Charge Transfer Emission Research Articles

An efficient thermally activated delayed fluorophore featuring an oxygen-locked azaryl ketone acceptor

Thermally activated delayed fluorescence (TADF) dyes are promising for applications like bioimaging, sensing, and optoelectronic devices due to their efficient exciton utilization without noble metals, making them ideal for organic light-emitting diodes (OLEDs). In this contribution, we introduce a new TADF emitter, 3-(9,9-dimethylacridin-10(9H)-yl)-12H-chromeno[2,3-b]quinolin-12-one (CQLO-DMAC), which incorporates an intramolecular oxygen-locked azaryl ketone acceptor, CQLO. This design enhances molecular rigidity and electron-accepting strength. The CQLO-DMAC compound, featuring a bulky donor, shows significant intramolecular charge transfer and TADF emission. Comprehensive analyses, including spectroscopic studies and theoretical calculations, reveal high luminescence efficiency and reduced non-radiative losses. Utilizing CQLO-DMAC in OLEDs, we achieved a green-yellow emission peak at 548 nm and a maximum external quantum efficiency of 17.4%. The findings highlight the potential of CQLQ as a highly effective acceptor in designing of TADF electroluminescent dyes.

Synthesis, nonlinear optical activity, solvents effect, β-cyclodextrin effect, and cytotoxic activity on skin fibroblast and breast cancer cell lines of a new chalcone derivative of nabumetone

The use of small organic molecules, such as chalcones, for efficient applications as organic luminescent materials has attracted increasing attention owing to their interesting optical, photophysical, and biological properties. In this study, a new chalcone, 1-(4-isopropylphenyl)-5-(6-methoxynaphthalen-2-yl)pent-1-en-3-one (INM), was synthesized via base condensation between nabumetone and cuminaldehyde. INM was subsequently identified and characterized by FT-IR, NMR spectroscopy (1H and 13C), mass spectrometry, elemental analysis, X-ray diffraction, thermogravimetric analysis, and FESEM studies. Investigation of the solvent effect revealed that the π → π* transition involved a bathochromic shift from hexane to water and a large Stokes-shifted, twisted intramolecular charge-transfer emission in water. Diffuse reflectance spectral studies confirmed the formation of transparent INM chalcones with excellent crystallinity, and photoluminescence studies substantiated the low recombination rate of electrons and holes. Tauc plot analysis with the Kubelka–Munk algorithm revealed higher direct (3.57 eV) and indirect (3.41 eV) bandgap energies of INM. Density functional theory calculations at B3LYP/6-31G(d,p) revealed that INM had promising nonlinear optical activity (β ≈ 30.504 × 10−30 compared to a reference material, urea. Cell biocompatibility was evaluated after culturing skin fibroblasts and breast cancer cells with INM using the MTT assay and fluorescence microscopy of the live/dead cell assay. It was observed that INM exhibited good NIH/3T3 cell adhesion and proliferation and the weak inhibiting ability of MDA-MB231.

Proton transfer induced persistent triplet charge transfer phosphorescence in molecule-doped polymer systems

The intermolecular interactions between small molecules and the matrix in a highly polar environment could easily lead to non-radiative transitions, which significantly hampers the development of high-performance persistent room-temperature phosphorescence (pRTP) materials in such condition. Herein, we introduce a novel strategy that integrates consecutive proton transfer and photo-induced charge transfer to facilitate triplet charge transfer emission in highly polar polymer matrix. In particular, doping aza-arene guests including 1,10-phenanthroline (1,10-Phen), 2,9-dimethyl-1,10-phenanthroline (DM-Phen), and 7,8-benzoquinoline (7,8-BQ) into polyacrylic acid (PAA) resulted in superior pRTP properties compared to other polymer hosts. Spectroscopic investigations and theoretical calculations elucidate that this is attributed to the proton transfer and triplet charge transfer characteristics between the host and guest. Moreover, due to the rigid environment and space confinement provided by PAA, pRTP with the lifetime up to 841 ms is achieved. Capitalizing on the excellent water solubility of the doped PAA films, the inkjet printings have been executed, and showcasing the promising the potential applications of these pRTP materials in the field of information encryption.

