Molecules

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11 pages, 1636 KiB

Open AccessArticle

Comparative Investigation of Ultrafast Excited-State Electron Transfer in Both Polyfluorene-Graphene Carboxylate and Polyfluorene-DCB Interfaces

by Amani A. Alsam

Molecules 2024, 29(3), 634; https://doi.org/10.3390/molecules29030634 - 29 Jan 2024

Cited by 1 | Viewed by 993

Abstract

The Photophysical properties, such as fluorescence quenching, and photoexcitation dynamics of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3′-N,N′-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and anionic graphene carboxylate (GC) have been discovered for the first time via steady-state and time-resolved femtosecond transient absorption (TA) [...] Read more.

The Photophysical properties, such as fluorescence quenching, and photoexcitation dynamics of bimolecular non-covalent systems consisting of cationic poly[(9,9-di(3,3′-N,N′-trimethyl-ammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) and anionic graphene carboxylate (GC) have been discovered for the first time via steady-state and time-resolved femtosecond transient absorption (TA) spectroscopy with broadband capabilities. The steady-state fluorescence of PFN is quenched with high efficiency by the GC acceptor. Fluorescence lifetime measurements reveal that the quenching mechanism of PFN by GC is static. Here, the quenching mechanisms are well proven via the TA spectra of PFN/GC systems. For PFN/GC systems, the photo electron transfer (PET) and charge recombination (CR) processes are ultrafast (within a few tens of ps) compared to static interactions, whereas for PFN/1,4-dicyanobenzene DCB systems, the PET takes place in a few hundreds of ps (217.50 ps), suggesting a diffusion-controlled PET process. In the latter case, the PFN^+•–DCB^−• radical ion pairs as the result of the PET from the PFN to DCB are clearly resolved, and they are long-lived. The slow CR process (in 30 ns time scales) suggests that PFN^+• and DCB^−• may already form separated radical ion pairs through the charge separation (CS) process, which recombine back to the initial state with a characteristic time constant of 30 ns. The advantage of the present positively charged polyfluorene used in this work is the control over the electrostatic interactions and electron transfers in non-covalent polyfluorene/quencher systems in DMSO solution. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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19 pages, 28245 KiB

Open AccessArticle

Excited State Dynamics of Alizarin Red S Nanoparticles in Solution

by Pascal Rauthe, Kristina Sabljo, Maike Kristin Vogelbacher, Claus Feldmann and Andreas-Neil Unterreiner

Molecules 2023, 28(15), 5633; https://doi.org/10.3390/molecules28155633 - 25 Jul 2023

Viewed by 1938

Abstract

Alizarin red S is a sulfonated, water-soluble derivative of alizarin. This work presents femtosecond studies of alizarin red S (ARS) nanoparticles in comparison to ARS in aqueous solution and to alizarin in DMSO. The femtosecond studies cover a probing spectral range of 350–750 [...] Read more.

Alizarin red S is a sulfonated, water-soluble derivative of alizarin. This work presents femtosecond studies of alizarin red S (ARS) nanoparticles in comparison to ARS in aqueous solution and to alizarin in DMSO. The femtosecond studies cover a probing spectral range of 350–750 nm using different excitation wavelengths, taking into account the variation of the absorption spectra with the pH values of the solvent. Stationary absorption spectra show slight differences between solution and nanoparticles. Excitation at 530 nm results in low and noisy responses, therefore, we additionally recorded transient spectra of the nanoparticles at λ_ex = 267 nm. While the results in DMSO are comparable to previous studies in non-aqueous solvents, we report a relatively fast relaxation of 14 ps in [La(OH)₂][ARS] nanoparticles in aqueous solution after excitation at 530 nm, which is similar to Na(ARS) solution (19 ps). The dynamics changed with lower pH, but still without significant differences between nanoparticles and solution. We propose [La(OH)₂][ARS] nanoparticles as a suitable alternative to dissolved molecules with similar spectroscopic properties, for example, with regard to biomarker applications. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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10 pages, 1904 KiB

Open AccessArticle

Intersystem Crossing of 2-Methlypyrazine Studied by Femtosecond Photoelectron Imaging

by Naipisai Wumaierjiang, Bumaliya Abulimiti, Fengzi Ling, Mei Xiang, Xulan Deng, Jie Wei and Bing Zhang

Molecules 2022, 27(19), 6245; https://doi.org/10.3390/molecules27196245 - 22 Sep 2022

Viewed by 1573

Abstract

2-methylpyrazine was excited to the high vibrational dynamics of the S₁ state with 260 nm femtosecond laser light, and the evolution of the excited state was probed with 400 nm light. Because it was unstable, the S₁ state decayed via intersystem [...] Read more.

