Spectroscopy and photophysics of dimethyl-substituted alloxazines

Tetramethylalloxazines as efficient singlet oxygen photosensitizers and potential redox-sensitive agents

Tetramethylalloxazines (TMeAll) have been found to have a high quantum yield of singlet oxygen generation when used as photosensitizers. Their electronic structure and transition energies (S0 → Si, S0 → Ti, T1 → Ti) were calculated using DFT and TD-DFT methods and compared to experimental absorption spectra. Generally, TMeAll display an energy diagram similar to other derivatives belonging to the alloxazine class of compounds, namely π,π* transitions are accompanied by closely located n,π* transitions. Photophysical data such as quantum yields of fluorescence, fluorescence lifetimes, and nonradiative rate constants were also studied in methanol (MeOH), acetonitrile (ACN), and 1,2-dichloroethane (DCE). The transient absorption spectra were also analyzed. To assess cytotoxicity of new compounds, a hemolytic assay was performed using human red blood cells (RBC) in vitro. Subsequently, fluorescence lifetime imaging experiments (FLIM) were performed on RBC under physiological and oxidative stress conditions alone or in the presence of TMeAll allowing for pinpointing changes caused by those compounds on the intracellular environment of these cells.

5-Deazaalloxazine as photosensitizer of singlet oxygen and potential redox-sensitive agent

Flavins are a unique class of compounds that combine the features of singlet oxygen generators and redox-dependent fluorophores. From a broad family of flavin derivatives, deazaalloxazines are significantly underdeveloped from the point of view of photophysical properties. Herein, we report photophysics of 5-deazaalloxazine (1a) in water, acetonitrile, and some other solvents. In particular, triplet excited states of 1a in water and in acetonitrile were investigated using ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The measured triplet lifetimes for 1a were all on the microsecond time scale (≈ 60 μs) in deoxygenated solutions. The quantum yield of S₁ → T₁ intersystem crossing for 1a in water was 0.43 based on T₁ energy transfer from 1a to indicaxanthin (5) acting as acceptor and on comparative actinometric measurements using benzophenone (6). 1a was an efficient photosensitizer for singlet oxygen in aerated solutions, with quantum yields of singlet oxygen in methanol of about 0.76, compared to acetonitrile ~ 0.74, dichloromethane ~ 0.64 and 1,2-dichloroethane ~ 0.54. Significantly lower singlet oxygen quantum yields were obtained in water and deuterated water (Ф_Δ ~ 0.42 and 0.44, respectively). Human red blood cells (RBC) were used as a cell model to study the antioxidant capacity in vitro and cytotoxic activity of 1a. Fluorescence-lifetime imaging microscopy (FLIM) data were analyzed by fluorescence lifetime parameters and distribution for different parts of the emission spectrum. Comparison of multidimensional fluorescent properties of RBC under physiological-like and oxidative-stress conditions in the presence and absence of 1a suggests its dual activity as probe and singlet-oxygen generator and opens up a pathway for using FLIM to analyze complex intracellular behavior of flavin-like compounds. These new data on structure-property relationship contribute to the body of information required for a rational design of flavin-based tools for future biological and biochemical applications.

Initial Excited State Dynamics of Lumichrome upon Ultraviolet Excitation

Lumichrome (LC) is the major photodegradation product of biologically important flavin cofactors. Since LC serves as a structural comparison to the flavins; understanding excited states of LC is fundamentally important to establish a connection with photophysics of different flavins, such as lumiflavin (LF), riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD). Herein, we deduce the initial excited state structural dynamics of LC using UV resonance Raman (UVRR) intensity analysis. The UVRR spectra at wavelengths across the 260 nm absorption band of LC were measured and resulting Raman excitation profiles and absorption spectrum were self consistently simulated using a time-dependent wave packet formalism to extract the initial excited state structural and solvent broadening parameters. These results are compared with those obtained for other flavins following UV excitations. We find that LC undergoes a very distinct instantaneous charge redistribution than flavins, which is attributed to the extended π-conjugation present in flavins but missing in LC. The homogeneous broadening linewidth of LC appears to be lower than that of LF, while the inhomogeneous broadening values are comparable, indicating greater solvent interaction with excited flavin on ultrafast timescale compared to LC, whereas on longer timescale these interactions are almost similar.

Photodynamics of biological active flavin in the presence of zwitterionic surfactants

In the flavin family of photoactive biomolecules, lumichrome (LM) is a very important compound. It contains a tri-cyclic structure with methyl groups at two sides. It formed by the partial decomposition and biodegradation of riboflavin in both acidic as well as in neutral medium. Herein, we have studied the photophysical properties of LM in the presence of two zwitterionic surfactants, namely dodecyldimethyl(3-sulfopropyl) ammonium hydroxide inner salt (DSB), and tetradecyldimethyl(3-sulfopropyl) ammonium hydroxide inner salt (TSB), having the same head group but a different tail part. We have used steady-state absorption, fluorescence emission, and time-resolved fluorescence emission measurements. We observed that in the presence of zwitterionic surfactant aggregates LM shows excitation and emission wavelength dependent emission properties, which demonstrate the structural changes that take place from one form to another prototropic form of LM molecule. The higher rotational relaxation time of LM in the case of DSB compared to TSB demonstrated that LM is facing more rigid environment in DSB micelles compared to TSB micelles.

