Conformational Effects on the Dynamics of Internal Conversion in Boron Dipyrromethene Dyes in Solution

Light-Harvesting Crystals Formed from BODIPY-Proline Biohybrid Conjugates: Antenna Effects and Excitonic Coupling

A boron dipyrromethene (BODIPY) derivative bearing a cis-proline residue at the meso-position crystallizes in the form of platelets with strong (i.e., Φ_F = 0.34) red fluorescence, but the absorption and emission spectra differ markedly from those for dilute solutions. A key building block for the crystal is a pseudo-dimer where hydrogen bonding aligns the proline groups and separates the terminal chromophores by ca. 25 Å. Comparison with a covalently linked bichromophore suggests that one-dimensional (1D) excitonic coupling between the terminals is too small to perturb the optical properties. However, accretion of the pseudo-dimer forms narrow channels possessing a high density of chromophores. The resultant absorption spectrum exhibits strong excitonic splitting, which can be explained quantitatively using the extended dipole approach and allowing for coupling between ca. 30 BODIPY units. Fluorescence, which decays with a lifetime of 2.2 ns, is assigned to a delocalized and (slightly) super-radiant BODIPY dimer situated at the interface and populated via electronic energy transfer from the interior.

Coherent internal conversion from high lying electronic states to S1 in boron-dipyrromethene derivatives

Internal conversion is the first step after photoexcitation to high lying electronic states, and plays a central role in many photoinduced processes. In this report, we demonstrate a truly ultrafast internal conversion (IC) in large molecules by time-resolved fluorescence (TF). Following photoexcitation to the S_n (n ≥ 2) state, TF of the S₁ state was recorded for two boron-dipyrromethene (BODIPY) derivatives in solution. IC to S₁ takes place nearly instantaneously within 20 fs for both molecules. Abundant nuclear wave packet motions in the S₁ state are manifest in the TF signals, which demonstrates that the IC in these BODIPY molecules is coherent with respect to most of the vibrational modes. Theoretical calculations assuming impulsive IC to S₁ account for the wave packet dynamics accurately.

Covalent Linkage of BODIPY-Photosensitizers to Anderson-Type Polyoxometalates Using CLICK Chemistry

The covalent attachment of molecular photosensitizers (PS) to polyoxometalates (POMs) opens new pathways to PS‐POM dyads for light‐driven charge‐transfer and charge‐storage. Here, we report a synthetic route for the covalent linkage of BODIPY‐dyes to Anderson‐type polyoxomolybdates by using CLICK chemistry (i. e. copper‐catalyzed azide‐alkyne cycloaddition, CuAAC). Photophysical properties of the dyad were investigated by combined experimental and theoretical methods and highlight the role of both sub‐components for the charge‐separation properties. The study demonstrates how CLICK chemistry can be used for the versatile linkage of organic functional units to molecular metal oxide clusters.

Tracking Ultrafast Structural Dynamics in a Dual-Emission Anti-Kasha-Active Fluorophore Using Femtosecond Stimulated Raman Spectroscopy

The anti-Kasha process provides the possibility of using high-energy excited states to develop novel applications. Our previous research (Nature communications, 2020, 11, 793) has demonstrated a dual-emission anti-Kasha-active fluorophore for bioimaging application, which exhibits near-infrared emissions from the S₁ state and visible anti-Kasha emissions from the S₂ state. Here, we applied tunable blue-side femtosecond stimulated Raman spectroscopy (FSRS) and transient absorption spectroscopy, assisted by quantum calculations, to reveal the anti-Kasha dual emission mechanism, in which the emergence of two fluorescing states is due to the retardation of internal conversion from the S₂ state to the S₁ state. It has been demonstrated that the facts of anti-Kasha high-energy emission are commonly attributed to a large energy gap between the two excited states, leading to a decrease in the internal conversion rate due to a poor Franck-Condon factor. In this study, analysis of the calculation and FSRS experimental results provide us further insight into the dual-emission anti-Kasha mechanism, where the observation of hydrogen out-of-plane Raman modes from FSRS suggested that, in addition to the energy-gap law, the initial photoinduced molecular conformational change plays a key role in influencing the rate of internal conversion.

