BODIPY-Based Semiconducting Materials for Organic Bulk Heterojunction Photovoltaics and Thin-Film Transistors

Interplay between Theory and Photophysical Characterization in Symmetric α-Substituted Thienyl BODIPY Molecule

4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based molecules have emerged as interesting materials for optoelectronic applications due to the possibility to easily fine-tune their photophysical and optical properties, dominated by two main absorption bands in the visible range. However, no studies have been reported on the nature of these spectral features. By means of ultrafast spectroscopy, we detect intramolecular energy transfer in a spin-coated film of di-thieno-phenyl BODIPY (DTPBDP) dispersed in a polystyrene matrix after pumping the high-energy absorption band. The same effect is not present upon pumping the lowest-energy band, which instead allows the achievement of efficient amplified spontaneous emission. Density functional calculations indicate the different nature of the two main absorption bands, explaining their different photophysical behavior.

Structural modification of A-C-A configured X-PCIC acceptor molecule for efficient photovoltaic properties with low energy loss in organic solar cells

Modification of terminal acceptors of non-fullerene organic solar cell molecule with different terminal acceptors can help in screening of molecules to develop organic photovoltaic cells with improved performance. Thus, in this work, seven new molecules with an unfused core have been designed and thoroughly investigated. DFT/TD-DFT simulations were performed on studied molecules to explore the ground and excited state characteristics. UV-Visible analysis revealed the red shift in the absorption spectrum (reaching 781 nm) owing to their smaller energy gap up to 1.94 eV. Furthermore, transition density matrix analysis demonstrated that peripheral acceptors extract the electron density from the core effectively. The effectiveness of our investigated molecules as materials for high-performing organic photovoltaic cells has been shown by an examination of their electron and hole mobilities for fast charge transfer. When combined with PTB7-Th, all molecules displayed high open circuit voltage. XP5 molecule exhibited highest open circuit voltage (1.70 eV) and lowest energy loss of 0.30 eV. All designed molecules exhibit the improved aforementioned parameters, which shows that these molecules can be used to develop competent solar devices in future.

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Highly crystalline and fluorescent BODIPY-labelled phenyl-triazole-coumarins as n-type semiconducting materials for OFET devices

In this work, the synthesis of BODIPY-phenyl-triazole labelled coumarins (BPhTCs) using a two-step approach is described. The influence of the BODIPY appending on the photophysical, electrochemical and thermal properties of the phenyl-triazole-coumarin precursors (PhTCs) was investigated. Band gap energies were measured by absorption spectroscopy (2.20 ± 0.02 eV in the solid and 2.35 ± 0.01 eV in solution) and cyclic voltammetry (2.10 ± 0.05 eV). The results are supported by DFT calculations confirming the presence of lowest LUMO levels that facilitate the electron injection and stabilize the electron transport. Their charge-transport parameters were measured in Organic Field-Effect Transistor (OFET) devices. BPhTCs showed an ambipolar transistor behavior with good n-type charge mobilities (10-2 cm2V-1s-1) allowing these derivatives to be employed as promising semiconducting crystalline and fluorescent materials with good thermal and air stability up to 250 °C.

BODIPY-Based Molecules for Biomedical Applications

BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives have attracted attention as probes in applications like imaging and sensing due to their unique properties like (1) strong absorption and emission in the visible and near-infrared regions of the electromagnetic spectrum, (2) strong fluorescence and (3) supreme photostability. They have also been employed in areas like photodynamic therapy. Over the last decade, BODIPY-based molecules have even emerged as candidates for cancer treatments. Cancer remains a significant health issue world-wide, necessitating a continuing search for novel therapeutic options. BODIPY is a flexible fluorophore with distinct photophysical characteristics and is a fascinating drug development platform. This review provides a comprehensive overview of the most recent breakthroughs in BODIPY-based small molecules for cancer or disease detection and therapy, including their functional potential.

