Intermolecular Charge Transfer Research Articles

The significance of methoxy substitution and π-spacer arrangements on carbazole donor and furofuran π-spacer based promising sensitizers for dye sensitized solar cells

Utilizing computational techniques, twenty-four dyes were designed based on carbazole and furofuran (unsubstituted and methoxy substituted), cyanovinyl, and cyanoacrylic acid and optimized for dye-sensitized solar cell (DSSC) applications. The effect of π-spacers arrangement and the effect of substitution of methoxy group on both donor and π-spacer were investigated. From DFT analysis, oscillator strength (f), band gap (Eg) and energy level values are calculated and they demonstrate the intermolecular charge transfer (ICT) and charge separation. The TDDFT analysis was done to find efficient sensitizers with maximum absorption in the visible region. The free energy calculation was also performed, which revealed that all the designed dyes exhibit quick electron injection (ΔGinj) and dye regeneration (ΔGreg) processes. The influence of π-spacer arrangement and addition of methoxy group on both donor and π-spacers in optoelectronic properties, electrochemical properties, and other properties were thoroughly discussed and also analyzed to predict efficient sensitizer for DSSC application.

Understanding the correlation between energy‐state mismatching and open‐circuit voltage loss in bulk heterojunction solar cells

AbstractPhotoinduced intermolecular charge transfer (PICT) determines the voltage loss in bulk heterojunction (BHJ) organic photovoltaics (OPVs), and this voltage loss can be minimized by inducing efficient PICT, which requires energy‐state matching between the donor and acceptor at the BHJ interfaces. Thus, both geometrically and energetically accessible delocalized state matching at the hot energy level is crucial for achieving efficient PICT. In this study, an effective method for quantifying the hot state matching of OPVs was developed. The degree of energy‐state matching between the electron donor and acceptor at BHJ interfaces was quantified using a mismatching factor (MF) calculated from the modified optical density of the BHJ. Furthermore, the correlation between the open‐circuit voltage (Voc) of the OPV device and energy‐state matching at the BHJ interface was investigated using the calculated MF. The OPVs with small absolute MF values exhibited high Voc values. This result clearly indicates that the energy‐state matching between the donor and acceptor is crucial for achieving a high Voc in OPVs. Because the MF indicates the degree of energy‐state matching, which is a critical factor for suppressing energy loss, it can be used to estimate the Voc loss in OPVs.

Intermolecular charge transfer and solid-state solvent effect synergistically induce near-infrared thermally activated delay fluorescence in the guest-host system.

The host matrix is an important means to tune emission color and improve luminescence efficiency of near-infrared (NIR) thermally activated delay fluorescence (TADF) light-emitting diodes. However, the mechanism of NIR TADF of the guest-host systems is still unclear. Namely, there is a controversy on whether the formation of J-aggregation, solid-state solvent effect, molecular polarization or intermolecular charge transfer (CT) is responsible for the NIR TADF. Here, the morphologies, geometrical and electronic structures, and photophysical properties are explored by combining molecular dynamics simulation, density functional theory and thermal vibration correlation function theory for the guest-host (TPAAP: TPBi) films with different concentrations. It is found that the red TADF is generated largely by the solid-state solvent effect in the low 1 wt% doped film while the NIR TADF is attributed to the synergistic effect of solid-state solvent and guest-guest intermolecular CT in the high 20 wt% film. These findings provide a deeper understanding of the mechanism of NIR-TADF of the guest-host systems.

Benchmarking DFT-based excited-state methods for intermolecular charge-transfer excitations.

Intermolecular charge-transfer is a highly important process in biology and energy-conversion applications where generated charges need to be transported over several moieties. However, its theoretical description is challenging since the high accuracy required to describe these excited states must be accessible for calculations on large molecular systems. In this benchmark study, we identify reliable low-scaling computational methods for this task. Our reference results were obtained from highly accurate wavefunction calculations that restrict the size of the benchmark systems. However, the density-functional theory based methods that we identify as accurate can be applied to much larger systems. Since targeting charge-transfer states requires the unambiguous classification of an excited state, we first analyze several charge-transfer descriptors for their reliability concerning intermolecular charge-transfer and single out the charge-transfer distance calculated based on the variation of electron density upon excitation (DCT) as an optimal choice for our purposes. In general, best results are obtained for orbital-optimized methods and among those, the maximum overlap method proved to be the most numerically stable variant when using the initial MOs as reference orbitals. Favorable error cancellation with optimally-tuned range-separated hybrid functionals and a rather small basis set can provide an economical yet reasonable wavefunction when using time-dependent density functional theory, which provides relevant information about the excited-state character to be used in the orbital-optimized methods. The qualitative agreement makes these fast calculations attractive for high-throughput screening applications.

