Synthesis and spectroscopic characterization of 1,8-naphthalimide derived "super" photoacids

Temperature Effects and Activation Barriers in Aqueous Proton-Uptake Reactions

Aqueous proton transfer reactions are fundamental in biology and chemistry, yet kinetics and mechanisms of strong base-weak acid reactions are not well understood. In this work, we present a temperature-dependent reaction kinetic study of the water-soluble photobase actinoquinol, in the presence and absence of succinimide, a weak acid reaction partner. We study the temperature dependence of the reaction and connect the observed dynamics to the reaction's thermodynamics. We find that actinoquinol reacts in associated complexes with water/succinimide, creating an intermediate complex that can undergo either dissociation to create products, or reverse proton transfer within the complex to recreate the initial reactants. We find that the intermediates' formation is energetically unfavorable with both reaction partners, which impacts the net reaction rates. We also find that the net reaction rate is additionally strongly influenced by the competition between the dissociation of the intermediates and their reverse reaction.

Free base porphyrin-cyanine dye conjugate: synthesis and optical properties

The covalent combination of a cyanine dye (IR-783) with a tetraphenyl porphyrin unit through an ether linkage results in a photoactive system capable of producing singlet oxygen. The synthesis, characterization and photophysical properties of the resulting novel free base porphyrin-cyanine conjugate named TPPO-IR-783 (TOI) is reported. Excited state properties were studied in various solvents with differing polarity. The fluorescence is strongly solvent dependent, however this is not the case for singlet oxygen phosphorescence, which is only observed in tetrahydrofuran (THF), when comparing 8 different polar, non-polar and medium-polarity solvents. This novel type of porphyrin-cyanine photosensitizer has the ability to produce singlet oxygen and absorbs light at NIR wavelengths.

Reliable experimental method for determination of photoacidity revealed by quantum chemical calculations

Photoacids are aromatic acids that exhibit significantly different acidities when they are electronically excited. Three experimental methods have been extensively used to determine the photoacidity, : fluorescence titration, the Förster cycle, and time-resolved experiments. However, the photoacidities determined by these experimental methods are not consistent. In this work, we used a theoretical method to evaluate the reliability of experimentally determined values. In particular, density functional theory (DFT) and time-dependent DFT calculations were used to obtain the changes in Gibbs free energy for acid dissociation reactions which are directly related to values. The Förster cycle, which is frequently used to experimentally determine the photoacidity due to its simplicity, yielded inconsistent results depending on how the transition energy was defined. We evaluated six empirical parameters extracted from the absorption and emission spectra of acidic and basic species of photoacids to adequately define the transition energy in the Förster cycle. And we found that the values obtained using the optical bandgap as the transition energy in the Förster cycle were in the best agreement with the results of quantum chemical calculations.

Theoretical Study on the Photoacidity of Hydroxypyrene Derivatives in DMSO Using ADC(2) and CC2

This work applies the thermodynamic Förster cycle to theoretically investigate the pK_a^*, i.e., excited-state pK_a values of pyranine-derived superphotoacids developed by Jung and co-workers. The latter photoacids are strong enough to transfer a proton to the aprotic solvent dimethyl sulfoxide (DMSO). The Förster cycle provides access to pK_a^* via the ground-state pK_a and the electronic excitation energies. We use the conductor-like screening model for real solvents (COSMO-RS) to compute the ground-state pK_a and the correlated wavefunction-based methods ADC(2) and CC2 with the continuum solvation model COSMO to calculate the pK_a change upon excitation. A comparison of the calculated UV/Vis absorption and fluorescence emission energies to the experimental results leads us to infer that this approach allows for a proper description of the electronic excitations. In particular, implicit solvation by means of the COSMO model appears to be sufficient for the treatment of these photoacids in DMSO. The calculations confirm the presumption that a charge redistribution from the hydroxy group to the aromatic ring and the electron-withdrawing substituents is the origin of photoacidity for these photoacids. Moreover, the calculations with the continuum solvation model predict that the pK_a jump upon excitation decreases with increasing solvent polarity, as rationalized based on the Förster cycle.

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

Excited-state proton transfer (ESPT) to solvent is often explained according to the two-step Eigen-Weller model including a contact ion pair (CIP*) as an intermediate, but general applicability of the model has not been thoroughly examined. Furthermore, examples of the spectral identification of CIP* are scarce. Here, we report on a detailed investigation of ESPT to protic (H₂O, D₂O, MeOH and EtOH) and aprotic (DMSO) solvents utilizing a broadband fluorescence technique with sub-200 fs time resolution. The time-resolved spectra are decomposed into contributions from the protonated and deprotonated species and a clear signature of CIP* is identified in DMSO and MeOH. Interestingly, the CIP* intermediate is not observable in aqueous environment although the dynamics in all solvents are multi-exponential. Global analysis based on the Eigen-Weller model is satisfactory in all solvents, but the marked mechanistic differences between aqueous and organic solvents cast doubt on the physical validity of the rate constants obtained.