Synthesis, Properties, and Semiconducting Characteristics of Bisbenzothieno[b]-Fused BODIPYs

A novel family of bisbenzothieno[b]-fused BODIPYs containing seven fused aromatic rings has been developed from readily available benzothieo[3,2-b]pyrroles through an efficient two-step synthetic route, exhibiting planar skeletons with excellent photostabilities, deep-red absorptions, and near-infrared emissions (up to 753 nm). Importantly, the thin-film transistors based on BTB with a meso-dimethylamino-phenyl group exhibit unipolar n-type charge transporting characteristics with a high electron mobility of 0.013 cm2 V-1 s-1.

Functionalization of BODIPY Dyes with Additional C-N Double Bonds and Their Applications

BODIPY (4-bora-3a,4a-diaza-s-indacene) dyes are regarded as highly useful compounds due to their rich photophysical properties, stability, and ease of functionalization. In recent years, hot topics studied with this class of compounds are targeted photodynamic therapy, photothermal therapy, fluorescent bioimaging agents, structural modification of the BODIPY core, synthesis of BODIPY analogs, and BODIPY-based supramolecular constructs. This review covers the advances in BODIPY structures substituted with additional carbon-nitrogen double bonds, namely imines, hydrazones, oximes, and related derivatives for various applications. Works based on fluorescent indicators of anions, cations, and neutral molecules are included in this review. In addition, the use of such structures for pharmaceutical applications, photodynamic therapy, fluorescent switches, and fluorescent building blocks are also investigated. In addition to covering the major literature within the mentioned subclass, design principles, working mechanisms, and outlooks are also provided to enlighten forthcoming promising efforts. With this work, we aim to provide insights about the synthesis, photophysical properties, contribution of C=N bonds to a class of dye, and possible areas of use and stimulate researchers to present new ideas and overcome the current problems using these derivatives.

Simple and Efficient Acceptor-Donor-Acceptor-Type Non-fullerene Acceptors for a BODIPY-Thiophene-Backboned Polymer Donor for High-Performance Indoor Photovoltaics

Herein, simple acceptor-donor-acceptor (A-D-A)-type small molecules denoted as DICTF and DRCTF with modification in terminal units were synthesized and used as electron acceptors. With the tuning of the electron-withdrawing units in electron acceptors, their photovoltaic properties were investigated when combined with low-band-gap BODIPY-thiophene-backboned donor material, named P(BdP-HT). The P(BdP-HT):DICTF-based organic solar cells (OSCs) displayed excellent efficiency of around 11.94%, which is superior to the P(BdP-HT):DRCTF counterpart (8.78%). Although the open-circuit voltage (V_OC) of the P(BdP-HT):DRCTF-based OSC is greater than that for the P(BdP-HT):DICTF counterpart, the rise in the short-circuit current density (J_SC) may be attributed to the fact that the P(BdP-HT):DICTF blend displayed impressive panchromatic absorption compared to P(BdP-HT):DRCTF. The improved fill factor (FF) is responsible for the balanced transport of charges in the P(BdP-HT):DICTF-based device. Moreover, the P(BdP-HT):DRCTF- and P(BdP-HT):DICTF-based OSCs showed 17.68 and 21.84%, respectively, under indoor illumination (1000 lx). To the best of our observation, this might be the first report on BODIPY-based donors with power conversion efficiency (PCE) of 21.84% under indoor illumination conditions.

Tuning the Properties of Donor-Acceptor and Acceptor-Donor-Acceptor Boron Difluoride Hydrazones via Extended π-Conjugation

