Direct observation of stepwise intermolecular proton and hydrogen transfer between alcohols and the triplet state of 4-nitro-1-naphthol

Transient spectroscopic insights into nitroindole's T1 state: Elucidating its intermediates and unique photochemical properties

Indoles are notable for their distinct photophysical and photochemical properties, making them useful indicators in biological systems and promising candidates for a variety of pharmaceutical applications. While some indoles exhibit room temperature phosphorescence, such a phenomenon has not been observed in nitroindoles. Typically, adding of a nitro group into aromatic compounds promotes ultrafast intersystem crossing and increases the formation quantum yield of the lowest excited triplet (T1). Therefore, understanding the reactivity of nitroindoles' T1 states is imperative. This study investigated the physical properties and chemical reactivities of the T1 state of 6-nitroindole (3HN-6NO2) in both polar aprotic and protic solvents, using transient absorption spectroscopy. Our results demonstrate the basicity and acidity of 3HN-6NO2, emphasizing its potential for protonation and dissociation in mildly acidic and basic conditions, respectively. Furthermore, 3HN-6NO2 has a high oxidizing capacity, participating in electron transfer reactions and proton-coupled electron transfer to produce radicals. Interestingly, in protic solvents like alcohols, 3HN-6NO2 dissociates at the -NH group and forms N-H…O hydrogen-bonded complexes with the nitro group. By identifying transient absorption spectra of intermediates and quantifying kinetic reaction rate constants, we illuminate the unique properties of the T1 state nitroindoles, enriching our understanding of their photophysical and photochemical behaviors. The results of this study have significant implications for their potential application in both biological systems and materials science.

Effect of electron-donating substitution on the triplet state reactivities of 1-nitronaphthalene

Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs), often found in polluted air, are carcinogenic and mutagenic. The nitro group increases the spin-orbit coupling and results in the lowest excited triplet (T₁) on the picosecond time scale with a high yield. The electron-donating substituents have a significant influence on the photophysics and photochemistry of nitro-PAHs. We used transient absorption spectroscopy and kinetic analysis to investigate the reactivities of the T₁ state 1-methoxy-4-nitronaphthalene (³MeO-NN) and 1-methyl-4-nitronaphthalene (³Me-NN). The results show that the methoxy and methyl substitutions have a minor effect on their hydrogen abstraction and electron accepting abilities. The main distinction is their reaction rates towards protons. The second order rate constant of ³MeO-NN towards protons is three orders of magnitude greater than that of ³Me-NN, indicating that ³MeO-NN has a stronger hydrogen bond accepting ability. The kinetic analysis reveals that the dimer of 2,2,2-trifluoroethanol participates in the reaction with ³MeO-NN. These results suggest that the formation of the hydrogen-bonded complex is responsible for the unusually short lifetime of ³MeO-NN in methanol solution and the lack of hydrogen abstraction radicals during the decay of ³MeO-NN in methanol.

Excited state proton transfer of triplet state p-nitrophenylphenol to amine and alcohol: a spectroscopic and kinetic study

Hydroxyaromatic compounds (ArOHs) have a wide range of applications in catalytic synthesis and biological processes due to their increased acidity upon photo-excitation. The proton transfer of ArOHs via the excited singlet state has been extensively studied. However, there has still been a debate on the unique type of ArOH that can undergo an ultrafast intersystem crossing. The nitro group in p-nitrophenylphenol (NO₂-Bp-OH) enhances the spin-orbit coupling between excited singlet states and the triplet manifold, enabling ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T₁) with a high yield. In this work, we used time-resolved transient absorption to investigate the excited state proton transfer of NO₂-Bp-OH in its T₁ state to t-butylamine, methanol, and ethanol. The T₁ state of the deprotonated form NO₂-Bp-O^- was first observed and identified in the case of t-butylamine. Kinetic analysis demonstrates that the formation of the hydrogen-bonded complex with methanol and ethanol as proton acceptors involves their trimers. The alcohol oligomer size required in the excited state proton transfer process is dependent on the excited acidity of photoacid.

Effect of the weak intermolecular C-H···O=C hydrogen bonding, solvent polarity and concentration on the frequency of the C=O stretch mode of 2-thiophenecarboxaldehyde

The vibrational frequency shift in the C=O stretch mode of 2-thiophenecarboxaldehyde (2TC) in the condensed phase is still not fully understood. In this paper, the vibrational spectra of 2TC were investigated using the FT-Raman, FT-IR and resonance Raman spectroscopies in conjunction with the density functional theory calculation. The pure compound (in the neat liquid) exhibits three vibrational bands 1658, 1672 and 1687 cm^-1 in the ν_C=O spectral region. It differs from the band pair 1672 and 1682 cm^-1 for 2-cyclohexene-1-one (CHO) and the single band 1700 cm^-1 for benzaldehyde. The relative intensities of observed bands vary with the polarity of aprotic solvents and the compound's concentration. In a diluted solution, the strongest band in the resonance Raman spectra of 2TC appears the C=O stretch mode at 1690 cm^-1 in cyclohexane and 1674/1675 cm^-1 in acetonitrile. The imparting factors that shift the C=O stretch mode frequency in the neat liquid and solvents with different polarities were examined. The spectral sources of the vibrational bands at 1658 and 1672 cm^-1 in the neat liquid and a dilute solution were determined, and the resonance Raman spectra were assigned. It is concluded that tetramers and monomer are the major sources of the bands at 1658 and 1672 cm^-1 in the neat liquid, respectively, while the monomer is the main source of the bands at 1674/1675 cm^-1 in acetonitrile and the band at 1690 cm^-1 in cyclohexane with a dilute concentration. The band's source at 1662/1663 cm^-1 in acetonitrile (a dilute concentration) can be either the dimers or 2TC-CH₃CN clusters. The C=O bond's electronic charge density is the main factor that shifts the vibrational frequency of the C=O stretch mode of 2TC monomer when an aprotic solvent is used. The larger the polarity of an aprotic solvent, the more negative the electronic charge-density of the C=O bond for the monomer, the lower the frequency of the C=O stretch mode.

Hydrogen bond configuration and protonation of ground and lowest excited triplet states of 4‑amino‑4'‑nitrobiphenyl based on nanosecond transient absorption spectroscopy

Intramolecular charge transfer (ICT) is an important photochemical process. In contrast to those in singlet manifold, triplet ICT states were less studied. In this paper, the lowest excited triplet state (T₁) of 4‑amino‑4'‑nitrobiphenyl (NH₂-Bp-NO₂) was recorded with nanosecond transient absorption spectroscopy in acidic acetonitrile and alcoholic solutions. By employing the Kamlet-Taft model to analyze the correlation between absorption maxima and alcohol solvent properties including polarity/polarizability, abilities of hydrogen bond donating and hydrogen bond accepting, hydrogen bond configuration in the ground state (S₀) and T₁ was resolved. The results suggest that the hydrogen bond between amino H and alcohol is dominant in S₀, while in T₁, hydrogen bonds between amino H and alcohol, between nitro O and alcohol have comparable contribution. By examination of the 1‑naphthol quench effect on T₁, the hydrogen bond between nitro O and alcohol was confirmed present. Theoretical calculation results on the model of NH₂-Bp-NO₂-(MeOH)₃ also indicate that hydrogen bonds between amino H and alcohol, between nitro O and alcohol are both much stronger in T₁ than in S₀. In acidic acetonitrile solution, in S₀ of NH₂-Bp-NO₂ the amino group is protonated with pK_a of 4.5, meanwhile in T₁ the nitro group is much easier to be protonated than in S₀. Its conjugated acid was measured to have a pK_a of 3.1.