Influence of π-conjugation structural changes on intramolecular charge transfer and photoinduced electron transfer in donor-π-acceptor dyads

Fluorescent metal nanoclusters: prospects for photoinduced electron transfer and energy harvesting

Research on noble metal nanoclusters (MNCs) (elements with filled electron d-bands) is progressing forward because of the extensive and extraordinary chemical, optical, and physical properties of these materials. Because of the ultrasmall size of the MNCs (typically within 1-3 nm), they can be applied in areas of nearly all possible scientific domains. The greatest advantage of MNCs is the tunability that can be imposed, not only on their structures, but also on their chemical, physical, and biological properties. Nowadays, MNCs are very effectively used as energy donors and acceptors under suitable conditions and hence act as energy harvesters in solar cells, semiconductors, and biomarkers. In addition, ultrafast photoinduced electron transfer (PET) can be practised using MNCs under various circumstances. Herein, we have focused on the energy harvesting phenomena of Au-, Ag-, and Cu-based MNCs and elaborated on different ways to apply them.

NIR-II emissive donor-acceptor-donor fluorophores for dual fluorescence bioimaging and photothermal therapy applications

Fluorescence bioimaging with near-infrared II (NIR-II) emissive organic fluorophores has proven to be a viable noninvasive diagnostic technique. However, there is still the need for the development of fluorophores that possess increased stability as well as functionalities that impart stimuli responsiveness. Through strategic design, we can synthesize fluorophores that possess not only NIR-II optical profiles but also pH-sensitivity and the ability to generate heat upon irradiation. In this work, we employ a donor-acceptor-donor (D-A-D) design to synthesize a series of NIR-II fluorophores. Here we use thienothiadiazole (TTD) as the acceptor, 3-hexylthiophene (HexT) as the π-spacer and vary the alkyl amine donor units: N,N-dimethylaniline (DMA), phenylpiperidine (Pip), and phenylmorpholine (Morp). Spectroscopic analysis shows that all three derivatives exhibit emission in the NIR-II region with λemimax ranging from 1030 to 1075 nm. Upon irradiation, the fluorophores exhibited noticeable heat generation through non-radiative processes. The ability to generate heat indicates that these fluorophores will act as theranostic (combination therapeutic and diagnostic) agents in which simultaneous visualization and treatment can be performed. Additionally, biosensing capabilities were supported by changes in the absorbance properties while under acidic conditions as a result of protonation of the alkyl amine donor units. The fluorophores also show minimal toxicity in a human mammary cell line and with murine red blood cells. Overall, initial results indicate viable NIR-II materials for multiple biomedical applications.

Donor-Acceptor-Donor Triads with Flexible Spacers: Deciphering Complex Photophysics for Targeted Materials Design

The complex photokinetics of donor-acceptor-donor triads with varying flexible spacer lengths (n = 4-10 carbon atoms) are investigated in liquid and solid solution, as well as in crystals, by steady-state and transient fluorescence spectroscopy combined with computational studies. For the short spacer (n = 4) in a liquid solution, dynamic charge-transfer (CT) state formation with subsequent, efficient exciplex emission is observed, effectively competing with quenching through electron transfer (eT) via a radical ion pair. In a solid solution, a fluorescent CT static complex is formed upon freezing for all spacer lengths. This allows the observations of a former seminal report on stimuli-responsive high-contrast fluorescence on/off switching in films of the triads to be reassigned (Adv. Mater. 2012, 24, 5487), now providing a holistic picture on varying spacer length. In fact, external stimuli of the film by modulating the geometry of the CT complex, which results in on/off fluorescence switching (for n > 4) or in a change of the emission color (n = 4). The work thus demonstrates how in-depth analysis of complex photophysics can be put to practical use in materials science.

A novel fluorophore with multichromic effect and its application in LED light for the volatile vapor detection

Successfully designed and synthesized diaryl maleimide DAM, and their structure was confirmed by mass spectroscopy and NMR techniques. They investigated their photophysical properties, such as solvatochromic and the aggregation effect of AIE/ACQ on the water/DMF ratio, and other studies of solid-state mechanofluorochromic, such as grinding, exposure to solvent fumes, hydrostatic pressures, and vapochromic. Interestingly, the solvent methods gave very similar results in both the dissolving phase and the vapor phase. For the solution state from hexane to CH₂Cl_2, a greenish-yellow to orange emission was observed. From hexane to dichloromethane, naked-eye colorimetric changes (from yellow to orange in a solid state) were observed. Very interesting results were obtained, the yellow solid was ground for 1 min, it turned into an orange color, and its wavelengths were red-shifted in both absorption and emission. Subsequently, the ground sample was exposed to diethyl ether vapors, which returned to the original green-yellow emission and absorption. The applied hydrostatic pressures (0-4Mpa) in DAM gave excellent red-shifted emission, and then their hydrostatic pressures were increased to 4-16 Mpa, and the red-shifted emission gradually decreased.

Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives

Photocatalytic systems for CO₂ reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO₂ conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO₂. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs^•-) with TiO₂ compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs^•- formed at adjacent TiO₂ without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.

Conformation-related excited-state charge transfer/separation of donor-π-acceptor chromophores

Understanding the excited-state charge transfer/separation (CT/CS) of donor-π-acceptor chromophores can provide guidance for designing and synthesizing advanced dyes to improve the performance of dye-sensitized solar cells (DSSCs) in practical applications. Herein, two newly synthesized electronic push-pull molecules, CS-14 and CS-15, that consist of carbazole donor and benzothiadiazole acceptor segments are chosen to explore the ultrafast dynamics of intramolecular CT/CS processes. The theoretical calculation results depict an excited-state intramolecular CT character for both dyes, while the dihedral angle between donor and acceptor of CS-14 is larger than that of CS-15, suggesting a more significant CT character of CS-14. Furthermore, compared to CS-14, the bond rotation of CS-15 between donor and π-bridge is restricted by employing the hexatomic ring, indicating the stronger molecular planarization of CS-15. Ultrafast spectroscopy clearly shows a solvent polarity-dependent excited-state species evolution from CT to CS-the CT character is observed in low-polar toluene solvent, while the feature of the CS state in polar tetrahydrofuran and acetone solvents is captured, which successfully proved a solvent polarity modulated excited-state CT/CS characters. We also found that though the generation of the CS state within CS-14 is slightly faster than that of CS-15, the charge recombination process of CS-15 with excellent planar conformation is much slower, providing enough time for a higher charge migration efficiency in DSSCs.

Revealing the sensing mechanism of a fluorescent pH probe based on a bichromophore approach

The pH sensing mechanism of TMARh is investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations, showing that this new type of sensor can be better understood using the bichromophore model.

Singlet Fission Dynamics of Colloidal Nanoparticles of a Perylenediimide Derivative in Solutions

Singlet fission (SF) is an intriguing process in which a singlet exciton produces two triplet excitons in molecular aggregates. Perylenediimide (PDI) derivatives are promising materials for SF-based photovoltaics, and the SF process in PDI aggregates is important to investigate for their applications. In this work, we studied the entire SF process occurring in the colloidal nanoparticles of a PDI derivative in solutions by using time-resolved fluorescence and transient absorption (TA) experiments. PE-PDI was found to form the colloidal nanoparticles of H- and J-aggregates in polar solvents. The TA signals of PE-PDI aggregates in solutions were selectively measured by wavelength-dependent excitation. The TA signals were analyzed by using a global fitting analysis, and all kinetic parameters involved in the entire SF process were determined. Our current investigation has confirmed that fast SF occurs on the surface of the colloidal nanoparticles of PDI aggregates via the charge transfer mediated mechanism, giving a high quantum yield of triplet excitons.

Insight into the role of copper in the promoted photocatalytic removal of NO using Zn2-xCuxCr-CO3 layered double hydroxide

In this work the ability of Zn_2-xCu_xCr-CO₃ layered double hydroxides (LDHs) as highly efficient DeNOx photocatalysts was studied. LDHs with x = 0, 0.2 and 0.4 were prepared using a coprecipitation method. The samples were characterized by different techniques such as XRD, XPS, FT-IR, ICP-MS, TG, SBET, SEM and Diffuse reflectance (DR). The increased amount of copper ions in the LDH layers gave rise to slight changes in the structure and morphology and an important variation of the optical properties of the LDHs. The prepared ZnCuCr-CO₃ photocatalysts exhibited favourable conversion efficiency (51%) and an extraordinary selectivity (97%) for the photochemical NO abatement. The photochemical mechanism was elucidated from DOS, EPR, Femtosecond transient absorption and in-situ DRIFTS studies. The results suggested that the presence of Cu²⁺ ions in the LDH framework introduced new states in the valence band states, thus favouring the production and mobility of e^-/h⁺ charge carriers and a greater production of ⋅O₂^- and ⋅OH.