Tight-binding model describes frontier orbitals of non-fullerene acceptors

HOMO and LUMO of conjugated molecules as superposition of HOMO and LUMO of individual moieties.

A reaction based carbazole-indolium conjugate probe for the selective detection of environmentally toxic ions.

A nucleophilic addition based chemodosimeter was designed and synthesized with a carbazole donor and an indole acceptor. The addition of a cyanide ion to an electron-deficient indole moiety disrupts the acceptor-donor relationship, resulting in noticeable color shifts and spectrum differences in both the absorption and emission profiles. The design has a D-π-A molecular arrangement. Selectivity was investigated in 90% aqueous DMSO solution of probe CI with various anions such as SCN-, PF6-, NO3-, N3-, I-, HSO4-, CN-, H2PO4-, F-, HS-, ClO4-, Cl-, Br-, and AcO-. An intermolecular charge transfer (ICT) band at 506 nm in the UV-visible spectra vanished and the intensity of emission was quenched at 624 nm upon the addition of CN- ions. These outcomes demonstrate the effective nucleophilic addition of cyanide ions to the electron-deficient indole moiety of the probe, resulting in the formation of a new adduct in which the ICT transition is interrupted when π conjugation is blocked. The Job plot, 1H NMR spectroscopy, and HRMS analysis confirmed the formation of a new product. An outstanding response was shown by paper test strips made using probe molecules for the easy detection of cyanide ions in aqueous solutions. Besides, the probe selectively senses cyanide ions in different water samples.

Enhancing UV Stability of Perovskite Solar Cells with Transparent Fluorinated Polyimide

Ultraviolet (UV)-induced degradation is one of the major problems in the field of perovskite solar cells (PSCs). Therefore, exploring materials and techniques to prevent UV light from penetrating into the device is urgently necessary. Here, we developed a special transparent fluorinated polyimide (FPI) resin, which can be directly spin-coated on the front side of conventional indium-doped tin oxide substrates (glass/ITO). Most aromatic polyimides strongly absorb visible light and are colored. The FPI we designed and synthesized bears electron-acceptor CF 3 - groups, which reduces the intra-/intermolecular charge-transfer (CT) effect, enabling FPI to possess high transmittance in the visible range while completely blocking UV light. As a result, the FPI coating slightly pulls down the initial power conversion efficiency (PCE) (21.02% to 20.19%). Remarkably, the coating significantly improves the PSC UV stability. Upon an 8-h enhanced UV aging test in air, the FPI/glass/ITO-based PSC is able to retain 85.0% of its initial PCE. In contrast, the control device (glass/ITO-based PSC) only keeps 40.9% of its initial PCE. The protective effect of FPI is even more prominent in current popular 3D/2D high-performance PSCs because UV light can seriously damage the 2D layer. The unencapsulated 3D/2D device based on FPI/glass/ITO substrate has a very high PCE retention of up to 80% after 12-h enhanced UV aging test in air, comparing to 36% for the control 3D/2D device without FPI. This work demonstrates that FPI and its possible derivatives could provide a feasible avenue to handle UV-induced degradation for PSCs effectively.

Boosting charge separation in organic photovoltaics: unveiling dipole moment variations in excited non-fullerene acceptor layers.

The power conversion efficiency (PCE) of organic photovoltaics (OPVs) has reached more than 19% due to the rapid development of non-fullerene acceptors (NFAs). To compete with the PCEs (26%) of commercialized silicon-based inorganic photovoltaics, the drawback of OPVs should be minimized. This drawback is the intrinsic large loss of open-circuit voltage; however, a general approach to this issue remains elusive. Here, we report a discovery regarding highly efficient NFAs, specifically ITIC. We found that charge-transfer (CT) and charge dissociation (CD) can occur even in a neat ITIC film without the donor layer. This is surprising, as these processes were previously believed to take place exclusively at donor/acceptor heterojunctions. Femtosecond time-resolved visible to mid-infrared measurements revealed that in the neat ITIC layers, the intermolecular CT immediately proceeds after photoirradiation (<0.1 ps) to form weakly-bound excitons with a binding energy of 0.3 eV, which are further dissociated into free electrons and holes with a time-constant of 56 ps. Theoretical calculations indicate that stacking faults in ITIC (i.e., V-type molecular stacking) induce instantaneous intermolecular CT and CD in the neat ITIC layer. In contrast, J-type stacking does not support such CT and CD. This previously unknown pathway is triggered by the larger dipole moment change on the excited state generated at the lower symmetric V-type molecular stacking of ITIC. This is in sharp contrast with the need of sufficient energy offset for CT and CD at the donor-acceptor heterojunction, leading to the significant voltage loss in conventional OPVs. These results demonstrate that the rational molecular design of NFAs can increase the local dipole moment change on the excited state within the NFA layer. This finding paves the way for a groundbreaking route toward the commercialization of OPVs.