Molecular materials with π-conjugated donor-acceptor (D-A) and acceptor-donor-acceptor (A-D-A) electronic structures have received significant attention due to their usage in organic photovoltaic materials, in organic light-emitting diodes, and as biological imaging agents. Boron-containing molecular materials have been explored as electron-accepting units in compounds with D-A and A-D-A properties as they often exhibit unique and tunable optoelectronic and redox properties. Here, we utilize Stille cross-coupling chemistry to prepare a series of compounds with boron difluoride hydrazones (BODIHYs) as acceptors and benzene, thiophene, or 9,9-dihexylfluorene as donors. BODIHYs with D-A and A-D-A properties exhibited multiple reversible redox waves, solid-state emission with photoluminescence quantum yields up to 10%, and aggregation-induced emission (AIE). Optical band gaps (or highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps) determined for these compounds (2.02-2.25 eV) agree well with those determined from cyclic voltammetry experiments (2.05-2.42 eV). The optoelectronic properties described herein are rationalized with density functional theory calculations that support the interpretation of the experimental findings. This work provides a foundation of understanding that will allow for the consideration of D-A and A-D-A BODIHYs to be incorporated into applications (e.g., organic electronics) where fine-tuning of band gaps is required.

Halogenated BODIPY photosensitizers: Photophysical processes for generation of excited triplet state, excited singlet state and singlet oxygen

We have systematically examined the formation of singlet oxygen O₂(¹Δ_g), the excited triplet state (T₁), and excited singlet state (S₁) for halogenated BODIPY photosensitizers (halogen = Cl, Br, and I) in eight solvents to understand how halogen atoms and solvent affect these properties. The phosphorescence spectra and lifetimes of singlet oxygen generated by these halogenated BODIPYs have been measured by steady state/time resolved NIR emission, while the formation quantum yield of singlet oxygen (Φ_Δ) has been determined by chemical method using diphenylisobenzofuran (DPBF) as the trapping agent. The formation quantum yield Φ_Δ of singlet oxygen can be as high as 0.96 for iodinated BODIPY and 0.71 for brominated BODIPY. The triplet state T₁ absorption spectra of brominated and iodinated BODIPYs have been recorded by laser flash photolysis method, in which T₁ shows high formation efficiency and long lifetime. The formation and decay of excited singlet state S₁ of four BODIPYs have been measured by ground state (S₀) absorption and steady state/time resolved fluorescence. The results show that larger halogen atoms on BODIPY core lead to smaller fluorescence quantum yield, shorter fluorescence lifetime and higher singlet oxygen formation quantum yield due to heavy atom effect that promotes the formation of triplet state. On the other hand, higher solvent polarity causes lower singlet oxygen formation quantum yield, smaller fluorescence quantum yield, and shorter fluorescence lifetime. This solvent effect is explained by the presence of photoinduced charge transfer (ICT) process from halogen atoms to BODIPY. The ICT efficiency has been estimated and the results are agreed with ICT theory. ICT process in halogenated BODIPYs has never been revealed in literature. HOMO/LUMO obtained from DFT calculation also supports the presence of ICT. The involvement of ICT in the photosensitizing process of halogenated BODIPYs provides new insights for designing BODIPY photosensitizers for photodynamic therapy of tumor.

BODIPY Cored A-D-A'-D-A Type Nonfused-Ring Electron Acceptor for Efficient Polymer Solar Cells

In this work, boron dipyrromethene (BODIPY) is for the first time employed as electron-deficient core (A') to construct an A-D-A'-D-A type nonfused-ring electron acceptor (NFREA) for polymer solar cells (PSCs). Among, cyclopentadithiophene (CPDT) and fluorinated dicyanoindanone (DFIC) are involved as electron-donating (D) bridges and terminal A groups, respectively. Bearing with the steric BODIPY core, tMBCIC exhibits twisted configuration with dihedral angles >45^o  between BODIPY and CPDT bridges. Thus, compared with the BODIPY-free planar A-D-D-A structured bCIC, reduced aggregation, weakened intramolecular D-A interactions with up-shifted LUMO by 0.4 eV as well as blue-shifted absorption by up to 150 nm is observed in tMBCIC. Moreover, owing to the intrinsic large molar extinction coefficient from BODIPY, promoted light-harvest ability is achieved for tMBCIC, particularly in its blend films. Therefore, PSCs by using PBDB-T as donor, tMBCIC as NFREA afford superior power conversion efficiency (PCE) of 9.22% and higher open-circuit voltage (V_oc ) of 0.954 V compared to 4.47% and 0.739 V from bCIC-devices. Moreover, compared to other BODIPY-flanked electron acceptors (<5%) reported so far, BODIPY-cored tMBCIC realizes a remarkable progress in PCE. This article is protected by copyright. All rights reserved.