Electron Transfer Enhanced by a Minimal Energetic Driving Force at the Organic‐Semiconductor Interface

AbstractThe energetic driving force for electron transfer must be minimized to realize efficient optoelectronic devices including organic light‐emitting diodes (OLEDs) and organic photovoltaics (OPVs). Exploring the dynamics of a charge‐transfer (CT) state at an interface leads to a comprehension of the relationship between energetics, electron‐transfer efficiency, and device performance. Here, we investigate the electron transfer from the CT state to the triplet excited state (T1) in upconversion OLEDs with 45 material combinations. By analyzing the CT emission and the singlet excited‐state emission from triplet–triplet annihilation via the dark T1, their energetics and electron‐transfer efficiencies are extracted. We demonstrate that the CT→T1 electron transfer is enhanced by the stronger CT interaction and a minimal energetic driving force (<0.1 eV), which is explained using the Marcus theory with a small reorganization energy of <0.1 eV. Through our analysis, a novel donor–acceptor combination for the OLED is developed and shows an efficient blue emission with an extremely low turn‐on voltage of 1.57 V. This work provides a solution to control interfacial CT states for efficient optoelectronic devices without energy loss.

Electron Transfer Enhanced by a Minimal Energetic Driving Force at the Organic-Semiconductor Interface.

The energetic driving force for electron transfer must be minimized to realize efficient optoelectronic devices including organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs). Exploring the dynamics of a charge-transfer (CT) state at an interface leads to a comprehension of the relationship between energetics, electron-transfer efficiency, and device performance. Here, we investigate the electron transfer from the CT state to the triplet excited state (T1) in upconversion OLEDs with 45 material combinations. By analyzing the CT emission and the singlet excited-state emission from triplet-triplet annihilation via the dark T1, their energetics and electron-transfer efficiencies are extracted. We demonstrate that the CT→T1 electron transfer is enhanced by the stronger CT interaction and a minimal energetic driving force (<0.1 eV), which is explained using the Marcus theory with a small reorganization energy of <0.1 eV. Through our analysis, a novel donor-acceptor combination for the OLED is developed and shows an efficient blue emission with an extremely low turn-on voltage of 1.57 V. This work provides a solution to control interfacial CT states for efficient optoelectronic devices without energy loss.

(Invited) Manipulation of Locally-Excited, Charge-Transfer, and Charge-Separated States at Organic-Inorganic Heterointerfaces

Precise engineering of excited-state interactions between an organic conjugated molecule and a two-dimensional semiconducting inorganic nanosheet, specifically the manipulation of locally-excited (LE), charge-transfer (CT), and charge-separated (CS) states, still remains a challenge for state-of-the-art photochemistry. In this study we successfully observed a long-lived, highly emissive CT state at structurally well-defined hetero-nanostructure interfaces of photoactive pyrene and two-dimensional (2D) MoS2 nanosheet via N-benzyl succinimide bridge (Py-Bn-MoS2). Spectroscopic measurements revealed that the LE state of pyrene in Py-Bn-MoS2 efficiently generates an unusual emissive CT state without further yielding the CS state. Theoretical studies supported the interaction of MoS2 vacant orbitals with the pyrene excited state to form a CT state. This is the first example of realizing a mixing luminescence property of organic–inorganic 2D hetero-nanostructures by an accurate design of bridge structure, and therefore represents an important step in their applications for energy conversion and optoelectronic devices and sensors.[1] J. Baek, T. Umeyama, W. Choi, Y. Tsutsui, H. Yamada, S. Seki, H. Imahori, Chem. Eur. J., 24, 1561 (2018).[2] T. Umeyama, T. Ohara, Y. Tsutsui, S. Nakano, S. Seki, H. Imahori, Chem. Eur. J., 26, 6726 (2020).[3] T. Umeyama, H. Xu, T. Ohara, Y. Tsutsui, S. Seki, and H. Imahori, J. Phys. Chem. C, 125, 13954 (2021).[4] T. Umeyama, D. Mizutani, Y. Ikeda, W. R. Osterloh, F. Yamamoto, K. Kato, A. Yamakata, M. Higashi, T. Urakami, H. Sato, H. Imahori, Chem. Sci., 14, 11914 (2023).