Molecular engineering and structure-property relationship based on D-A chlorophyll derivative and the application in organic solar cells.

The photoactive layer materials of organic solar cells (OSCs) play a critical role in achieving excellent performance. Chlorophyll derivatives are commonly used due to their environmental friendliness, low cost, and easy accessibility. However, the efficiency of OSCs based on chlorophyll is limited by their photoelectric properties. Here, we focused on the D-A structure of chlorophyll derivative ZnChl-1 and designed four molecules through rational molecular engineering. The photoelectric properties at the microscopic level were systematically studied using density functional theory (DFT). Our findings reveal that T-ZnChl-1 with triphenylamine donor unit has a smaller energy gap and ionization energy, as well as a larger spectral red shift and absorption range. This suggests that intramolecular charge transfer will be enhanced, leading to an improvement in short-circuit current. Furthermore, Y6 is used as the acceptor to construct the heterojunction interfaces. The results indicate that the T-ZnChl-1/Y6 interface exhibits more charge transfer states and higher exciton dissociation rate KCS, which will promote charge separation and lead to excellent photovoltaic performance. This work clarifies the structure-property relationship of chlorophyll derivatives and the photo-response mechanism of intermolecular charge transfer, providing a theoretical basis for developing valuable chlorophyll-based OSCs.

The mechanism governing the formation of intermolecular charge transfer bands: a series of polyoxomolybdates as a case study.

A series of proof-of-concept models of polyoxomolybdates with different protonated disubstituted aniline counterions and the same β-Mo8O26 polyanion were synthesized to study the mechanism governing the formation of the intermolecular charge transfer (inter-CT) band.

P3HT∶Y6-Based Near-Infrared Organic Photomultiplication Photodetectors by Intermolecular Charge Transfer Effects

P3HT∶Y6-Based Near-Infrared Organic Photomultiplication Photodetectors by Intermolecular Charge Transfer Effects

Highly selective detection of 2,4-dinitrophenol by fluorescent NH2-MIL-125(Ti) via dual-parameter sensing technology.

The organic contaminant 2,4-dinitrophenol (2,4-DNP) is widely prevalent and poses significant risks to human health. Although numerous in-depth studies having been reported on the highly sensitive detection of 2,4-DNP, there are still challenges to its selective detection. Here, the fluorescence intensity ratio (I0/I) and emission peak shift (Δλ) were utilized for selective detection of 2,4-DNP by NH2-MIL-125(Ti). Notably, the emission peak of the NH2-MIL-125(Ti) suspension exhibited a remarkable red shift in the presence of 2,4-DNP (Δλ = 26 nm), accompanied by the blue shift or weak red shift of analogs, which provided a solid basis for selective detection of 2,4-DNP. Meanwhile, the I0/I ratio of the NH2-MIL-125(Ti) suspension exhibited a robust linear correlation with 2,4-DNP at the low concentration range (0-70 μM). The interaction of the analyte with NH2-MIL-125(Ti) was revealed to involve intermolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) through XPS, FTIR, and UV-vis absorption spectroscopy. Additionally, we achieved the detection of 2,4-DNP using a smartphone by recognizing both the blue (B) values and the luminance (L) values. The obtained results demonstrated that the NH2-MIL-125(Ti) probe based on dual-parameter sensing technology exhibited excellent potential for selectively detecting 2,4-DNP in water environments, thereby offering significant prospects for its application in water quality assessment.

Europium doping promoted intermolecular charge transfer in UiO-67-CDC for ultrasensitive turn-on colorimetric detection of an anthrax biomarker

We have synthesised a lanthanide MOF, Eu@UiO-67-CDC, for the sensitive detection of DPA, offering a practical, instrument-free sensing solution.

Atomic-layer tailored organic photodetectors: harnessing intermolecular charge-transfer absorption for expanded spectral sensitivity up to the telecommunication band

The ZnPc:C60 OPD based on intermolecular charge transfer absorption can sense light up to the telecommunication band by incorporating an atomic-thick Al2O3 layer.

Role of intermolecular charge transfer towards fluorometric detection of fluoride ions with anthrapyrazolone derivatives

Anion detection using small molecules as chemosensors holds significant importance in the biological and environmental fields, offering several advantages over conventional methods.

Unraveling non-radiative decay channels of exciplexes to construct efficient red emitters for organic light-emitting diodes.

