Photophysics, Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

Abstract

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.

Study of photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine in dependence of pH using different spectral techniques

“…On the basis of our recent work, we conclude that the behaviour of an alloxazine derivative (lumichrome) 17 is similar to that of 5-deazaalloxazine (9-methyl-5-deazaalloxazine) 18 in aqueous solutions at different pH values; however, the values of the absorption and emission maxima of the corresponding species (cationic, neutral and anionic forms) of these compounds a Faculty of Chemistry, Adam Mickiewicz University in Poznań, Umultowska 89b, are shifted relative to each other. The acid-base properties of alloxazines (mainly lumichromes) have been studied quite intensively, 17,[19][20][21] while less information is available on the acid-base equilibria of the derivatives of 5-deazalloxazine.…”

“…3,4 Some derivatives of 5-deazaflavin and its analogs with the 5-deazaalloxazine structure were reported as showing antitumor activity in vitro; 5-9 they show antimalarial, antibacterial, analgesic and antitumor activities. 11,12 Such behavior is similar to that of alloxazines (derivatives of flavins) which also undergo excited state double proton transfer, catalyzed by carboxylic acids, such as acetic acid, and other organic compounds. 11,12 Such behavior is similar to that of alloxazines (derivatives of flavins) which also undergo excited state double proton transfer, catalyzed by carboxylic acids, such as acetic acid, and other organic compounds.…”

“…The acid-base properties of alloxazines (mainly lumichromes) have been studied quite intensively, 17,[19][20][21] while less information is available on the acid-base equilibria of the derivatives of 5-deazalloxazine. 18 According to our previous work, 17 lumichrome (7,8-dimethylalloxazine) exists as the neutral form in the 1 to 8 pH range, similarly to 9-methyl-5-deazalloxazine, which exists in its neutral form in the pH range from 1.2 to about 9. 18 Next, at even higher pH values, these compounds undergo deprotonation in a similar way.…”

“…18 According to our previous work, 17 lumichrome (7,8-dimethylalloxazine) exists as the neutral form in the 1 to 8 pH range, similarly to 9-methyl-5-deazalloxazine, which exists in its neutral form in the pH range from 1.2 to about 9. 18 Next, at even higher pH values, these compounds undergo deprotonation in a similar way. Namely, they dissociate their both hydrogen atoms from the N(1) and N(3) positions in the same pH range, forming two monoanions.…”

“…18 Next, at even higher pH values, these compounds undergo deprotonation in a similar way. Also, at high pH values (12)(13)(14), the isoalloxazinic dianion is formed for both compounds. The N(1) monoanion has the isoalloxazinic form and the N(3) monoanion has the alloxazinic form, for both compounds.…”