Comprehensive Study of Ultrafast Excited-State Proton Transfer in Water and D₂O Providing the Missing RO^–···H⁺ Ion-Pair Fingerprint

Abstract

Steady-state and time-resolved optical techniques were employed to study the photoprotolytic mechanism of a general photoacid. Previously, a general scheme was suggested that includes an intermediate product that, up until now, had not been clearly observed experimentally. For our study, we used quinone cyanine 7 (QCy7) and QCy9, the strongest photoacids synthesized so far, to look for the missing intermediate product of an excited-state proton transfer to the solvent. Low-temperature steady-state emission spectra of both QCy7 and QCy9 clearly show an emission band at T < 165 K in H2O ice that could be assigned to ion-pair RO(-)*···H3O(+), the missing intermediate. Room-temperature femtosecond pump-probe spectroscopy transient spectra at short times (t < 4 ps) also shows the existence of transient absorption and emission bands that we assigned to the RO(-)*···H3O(+) ion pair. The intermediate dissociates on a time scale of 1 ps and about 1.5 ps in H2O and D2O samples, respectively.

Panoramic portrait of primary molecular events preceding excited state proton transfer in water

“… 7 , 8 , 18 – 21 Previous results showed multi-stage proton transfer processes involving the formation and separation of the charge-separated contact ion pair on the ps timescale. 19 , 20 , 22 – 25 In contrast to direct proton transfer from HPTS to carboxylate ions which can occur within ∼200 femtoseconds (fs) at high base concentrations, 8 , 23 , 26 pure water provides a general environment with moderate H-bonding strength that can elucidate aqueous proton transfer pathways with different number of intervening water molecules, 19 , 21 , 27 revealing characteristic time constants of ∼3 and 90 ps in the commonly reported kinetic scheme for ESPT. Based on femtosecond transient absorption results, we select a strategic Raman pump wavelength at 580 nm that pre-resonantly enhances both the photoexcited protonated and deprotonated chromophore through the entire time window, directly tracking molecular structural evolution of the photoacid along multiple atomic motion trajectories.…”

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

“…[18][19][20][21][22] However, examples of a clear identication of the CIP* spectral signatures are scarce. [23][24][25][26][27][28] Few studies reported an intermediate uorescence between the ROH* and RO À * bands that was attributed to the contact ion pairs but this was observed only in supercritical, 23 frozen or strongly acidic aqueous solutions 24,25 and aprotic organic solvents. 26,27 Therefore, it remains unclear whether the CIP* emission would be detectable in protic solvents at a moderate pH range.…”

How Does Interfacial Hydration Alter during Rod to Sphere Transition in DDAB/Water/Cyclohexane Reverse Micelles? Insights from Excited State Proton Transfer and Fluorescence Anisotropy

“…In water, HPTS dynamics is highly nonexponential having contributions from at least three different time constants: <1, ∼3, and ∼90 ps. 48 …”