Novel benzo[b]thieno[2,3-d]thiophene derivatives with an additional alkyl-thiophene core: synthesis, characterization, and p-type thin film transistor performance

Newly synthesized benzo[b]thieno[2,3-d]thiophene derivatives were employed as active layers of organic field effect transistors, and these transistors showed decent electrical performance.

Development of Dithieno[3,2-b:2′,3′-d]thiophene (DTT) Derivatives as Solution-Processable Small Molecular Semiconductors for Organic Thin Film Transistors

Novel solution-processable dithieno[3,2-d:2′,3′-d]thiophene (DTT) derivatives with alkylated thiophene or alkyl chain substituents, 2,6-bis(5-octylthiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 1), 2,6-bis(5-(2-ethylhexyl)thiophen-2-yl)dithieno[3,2-b:2′,3′-d]thiophene (compound 2), and 2,6-dioctyldithieno[3,2-b:2′,3′-d]thiophene (compound 3), have been synthesized and employed as small molecular organic semiconductors for organic field-effect transistors (OFETs). All compounds exhibited good thermal stability over 290 °C, while different side groups of DTT compounds afforded different melting temperatures. The molecular orbital energy levels were experimentally and theoretically calculated, and their trend was almost the same. The developed compounds were employed as active layers for top-contact/bottom-gate OFETs with average charge carrier mobility as high as 0.10 cm2/Vs and current on/off ratio > 107 in ambient atmosphere. Notably, DTT derivative with linear alkyl chain (-octyl) substituents showed the best device performance. High device performance could be attributed to the large grains and continuous surface coverages as well as high film texture of the corresponding semiconductor films.

Modulating the Photophysical Properties and Electron Transfer Rates in Diketopyrrolopyrrole-Based Coordination Polymers

Molecular self-assembly through noncovalent interactions is a particularly efficient approach to fine-tune the optoelectronic and photophysical properties of electroactive materials. In metal-ligand coordination polymers, the final properties of the assemblies are directly related to the nature of the metal-ligand interaction. To probe for such influence on the photophysical properties of electroactive materials, a series of coordination polymers based on a well-known organic dye, diketopyrrolopyrrole, was prepared through coordination of a terpyridine-containing monomer with various metal sources, including iron, cobalt, zinc, and manganese. The resulting supramolecular polymers were characterized through multiple techniques, including UV-vis and fluorescence spectroscopy, time-correlated single-photon counting, and femtosecond transient absorption spectroscopy to reveal the impact of the metal source on the final photophysical properties of coordination polymers. As expected, important variations were found between different coordination polymers in terms of absorption, fluorescence kinetics, and electron transfer rate. While iron and cobalt-containing polymers showed ultrafast electrons transfer rates, assemblies from manganese were shown to be much less efficient, confirming the importance of metal centers. This detailed fundamental study unravels some important relationships between metal-ligand interactions, supramolecular self-assembly, and photophysical properties, ultimately leading to new avenues for the design of functional polymers based on organic dyes.

The TDDFT Excitation Energies of the BODIPYs; The DFT and TDDFT Challenge Continues

The derivatives of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) are pivotal ingredients for a large number of functional, stimuli-responsive materials and therapeutic molecules based on their photophysical properties, and there is a urgent need to understand and predict their optical traits prior to investing a large amount of resources in preparing them. Density functional theory (DFT) and time-dependent DFT (TDDFT) computations were performed to calculate the excitation energies of the lowest-energy singlet excited state of a large series of common BODIPY derivatives employing various functional aiming at the best possible combination providing the least deviations from the experimental values. Using the common "fudge" correction, a series of combinations was investigated, and a methodology is proposed offering equal or better performances than what is reported in the literature.