Fluorescent properties of chitosan-stabilized bisdemethoxycurcumin-silver nanoparticles: Synthesis and characterization

The extraction and modification of naturally-occurring compounds are widely performed due to their valuable medicinal and physical properties. Herin, we reported the extraction of bis-demethoxycurcumin from Ragbrai turmeric root. The isolated derivative was characterized by X-ray diffraction, IR, and NMR spectroscopy. Also, the green synthesis of bis-demethoxycurcumin chitosan silver BDMC-Ag-Ch nanoparticles is illustrated. These nanoparticles are identified by atomic force microscopy AFM and exhibit average particle size ∼ 53.41 nm The crystalline structure was confirmed by XRD analysis, which showed distinct peaks corresponding to silver oxide nanoparticles and Zeta power measurements revealed stable dispersion of the nanoparticles, indicating a good chitosan coating on the surface of bis-demethoxycurcumin Ag nanoparticles. The absorption bands at 240 and 405 nm are due to oxygen atom (n → π*) and aromatic (π → π*) transitions in curcumin and Chitosan molecules. The photoluminescence (PL) study of synthesized BDMC-Ag-Ch nanoparticles demonstrated an existence of two emission excited states in light-emitting, namely local excited (LE) and intramolecular charge transfer (ICT) emission states, due to the aromatic conjugation system within BDMC units and intra charge transfer between the silver ion and organic part.

The Delayed Box: Biphenyl Bisimide Cyclophane, a Supramolecular Nano-environment for the Efficient Generation of Delayed Fluorescence.

Activating delayed fluorescence emission in a dilute solution via a non-covalent approach is a formidable challenge. In this report, we propose a strategy for efficient delayed fluorescence generation in dilute solution using a non-covalent approach via supramolecularly engineered cyclophane-based nanoenvironments that provide sufficient binding strength to π-conjugated guests and that can stabilize triplet excitons by reducing vibrational dissipation and lowering the singlet-triplet energy gap for efficient delayed fluorescence emission. Toward this goal, a novel biphenyl bisimide-derived cyclophane is introduced as an electron-deficient and efficient triplet-generating host. Upon encapsulation of various carbazole-derived guests inside the nanocavity of this cyclophane, emissive charge transfer (CT) states close to the triplet energy level of the biphenyl bisimide are generated. The experimental results of host-guest studies manifest high association constants up to 104 M-1 as the prerequisite for inclusion complex formation, the generation of emissive CT states, and triplet-state stabilization in a diluted solution state. By means of different carbazole guest molecules, we could realize tunable delayed fluorescence emission in this carbazole-encapsulated biphenyl bisimide cyclophane in methylcyclohexane/carbon tetrachloride solutions with a quantum yield (QY) of up to 15.6%. Crystal structure analyses and solid-state photophysical studies validate the conclusions from our solution studies and provide insights into the delayed fluorescence emission mechanism.

Unravelling the optoelectronic and biological properties of phenanthroimidazo [1,2‐c] quinazoline‐based donor‐acceptor materials

Imidazo[1,2‐c]quinazoline is widely established as biologically spectral active materials, while their optoelectronic properties were seldom investigated in the literature. In this context, this research work introduced two donors of varying strength, such as triphenylamine (TP) and phenothiazine (PZ) units, into the phenanthroimidazo [1,2‐c] quinazoline acceptor unit to form donor‐acceptor type luminescence materials such as TPQZ and PZQZ, respectively and were characterized by NMR and mass spectroscopy. Both these materials exhibited intramolecular charge transfer (ICT) type absorption (∼380−450 nm) and emission (∼540−600 nm) characteristics, which attributed to the electronic transition occurring from the HOMO of the PZ/TP donor to the LUMO+1 and LUMO+2 of the imidazo [1,2‐c] quinazoline acceptor unit, as predicted using DFT calculations. Increasing the electron donor strength was not only limited to fine‐tuning the π→π* based localized (∼400−450 nm) to ICT (∼450−650 nm) emission characteristics in both the solution and solid‐state conditions but also found to improve the zone of inhibition to 16 mm against Staphylococcus aureus/Bacillus subtilis bacterial species. The scope of realizing the luminescence nature of this acceptor unit is further expanded towards tagging biological samples such as E. coli. This work opens up a new paradigm in developing luminescent materials for optoelectronic and biological applications.

