Aza‐BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts

A new series of aza‐BODIPY derivatives ( 4 a – 4 c , 5 a , c , and 6 b , c ) were synthesized and their excited‐state properties, such as their triplet excited state and the yield of singlet‐oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core‐substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet‐oxygen generation when compared to the peripheral‐substituted systems. The dye 6 c , which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ_T=0.86) and of the efficiency of singlet‐oxygen generation (Φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza‐BODIPY systems is that they are stable under irradiation conditions, possess strong red‐light absorption (620–680 nm), exhibit high yields of singlet‐oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.     

Structural Dynamics of Phenyl Azide in Light-Absorbing Excited States:Resonance Raman and Quantum Mechanical Calculation Study

The excited state structural dynamics of phenyl azide (PhN₃) after excitation to the light absorbing S₂(A'), S₃(A'), and S₆(A') states were studied using the resonance Raman spectroscopy and complete active space self-consistent field calculations. The vibrational spectra and the UV absorption bands were assigned on the basis of the Fourier transform (FT)-Raman, FT-infrared measurements, the density-functional theory computations and the normal mode analysis. The A-, B-, and C-bands resonance Raman spectra in cyclohexane, acetonitrile, and methanol solvents were, respectively, obtained at 273.9, 252.7, 245.9, 228.7, 223.1, and 208.8 nm excitation wavelengths to probe the corresponding structural dynamics of PhN₃. The results indicated that the structural dynamics in the S₂(A'), S₃(A'), and S₆(A') states were significantly different. The crossing points of the potential energy surfaces, S₂S₁(1) and S₂S₁(2), were predicted to play a key role in the low-lying excited state decay dynamics, in accordance with Kasha's rule, and N7=N8 dissociation. Two decay channels initiated from the Franck-Condon region of the S₂(A') state were predicted: the radiative S_2,min→S₀ radiative decay and the S₂→S₁ internal conversion through the crossing points S₂S₁(1)/S₂S₁(2).     

Analysis of a General Theorem Concerning Two Non-Commuting Hermitian Matrices: Quantum Mechanical Implications for Ground and Excited States

A general theorem concerning the spectral relationship of two non-commuting Hermitian matrices is demonstrated. Discussion and analysis of such finding leads to consider its tight connection with respect of the Hohenberg–Kohn theorem (HKT), cornerstone of DFT theory. The present analysis shows that not only HKT can be considered a particular case of the proposed theorem, but also the validity of the studied spectral relationship can be extended from quantum mechanical ground state to excited states as well.     

Solvatochromism and Nonradiative Decay of Intramolecular Charge‐Transfer Excited States: Bands‐of‐Energy Model,Thermodynamics, and Self‐Organization

The fluctuations of orientation and induction interactions in solution and their impact on the broadening of absorption and fluorescence spectra are considered in terms of a bands‐of‐energy model. Also covered is the application of principles of thermodynamics and self‐organization of systems for calculation of solvatochromic shift, among them a component owing to the work on electronic polarization of solvent at the instant of electronic transition in the solute. The findings on solvatochromic shift and spectral broadening open the way to the calculation of solvent effects on the rate constant of nonradiative transitions. As demonstrated herein for 15 fluorophores, the novel theory of nonradiative decay of the intramolecular charge‐transfer excited states is carried out for dyes and organic compounds of different nature, both for polar and nonpolar media.     

Photoinduced Electron Transfer‐based Halogen‐free Photosensitizers: Covalent meso‐Aryl (Phenyl,Naphthyl, Anthryl,and Pyrenyl) as Electron Donors to Effectively Induce the Formation of the Excited Triplet State and Singlet Oxygen for BODIPY Compounds

Pristine BODIPY compounds have negligible efficiency to generate the excited triplet state and singlet oxygen. In this report, we show that attaching a good electron donor to the BODIPY core can lead to singlet oxygen formation with up to 58 % quantum efficiency. For this purpose, BODIPYs with meso ‐aryl groups (phenyl, naphthyl, anthryl, and pyrenyl) were synthesized and characterized. The fluorescence, excited triplet state, and singlet oxygen formation properties for these compounds were measured in various solvents by UV/Vis absorption, steady‐state and time‐resolved fluorescence methods, as well as laser flash photolysis technique. In particular, the presence of anthryl and pyrenyl showed substantial enhancement on the singlet oxygen formation ability of BODIPY with up to 58 % and 34 % quantum efficiency, respectively, owing to their stronger electron‐donating ability. Upon the increase in singlet oxygen formation, the fluorescence quantum yield and lifetime values of the aryl‐BODIPY showed a concomitant decrease. The increase in solvent polarity enhances the singlet oxygen generation but decreases the fluorescence quantum yield. The results are explained by the presence of intramolecular photoinduced electron transfer from the aryl moiety to BODIPY core. This method of promoting T₁ formation is very different from the traditional heavy atom effect by I, Br, or transition metal atoms. This type of novel photosensitizers may find important applications in organic oxygenation reactions and photodynamic therapy of tumors.