Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

Abstract

We investigate with femtosecond mid‐infrared spectroscopy the vibrational‐mode characteristics of the electronic states involved in the excited‐state dynamics of pyranine (HPTS) that ultimately lead to efficient proton (deuteron) transfer in H2O (D2O). We also study the methoxy derivative of pyranine (MPTS), which is similar in electronic structure but does not have the photoacidity property. We compare the observed vibrational band patterns of MPTS and HPTS after electronic excitation in the solvents: deuterated dimethylsulfoxide, deuterated methanol and H2O/D2O, from which we conclude that for MPTS and HPTS photoacids the first excited singlet state appears to have charge‐transfer (CT) properties in water within our time resolution (150 fs), whereas in aprotic dimethylsulfoxide the photoacid appears to be in a nonpolar electronic excited state, and in methanol (less polar and less acidic than water) the behaviour is intermediate between these two extremes. For the fingerprint vibrations we do not observe dynamics on a time scale of a few picoseconds, and with our results obtained on the OH stretching vibration we argue that the dynamic behaviour observed in previous UV/Vis pump–probe studies is likely to be related to solvation dynamics.

Deprotonation Dynamics and Stokes Shift of Pyranine (HPTS)

“…The results presented here are in contrast to visiblepump IR-probe studies, which do not observe the fast component of the deprotonation kinetics and only see the slower ∼90 ps decay. 9 Because the experiments and analysis given above unambiguously separate the time dependence of the Stokes shift from the deprotonation dynamics, it is clear that the ∼3 ps component of the spectral dynamics is not part of the Stokes shift (∼1 ps) and that the deprotonation is biexponential on times short (hundreds of picoseconds) compared with diffusive contributions. …”

Parallel proton transfer pathways in aqueous acid-base reactions

“…With increasing delay, these absorption bands decay and a new absorption at 1503 cm −1 grows in that is assigned to the conjugated photobase PTS * − . [14][15][16][17][18][19][20][21] Therefore, the decay and rise of these bands reflect the release of the proton by HPTS * . [14][15][16][17][18][19][20][21] The transient spectra in the frequency region between 1600 and 2100 cm −1 are shown in Fig.…”

“…13 Recently, the proton transfer between HPTS and different carboxylate bases dissolved in water has been studied by probing the vibrations of the photoacid and an accepting base with femtosecond midinfrared laser pulses. [14][15][16][17][18][19][20][21] These studies showed that the SCK model does not provide an adequate description of the data at all delay times. It was also found that there is not a single well-defined reaction distance at which proton transfer takes place.…”

“…12,[14][15][16][17][18][19][20][21] If the strength and concentration of the base are sufficient, the release of the proton by HPTS * is strongly accelerated. In this acid-base reaction, the diffusion of the reactants through the water medium can play an essential role, as the reactants have to come sufficiently close to let the proton transfer take place.…”

“…14,15,20,21 Additionally, at high base concentrations, most of the proton transfers occur in complexes that are already present in the solution before the HPTS molecule is excited. [14][15][16][17][18][19][20][21] Of course, at low base concentrations, diffusion is important as it is required to generate the hydrogenbonded reaction complexes.…”