Synthesis, Photophysical and Electronic Properties of a D-π-A Julolidine-Like Pyrenyl-o-Carborane.

We synthesized 2-(1-1,2-dicarbadodecaboranyl(12))-6,6,12,12-tetramethyl-7,8,11,12-tetrahydro-6H,10H-phenaleno[1,9-fg]pyrido[3,2,1-ij]quinoline (4), a julolidine-like pyrenyl-o-carborane, with pyrene substituted at the 2,7-positions on the HOMO/LUMO nodal plane. Using solid state molecular structures, photophysical data, cyclic voltammetry, DFT and LR-TDDFT calculations, we compare o-carborane and B(Mes)2 (Mes=2,4,6-Me3C6H2) as acceptor groups. Whereas the π-acceptor strength of B(Mes)2 is sufficient to drop the pyrene LUMO+1 below the LUMO, the carborane does not do this. We confirm the π-donor strength of the julolidine-like moiety, however, which raises the pyrene HOMO-1 above the HOMO. In contrast to the analogous pyrene-2-yl-o-carborane, 2-(1-1,2-dicarbadodecaboranyl(12))-pyrene VI, which exhibits dual fluorescence, because the rate of internal conversion between locally-excited (LE) and charge transfer (CT) (from the pyrene to the carborane) states is faster than the radiative decay rate, leading to a thermodynamic equilibrium between the 2 states, 4 shows only single fluorescence, as the CT state involving the carborane as the acceptor moiety in not kinetically accessible, so a more localized CT emission involving the julolidine-like pyrene moiety is observed.

Harnessing synergistic effects in GQD@Pt(II) nanocomposites for enhanced photovoltaic performance: a computational study.

The development of efficient solar energy conversion technologies is crucial for addressing global energy challenges and reducing reliance on fossil fuels. Platinum(II) complexes are promising materials for photovoltaic applications due to their strong light absorption and long-lived excited states. However, their narrow absorption in the visible spectrum and stability issues limit their performance. Combining platinum(II) complexes with graphene quantum dots (GQDs) can enhance photovoltaic performance by leveraging the complementary light harvesting and charge transfer characteristics of the two components. This study utilizes density functional theory (DFT) calculations to explore their electronic structures, charge transfer dynamics, and photoelectric performance. Specifically, it investigates the effects of incorporating different substituents, either electron-donating or electron-withdrawing, onto the fluorene motif of the Pt(II) complex. The findings reveal that combining GQDs with Pt(II) complexes extends light absorption into the UV range, enabling comprehensive solar utilization. Upon photoexcitation, electrons migrate between the GQD conduction band and the Pt(II) complex, stabilizing charges and enhancing extraction. Substituents significantly influence charge transfer dynamics: electron-withdrawing groups promote transfer to the GQD, while electron-donating groups encourage charge separation and delocalization. Nanocomposites featuring electron-donating substituents achieve the highest energy conversion efficiencies, with GQD@Pt(II)-NPh2 reaching 24.6%. This is attributed to improved light harvesting, efficient charge injection, and reduced recombination. These insights guide the rational design of GQD-Pt(II) nanocomposites, optimizing charge separation and transfer processes for enhanced photovoltaic performance. The computational approach employed here provides a robust tool for developing advanced materials in renewable energy technologies. The computational studies reported in this work were performed using the DFT approach, specifically employing the hybrid functional PBE0. The PBE0 functional's accuracy in describing electronic structures and excited-state properties is essential for understanding charge transfer processes, photoabsorption, and emission characteristics in metal-organic complexes. Geometry optimizations and time-dependent DFT (TD-DFT) calculations were carried out to investigate the properties of the nanocomposites. The effects of solvents were replicated using the conductor-like polarizable continuum model (CPCM). The charge transfer length (ΔL) and interfragment charge transfer (ΔQ) were calculated using the Multiwfn software package, and all calculations were performed using the BDF software package.