Optical spectroscopic properties recorded for simple BOPHY dyes in condensed media: The mirror-symmetry factor

The BOPHY structural scaffold provides opportunities for the synthesis of innumerable derivatives with linear geometries and well-controlled π-conjugation pathways. The simpler BOPHY chromophores are highly fluorescent but exhibit poor mirror symmetry between absorption and fluorescence spectra at ambient temperature. In particular, the absorption (and excitation) spectra are broad and appear as two overlapping bands of comparable intensity. In constrained media, such as low-temperature rigid glasses or stretched poly(ethylene) films, mirror symmetry is restored. Analysis of the temperature dependence recorded for simple BOPHY derivatives indicates that the vibronic envelope accompanying the electronic transitions can be well described in terms of low- and medium-frequency modes. Whereas the fluorescence spectral profile is only weakly dependent on temperature, the excitation spectrum is far more affected. The magnitude of the low-frequency mode, and the associated electron-phonon coupling, increase substantially with increasing temperature and is responsible for temperature broadening and distortion of the excitation spectrum in solution. This critical low-frequency vibronic mode is associated with out-of-plane torsional bending of the BOPHY unit. Variable temperature NMR studies failed to provide unequivocal evidence for conformational changes of one of the derivatives over the temperature range 193-353 K.

BODIPYs to the rescue: Potential applications in photodynamic inactivation

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives have been proposed in several potential biomedical applications. BODIPYs absorb strongly in blue-green region with high fluorescence emission, properties that convert them in effective fluorophores in the field of biological labeling. However, BODIPY structures can be conveniently modified by heavy atoms substitution to obtain photosensitizers with applications in photodynamic therapy. Also, external heavy atoms effect can be used to increase the photodynamic activity of these compounds. In recent years, BODIPYs have been proposed as phototherapeutic agents for the photodynamic inactivation of microorganisms. Therefore, BODIPY structures need to be optimized to produce an efficient photocytotoxic activity. In this way, amphiphilic cationic BODIPYs can selectively bind to microbial cells, inducing an effective photokilling of pathogenic microbial cells. This review summarizes the attributes of BODIPY derivatives for applications as antimicrobial photosensitizing agents.

Ultrafast dynamics and solvent-dependent deactivation kinetics of BODIPY molecular rotors

The fluorescent excited state of a molecular rotor based on the meso-substituted boron-dipyrromethane (BODIPY) core decays rapidly to the ground state via a conical intersection. The fluorescence is strongly increased in viscous solvents, but solvent polarity has only a small effect.

Fluorescent Properties of 8-Substituted BODIPY Dyes: Influence of Solvent Effects

Three boron-dipyrrine (BODIPY) based dyes with bulky substituents in 8-position of dipyrrin ligand have been synthesized and characterized. Photophysical properties of the obtained compounds have been investigated in different individual solvents and solvent mixtures. Investigated compounds was found to be intensive fluorescent molecular rotors. The influence of different solvent parameters and the substituent nature on rotor characteristics have been observed and discussed. Minor changes in the nature of 8-substituent does not influence the spectral properties, but the presence of nitrogen donor atom in the phenyl substituent could be used for the sensing of the donor-acceptor interactions with solvent or dissolved compounds. The new approach of spectral properties correlation with solvent parameters was proposed, the viscosity parameter should be taken into account in case of BODIPYs with bulky substituents. The intensity of fluorescence molecular rotor properties decrease gradually with the viscosity increase above 1 cP.

Fluorescent properties of 8-phenylBODIPY in ethanol - ethylene glycol mixed solutions

Fluorescent characteristics of two BODIPY dyes in solutions were investigated under the viscosity variations. Investigated dyes differ in the nature of 8-substituent of ligand molecule. Viscosity variation was achieved by changing of solvent composition in the binary system ethanol - ethylene glycol and solvent temperature. 8-Phenyl substituted BODIPY is found to exhibit properties of molecular rotor, i.e. its fluorescent characteristics are highly dependent on the viscosity or rigidity of the microenvironment. Increased temperature leads to a linear decrease of fluorescence parameters unlike the solvent composition variation where fluorescence change is nonlinear. The highest specific change in the fluorescence was observed in pure ethanol. Change in the nonradiative decay rate constant values with the increasing of viscosity indicates that the analyzed phenyl substituted complex is a true molecular rotor. Investigated 8-phenylBODIPY could be recommended as promising viscosity sensor in different organic media.

