Solvent Effects on the Singlet-Triplet Couplings in Nitroaromatic Compounds

Hydrogen-bonding interaction between the T1 state of 2-nitrofluorene and alcoholic molecules: Enhancing the oxidative reactivity of the triplet state

2-Nitrofluorene (2-NF) is one of the most abundant nitro polycyclic aromatic hydrocarbons (NPAHs), airborne pollutants, and is known for their enhanced toxicity and potential health impacts. This study investigates the reactivity of the T1 state of 2-nitrofluorene, denoted as 2-3NF, toward proton, electron, and hydrogen donors in both polar aprotic and protic solvents, using nanosecond transient absorption and resonance Raman spectroscopies. For the first time, we characterize 2-3NF in various solvents using these techniques. Our results reveal an acid-base equilibrium constant pKa of 1.8 for 2-3NF, indicating much stronger basicity compared to other NPAHs in their T1 state. Resonance Raman and TA spectroscopic analyses confirm the presence of hydrogen-bond interaction between the 2-3NF and alcohol solvent molecules. This interaction significantly enhances the reduction potential of 2-3NF and influences its photoreduction reactivity with substrates such as 1,4-cyclohexadiene and 1-naphthol, where the electron transfer coupled with proton transfer is the primary mechanism. These results provide valuable insight into the role hydrogen bonds play in modulating the reduction reactivity and oxidation potential of NPAHs in their excited states. Furthermore, this study highlights the complex influence of solvent interactions on photochemical processes, emphasizing the importance of incorporating these effects in atmospheric chemistry studies.

Relaxation and Photochemistry of Nitroaromatic Compounds: Intersystem Crossing through 1ππ* to Higher 3ππ* States, and NO• Dissociation in 9-Nitroanthracene─A Theoretical Study

Determination of the photodegradation pathways of nitroaromatic compounds, known for their mutagenic properties and toxicity, is a relevant topic in atmospheric chemistry. In the present theoretical study, mechanisms for the photophysical relaxation and NO• dissociation of 9-nitroanthracene (9-NA) are proposed that challenge the commonly assumed pathways based on the El-Sayed rules. The analysis of the stationary points on the potential energy surfaces obtained with multiconfigurational methods indicates that after light absorption and subsequent relaxation of the S1 state, the system undergoes ultrafast intersystem crossing to T2, which serves as a gate-state to the triplet manifold due to favorable energetic couplings. This occurs despite the nature of the singlet and triplet states being 1ππ* and 3ππ*, where the receiver triplet involves NO2 orbitals that are tilted from the polyaromatic plane, with no involvement of the 3nπ state in the process. After the singlet to triplet manifold crossing, the system evolves along two possible trajectories. One leads to the global minimum of T1 (phosphorescent end state) and the other involves the dissociation into antryloxy and NO• radicals. Overall, the information obtained is in agreement with steady-state and time-resolved spectroscopic data reported for 9-NA. Furthermore, it suggests that the deactivation mechanism of nitroaromatic compounds can take place between 1ππ* and 3ππ* states, which opens a new landscape for the rationalization of the photophysics of these and other systems.

Wavelength-dependent intersystem crossing dynamics of phenolic carbonyls in wildfire emissions

Quantum chemical calculations were employed to construct Jablonski diagrams for a series of phenolic carbonyls, including vanillin, iso-vanillin, 4-hydroxybenzaldehyde, syringaldehyde, and coniferyl aldehyde. These molecules can enter the Earth's atmosphere from forest fire emissions and participate in photochemical reactions within the atmospheric condensed phase, including cloud and fog droplets and aqueous aerosol particles. This photochemistry alters the composition of light-absorbing organic content, or brown carbon, in droplets and particles through the formation and destruction of key chromophores. This study demonstrates that following photon absorption, phenolic carbonyls efficiently transition to triplet states via intersystem crossings (ISC), with rate coefficients ranging from 109 to 1010 s-1. Despite the presence of multiple potential ISC pathways due to several lower-lying triplet states, a single channel is found to dominate for each system. We further investigated the dependence of the ISC rate constant (kISC) on the vibrational excitation energy of the first accessible (ππ*) singlet excited state (S1 or S2, depending on the molecule), and compared it with the measured wavelength dependence of the photochemical quantum yield (Φloss). Although our model only accounts for intramolecular nonradiative electronic transitions, it successfully captures the overall trends. All studied molecules, except coniferyl aldehyde, exhibit saturation in the dependence of both kISC and Φloss on the wavelength (or vibrational excitation energy). In contrast, coniferyl aldehyde displays a single maximum, followed by a monotonic decrease as the excitation energy increases (wavelength decreases). This distinct behavior in coniferyl aldehyde may be attributed to the presence of a double-bonded substituent, which enhances π-electron conjugation, and reduces the exchange energy and thus the adiabatic energy gap between the S1(ππ*) state and the target triplet state. For small energy gaps, the classical acceptor modes of the ISC process are less effective, leading to a low effective density of final states. Larger gaps enhance the effective density of states, making the wavelength dependence of the ISC more pronounced. Our calculations show that while all the studied phenolic carbonyls have similar acceptor modes, coniferyl aldehyde has a substantially smaller adiabatic gap (1700 cm-1) than the other molecules. The magnitude of the adiabatic energy gap is identified as the primary factor determining the energy/wavelength dependence of the ISC rate and thus Φloss.