Tandem Four-Component Reaction to Access Fused Polycycles Exhibiting Aggregation-Enhanced Through-Space Charge Transfer Emission.

Rapid construction of new fluorescence emitters is essential in advancing synthetic luminescent materials. This study illustrated a piperidine-promoted reaction of chiral dialdehyde with benzoylacetonitrile and malonitrile, leading to the formation of the 6/6/7 fused cyclic product in good yield. The proposed reaction mechanism involves a dual condensation/cyclization process, achieving the formation of up to six bonds for fused polycycles. The single crystal structure analysis revealed that the fused cyclic skeleton contains face-to-face naphthyl and cyanoalkenyl motifs, which act as the electronic donor and acceptor, respectively, potentially resulting in through-space charge transfer (TSCT) emission. While the TSCT emissions were weak in solution, a notable increase in luminescence intensity was observed upon aggregation, indicating bright fluorescent light. A series of theoretical analyses further supported the possibility of spatial electronic communication based on frontier molecular orbitals, the distance of charge transfer, and reduced density gradient analysis. This work not only provides guidance for the one-step synthesis of complex polycycles, but also offers valuable insights into the design of aggregation-enhanced TSCT emission materials.

Intramolecular Charge Transfer and Stimuli-Responsive Emission in Cholesterol-Appended Phenothiazine-Cyanostyryl-Based Donor-Acceptor Systems.

Organic fluorescent molecules have received considerable attention owing to their various optoelectronic applications. Herein, we report the design and synthesis of two cholesterol-functionalized cyanostyrene-phenothiazine-based D-π-A systems that are emissive in both the solution and solid states. The newly synthesized cholesterol-appended phenothiazine-cyanostyrene diads PTCS-1 and PTCS-2 vary in the N-alkylation of phenothiazine, respectively, with─octyl and─hexyl chains. Both molecules are highly fluorescent and show reasonably good quantum yields in nonpolar solvents because of twisted intramolecular charge transfer (TICT). The molecules exhibit aggregation-induced emission in the solid state. Due to the presence of flexible alkyl chains in the phenothiazine and cholesterol moieties, PTCS-1 and PTCS-2 show mechanochromic luminescence switching in response to external shear stress and emission recovery under methanol vapor. Powder X-ray diffraction studies prove that the emission switching on the applied stimuli in both PTCS-1 and PTCS-2 is attributed to the reversible transformation between the crystalline and amorphous states. Time-dependent density functional theory (TD-DFT) studies are carried out to gain insight into the ICT interactions. TD-DFT analysis at the TD-M06-2X/def2-TZVP level further revealed that in both molecules, the lowest unoccupied molecular orbital (LUMO) + 2, LUMO, highest occupied molecular orbital (HOMO), and HOMO - 1 orbitals are responsible for the charge transfer interactions. These ICT interactions are identified as π-π* type interactions.

Two-Photon-Responsive "TICT + AIE" Active Naphthyridine-BF2 Photoremovable Protecting Group: Application for Specific Staining and Killing of Cancer Cells.

The combined effects of twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE) phenomena have demonstrated a significant influence on excited-state chemistry. These combined TICT and AIE features have been extensively utilized to enhance photodynamic and photothermal therapy. Herein, we demonstrated the synergistic capabilities of TICT and AIE phenomena in the design of the photoremovable protecting group (PRPG), namely, NMe2-Napy-BF2. This innovative PRPG incorporates TICT and AIE characteristics, resulting in four remarkable properties: (i) red-shifted absorption wavelength, (ii) strong near-infrared (NIR) emission, (iii) viscosity-sensitive emission property, and (iv) accelerated photorelease rate. Inspired by these intriguing attributes, we developed a nanodrug delivery system (nano-DDS) using our PRPG for cancer treatment. In vitro studies showed that our nano-DDS manifested effective cellular internalization, specific staining of cancer cells, high-resolution confocal imaging of cancerous cells in the NIR region, and controlled release of the anticancer drug chlorambucil upon exposure to light, leading to cancer cell eradication. Most notably, our nano-DDS exhibited a substantially increased two-photon (TP) absorption cross section (435 GM), exhibiting its potential for in vivo applications. This development holds promise for significant advancements in cancer treatment strategies.

