Steric effects in light-induced solvent proton abstraction

Lahiri, Jurick; Moemeni, Mehdi; Magoulas, Ilias; Yuwono, Stephen H.; Kline, Jessica; Borhan, Babak; Piecuch, Piotr; Jackson, James E.; Blanchard, G. J.; Dantus, Marcos

doi:10.1039/d0cp03037f

Cited by 5 publications

(11 citation statements)

References 32 publications

(40 reference statements)

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“…For the time-resolved spectral evolution seen with FR0 -SB, there were well-resolved emission bands corresponding to unprotonated and protonated species, − but for FR0 there is no corresponding imine and, consequently, no analogous spectrally resolved features. It is thus important to consider first whether the data shown in Figure a,b represent spectral shifts of a single electronic state in time or the evolution of populations in multiple spectrally overlapped bands.…”

Section: Resultsmentioning

confidence: 99%

“…As mentioned in the Introduction section, the super photobase FR0 -SB exhibits an anomalously large change in its excited-state p K b , and among the issues that remain to be resolved is the role that the fluorene derivative chromophore structure plays in producing this behavior. In this work, we investigate the precursor to FR0 -SB, the aldehyde FR0 (Figure b), which exhibits unusual fluorescence relaxation dynamics in protic solvents.…”

Section: Resultsmentioning

confidence: 99%

“…The ability to alter the acidity or basicity of certain compounds through photoexcitation has been explored extensively for photoacids, which are photoactivated proton donors, , but the development of their photobase counterparts, which are able to abstract available protons from their local environment upon photoexcitation, has been much more limited. , Recently, we have focused on the design and characterization of the super photobase FR0 -SB − that is capable of exhibiting a remarkable increase in K b of 14 orders of magnitude (or Δp K b ∼ 14) upon photoexcitation. The behavior of FR0 -SB (Figure a) in protic and aprotic media has been reported recently, − providing fundamental insights into the proton abstraction reaction that occurs in protic solvents. Among the issues that require further investigation is the chemical structural basis for such a large change in the basicity of the imine nitrogen upon photoexcitation.…”

Section: Introductionmentioning

confidence: 99%

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Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent

et al. 2021

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Substituted fluorene structures have demonstrated unusual photochemical properties. Previous reports on the substituted fluorene Schiff base FR0-SB demonstrated super photobase behavior with a ΔpK b of ∼14 upon photoexcitation. In an effort to understand the basis for this unusual behavior, we have examined the electronic structure and relaxation dynamics of the structural precursor of FR0-SB, the aldehyde FR0, in protic and aprotic solvents using time-resolved fluorescence spectroscopy and quantum chemical calculations. The calculations show three excited singlet states in relatively close energetic proximity. The spectroscopic data are consistent with relaxation dynamics from these electronic states that depend on the presence and concentration of solvent hydroxyl functionality. These results underscore the central role of solvent hydrogen bonding to the FR0 aldehyde oxygen in mediating the relaxation dynamics within this molecule.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent

et al. 2021

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“…Two-photon excitation of the super photobase FR0 -SB leads to the electronically excited S 1 state FR0 -SB*, which shows emission at ∼21,000 cm –1 . In protic solvents, excited-state proton transfer from the solvent to the photobase FR0 -SB* leads to the formation of the excited protonated photobase FR0 -HSB + *, which emits at ∼15,000 cm –1 . , Because the SHG spectrum provides an accurate measurement of | E (2) (ω)| 2 , which, as shown by comparing panels b and c of Figure , can be accurately modeled using eq , it serves in this study as a reference. In this study, we collected the frequency integrated TPEF signals using shaped and TL pulses for FR0 -SB in methanol and compared them to the SHG reference data.…”

Section: Resultsmentioning

confidence: 99%

“…Here, we control the TPE pathways of the fluorescent Schiff photobase ( E )-7-((butylimino)methyl)- N , N -diethyl-9,9-dimethyl-9 H -fluoren-2-amine ( FR0 -SB, Scheme ), in solution. − This compound, in addition to exhibiting an increase in p K a by 14 units upon photoexcitation, was recently found to become up to 62% more reactive when the first excited singlet (S 1 ) state was populated via TPE instead of single-photon excitation . This is a consequence of the very large differences between the ground- and excited-state dipole moments Δμ 10 = μ 1 – μ 0 and the large values of the corresponding transition dipole moments μ 10 (for example, Δμ 10 = 15.6 D and μ 10 = 9.6 D for FR0 -SB in methanol) .…”