Dual Emission of meso-Phenyleneethynylene-BODIPY Oligomers: Synthesis, Photophysics, and Theoretical Optoelectronic Study

Two series of 2,5-di(butoxy)phenyleneethynylenes, one halogenated (nPEC4-X; n=2, 3, or 4) and the other boron-dipyrromethene (BODIPY) terminated (nPEC4-By; n=3, 4, or 5; By=BODIPY), were synthesized monodirectionally by the step-by-step approach and the molecular structure was corroborated by NMR spectroscopy (¹ H, ¹³ C-DEPTQ-135, COSY, HSQC, HMBC, ¹¹ B, ¹⁹ F) and MALDI-TOF mass spectrometry. The multiplicity and J-coupling constants of ¹ H, ¹¹ B, and ¹⁹ F/¹¹ B NMR signals revealed, in the nPEC4-By series, that the phenyl in the meso position of BODIPY becomes electronically part of the conjugation of the phenyleneethynylene chain, whereas BODIPY is electronically isolated. The photophysical, electrochemical, and theoretical studies confirm this finding because the properties of nPEC4-By are comparable to those of the nPEC4-X oligomers and BODIPY, indicating negligible electron communication between BODIPY and the nPEC4 moieties. Nevertheless, energy transfer (ET) from nPEC4 to BODIPY was rationalized by spectroscopy and theoretical calculations. Its yield decreases with the nPEC4 conjugation length, according to the increase in distance between the two chromophores, resulting in dual emission for the longest oligomer in which ET is quenched.

BODIPY-Based Molecules, a Platform for Photonic and Solar Cells

The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based molecules have emerged as interesting material for optoelectronic applications. The facile structural modification of BODIPY core provides an opportunity to fine-tune its photophysical and optoelectronic properties thanks to the presence of eight reactive sites which allows for the developing of a large number of functionalized derivatives for various applications. This review will focus on BODIPY application as solid-state active material in solar cells and in photonic devices. It has been divided into two sections dedicated to the two different applications. This review provides a concise and precise description of the experimental results, their interpretation as well as the conclusions that can be drawn. The main current research outcomes are summarized to guide the readers towards the full exploitation of the use of this material in optoelectronic applications.

Synthesis and Opto-electronic Properties of BODIPY o-OPhos Systems

Herein, we report the versatile synthetic strategy and opto-electronic properties for the phosphorylation of BODIPY derivatives 5aa-5ak by substituting with an electron-donating/withdrawing group at the ortho position. Nevertheless, this new methodology relatively promotes the tolerance of the aldehyde moiety and the high yield for the synthesis of BODIPY o-OPhos derivatives. The photophysical studies suggest improved optical properties due to the inductive effect of various electron-donating/withdrawing groups. The UV-visible and the emission data suggest that BODIPY o-OPhos derivatives emphasize the property of the excited states with an increase in fluorescence intensity and high quantum yields due to the presence of bulky phospsho-triester at the meso- position which hinders the free rotation around the C-Ar bond and facilitates the development of OLEDs and various organophosphorus warfare agents. Electrochemical studies reveal 5ak depicts the ease of redox activity amongst the 5aa-5ak derivatives. The density functional theory indicates the highest occupied molecular orbital on the BODIPY moiety whereas the lowest unoccupied molecular orbital delocalized on BODIPY and the phospho-triester moieties. Thus, the unique development of the novel BODIPY derivatives with improved optical and redox properties pave the way for fluorescent probes and bioimaging techniques.

Synthesis and Photovoltaic Properties of Boron β-Ketoiminate Dyes Forming a Linear Donor-π-Acceptor Structure

Organoboron complexes are of interest as chromophores for dye sensitizers owing to their light-harvesting and carrier-transporting properties. In this study, compounds containing boron β-ketoiminate (BKI) as a chromophore were synthesized and used as dye sensitizers in dye-sensitized solar cells. The new dyes were orange or red crystals and showed maximum absorptions in the 410-450 nm wavelength region on titanium dioxide substrates. These electrodes exhibited maximum efficiencies of over 80% in incident photon-to-current conversion efficiency spectra, suggesting that the continuous process of light absorption-excitation-electron injection was effectively performed. Open-circuit photovoltages were relatively high owing to the large dipole moments of the BKI dyes with a linear molecular structure. Thus, a maximum power conversion efficiency of 5.3% was successfully observed. Comparison of BKI dyes with boron β-diketonate dyes revealed certain differences in solution stability, spectral properties, and photovoltaic characteristics.