Bodipy-based dyes for ionic-liquid-based optical carbon dioxide sensors

When ionic liquids come in contact with CO2 gas, their polarity and viscosity increase. Owing to these properties, ionic liquids containing organic dyes, which can change their optical properties depending on their surrounding polarity and viscosity, have recently been applied in portable CO2 optical sensors. Although ionic–liquid-based CO2 optical sensors have excellent sensing properties, the optimal organic dyes for ionic–liquid-based CO2 optical gas sensors have not been extensively investigated. Hence, we synthesized and characterized four 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based dyes (PTPBDP, MTPBDP, TPABDP, and TPAMBDP) to explore the optimal structure for liquid-based CO2 optical sensors, with intramolecular charge transfer (ICT) and aggregation-induced emission (AIE) investigated. In addition, the synthesized dyes were applied to an ionic liquid, with their optical properties measured to analyze and compare their detection abilities and selectivity toward CO2 gas. As the CO2 concentration increased, all sensors, except TPAMBDP, showed changes in the optical properties in the visual and fluorescence channels. This approach facilitates the development of a portable sensor capable of detecting CO2 across diverse environmental conditions, showcasing its potential for versatile application in monitoring and analysis tasks. We believe these results provide insights into the design of organic dye structures for ionic liquid-based CO2 optical sensors.

A TICT-AIE activated dual-channel fluorescence-on probe to reveal the dynamics mechanosensing of lipid droplets during ferroptosis

Mechanical forces play a crucial role in cellular processes, including ferroptosis, a form of regulated cell death associated with various diseases. However, the mechanical aspects of organelle lipid droplets (LDs) during ferroptosis are poorly understood. In this study, we designed and synthesized a fluorescent probe, TPE-V1, to enable real-time monitoring of LDs' viscosity using a dual-channel fluorescence-on model (red channel at 617 nm and NIR channel at 710 nm). The fluorescent imaging of using TPE-V1 was achieved due to the integrated mechanisms of the twisted intramolecular charge transfer (TICT) and aggregation-induced emission (AIE). Through dual-emission channel fluorescence imaging, we observed the enhanced mechanical energy of LDs triggering cellular mechanosensing, including ferroptosis and cell deformation. Theoretical calculations confirmed the probe's behavior, showing that high-viscosity media prevented the rotation processes and restored fluorescence quenching in low viscosity. These findings suggest that our TICT-TPE design strategy provides a practical approach to study LDs' mechanical properties during ferroptosis. This development enhances our understanding of the interplay between mechanical forces and LDs, contributing to the knowledge of ferroptotic cell death and potential therapeutic interventions targeting dysregulated cell death processes.

Mechanofluorochromic behaviors and data security protection properties of salicylaldimine-based difluoroboron complexes with different aryl substituents.

To further explore the relationship between aryl substituents and mechanofluorochromic (MFC) behaviors, four salicylaldimine-based difluoroboron complexes (ts-Ph BF2, ts-Ph-NA BF2, ts-2NA BF2, and ts-triphenylamine [TPA] BF2), including aromatic substituents with different steric hindrance effects, were designed and successfully synthesized. Four complexes with twisted molecular conformation displayed intramolecular charge transfer and aggregation-induced emission properties. Under external mechanical stimuli, the as-synthesized powders of ts-Ph BF2, ts-Ph-NA BF2, and ts-TPA BF2 exhibited redshift fluorescence emission behaviors, and ts-Ph BF2 and ts-TPA BF2 could be recovered to original shifts by fuming, but ts-Ph-NA BF2 displayed irreversible switching. ts-2NA BF2 had no change during the grinding and fuming processes. The results indicated that the MFC behaviors could be attributed to the phase transformation between the well-defined crystalline and disordered amorphous states by X-ray diffraction measurement. Further research illustrated that ts-TPA BF2 with the most significant MFC phenomenon could be applied in data security protection in ink-free rewritable paper.

