Origin of intersystem crossing in highly distorted organic molecules: a case study with red light-absorbing N,N,O,O-boron-chelated Bodipys

To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, Φ_Δ = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔE_S1/T1 = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which Φ_Δ is increased to 40%. The improved ISC yield is rationalized by the presence of a T₂ state, localized on the anthryl unit, with energy close to that of the S₁ state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the T_z sublevel of the T₁ state overpopulated. The small zero-field splitting D parameter (-1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S₁/T_n energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.

Application of meso-CF3-Fluorophore BODIPY with Phenyl and Pyrazolyl Substituents for Lifetime Visualization of Lysosomes

A bright far-red emitting unsymmetrical meso-CF₃-BODIPY fluorescent dye with phenyl and pyrazolyl substituents was synthesized by condensation of trifluoropyrrolylethanol with pyrazolyl-pyrrole, with subsequent oxidation and complexation of the formed dipyrromethane. This BODIPY dye exhibits optical absorption at λ_ab ≈ 610-620 nm and emission at λ_em ≈ 640-650 nm. The BODIPY was studied on Ehrlich carcinoma cells as a lysosome-specific fluorescent dye that allows intravital staining of cell structures with subsequent real-time monitoring of changes occurring in the cells. It was also shown that the rate of uptake by cells, the rate of intracellular transport into lysosomes, and the rate of saturation of cells with the dye depend on its concentration in the culture medium. A concentration of 5 μM was chosen as the most suitable BODIPY concentration for fluorescent staining of living cell lysosomes, while a concentration of 100 μM was found to be toxic to Ehrlich carcinoma cells.

Radiative lifetime of a BODIPY dye as calculated by TDDFT and EOM-CCSD methods: solvent and vibronic effects

The radiative emission lifetime and associated S₁ excited state properties of a BODIPY dye are investigated with TDDFT and EOM-CCSD calculations. The effects of a solvent are described with the polarizable continuum model using the linear response (LR) approach as well as state-specific methods. The Franck-Condon (FC), Herzberg-Teller (HT) and Duschinsky vibronic effects are evaluated for the absorption and emission spectra, and for the radiative lifetime. The transition energies, spectra shapes and radiative lifetime are assessed with respect to experimental results. It is found that the TDDFT transition energies are overestimated by about 0.4-0.5 eV, whereas EOM-CCSD improves the vertical emission energy by about 0.1 eV in comparison to TDDFT. The solvatochromic and Stokes shifts are better reproduced by the state-specific solvation methods, which show that these methods are more suited than the LR model to describe the solvent effects on the BODIPY dye. The vibronic effects lead to an increase of the radiative lifetime of about 0.4 to 1.0 ns depending on the theoretical approach, which highlights the importance of such effects. Moreover, the HT effects are negligible on both the spectra and lifetime, which demonstrates that the FC approximation is accurate for the BODIPY dye. Finally, the comparison with experimental data shows that the radiative lifetimes predicted by EOM-CCSD and TDDFT have comparable accuracy.

Optical and electrochemical effects of triarylamine inclusion to alkoxy BODIPY-based derivatives

Three new triphenylamine-BODIPY dyads BDPT1–3 have been designed and synthesized.

A Time-Resolved Spectroscopic Investigation of a Novel BODIPY Copolymer and Its Potential Use as a Photosensitiser for Hydrogen Evolution

A novel 4,4-difuoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) copolymer with diethynylbenzene has been synthesised, and its ability to act as a photosensitiser for the photocatalytic generation of hydrogen was investigated by time-resolved spectroscopic techniques spanning the ps- to ns-timescales. Both transient absorption and time-resolved infrared spectroscopy were used to probe the excited state dynamics of this photosensitising unit in a variety of solvents. These studies indicated how environmental factors can influence the photophysics of the BODIPY polymer. A homogeneous photocatalytic hydrogen evolution system has been developed using the BODIPY copolymer and cobaloxime which provides hydrogen evolution rates of 319 μmol h⁻¹ g⁻¹ after 24 h of visible irradiation.

