Long living excited state of protonated adenosine unveiled by ultrafast fluorescence spectroscopy and density functional theoretical study.

Adenosine (Ado) possesses ultrafast nonradiative dynamics accounting for its remarkably high photostability. The deactivation dynamics of Ado after protonation in an aqueous solution remains an elusive issue. Herein we report an investigation of the excited state dynamics of protonated Ado (AdoH+) performed using ultrafast time-resolved fluorescence spectroscopy combined with density functional theoretical calculation. The result obtained from comparison of conformers with protonation at different sites revealed that the syn-conformer with protonation occurring at the N3 position (syn-N3) is the predominant form of AdoH+ in the ground state, similar to that of Ado. In contrast, the fluorescence of AdoH+ with maximum intensity at 385 nm, significantly red-shifted from that of Ado, displaying decay dynamics composed of an ultrafast component with the lifetime of ∼0.5 ps and a slower one of ∼2.9 ns. The former is because of the decay of the syn-N3 conformer, similar to that reported for AdoH+ under the gas phase condition. The latter is due to the syn-N1 conformer formed via ultrafast proton transfer of the syn-N3. The excited state of syn-N1 has a peculiar nonplanar conformation over the purine molecule, which is responsible for the substantial Stokes shift showed in the fluorescence spectrum and correlates with a large energy barrier for nonradiative decay likely involving a reversed proton transfer. This study demonstrates the importance of protonation and solvent environment in altering dramatically the excited states of Ado, providing insight for better understanding nonradiative dynamics of both the monomeric bases and the oligomeric or polymeric DNAs.

A long lasting sunscreen controversy of 4-aminobenzoic acid and 4-dimethylaminobenzaldehyde derivatives resolved by ultrafast spectroscopy combined with density functional theoretical study.

4-Aminobenzoic acid (PABA) is one of the earliest patented and most commonly used sunscreen components. There is however a long-lasting controversy on its photo-protective efficacy owing to the lack of information on its protolytic equilibrium and photo-dynamics after absorption of ultraviolet radiation in physiologically relevant aqueous solution. The excitation dynamics in water also remains largely unknown for analogs of PABA such as 4-dimethylaminoacetophenone (DMAAP) and 4-dimethylaminobenzaldehyde (DMABA) which are recognized as prototypes for photo-induced twisted intramolecular charge transfer (TICT). Herein we report a combined application of femtosecond broadband time-resolved fluorescence and transient absorption coupled with density functional theoretical study for PABA, DMAAP, and DMABA under several solvent conditions with representative properties in terms of the pH, polarity and hydrogen bonding capacity. The results we gained demonstrate that, in a neutral aqueous solution, PABA taking the deprotonated anion form in the ground state undergoes rapid protonation after excitation, producing excited state species in the neutral form that may shift effectively by intersystem crossing (ISC) to the long-lasting triplet state capable of damaging nucleic acids. This provides evidence at the molecular level for the detrimental effect of PABA if used as a sunscreen ingredient. In contrast, our investigation on DMAAP and DMABA unveils an unusual solvent controlled deactivation dynamics rendered by the participation of the carbonyl oxygen associated nOπ* state featuring energy and structure strongly responsive to solvent properties. In particular, these molecules in water exhibit solute-solvent hydrogen bonding at the sites of the carbonyl oxygen and the amino nitrogen which is, respectively, weakened and strengthened after the excitation, leading to state reversal and formation of a nOπ* state with a peculiar non-planar structure. This quenches strongly the excitation, eliminates the TICT, suppresses the ISC and opens up the otherwise inaccessible internal conversion (IC) to account for ∼80% of the entire deactivation. The IC, observed to proceed at a rate of ∼2.5 ps, allows the effective recovery of the ground state, providing substantial protection against ultraviolet irradiation. Moreover, the revelation of highly solvent sensitive fluorescence emission from DMABA and DMAAP implies the potential application of these molecules as the functional element in the design of sensory materials for probing the polarity and hydrogen bonding character of the surrounding environment.