Aryl-substituted acridanes as hosts for TADF-based OLEDs

Four aryl-substituted acridan derivatives were designed, synthesized and characterized as electroactive materials for organic light emitting diodes based on emitters exhibiting thermally activated delayed fluorescence. These compounds possessed relatively high thermal stability with glass-transition temperatures being in the range of 79–97 °C. The compounds showed oxidation bands arising from acridanyl groups in the range of 0.31–038 V. Ionization potentials of the solid films ranged from 5.39 to 5.62 eV. The developed materials were characterized by triplet energies higher than 2.5 eV. The layer of 10-ethyl-9,9-dimethyl-2,7-di(naphthalen-1-yl)-9,10-dihydroacridine demonstrated hole mobilities reaching10⁻³ cm²/V·s at electric fields higher then ca. 2.5 × 10⁵ V/cm. The selected compounds were used as hosts in electroluminescent devices which demonstrated maximum external quantum efficiencies up to 3.2%.

Devising Efficient Red-Shifting Strategies for Bioimaging: A Generalizable Donor-Acceptor Fluorophore Prototype

Long emission wavelengths, high fluorescence quantum yields (FQYs), and large Stokes shifts are highly desirable features for fluorescent probes in biological imaging. However, the current development of many fluorescent probes remains largely trial-and-error and lacks efficiency. Moreover, to achieve far-red/near-infrared emission, a significant extension in the π -conjugation is usually adopted but accompanied by other drawbacks such as fluorescence loss. In this review, we discuss an effective red-shifting strategy built upon the green fluorescent protein chromophore, which enables a synergistic tuning of both the electronic ground and excited states. This approach could shorten the path toward redder emission in comparison to the conventional intramolecular charge transfer (ICT) strategy. We envision that this spectroscopy and computation-aided strategy may advance the noncanonical fluorescent protein design and be generalized to various fluorophore scaffolds for redder emission while preserving other superior properties such as high FQYs.

Charge separation, recombination and intersystem crossing of directly connected perylenemonoimide-carbazole electron donor/acceptor dyads

Perylenemonoimide (PMI)-carbazole (Cz) compact electron donor/acceptor dyads were prepared to study the relationship between the mutual orientation of the electron donor/acceptor in the dyads and the spin-orbit charge transfer intersystem crossing (SOCT-ISC) efficiency. The PMI and the Cz units are connected via either a C-C or C-N bond, or with an intervening phenyl moiety. The photophysical properties of the dyads were studied with steady state and time-resolved optical spectroscopies. The fluorescence of the PMI unit in the dyads was generally quenched, due to photo-induced electron transfer, especially in polar solvents (the fluorescence has a biexponential decay in acetonitrile, τ_F = 1.4 ns/population ratio: 98.9%, and 9.6 ns/population ratio: 1.1%). The triplet state (lifetime τ_T = 14.7 μs) formation of the dyads is dependent on the solvent polarity, which is characteristic for SOCT-ISC. Femtosecond transient absorption spectra show that the charge separation takes 0.28 ps and the charge recombination takes 1.21 ns. Reversible photo-reduction of the PMI-Cz dyads and generation of the near IR-absorbing (centered at 604 nm and 774 nm) PMI radical anion (PMI^-˙) were observed in the presence of a sacrificial electron donor (triethylamine). These results are useful for study of the fundamental photochemistry of compact electron donor/acceptor dyads and for design of new heavy atom-free triplet photosensitizers.

Synergistic Approach of Ultrafast Spectroscopy and Molecular Simulations in the Characterization of Intramolecular Charge Transfer in Push-Pull Molecules