Section: Introductionmentioning

confidence: 99%

Controlling Quantum Interference between Virtual and Dipole Two-Photon Optical Excitation Pathways Using Phase-Shaped Laser Pulses

et al. 2021

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Two-photon excitation (TPE) proceeds via a “virtual” pathway, which depends on the accessibility of one or more intermediate states, and, in the case of non-centrosymmetric molecules, an additional “dipole” pathway involving the off-resonance dipole-allowed one-photon transitions and the change in the permanent dipole moment between the initial and final states. Here, we control the quantum interference between these two optical excitation pathways by using phase-shaped femtosecond laser pulses. We find enhancements by a factor of up to 1.75 in the two-photon-excited fluorescence of the photobase FR0-SB in methanol after taking into account the longer pulse duration of the shaped laser pulses. Simulations taking into account the different responses of the virtual and dipole pathways to external fields and the effect of pulse shaping on two-photon transitions are found to be in good agreement with our experimental measurements. The observed quantum control of TPE in the condensed phase may lead to an enhanced signal at a lower intensity in two-photon microscopy, multiphoton-excited photoreagents, and novel spectroscopic techniques that are sensitive to the magnitude of the contributions from the virtual and dipole pathways to multiphoton excitations.

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Excited State Proton Transfer from Acidic Alcohols to a Quinoline Photobase Can Be Solvated by Non-Acidic Alcohol Solvents

Hunt,

Hecht,

Goolsby

et al. 2024

J. Phys. Chem. A

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Photobases are a type of molecule that become more basic upon photoexcitation and can therefore be used to control proton transfer reactions with light. The solvation requirements for excited state proton transfer (ESPT) in photobase systems is poorly understood, which limits their applicability. Here, we investigate the solvation of the ESPT reaction using 5-methoxyquinoline (MeOQ), a well-studied photobase with an excited state pK a (pK a *) of approximately 15.1, as a model system. Previous studies have shown that, in addition to the acidic donor that donates a proton to the photoexcited MeOQ, an additional "auxiliary donor" is necessary to solvate the resulting alkoxide. We investigate whether a nonacidic hydrogen bond donor (an alcohol solvent that MeOQ cannot deprotonate in bulk) can act as the auxiliary donor for the MeOQ ESPT reaction. First, we use steady state spectroscopy, TCSPC, and electronic structure calculations to show that MeOQ can deprotonate the acidic donor 2,2,2-trifluoroethanol (TFE, pK a = 12.5) using ethanol as the auxiliary donor. We show that the degree of ESPT is largely predicted by the degree of ground state hydrogen bonding between the photobase and the acidic donor. Next, we study the deprotonation of the acidic donors TFE and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP, pK a = 9.3) with MeOQ in a variety of nonacidic alcohol solvents of varying chain length and branching. MeOQ ESPT occurs to varying extents in all solvents, suggesting that all studied nonacidic alcohols can function as auxiliary donors. We show that the concentration of the acidic donor is strongly correlated with the degree of ESPT. These results are necessary fundamental steps toward the understanding of the photobase ESPT reaction and its wide application in a variety of chemical systems.

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Steric effects in light-induced solvent proton abstraction

Abstract: The significance of solvent structural factors in the excited-state proton transfer (ESPT) reactions of Schiff bases with alcohols is reported here. We use the super photobase FR0-SB and a series...

Cited by 5 publications

References 32 publications

Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent

Excited-State Dynamics of a Substituted Fluorene Derivative. The Central Role of Hydrogen Bonding Interactions with the Solvent

Controlling Quantum Interference between Virtual and Dipole Two-Photon Optical Excitation Pathways Using Phase-Shaped Laser Pulses

Excited State Proton Transfer from Acidic Alcohols to a Quinoline Photobase Can Be Solvated by Non-Acidic Alcohol Solvents

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