Improved Performance of Ternary Solar Cells by Using BODIPY Triads

Two boron dipyrromethene (BODIPY) triads, namely BODIPY-1 and BODIPY-2, were synthesized and incorporated with poly-3-hexyl thiophene: (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) P3HT:PCBM. The photovoltaic performance of BODIPY:P3HT:PCBM ternary solar cells was increased, as compared to the control binary solar cells (P3HT:PCBM). The optimized power conversion efficiency (PCE) of BODIPY-1:P3HT:PCBM was improved from 2.22% to 3.43%. The enhancement of PCE was attributed to cascade charge transfer, an improved external quantum efficiency (EQE) with increased short circuit current (Jsc), and more homogeneous morphology in the ternary blend.

Pyridyl-Anchored Type BODIPY Sensitizer-TiO2 Photocatalyst for Enhanced Visible Light-Driven Photocatalytic Hydrogen Production

Dye-sensitized photocatalytic hydrogen production using a boron-dipyrromethene (BODIPY) organic material having a pyridyl group at the anchor site was investigated. Phenyl, carbazole, and phenothiazine derivatives were introduced into BODIPY dyes, and their photocatalytic activities were examined. Identification was performed by nuclear magnetic resonance (NMR), infrared (IR), mass (MS) spectra, and absorption spectra, and catalyst evaluation was performed by using visible-light irradiation and photocatalytic hydrogen production and photocurrent. These dyes have strong absorption at 600–700 nm, suggesting that they are promising as photosensitizers. When the photocatalytic activity was examined, stable catalytic performance was demonstrated, and the activity of the Pt-TiO2 photocatalyst carrying a dye having a carbazole group was 249 μmol/gcat·h. Photocurrent measurements suggest that dye-sensitized photocatalytic activity is occurring. This result suggests that BODIPY organic materials with pyridyl groups as anchor sites are useful as novel dye-sensitized photocatalysts.

Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes

(aza-)BODIPY dyes (boron dipyrromethene dyes) are well-established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza-)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the β-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH2 -rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT-Calculations confirm the [12+2] π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm-1 in BODIPY versus >4170 cm-1 in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.

A Solution-Processable meso-Phenyl-BODIPY-Based n-Channel Semiconductor with Enhanced Fluorescence Emission

The molecular design, synthesis, and characterization of an acceptor-donor-acceptor (A-D-A) semiconductor BDY-Ph-2T-Ph-BDY comprising a central phenyl-bithiophene-phenyl π-donor and BODIPY π-acceptor end-units is reported. The semiconductor shows an optical band gap of 2.32 eV with a highly stabilized HOMO/LUMO (-5.74 eV/-3.42 eV). Single-crystal X-ray diffraction (XRD) reveals D-A dihedral angle of ca. 66° and strong intermolecular "C-H ⋅⋅⋅ π (3.31 Å)" interactions. Reduced π-donor strength, increased D-A dihedral angle, and restricted intramolecular D-A rotations allows for both good fluorescence efficiency (Φ_F =0.30) and n-channel OFET transport (μ_e =0.005 cm² /V ⋅ s; I_on /I_off =10⁴ -10⁵ ). This indicates a much improved (6-fold) fluorescence quantum yield compared to the meso-thienyl BODIPY semiconductor BDY-4T-BDY. Photophysical studies reveal important transitions between locally excited (LE) and twisted intramolecular charge-transfer (TICT) states in solution and the solid state, which could be controlled by solvent polarity and nano-aggregation. This is the first report of such high emissive characteristics for a BODIPY-based n-channel semiconductor.