The comprehensive characterization of Intramolecular Charge Transfer (ICT) stemming in push-pull molecules with a delocalized π-system of electrons is noteworthy for a bespoke design of organic materials, spanning widespread applications from photovoltaics to nanomedicine imaging devices. Photo-induced ICT is characterized by structural reorganizations, which allows the molecule to adapt to the new electronic density distribution. Herein, we discuss recent photophysical advances combined with recent progresses in the computational chemistry of photoactive molecular ensembles. We focus the discussion on femtosecond Transient Absorption Spectroscopy (TAS) enabling us to follow the transition from a Locally Excited (LE) state to the ICT and to understand how the environment polarity influences radiative and non-radiative decay mechanisms. In many cases, the charge transfer transition is accompanied by structural rearrangements, such as the twisting or molecule planarization. The possibility of an accurate prediction of the charge-transfer occurring in complex molecules and molecular materials represents an enormous advantage in guiding new molecular and materials design. We briefly report on recent advances in ultrafast multidimensional spectroscopy, in particular, Two-Dimensional Electronic Spectroscopy (2DES), in unraveling the ICT nature of push-pull molecular systems. A theoretical description at the atomistic level of photo-induced molecular transitions can predict with reasonable accuracy the properties of photoactive molecules. In this framework, the review includes a discussion on the advances from simulation and modeling, which have provided, over the years, significant information on photoexcitation, emission, charge-transport, and decay pathways. Density Functional Theory (DFT) coupled with the Time-Dependent (TD) framework can describe electronic properties and dynamics for a limited system size. More recently, Machine Learning (ML) or deep learning approaches, as well as free-energy simulations containing excited state potentials, can speed up the calculations with transferable accuracy to more complex molecules with extended system size. A perspective on combining ultrafast spectroscopy with molecular simulations is foreseen for optimizing the design of photoactive compounds with tunable properties.

Synthesis and photophysical studies of an indigo derivative: N-octyl-7,7′-diazaindigo

In this paper, we explore the synthesis, characterization, and photophysical properties of a novel indigo derivative, N-octyl-7,7′-diazaindigo, being the first time that diazaindigos have been studied as photophysically-active chemical entities. Reduction of the neutral “keto-form” to the so-called “leuco-form” changes the global spectroscopic and photophysical behaviors. Both species have been investigated by different photophysical studies, such as analysis of absorption and emission spectra, fluorescence quantum yields (Φ_F) and lifetimes. Finally, to appraise in depth the deactivation of the excited state of the keto form, femtosecond transient absorption (TA) experiments and Density Functional Theory (DFT) and Time Dependent (TD)-DFT calculations were performed. In an organic aprotic solvent (N,N-dimethylformamide), TA experiments showed a fast deactivation channel (τ₁ = 2.9 ps), which was ascribed to solvent reorganization, and a longer decay component (τ₂ = 86 ps) associated with an internal conversion (IC) process to the ground-state, in opposition to the excited state proton transfer (ESPT) mechanism that takes place in the indigo molecules but in protic solvents. A comparative study was also carried out on the parent molecule, 7,7′-diazaindigo, corroborating the previous conclusions obtained for the alkyl derivative. In agreement with experimental observations, DFT and TD-DFT calculations revealed that the deactivation of the S₁ state of the keto form takes place through an internal conversion process.

We report the synthesis, characterization, photophysical properties, and theoretical calculations of a novel indigo derivative, N-octyl-7,7′-diazaindigo.

Terphenyl backbone-based donor–π–acceptor dyads: geometric isomer effects on intramolecular charge transfer

The molecular geometry effects of ortho, meta, and para-terphenyl based donor–π–acceptor (D–π–A) dyads on intramolecular charge transfer (ICT) were studied to investigate structure-ICT relationships.

Intramolecular charge transfer and solvation dynamics of push-pull dyes with different π-conjugated linkers

The solvation-dependent excited state dynamics of two push-pull fluorophores with donor-π-acceptor (D-π-A) structures were investigated using steady-state and ultrafast transient absorption (TA) spectroscopy, backed by theoretical calculations. Identical D and A groups were present in both dyes, which differed only in the structure of their central π-conjugated linkers. Dye 1 features a p-phenylenediethynyl linker, while dye 2 contains a 2,5-diethynylthiophene linker. From the steady-state spectra, no appreciable shifts in absorption bands were observed, whereas large red-shifts in emission were seen with increasing solvent polarity, which indicated that the excited states were more polar than the ground state. Theoretical calculations support charge transfer from the triphenylamine (TPA) donor to the pentafluorosulfanyl (SF₅) acceptor viaπ-conjugated linkers to form an intramolecular charge transfer (ICT) state. TA spectra revealed that a solvation-stabilized conformationally relaxed intramolecular charge transfer (ICT') state was formed in polar solvents, but only an ICT state was observed in nonpolar solvent. The SE band was quenched within 1 ps in high-polarity solvent, which corresponds to the low fluorescence quantum yield. It can be concluded that the dye with the p-phenylenediethynyl π-linker (i.e., dye 1) exhibits a larger degree of ICT than the thiophene analogue (i.e., dye 2). These findings demonstrate how solvation can fine-tune the photophysical properties of push-pull dyes, and this study highlights the importance of π-conjugated linkers in the excited state ICT process.

Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing an Asymmetric Oxadiazole-Difluoroboron Double Acceptor

Difluoroboron (BF₂)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF₂-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF₂ and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF₂-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules.

Interplay between H-Bonding and Preorganization in the Evolution of Self-Assembled Systems

Cooperative π-π interactions and H-bonding are frequently exploited in supramolecular polymerization; however, close scrutiny of their mutual interplay has been largely unexplored. Herein, we compare the self-assembly behavior of a series of C2 - and C3 -symmetrical oligophenyleneethynylenes differing in their amide topology (N- or C-centered). This subtle structural modification brings about drastic changes in their photophysical and supramolecular properties, highlighting the reciprocal impact of H-bonding vs. preorganization on the evolution and final outcome of supramolecular systems.

Photoinduced electron and hole transfers in carbazole dendrimers with heteroleptic Ir-complex cores

We synthesised carbazole (Cz) dendrimers with heteroleptic Ir-complex cores. Upon excitation of the carbazole (Cz) dendrons, the phosphorescence of the core Ir(iii) complex was quenched due to the photoinduced electron transfer (PET) process. The PET dynamics of the excited Cz-dendrons were investigated using the femtosecond time-resolved transient absorption technique. A broad transient absorption (TA) band attributed to the S1-Sn transition of the 1Cz*-dendron was observed at around 630 nm in the first generation Cz-dendrimer (G1). This TA band in the second-generation dendrimer (G2) decayed with a longer lifetime of 55.5 ps compared to that of G1 (9.8 ps), because G2 has a larger distance between the Cz-dendron and Ir-complex core than that of G1. The decay time of the free 1Cz*-dendron was 6.3 ns, and thus, the reduced decay time in Gn corresponds to the PET dynamics. As a result of the PET process, the Cz cationic radical species (Cz˙+) was observed at around 780 nm. Interestingly, when the core Ir-complex in the dendrimer was excited with a 400 nm pulse selectively, the TA band of Cz˙+ was also detected at around 780 nm. This may be due to the photoinduced hole transfer (PHT) from the highest occupied molecular orbital (HOMO) energy state of Cz to the lowest singly occupied molecular orbital (LSOMO) energy state of the excited Ir-complex. The oxidation potential of Cz is lower than that of the Ir-complex, indicating that the HOMO of the Cz-dendron is located at a higher energy state than that of the Ir-complex. To investigate the relative order of the energy states and their orbital shapes, we performed theoretical calculations using density functional theory. The TA spectra were globally deconvoluted to generate the decay-associated spectra (DAS), from which the species-associated spectra (SAS) were calculated. The SAS can distinguish the individual intermediate species participating in the PET and PHT processes. The analysed rate constants of SAS were consistent with the results determined by the TA decays.

Design and synthesis of solution processable green fluorescent D–π–A dyads for OLED applications

Synthesized solution processable green fluorescent donor–acceptor dyads and their investigated photophysical, electrochemical, and morphological properties for OLED applications.

Time-resolved spectroscopic analysis of the light-energy harvesting mechanism in carbazole-dendrimers with a blue-phosphorescent Ir-complex core

In order to understand the light energy-harvesting mechanism, singular value decomposition analysis was performed to classify the temporal and spectral species in transient absorption spectra.

The effect of interligand energy transfer on the emission spectra of heteroleptic Ir complexes

Heteroleptic Ir³⁺ complexes with luminescent ancillary ligands showed broad and red-shifted emissions owing to ILET, which was affected by the energy differences between the main and ancillary ligands, the temperature, and the rigidity of the environment.

Direct observation of the photoinduced electron transfer processes of bis(4-arylphenylamino benzo)-ortho-carborane using transient absorption spectroscopic measurements

The characteristic Cb˙⁻ and Ar˙⁺ species of ortho-carborane triads generated by the photoinduced electron transfer are investigated by transient absorption measurement.