Propyl acetate/butyronitrile mixture is ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions

Characterization of propyl acetate/butyronitrile (PA/BuCN) mixtures by various spectroscopic techniques is described. The neat solvents have identical viscosities and refractive indices but their dielectric constants differ significantly. Detailed solvatochromic and titration data show that the mixtures do not exhibit specific solute-solvent interactions or significant dielectric enrichment effects. Therefore, the mixtures are ideally suited for investigating the effect of dielectric stabilization on (photo)chemical reactions. Dynamic Stokes shift experiments performed on two push-pull probes demonstrate that the solvation dynamics are significantly decelerated in the mixtures as compared to the neat solvents. Therefore, the mixtures allow for varying both the extent and time scale of the dielectric stabilization in a predictable manner.

Involving Synergy of Green Light and Acidic Responses in Control of Unimolecular Multicolor Luminescence

Conversion of multicolor luminescence is one of desirable goals in study and development of next-generation molecular emitters, whereas involving visible light into the control of the above-mentioned ability has been poorly addressed due to the need of a relatively complicate molecular design. In this work, we present a novel dyad with a linkage of 4-piperazinyl-1,8-naphthalimide and cyanostyryl-modified azulene moiety, upon which the luminescence signal can be orthogonally controlled by protonation and green light irradiation. The superior features of the protonation induced excited state energy alteration, followed by green light driven photoisomerization led to a progressive luminescent color conversion among blue, yellow and green at the single molecular level. This strategy may bring in novel insights for preparing advanced function-integrated optoelectronic materials.

Halogenated earth abundant metalloporphyrins as photostable sensitizers for visible-light-driven water oxidation in a neutral phosphate buffer solution

Very photostable tetrachloro-metalloporphyrins were developed as sensitizers for visible-light-driven water oxidation coupled to cobalt based water-oxidation catalysts in concentrated (0.1 M) phosphate buffer solution. Potassium persulfate (K2S2O8) acts as a sacrificial electron acceptor to oxidize the metalloporphyrin photosensitizers in their excited states. The radical cations thus produced drive the cobalt based water-oxidation catalysts: Co4O4-cubane and Co(NO3)2 as pre-catalyst for cobalt-oxide (CoOx) nanoparticles. Two different metalloporphyrins (Cu(ii) and Ni(ii)) both showed very high photostability in the photocatalytic reaction, as compared to non-halogenated analogues. This indicates that photostability primarily depends on the substitution of the porphyrin macrocycle, not on the central metal. Furthermore, our molecular design strategy not only positively increases the electrochemical potential by 120-140 mV but also extends the absorption spectrum up to ∼600 nm. As a result, the solar photon capturing abilities of halogenated metalloporphyrins (Cu(ii) and Ni(ii)) are comparable to that of the natural photosynthetic pigment, chlorophyll a. We successfully demonstrate long-term (>3 h) visible-light-driven water oxidation using our molecular system based on earth-abundant (first-row transition) metals in concentrated phosphate buffer solution.

Influence of Solvent Relaxation on Ultrafast Excited-State Proton Transfer to Solvent

A thorough understanding of the microscopic mechanism of excited-state proton transfer (ESPT) and the influence of the solvent environment on its dynamics are of great fundamental interest. We present here a detailed investigation of an ESPT to solvent (DMSO) using time-resolved broadband fluorescence and transient absorption spectroscopies. All excited-state species are resolved spectrally and kinetically using a global target analysis based on the two-step Eigen-Weller model. Reversibility of the initial short-range proton transfer producing excited contact ion pairs (CIP*) is observed unambiguously in fluorescence and must be explicitly considered to obtain the individual rate constants. Close inspection of the early dynamics suggests that the relative populations of the protonated form (ROH*) and CIP* are governed by solvent relaxation that influences the relative energies of the excited states. This constitutes a breakdown of the Eigen-Weller model, although the overall agreement between the data and the analysis using classical rate equations is excellent.

Complexes of a naphthalimide photoacid with organic bases, and their excited-state dynamics in polar aprotic organic solvents

Complex formation and intermolecular excited-state proton transfer (ESPT) between a dihydroxy-1,8-naphthalimide photoacid and organic bases are investigated in polar aprotic solvents. First, quantum chemical calculations are used to explore the acid-base and spectroscopic properties and to identify energetically favorable complexes. The two hydroxyl groups of the photoacid enable stepwise formation of 1 : 1 and 1 : 2 complexes. Weak bases exhibit only hydrogen-bonding interactions whereas strong bases are able to deprotonate one of the hydroxyl groups resulting in strong negative cooperativity (K1≫ 4K2) in the formation of the 1 : 2 complex. Time-resolved fluorescence studies of the complexes provide strong indications of a three-step dissociation process. The species involved in the model are: a hydrogen-bonded complex, a hydrogen-bonded ion pair, a solvent separated ion pair, and a free ion pair.

Nanosecond photochromic molecular switching of a biphenyl-bridged imidazole dimer revealed by wide range transient absorption spectroscopy

We demonstrate that a biphenyl-bridged imidazole dimer exhibits fast photochromism with a thermal recovery time constant of ∼100 ns, which is the fastest thermal back reaction in all reported imidazole dimers.