Synthesis and photophysical properties of difluoroboron complexes of curcuminoid derivatives bearing different terminal aromatic units and a meso-aryl ring

The synthesis of nine curcuminoids and their difluoroboron complexes is described, with seven of them containing a meso-phenyl ring. Dynamic (19)F NMR confirmed the fact that rotation of that meso-aryl fragment is restricted in the latter systems at room temperature and become allowed at higher temperature (>45 °C). The molecular structure of a meso-substituted derivative in the solid state showed that the phenyl ring lies in a highly twisted plane with respect to the mean curcuminoid plane. The photophysical properties of the nine compounds were investigated in solvents of different polarity. Meso-substitution with a phenyl ring has little influence on fluorescence emission properties in solution, radiative and nonradiative kinetic constants being similar for meso- and nonsubstituted compounds, which is in contrast to the case of BODIPY derivatives. However, introduction of an electron donor p-methoxy group at the meso-phenyl ring leads to small perturbation of the curcuminoid π-system fluorescence emission. We also report the influence of the meso-phenyl group on the emission properties of the aggregated solids.

Consequences of the one-electron reduction and photoexcitation of unsymmetric bis-imidazolium salts

Coupling of uronium salts with in situ generated N-heterocyclic carbenes provides straightforward access to symmetrical [4](2+) and unsymmetrical bis-imidazolium salts [6](2+) and [9](2+) . As indicated by cyclic and square-wave voltammetry, [6](2+) and [9](2+) can be (irreversibly) reduced by one electron. The initially formed radicals [6](.+) and [9](.+) undergo further reactions, which were probed by EPR spectroscopy and density functional calculations. The final products of the two-electron reduction are the two carbenes. Upon irradiation with UV light both [6](2+) and [9](2+) emit at room temperature in solution but with dramatically different characteristics. The different fluorescence behavior is analyzed by emission spectroscopy and interpreted by using time-dependent density functional calculations as largely due to different excited-state dynamics of [6](2+) and [9](2+) . The geometries of both radicals [6](.+) and [9](.+) and excited states {[6](2+) }* and {[9](2+) }* are substantially different from those of the parent ground-state molecules.

Modulating the singlet oxygen generation property of meso-β directly linked BODIPY dimers

We report the synthesis, X-ray analysis and singlet oxygen generation properties of a set of meso-β directly linked BODIPYs with the meso-aryl group (φ(1)) and meso-BODIPY component (φ(2)) free to rotate or constrained.

Excited state properties and energy transfer within dipyrrin-based binuclear iridium/platinum dyads: the effect of ortho-methylation on the spacer

Luminescent cyclometalated iridium complexes based on pyridyl appended dipyrrin ligands were prepared and characterized both in the solid state and in solution. The functionalization of the peripheral pyridyl moiety causes dramatic changes on the emission properties of both mono- and hetero- binuclear complexes. A detailed photophysical investigation of the two mononuclear derivatives of the [(Ppy)(2)Ir(dpm-py)] family (Ppy=2-phenylpyridine, dpm-py=5-(4-pyridyl)dipyrrin) was carried out. Introduction of methyl groups at the 3 and 5 positions on the pyridyl unit diminishes the non-radiative rate constant by locking the peripheral pyridyl group orthogonally to the dipyrrinato plane. Thus, they limit the rotational degree of freedom, as well as the charge-transfer character of the excited state. The coordination of these two complexes to a cyclometalated [(dppy)Pt] fragment (dppy=2,6-diphenylpyridine) led to the formation of binuclear species in which the iridium and platinum complexes behave as acceptors and donors, respectively. In these heterobinuclear compounds, the methyl groups do not influence the energy transfer efficiency, which is estimated to be above 90 %. However, they do limit the charge-transfer character of the acceptor's excited state, as well as its rotational degree of freedom, thus avoiding the detrimental effect upon the photophysical performance.