The odyssey of cobaloximes for catalytic H2 production and their recent revival with enzyme-inspired design

Cobaloxime complexes gained attention for their intrinsic ability of catalytic H₂ production despite their initial emergence as a vitamin B12 model. The simple, robust, and synthetically manoeuvrable cobaloxime core represents a model catalyst molecule for the investigation of optimal conditions for both photo- and electrocatalytic H₂ production catalytic assemblies. Cobaloxime is one of the rare catalysts that finds equal applications in the analysis of homogeneous and heterogeneous catalytic conditions. However, the poor aqueous solubility and long-term instability of cobaloximes have severely impeded their growth. Lately, interest in the cobaloxime-based catalysts has been resuscitated with the rational use of extended enzymatic features. This unique enzyme-inspired catalyst design strategy has instigated the formation of a new genre of cobaloxime molecules that exhibit enhanced photo- and electrocatalytic H₂ evolution with improved aqueous and air stability.

Orthogonal Supramolecular Assembly Triggered by Inclusion and Exclusion Interactions with Cucurbit[7]uril for Photocatalytic H2 Evolution

The fabrication of efficient and convenient photocatalytic H₂ evolution systems is a fascinating research topic in the field of solar energy conversion. A ternary self-assembled photocatalytic H₂ evolution system was fabricated through supramolecular host-guest chemistry. The system consisted of the H₂ evolution catalyst [Co(dmgH)₂ (4-ppy)₂ ]NO₃ (1; dmgH₂ =dimethylglyoxime, 4-ppy=4-phenylpyridine) and the photosensitizer Eosin Y (EY) assembled with the macrocyclic compound cucurbit[7]uril (CB[7]) to form the 1@CB[7]/EY complex through inclusion and exclusion interactions, respectively. The synchronous self-assembly drives an orthogonal arrangement of the 1@CB[7]/EY system. The inclusion complex 1@CB[7] was successfully characterized by ¹ H NMR spectroscopy and single-crystal XRD. The exclusion process of CB[7] with EY was identified by NMR titration and the optimized geometry of the exclusion structure was determined by DFT calculations. The use of CB[7] resulted in a 6-fold increase in turnover number, a 3-fold increase in turnover frequency, and a 3-fold extension of lifetime for photocatalytic H₂ evolution as compared with the system in the absence of CB[7]. The improvement of the light-driven H₂ evolution activity was ascribed to the ability of CB[7] to link the photosensitizer and catalyst.

Conjugated nanoporous polycarbazole bearing a cobalt complex for efficient visible-light driven hydrogen evolution

A conjugated nanoporous polycarbazole (CNP) cross-linked by pyridine and coordinated to Co(iii) displays high catalytic performance for visible light-driven H₂ generation.

Tethered sensitizer-catalyst noble-metal-free molecular devices for solar-driven hydrogen generation

Inspired by natural photosynthesis in an organized assembly, compact H2-evolving molecular devices, which tether sensitizer and catalyst modules in one single molecule, present an opportunity to overcome the diffusion limit required for multi-component molecular systems, and increase intramolecular electron transfer rates from the photoactivated unit to the catalytic center to improve H2-evolving efficiency. Thereinto absolutely noble-metal free H2-evolving molecular devices are of particular interest because they don't contain precious and scarce noble-metal based components. This Frontier article focuses specifically on the recent advances in the design, synthesis, and photocatalytic properties of all-abundant-element molecular devices for photoinduced H2 generation via intramolecular processes. Some challenges and suggestions for future directions in this field are also illustrated.

Photo-Induced Charge Separation vs. Degradation of a BODIPY-Based Photosensitizer Assessed by TDDFT and RASPT2

A meso-mesityl-2,6-iodine substituted boron dipyrromethene (BODIPY) dye is investigated using a suite of computational methods addressing its functionality as photosensitizer, i.e., in the scope of light-driven hydrogen evolution in a two-component approach. Earlier reports on the performance of the present iodinated BODIPY dye proposed a significantly improved catalytic turn-over compared to its unsubstituted parent compound based on the population of long-lived charge-separated triplet states, accessible due to an enhanced spin-orbit coupling (SOC) introduced by the iodine atoms. The present quantum chemical study aims at elucidating the mechanisms of both the higher catalytic performance and the degradation pathways. Time-dependent density functional theory (TDDFT) and multi-state restricted active space perturbation theory through second-order (MS-RASPT2) simulations allowed identifying excited-state channels correlated to iodine dissociation. No evidence for an improved catalytic activity via enhanced SOCs among the low-lying states could be determined. However, the computational analysis reveals that the activation of the dye proceeds via pathways of the (prior chemically) singly-reduced species, featuring a pronounced stabilization of charge-separated species, while low barriers for carbon-iodine bond breaking determine the photostability of the BODIPY dye.

