Early events associated with the excited state proton transfer in 2-(2′-pyridyl)benzimidazole

Abstract

2-͑2Ј-pyridyl͒benzimidazole ͑2PBI͒ undergoes excited state proton transfer ͑ESPT͒ in acidic solutions, leading to a tautomer emission at 460 nm. This photoprocess has been studied using ultrafast fluorescence spectroscopic techniques in acidic neat aqueous solutions, in viscous mixtures of glycerol with water, as well as in sucrose solutions. The tautomer is found to be stabilized in the more viscous medium, leading to a greater relative quantum yield as well as lifetime. The long rise time in tautomer emission is not affected by viscosity though. Rather, it appears to have the same value as the long component of the decay of the cationic excited state ͑C ‫ء‬ ͒. In addition to the subnanosecond lifetime reported earlier, C ‫ء‬ is found to exhibit a decay time of 2 ps. This is assigned to its protonation to form the nonfluorescent dication in its excited state ͑D ‫ء‬ ͒ considering the ground and excited state pK a values reported earlier. An additional rising component of 100 ps is observed in the region of C ‫ء‬ emission. This is likely to arise from a structural change or charge redistribution in C ‫ء‬ immediately after its creation and before the phototautomerization.

Femtosecond Hydration Map of Intrinsically Disordered α-Synuclein

“…The instrument response function was measured by measuring the cross correlation between the gate light and the Raman signal of buffer, and the full width at half-maximum was $250 fs. The emission decay traces were fitted by reconvolution in a desired number of exponentials in a homebuilt program in Igor Pro 6.37 (33). The data were then plotted in Origin Pro 8.5.…”

Interplay of Hydrophobic and Electrostatic Interactions in Modulation of Protonation–Deprotonation Equilibria of Two Positional Isomers in Their Complexes with Cucurbiturils

“…(Figures S1 and S3). Interestingly, such an excited state deprotonation is not observed for 2PBI, indicating that the intramolecular hydrogen bond between benzimidazolium hydrogen and pyridyl nitrogen in 2PBI is too strong to be disrupted (Scheme S3), even in the excited state, by the ion-dipole interactions with the rim of CB5. This is in stark contrast to the excited state deprotonation of another fluorophore, (2,2′-bipyridyl)-3-3′-diol in nafion .…”

“…In summary, 4PBI undergoes protonation in the ground state and deprotonation in the excited state, in its exclusion complex formed with CB5, as the pyridyl nitrogen is isolated. Moreover, the more exposed pyridyl nitrogen is selectively protonated to form T. On the other hand, there is no such modulation of protonation–deprotonation equilibria for 2PBI, as the pyridyl nitrogen is hydrogen bonded to the benzimidazolium cation …”

“…This has been rationalized in the light of a lower local pH of such interfaces (Scheme S3). − In its inclusion complexes with cyclodextrins (CD), orientation of 4PBI molecule is such that it can form hydrogen bonds with primary/secondary −OH groups of the macrocycle. Hence, by virtue of its small p K a * value, the pyridyl nitrogen of 4PBI can abstract protons from the hydroxyl groups inside the CD cavity.…”

“…Moreover, the more exposed pyridyl nitrogen is selectively protonated to form T. On the other hand, there is no such modulation of protonation− deprotonation equilibria for 2PBI, as the pyridyl nitrogen is hydrogen bonded to the benzimidazolium cation. 38 CB6 and CB7 affect the ground state protonation− deprotonation equilibrium of 4PBI in an exactly opposite manner. They do not promote the T absorption band at pH = 9, while it gets suppressed at pH = 4.…”