Study of Intrachain Charge Transfer in a Blue Emissive Polyfluorene Random Copolymer.

Photophysics of a blue light-emitting fluorescent random copolymer, consisting of arylated polydioctylfluorene (aryl-F8), polydioctylfluorene (F8), and amine comonomers in a ratio of 80:15:5 is reported. In a solution of 10-6 M, solvatochromism in absorption and photoluminescence (PL) is observed with an increased lifetime of PL as the polarity of the solvent increases. Dual fluorescence is observed in the 10-9 M diluted solution, comprising a structured emission from a localized state in the aryl-F8 comonomer and a broad emission peak from the charge-transfer (CT) state at a lower energy. Emission wavelength-dependent time-resolved photoluminescence studies in different polar media confirm the presence of the emissive intrachain CT state in this copolymer. Analyzing the PL decay kinetics, we calculated the formation rate of the intrachain CT state to be ∼3.0 × 109 s-1. Repopulation of the localized state from the CT state is observed in the lower polarity medium with a rate of 7 × 108 s-1, which is almost absent for the large Stokes-shifted CT emission in the higher polarity medium. Along with the fundamental understanding of the photophysics of the random copolymer, this study suggests that the emission spectrum can be tailored by the concentration of polymer and the polarity of surrounding media.

Bridge-induced taming of the visible electronic circular dichroism signatures of helicoBODIPYs

Helically-flexible bis(BODIPY)s based on chiral ethane-1,2-diamine or −1,2-diol bridges (named helicoBODIPYs) are an interesting family of accessible and tunable small multichromophoric organic dyes enabling visible electronic circular dichroism (ECD) and, even visible circularly polarized luminescence (CPL). However, their CPL brightness (BCPL) is still limited (up to ca. 10 M−1cm−1), due to low fluorescence efficiency owing to a non-optimal participation of intramolecular charge transfer (CT) emission. Herein, we analyze in depth the origin and factors controlling this CT, demonstrating the key participation of a BODIPY-to-BODIPY symmetry-breaking induced CT (SBCT), beyond the expected CT within each single BODIPY chromophore due to their push-pull character. It is also demonstrated that increasing the overcrowding at the flexible helicoBODIPY ethane bridges leads to BODIPY-BODIPY faced dispositions, promoting the SBCT and enabling the detection of both CT absorption and CT emission bands under specific conditions. Interestingly, this SBCT enhances the visible ECD (absolute gabs values), supporting the key role of the involved forbidden CT transitions in the chiroptical activity of these dyes, as well as the possibility of modulating their valuable visible chiroptical signatures (e.g., visible CPL brightness) by easily playing with steric factors at the dye bridge.

Tuning the Absorption, Fluorescence, Intramolecular Charge Transfer, and Intersystem Crossing in Spiro[fluorene]acridinone.

In this work, we prepared a series of electron donor-acceptor systems based on spiro[fluorene-9,7'-dibenzo[c,h]acridine]-5'-one (SFDBAO). Our SFDBAOs consist of orthogonally positioned fluorenes and aromatic ketones. By fine-tuning the substitution of electron-donating pyrenes, the complex interplay among different excited-state decay channels and the overall impact of solvents on these decay channels were uncovered. Placing pyrene, for example, at the aromatic ketones resulted in a profound solvatochromism in the form of a bright charge-transfer (CT) emission spanning from yellow to red-NIR. In contrast, a dark non-emissive CT was noted upon pyrene substitution at the fluorenes. In apolar solvents, efficient triplet-excited state generation was observed for all SFDBAOs. Either charge transfer was concluded to mediate the intersystem crossing (ISC) in the case of pyrene substitution or the El-Sayed rule was applicable when lacking pyrene substitution as in the case of SFABAO. In polar solvents, charge separation is the sole decay upon pyrene substitution. Moreover, competition between ISC and CT lowered the triplet-excited state generation in SFDBAO.