighty in force” – The first principle study of the impact of A/D units in A/D-phenyl-π-phenothiazine-π-dicyanovinyl systems on photophysical and optoelectronic properties

A set of N-octylphenothiazine malononitriles (P1–P4) containing (p-methoxyphenyl, p-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl, or p-cyanophenyl) substituents connected via acetylene linker were designed and synthesized. The obtained structures, which exhibit A/D-π-D-A architecture, were characterized by NMR and mass spectroscopy, and elemental analyses. Photophysical, thermal, and electrical properties of these novel dyes P1–P4 were comprehensively studied and confronted with theoretical calculations. They exhibit high thermal stability of up to 306–369 °C. Compounds showed an intramolecular charge transfer (ICT) absorption band at about 480 nm and emission band at around 620 nm with quantum yields between 31 and 50% in chloroform solution. Comparison of the solution and thin film photoluminescence (PL) spectra of P1–P4 exhibit a significant red-shift of 40–70 nm in the thin film PL spectrum. Furthermore, the impact of the end-capped groups in P1–P4 on optical and electrochemical properties was investigated in detail with the support of TD/DFT calculations. The diodes based on P1–P4 with configuration ITO/PDOT:PSS/PBK:PBD:compound/Al were constructed and their electroluminescence (EL) spectra were registered. Considering the preliminary EL investigations, the beneficial effect of the presence of p-cyanophenyl as a substituent (P4) was demonstrated.

Novel thieno[3,2-b]thiophene-embedded small-molecule donors for highly efficient and photostable vacuum-processed organic photovoltaics

Four thieno[3,2-b]thiophene (TT)-based donor-π-acceptor–type small-molecule donors DTCPTT, DTCPTT-2CN, DTDCPTT, and DTDCPTT-2CN were synthesized and characterized. A clear structure-property relationship correlating the effects of introducing cyano groups onto the TT and the end-capping groups has been successfully established. DTDCPTT and DTDCPTT-2CN were adopted as electron donors for organic photovoltaics (OPVs) mainly due to the strong intramolecular charge transfer absorption in the visible light region, sufficiently high thermal stability, and appropriate energy level alignment with C70. Moreover, the analyses of crystal structures reveal that DTDCPTT and DTDCPTT-2CN form antiparallel dimer pairs through strong dipolar and non-covalent intermolecular interactions. X-ray structures also indicate that introducing cyano groups onto the TT moiety does not affect the backbone planarity significantly. Compared with DTDCPTT-2CN, the more compact and uniform crystal packing renders DTDCPTT advantageous for exciton dissociation and charge carrier transport. DTDCPTT-based device achieves power conversion efficiency as high as 7.81% and 16.89%, respectively, under simulated AM 1.5G illumination and 500 lux TLD-840 fluorescent lamp illumination, which could be attributed to stronger absorption and better photocurrent extraction. In addition, both devices exhibit high stability under continuous simulated 1 sun illumination for more than 300 h. As far as we know, this is the best performance for vacuum-processed small-molecule indoor OPVs to date.

Cu(ii)-Porphyrin based near-infrared molecules: synthesis, characterization and photovoltaic application

Three novel Cu(ii)-porphyrin-based near-infrared non-fullerene acceptors were developed, which show strong intramolecular charge transfer absorption spectra.

Novel 1,5-naphthyridine-chromophores with D-A-D architecture: Synthesis, synthesis, luminescence and electrochemical properties

A series of symmetrical 1,5-naphthyridine compounds with donor-acceptor-donor (D-A-D) architecture and different chromophores were synthesized and characterized. Their thermal, optical and electrochemical properties were investigated systematically. These compounds show robust thermal property with high phase transition temperatures (above 247 °C). In addition, all these compounds exhibit intense absorption bands in UV and visible region, which arise from the intramolecular charge transfer (ICT) transition between the donor and acceptor segments. The electron-donating chromophores on the central naphthyridine cause significant bathochromic shifts of the low energy ICT absorption bands. Meanwhile, all compounds are emissive in solution at room temperature (λem = 450–520 nm). When the electron-donating ability of the attached chromophores increases, the fluorescent quantum yields of these compounds increase. The calculated ground-state geometries further demonstrate the ICT transition for these molecules from HOMO to LUMO. Their luminescent and electrochemical properties could be used for understanding the structure-property relationship of naphthyridine compounds and developing optical functional materials.

An Azo Dye Based D‐π‐A Chromogenic Probe for Selective Naked‐Eye Detection of Hg2+ Ion: Application in Logic Gate Operation

AbstractThe current work describes the selective colorimetric detection of metal ions using a simple azo dye (L1) containing donor‐π‐acceptor framework. In this chromogenic probe, salicylaldehyde is used as donor and nitrobenzene group as acceptor unit. L1 is found to show selective recognition of mercuric ion both visually and spectrophotometrically with no interference from environmentally relevant metal ions in DMSO‐water mixture. It exhibits reddish‐pink colour in DMSO‐water medium as a result of intramolecular charge transfer (ICT) from the electron donating phenolic group to electron withdrawing nitrobenzene group. It selectively binds with Hg2+ even in presence of other interfering metal ions and form a 1 : 1 L1‐Hg2+ adduct. It is evident from a change in colour along with a decrease of ICT absorption band and hypsochromic shift of π‐π* band. The binding of the chemosensor and mercuric ion through the salicylaldehyde unit is proved from 1H‐NMR data and DFT calculations. Using UV‐Vis spectroscopic technique the detection limit and binding constant values are calculated to be 6 μM and 3.3 x 103 M−1 respectively. Hg2+ probing process displays reversibility upon addition of fluoride ion. The on‐off‐on colorimetric change on sequential addition of Hg2+ and F− mimics an IMPLICATION type logic gate.