2-methylpyrazine was excited to the high vibrational dynamics of the S₁ state with 260 nm femtosecond laser light, and the evolution of the excited state was probed with 400 nm light. Because it was unstable, the S₁ state decayed via intersystem crossing to the triplet state T₁, and it may have decayed to the ground state S₀ via internal conversion. S₁-to-T₁ intersystem crossing was observed by combining time-resolved mass spectrometry and time-resolved photoelectron spectroscopy. The crossover time scale was 23 ps. Rydberg states were identified, and the photoelectron spectral and angular distributions indicated accidental resonances of the S₁ and T₁ states with the 3s and 3p Rydberg states, respectively, during ionization. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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12 pages, 1789 KiB

Open AccessArticle

Singlet Fission, Polaron Generation and Intersystem Crossing in Hexaphenyl Film

by Wenjun Ni, Tianjiao Li, Christian Kloc, Licheng Sun and Gagik G. Gurzadyan

Molecules 2022, 27(16), 5067; https://doi.org/10.3390/molecules27165067 - 9 Aug 2022

Cited by 1 | Viewed by 2484

Abstract

The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps [...] Read more.

The ultrafast dynamics of triplet excitons and polarons in hexaphenyl film was investigated by time-resolved fluorescence and femtosecond transient absorption techniques under various excitation photon energies. Two distinct pathways of triplet formation were clearly observed. Long-lived triplet states are populated within 4.5 ps via singlet fission-intersystem crossing, while the short-lived triplet states (1.5 ns) are generated via singlet fission from vibrational electronic states. In the meantime, polarons were formed from hot excitons on a timescale of <30 fs and recombined in ultrafast lifetime (0.37 ps). In addition, the characterization of hexaphenyl film suggests the morphologies of crystal and aggregate to wide applications in organic electronic devices. The present study provides a universally applicable film fabrication in hexaphenyl system towards future singlet fission-based solar cells. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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16 pages, 3455 KiB

Open AccessArticle

Conformer-Specific Dissociation Dynamics in Dimethyl Methylphosphonate Radical Cation

by Vaibhav Singh, Hugo A. López Peña, Jacob M. Shusterman, Patricia Vindel-Zandbergen, Katharine Moore Tibbetts and Spiridoula Matsika

Molecules 2022, 27(7), 2269; https://doi.org/10.3390/molecules27072269 - 31 Mar 2022

Cited by 1 | Viewed by 2046

Abstract

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent [...] Read more.

The dynamics of the dimethyl methylphosphonate (DMMP) radical cation after production by strong field adiabatic ionization have been investigated. Pump-probe experiments using strong field 1300 nm pulses to adiabatically ionize DMMP and a 800 nm non-ionizing probe induce coherent oscillations of the parent ion yield with a period of about 45 fs. The yields of two fragments, PO₂C₂H₇⁺ and PO₂CH₄⁺, oscillate approximately out of phase with the parent ion, but with a slight phase shift relative to each other. We use electronic structure theory and nonadiabatic surface hopping dynamics to understand the underlying dynamics. The results show that while the cation oscillates on the ground state along the P=O bond stretch coordinate, the probe excites population to higher electronic states that can lead to fragments PO₂C₂H₇⁺ and PO₂CH₄⁺. The computational results combined with the experimental observations indicate that the two conformers of DMMP that are populated under experimental conditions exhibit different dynamics after being excited to the higher electronic states of the cation leading to different dissociation products. These results highlight the potential usefulness of these pump-probe measurements as a tool to study conformer-specific dynamics in molecules of biological interest. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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12 pages, 1977 KiB

Open AccessCommunication

On the Photostability of Cyanuric Acid and Its Candidature as a Prebiotic Nucleobase

by Luis A. Ortiz-Rodríguez, Sean J. Hoehn and Carlos E. Crespo-Hernández

Molecules 2022, 27(4), 1184; https://doi.org/10.3390/molecules27041184 - 10 Feb 2022

Cited by 1 | Viewed by 2303

Abstract

Cyanuric acid is a triazine derivative that has been identified from reactions performed under prebiotic conditions and has been proposed as a prospective precursor of ancestral RNA. For cyanuric acid to have played a key role during the prebiotic era, it would have [...] Read more.