Investigating the excited state optical properties and origin of large stokes shift in Benz[c,d]indole N-Heteroarene BF2 dyes with ab initio tools

Density Functional Theory and Time Dependent Density Functional Theory computations reveal experimental trends accurately and provide an insight into the origin of red shifted optical spectra, large Stokes shift, nonlinear optical responses and quantitative description of the singlet-triplet energy gap of recently synthesized highly fluorescent membrane permeable benz[c,d]indole N-heteroarene BF₂ rigidified BBN and BBC dyes. Our results benchmark the importance of the DFT functionals and polarization functions using the accurate basis set for the calculations of excitation energies, band gaps, dipole moment, non-linear optical properties and geometrical parameters. The anomalous increase in dipole moment in the excited state and pronounced geometric distortion are found to be responsible for the increased Stokes shift in BBN dyes. The resultant decrease in the HOMO-LUMO energy gap was found to be responsible for the red shift in BBC dyes. Enhanced nonlinear optical properties and inverse relationship between the Stokes shift and static hyperpolarizability were found in these dyes. Their possible potential therapeutic use as a photosensitizer in photodynamic therapy is proposed on the basis of vertical triplet energies resulting in the range of 0.78 to 0.88eV. The singlet-triplet energetic gaps suggest that the simple variation of the N-heteroarenes or their post functionalization can lead to the efficient generation of singlet oxygen. Further, we compared these new class of dyes with conventional aza-BODIPY and BODIPY dyes, thereby confirming their use as alternative promising candidates for nonlinear optics, triplet sensitizers, and imaging.

BODIPYs for Dye-Sensitized Solar Cells

BODIPY, abbreviation of boron-dipyrromethene, is one class of robust organic molecules that has been used widely in bioimaging, sensing, and logic gate design. Recently, BODIPY dyes have been explored for dye-sensitized solar cells (DSCs). Studies demonstrate their potential as light absorbers for the conversion of solar energy to electricity. However, their photovoltaic performance is inferior to many other dyes, including porphyrin dyes. In this review, several synthetic strategies of BODIPY dyes for DSCs and their further functionalization are described. The photophysical properties of dye molecules and their photovoltaic performances in DSCs are summarized. We aim to provide readers a clear picture of the field and expect to shed light on the next generation of BODIPY dyes for their applications in solar energy conversion.

Novel distyryl BODIPY–fullerene dyads: preparation, characterization and photosensitized singlet oxygen generation efficiency

Novel types of mono- and bis-distyryl-BODIPY–fullerene dyads, 6 and 7, were prepared by Bingel cyclopropanation.

Synthesis of a dibenzo-BODIPY-incorporating phenothiazine dye as a panchromatic sensitizer for dye-sensitized solar cells

The introduction of dibenzo-BODIPY into a phenothiazine dye led to the production of a panchromatic sensitizer for dye-sensitized solar cells, a cell with the dye showing a PCE (η) value of 7.69%.

Demonstrating the role of anchoring functionality in interfacial electron transfer dynamics in the newly synthesized BODIPY–TiO2 nanostructure composite

Scheme illustrating the extent of coupling and charge recombination dynamics between BODIPY and NS-TiO₂ anchoring through the catechol and resorcinol binding group.

Photocatalytic water reduction from a noble-metal-free molecular dyad based on a thienyl-expanded BODIPY photosensitizer

A noble-metal-free molecular dyad was constructed by anchoring a thienyl-expanded BODIPY photosensitizer (PS) to a cobaloxime catalyst, which gives a 2.5-fold increase in the TON, and a 3-fold enhancement in the quantum efficiency as compared to the multicomponent catalytic system for the generation of hydrogen via the reduction of water. The stability of PS was expected to improve by introducing the thienyl moiety into the BODIPY core.

Light-driven generation of hydrogen: New chromophore dyads for increased activity based on Bodipy dye and Pt(diimine)(dithiolate) complexes

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF2) dye and a platinum diimine dithiolate (PtN2S2) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN2S2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN2S2 moieties), indicating little electronic coupling between them. Connection to TiO2 nanoparticles is carried out by sonication leading to in situ attachment to TiO2 without prior hydrolysis of the ester linking groups to acids. For H2 generation studies, the TiO2 particles are platinized (Pt-TiO2) so that the light absorber (the dyad), the electron conduit (TiO2), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H2O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO2 via a phosphonate or carboxylate attachment shows excellent light-driven H2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼ 40,000 turnover numbers of H2 over 12 d (with respect to dye).

The triplet excited state of Bodipy: formation, modulation and application

The accessing of the triplet excited state of one of the most popular fluorophores, boron-dipyrromethene (Bodipy), was summarized.