Exciplex emitters naturally have thermally activated delayed fluorescence characteristics due to their spatially separated molecular orbitals. However, the intermolecular charge transfer potentially induces diverse non-radiative decay channels, severely hindering the construction of efficient red exciplexes. Thus, a thorough comprehension of this energy loss is of paramount importance. Herein, different factors, including molecular rigidity, donor-acceptor interactions and donor-donor/acceptor-acceptor interactions, that impact the non-radiative decay were systematically investigated using contrasting exciplex emitters. The exciplex with rigid components and intermolecular hydrogen bonds showed a photoluminescence quantum yield of 84.1% and a singlet non-radiative decay rate of 1.98 × 106 s-1 at an optimized mixing ratio, respectively, achieving a 3.3-fold increase and a 70% decrease compared to the comparison group. In the electroluminescent device, a maximum external quantum efficiency of 23.8% was achieved with an emission peak of 608 nm, which represents the state-of-the-art organic light-emitting diodes using exciplex emitters. Accordingly, a new strategy is finally proposed, exploiting system rigidification to construct efficient red exciplex emitters that suppress non-radiative decay.

External electric field driven conformation transition between lithium salts and electride-like molecules: intriguing NLO switches in Li@corannulene.

The external electric field has emerged as a powerful tool for building molecular switches with excellent properties. In this work, we investigate the impact of an external electric field on the transition between lithium salt and electride-like molecule conformations in Li@corannulene. Remarkably, the distance between the Li atom and the corannulene bottom displays a sharp increase under the influence of an external electric field strength of F-z = 110 × 10-4 a.u. As the external electric field strength increases, the Li atom brings about different directions of charge transfer (CT). The natural population analysis (NPA) charge and the molecular electrostatic potential (ESP) results show that the intermolecular CT occurs from the Li atom to the corannulene with the F-z ranging from 0 to 100 × 10-4 a.u. Interestingly, when the external electric field reaches F-z = 110 × 10-4 a.u., the CT is oriented from the corannulene to the Li atom. Moreover, electron localization function (ELF) basins are presented under an F-z of 110 × 10-4 a.u., which indicates that Li@corannulene exhibits electride-like (e-⋯[Li@corannulene]+) molecules and lithiation salt (Li+[corannulene]-) under an F-z of 0 to 100 × 10-4 a.u. Significantly, the differences in charge transfer also contribute to a significant improvement in hyperpolarizabilities (βtot) during the conformation transition from lithiation salt (Li+[corannulene]-) to electride-like (e-⋯[Li@corannulene]+) molecules. This study explores the potential of Li@corannulene as a promising second-order NLO material, and the external electric field provides an efficient strategy for designing and developing NLO switching devices.

Molecularly induced order promotes charge separation through delocalized charge-transfer states at donor-acceptor heterojunctions.

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer (CT) states into free charge carriers (FCC) in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by molecular arrangement. We experimentally determine the CT dissociation properties of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors above 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher structural order of B4PYMPM molecules leads to more delocalized CT states. Furthermore, we find no correlation between the quantum efficiency of FCC radiative recombination and the bound or unbound nature of the CT states. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient FCC generation and shows that less bound CT states do not preclude efficient radiative recombination.

Acid/base responsive pseudo[3]rotaxanes from amine naphthotubes and bis-pyridinium/isoquinolinium guests.

A novel cooperative pseudo[3]rotaxane system was successfully constructed by the inclusion complexation of two identical amine naphthotubes with a bis-pyridinium/isoquinolinium guest. Single crystal structure analysis revealed that weak Csp3-H⋯O hydrogen bonds between the two hosts are responsible for the positive cooperativity during the formation of pseudo[3]rotaxanes. Moreover, intermolecular charge-transfer interactions between the electron-rich host and the electron-poor guests were observed. The pseudo[3]rotaxanes showed pH-controllable association/dissociation processes with naked-eye color changes in solution.

Stimulating Phonon Bottleneck Effect in Organic Semiconductors by Charge-Transfer-Mediated J-Aggregation.

Hot carriers rapidly lose kinetic energies on a subpicosecond time scale, posing significant limitations on semiconductors' photon-conversion efficiencies. To slow the hot carrier cooling, the phonon bottleneck effect is constructed prevalently in quantum-confined structures with discrete energy levels. However, the maximum energy separation (ΔEES) between the energy levels is in a range of several hundred meV, leading to unsatisfactory cooling time. To address this, we design a novel organic semiconductor capable of forming intermolecular charge transfer (CT) in J-aggregates, where the lowest singlet excited state (S1) splits into two states due to the significant interplay between the Coulomb interaction and intermolecular CT coupling. The ΔEES between the two states can be adjusted up to 1.02 eV, and an extremely slow carrier cooling process of ∼72.3 ps was observed by femtosecond transient absorption spectroscopy. Moreover, the phonon bottleneck effect was identified in organic materials for the first time, and CT-mediated J-aggregation with short-range interactions was found to be the key to achieving large ΔEES. The significantly prolonged carrier cooling time, compared to <100 fs in the isolated molecule (10-6 M), highlights the potential of organic molecules with diversified aggregation structures in achieving long-lived hot carriers. These findings provide valuable insights into the intrinsic photophysics of electron-phonon scattering in organic semiconductors.