Cyanuric acid is a triazine derivative that has been identified from reactions performed under prebiotic conditions and has been proposed as a prospective precursor of ancestral RNA. For cyanuric acid to have played a key role during the prebiotic era, it would have needed to survive the harsh electromagnetic radiation conditions reaching the Earth’s surface during prebiotic times (≥200 nm). Therefore, the photostability of cyanuric acid would have been crucial for its accumulation during the prebiotic era. To evaluate the putative photostability of cyanuric acid in water, in this contribution, we employed density functional theory (DFT) and its time-dependent variant (TD-DFT) including implicit and explicit solvent effects. The calculations predict that cyanuric acid has an absorption maximum at ca. 160 nm (7.73 eV), with the lowest-energy absorption band extending to ca. 200 nm in an aqueous solution and exhibiting negligible absorption at longer wavelengths. Excitation of cyanuric acid at 160 nm or longer wavelengths leads to the population of S_5,6 singlet states, which have ππ* character and large oscillator strengths (0.8). The population reaching the S_5,6 states is expected to internally convert to the S_1,2 states in an ultrafast time scale. The S_1,2 states, which have nπ* character, are predicted to access a conical intersection with the ground state in a nearly barrierless fashion (ca. ≤ 0.13 eV), thus efficiently returning the population to the ground state. Furthermore, based on calculated spin–orbit coupling elements of ca. 6 to 8 cm⁻¹, the calculations predict that intersystem crossing to the triplet manifold should play a minor role in the electronic relaxation of cyanuric acid. We have also calculated the vertical ionization energy of cyanuric acid at 8.2 eV, which predicts that direct one-photon ionization of cyanuric acid should occur at ca. 150 nm. Collectively, the quantum-chemical calculations predict that cyanuric acid would have been highly photostable under the solar radiation conditions reaching the Earth’s surface during the prebiotic era in an aqueous solution. Of relevance to the chemical origin of life and RNA-first theories, these observations lend support to the idea that cyanuric acid could have accumulated in large quantities during the prebiotic era and thus strengthens its candidature as a relevant prebiotic nucleobase. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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10 pages, 4184 KiB

Open AccessArticle

Unravelling the Photoprotection Properties of Garden Cress Sprout Extract

by Temitope T. Abiola, Nazia Auckloo, Jack M. Woolley, Christophe Corre, Stéphane Poigny and Vasilios G. Stavros

Molecules 2021, 26(24), 7631; https://doi.org/10.3390/molecules26247631 - 16 Dec 2021

Cited by 4 | Viewed by 3904

Abstract

Plants, as with humans, require photoprotection against the potentially damaging effects of overexposure to ultraviolet (UV) radiation. Previously, sinapoyl malate (SM) was identified as the photoprotective agent in thale cress. Here, we seek to identify the photoprotective agent in a similar plant, garden [...] Read more.

Plants, as with humans, require photoprotection against the potentially damaging effects of overexposure to ultraviolet (UV) radiation. Previously, sinapoyl malate (SM) was identified as the photoprotective agent in thale cress. Here, we seek to identify the photoprotective agent in a similar plant, garden cress, which is currently used in the skincare product Detoxophane nc. To achieve this, we explore the photodynamics of both the garden cress sprout extract and Detoxophane nc with femtosecond transient electronic absorption spectroscopy. With the assistance of liquid chromatography-mass spectrometry, we determine that the main UV-absorbing compound in garden cress sprout extract is SM. Importantly, our studies reveal that the photoprotection properties of the SM in the garden cress sprout extract present in Detoxophane nc are not compromised by the formulation environment. The result suggests that Detoxophane nc containing the garden cress sprout extract may offer additional photoprotection to the end user in the form of a UV filter booster. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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15 pages, 3381 KiB

Open AccessArticle

Experimental and Computational Analysis of Para-Hydroxy Methylcinnamate following Photoexcitation

by Jack Dalton, Gareth W. Richings, Jack M. Woolley, Temitope T. Abiola, Scott Habershon and Vasilios G. Stavros

Molecules 2021, 26(24), 7621; https://doi.org/10.3390/molecules26247621 - 15 Dec 2021

Cited by 3 | Viewed by 2234

Abstract

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S₁(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, [...] Read more.

Para-hydroxy methylcinnamate is part of the cinnamate family of molecules. Experimental and computational studies have suggested conflicting non-radiative decay routes after photoexcitation to its S₁(ππ*) state. One non-radiative decay route involves intersystem crossing mediated by an optically dark singlet state, whilst the other involves direct intersystem crossing to a triplet state. Furthermore, irrespective of the decay mechanism, the lifetime of the initially populated S₁(ππ*) state is yet to be accurately measured. In this study, we use time-resolved ion-yield and photoelectron spectroscopies to precisely determine the S₁(ππ*) lifetime for the s-cis conformer of para-hydroxy methylcinnamate, combined with time-dependent density functional theory to determine the major non-radiative decay route. We find the S₁(ππ*) state lifetime of s-cis para-hydroxy methylcinnamate to be ∼2.5 picoseconds, and the major non-radiative decay route to follow the [¹ππ*→¹nπ*→³ππ*→S₀] pathway. These results also concur with previous photodynamical studies on structurally similar molecules, such as para-coumaric acid and methylcinnamate. Full article

(This article belongs to the Special Issue Ultrafast Dynamics in Chemical Processes)

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