Double proton transfer in a polar nano-droplet: Phototautomerization of alloxazine in AOT/alkane reverse micelles containing water or glycerol

Alloxazine phototautomerization is believed to occur through an excited state double proton transfer (ESDPT) mechanism involving cyclic intermolecular H-bonded complexes between Alloxazine and hydroxylic solvents like water and alcohols. In AOT/alkane dispersions in the absence of any polar liquid, Alloxazine molecules reside inside the polar core of the AOT reverse micelle nanoparticles, where they involve in H-bonding with the anionic sulfonate head-groups of the AOT molecules, but are unable to generate the appropriate cyclic intermolecular H-bonded complexes conducive to ESDPT. However, tautomerization is switched on with addition of water and formation ofwater nano-droplet at the core of reverse micelle. Evidently, the Alloxazine⋅⋅⋅⋅AOT H-bonds are now replaced by Alloxazine⋅⋅⋅⋅Water H-bonds, promotingESDPT. On the other hand, Alloxazine phototautomerization is hindered in Glycerol, irrespective of whether the latter is in the bulk liquid state or in the form of a polar nano-droplet. This may be explained by steric considerations.

Photophysical properties of N-methyl and N-acetyl substituted alloxazines: a theoretical investigation

The electronic structure and photophysical properties of a series of N-methyl and N-acetyl substituted alloxazines (AZs) were investigated with extensive density functional theory (DFT) and time-dependent density functional theory (TD-DFT) based calculations. We showed that non-radiative decays from the lowest singlet and triplet excited states of these AZs are dominant over their radiative counterparts. The fast non-radiative decays of the excited AZs can be attributed to the energy consumption (Ereorg) through structural reorganization facilitated by the intrinsic normal modes of the alloxazine framework, as well as their coupling with those of the functional groups. Substitution with functional groups may lead to further perturbation of the electronic structure of the AZ chromophore, which may enhance intersystem crossing with the ππ* states of the AZs. Due to the different bonding of N1 and N3 within the alloxazine framework, substitution may result in AZs with different photophysical properties. Specifically, functionalization at N1 may help in maintaining or even reducing Ereorg and would promote the absorption and radiative decay from the excited AZs. However, the strong coupling of the vibrational modes of acetyl at N3 with the intrinsic normal modes of the alloxazine framework would contribute significantly to Ereorg, and benefit the non-radiative decay of the excited AZs. We expect that the findings would pave the way for rational design of novel AZs with extraordinary photophysical properties.

Efficient Photooxidation of Sulfides with Amidated Alloxazines as Heavy-atom-free Photosensitizers

Photooxidation utilizing visible light, especially with naturally abundant O₂ as the oxygen source, has been well-accepted as a sustainable and efficient procedure in organic synthesis. To ensure the intersystem crossing and triplet quantum yield for efficient photosensitization, we prepared amidated alloxazines (AAs) and investigated their photophysical properties and performance as heavy-atom-free triplet photosensitizers and compared with those of flavin (FL) and riboflavin tetraacetate (RFTA). Because of the difference in the framework structure of AAs and FL and the introduction of carbonyl moiety, the absorption of FL at ∼450 nm is blue-shifted to ∼380 nm and weakened (ε = 8.7 × 10³ for FL to ∼6.8 × 10³ M^-1 cm^-1), but the absorption at ∼340 nm is red-shifted to ∼350 nm and enhanced by ∼50% (from ε = 6.4 × 10³ for FL to ∼9.9 × 10³ M^-1 cm^-1) in AAs. The intersystem crossing rates from the S₁ to T₁ are also enhanced in these AAs derivatives, while the fluorescence quantum yield decreases from ∼30 to ∼7% for FL and AAs, respectively, making the triplet excited state lifetime and the singlet oxygen quantum yield of AAs at least comparable to those of FL and RFTA. We examined the performance of these heave-atom-free chromophores in the photooxidation of sulfides to afford sulfoxides. In accordance with the prolonged triplet excited state lifetime and enhanced triplet quantum yield, 2-5-fold performance enhancements were observed for AAs in the photooxidation of sulfides with respect to FL. We proposed that the key reactive oxygen species of AA-sensitized photooxidation are singlet oxygen and superoxide radical anion based on mechanistic investigations. The research highlights the superior performance of AAs in photocatalysis and would be helpful to rationalize the design of efficient heavy-atom-free organic photocatalysts.

Photophysics, excited-state double-proton transfer and hydrogen-bonding properties of 5-deazaalloxazines

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0 -Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM-CCSD] with the correlation-consistent basis sets. Also the time-dependent density functional theory (TD-DFT) has been employed. The lowest singlet excited states of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close-lying π, π* and n, π* transitions. Experimental steady-state and time-resolved spectral studies indicate formation of an isoalloxazinic excited state via excited-state double-proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5-deazaalloxazine molecule. Solvatochromism of both 5-deazaalloxazine and its 1,3-dimethyl substituted derivative was analyzed using the Kamlet-Taft scale and four-parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute-solvent hydrogen-bonding interactions in their excited state.