Competing Nonadiabatic Relaxation Pathways for Near-UV Excited ortho-Nitrophenol in Aqueous Solution

Nitrophenols are atmospheric pollutants found in brown carbon aerosols produced by biomass burning. Absorption of solar radiation by these nitrophenols contributes to atmospheric radiative forcing, but quantifying this climate impact requires better understanding of their photochemical pathways. Here, the photochemistry of near-UV (λ = 350 nm) excited ortho-nitrophenol in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy over the fs to μs time scale to characterize the excited states, intermediates, and photoproducts. Interpretation of the transient spectroscopy data is supported by quantum chemical calculations using linear-response time-dependent density functional theory (LR-TDDFT). Our results indicate efficient nonradiative decay via an S1(ππ*)/S0 conical intersection leading to hot ground state ortho-nitrophenol which vibrationally cools in solution. A previously unreported minor pathway involves intersystem crossing near an S1(nπ*) minimum, with decay of the resulting triplet ortho-nitrophenol facilitated by deprotonation. These efficient relaxation pathways account for the low quantum yields of photodegradation.

Ultrafast Intersystem Crossing in Benzanthrone: Effect of Hydrogen Bonding and Viscosity

Understanding the intricate factors governing intersystem crossing (ISC) in aromatic carbonyl compounds remains a long-standing interest among researchers. This study unveils the crucial roles of vibration in influencing the ISC of a typical aromatic carbonyl chromophore, benzanthrone, and how hydrogen bonding and solvent viscosity affect these vibrations and, thus, the associated ISC kinetics. We demonstrate that for benzanthrone, the ISC is exceedingly facile in an aprotic solvent, while in protic solvents, the ISC is significantly suppressed through the formation of the hydrogen-bonded state. Moreover, in a high-viscosity medium, ISC is further retarded due to restrictions of volume-changing motions, which may assist ISC. Theoretical calculations revealed that the C═O bond vibration and specific out-of-plane vibrations accompanying a volume change could be the probable coordinates for ISC. These findings provide valuable insights for tailoring the excited-state behavior of carbonyl-functionalized materials for diverse applications in photocatalysis, organic electronics, and biomedicine.

Unexpected longer T1 lifetime of 6-sulfur guanine than 6-selenium guanine: the solvent effect of hydrogen bonds to brake the triplet decay

The decay of the T1 state to the ground state is an essential property of photosensitizers because it decides the lifetime of excited states and, thus, the time window for sensitization. The sulfur/selenium substitution of carbonyl groups can red-shift absorption spectra and enhance the triplet yield because of the large spin-orbit coupling, modifying nucleobases to potential photosensitizers for various applications. However, replacing sulfur with selenium will also cause a much shorter T1 lifetime. Experimental studies found that the triplet decay rate of 6-seleno guanine (6SeGua) is 835 times faster than that of 6-thio guanine (6tGua) in aqueous solution. In this work, we reveal the mechanism of the T1 decay difference between 6SeGua and 6tGua by computing the activation energy and spin-orbit coupling for rate calculation. The solvent effect of water is treated with explicit microsolvation and implicit solvent models. We find that the hydrogen bond between the sulfur atom of 6tGua and the water molecule can brake the triplet decay, which is weaker in 6SeGua. This difference is crucial to explain the relatively long T1 lifetime of 6tGua in an aqueous solution. This insight emphasizes the role of solvents in modulating the excited state dynamics and the efficiency of photosensitizers, particularly in aqueous environments.

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.