Monomolecular pyrenol-derivatives as multi-emissive probes for orthogonal reactivities.

Photoacids on the basis of pyrenol have been extensively studied in the past 60 years. As their photophysical properties strongly depend on the substituents at the aromatic scaffold, we introduced two reactive moieties with different electronic coefficients thus creating multi-wavelength fluorescent probes. One probe is capable of monitoring two orthogonal transformations by four fluorescence colors, distinguishable even by the naked human eye. Another derivative can act as a three-color sensor for a wide range of different pH values. Both the presented compounds allow for mimicking of fundamental and advanced two-input logic operations due to the multi-wavelength emission. Furthermore, these compounds can process information in a logically reversible way (Feynman gate).

Biexponential photon antibunching: recombination kinetics within the Förster-cycle in DMSO.

Time-resolved experiments with pulsed-laser excitation are the standard approach to map the dynamic evolution of excited states, but ground-state kinetics remain hidden or require pump-dump-probe schemes. Here, we exploit the so-called photon antibunching, a purely quantum-optical effect related to single molecule detection to assess the rate constants for a chemical reaction in the electronic ground state. The measurement of the second-order correlation function g((2)), i.e. the evaluation of inter-photon arrival times, is applied to the reprotonation in a Förster-cycle. We find that the antibunching of three different photoacids in the aprotic solvent DMSO significantly differs from the behavior in water. The longer decay constant of the biexponential antibunching tl is linked to the bimolecular reprotonation kinetics of the fully separated ion-pair, independent of the acidic additives. The value of the corresponding bimolecular rate constant, kp = 4 × 10(9) M(-1) s(-1), indicates diffusion-controlled reprotonation. The analysis of tl also allows for the extraction of the separation yield of proton and the conjugated base after excitation and amounts to approximately 15%. The shorter time component ts is connected to the decay of the solvent-separated ion pair. The associated time constant for geminate reprotonation is approximately 3 ± 1 ns in agreement with independent tcspc experiments. These experiments verify that the transfer of quantum-optical experiments to problems in chemistry enables mechanistic conclusions which are hardly accessible by other methods.

Solvent dependence of excited-state proton transfer from pyranine-derived photoacids.

Steady-state and time-resolved techniques were employed to study the excited-state proton-transfer (ESPT) rate of two newly synthesized 8-hydroxy-1,3,6-pyrenetrisulfonate (pyranine, HPTS) derived photoacids in three protic solvents, water, methanol and ethanol. The ESPT rate constant k(PT) of tris(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate, 1a, whose pK(a)* ~ -4, in water, methanol and ethanol is 3 × 10(11) s(-1), 8 × 10(9) s(-1) and 5 × 10(9) s(-1) respectively. (8-Hydroxy-N1,N3,N6-tris(2-hydroxyethyl)-N1,N3,N6-trimethylpyrene-1,3,6 trisulfonamide, 1b) is a weaker acid than 1a but still a strong photoacid with pK(a)* ~ -1 and the ESPT rate in water, methanol and ethanol is 7 × 10(10) s(-1), 4 × 10(8) s(-1) and 2 × 10(8) s(-1). We qualitatively explain our kinetic results by a Marcus-like free-energy correlation which was found to have a general form suitable for describing proton transfer reactions in both the proton-adiabatic and the proton-non-adiabatic limits.

Highly photostable "super"-photoacids for ultrasensitive fluorescence spectroscopy.

The photoacid 8-hydroxypyren-1,3,6-trisulfonic acid (HPTS, pyranine) is a widely used model compound for the examination of excited state proton transfer (ESPT). We synthesized five "super"-photoacids with varying hydrophilicity and acidity on the basis of HPTS. By chemical modification of the three sulfonic acid substituents, the photoacidity is enhanced by up to more than five logarithmic units from pK*≈ 1.4 to ∼-3.9 for the most acidic compound. As a result, nearly quantitative ESPT in DMSO can be observed. The novel photoacids were characterized by steady-state and time-resolved fluorescence techniques showing distinctively red shifted spectra compared to HPTS while maintaining a high quantum yield near 90%. Photostability of the compounds was checked by fluorescence correlation spectroscopy (FCS) and was found to be adequately high for ultrasensitive fluorescence spectroscopy. The described photoacids present a valuable palette for a wide range of applications, especially when the properties of HPTS, i.e. highly charged, low photostability and only moderate excited state acidity, are limiting.

Solvatochromism of pyranine-derived photoacids.

Photoacidity is frequently found in aromatic alcohols where the equilibrium dissociation constant increases by some orders of magnitude upon electronic excitation. In this study we investigated the solvatochromism of a family of recently synthesized super-photoacids and their methylated counterparts based on pyrene. The chemical similarity of these molecules on the one hand and their differing photoacidity with pKa* values between -0.8 and -3.9 on the other allow for gaining insights into the mechanisms contributing to excited-state proton transfer. Three different solvent scales, namely Lippert-Mataga, Kamlet-Taft and Catalán, were independently employed in this study and gave consistent results. We found the strongest correlation of the excited-state acidity with the dipolarity of the excited state, pem ranging from -1775 cm(-1) to -2500 cm(-1), and a concomitant change in the permanent dipole moment of roughly 14 Debye. Spectral changes due to varying basicity of the solvent, which probes the conjugated property of the solute, are found to be less indicative of the graduation of excited-state acidity, i.e. bem values between -700 and -1200 cm(-1). The solvent acidity is the only parameter with a distinct influence on the electronic spectra of the deprotonated species. The low values of aem ~ 400 cm(-1), which are 3-4× smaller than aabs and aexc, indicate the low basicity of these species in the excited state. Triggered by semiempirical theoretical calculations, the energetic splitting between the two lowest excited states could be related to the excited-state acidity and points to alterations in the electronic mixing of locally excited and charge-transfer states, caused by the substituents. Differences between the threefold negatively charged pyranine and the new neutral photoacids are also discussed.