Understanding the Impact of Symmetrical Substitution on the Photodynamics of Sinapate Esters Using Gas-Phase Ultrafast Spectroscopy

Two model biomimetic systems, ethyl sinapate (ES) and its symmetrical analogue, diethyl 2-(4-hydroxy-3,5-dimethoxybenzylidene)malonate (or diethyl sinapate, DES), are stripped to their core fundamentals through gas-phase spectroscopy to understand the underlying photophysics of photothermal materials. Following photoexcitation to the optically bright S1(ππ*) state, DES is found to repopulate the electronic ground state over three orders of magnitude quicker than its non-symmetrical counterpart, ES. Our XMS-CASPT2 calculations shed light on the experimental results, revealing crucial differences in the potential energy surfaces and conical intersection topography between ES and DES. From this work, a peak conical intersection, seen for DES, shows vital importance for the non-radiative ground state recovery of photothermal materials. This fundamental comparative study highlights the potential impact that symmetrical substitution can have on the photodynamics of sinapate esters, providing a blueprint for future advancement in photothermal technology

Implications of flexible spacer rotational processes on the liquid crystal behavior of 4,5-dihydroisoxazole benzoate dimers

The synthesis of some novel non-symmetric liquid crystal dimers, {3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-(decyloxy)benzoates (5a–d) and 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-{[6-(octyloxy)naphthalen-2-yl]ethynyl}benzoate (9a–d), are reported. The liquid-crystalline properties, theoretical calculations based on the conformational aspects of the flexible alkyl spacer and X-ray experiments are discussed. The syntheses of the key intermediates, 2-{3-[4-(octyloxy)phenyl]-4,5-dihydroisoxazol-5-yl}alkanol (3a–d), presenting the flexible alkyl spacer were achieved through [3+2] cycloaddition reactions between nitrile oxides, which were generated in situ by oxidation of the respective aromatic oximes, and dipolarophile alkenols (CH2[double bond, length as m-dash]CH(CH2)nOH, n = 1, 2, 3, and 4). The benzoates 5a–d were synthesized through esterification of 3a–d and p-n-decyloxybenzoic acid (4). The esters 9a–d were synthesized through derivatization of isoxazolines 3a–d into 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-bromobenzoate (7a–d) followed by a Sonogashira reaction with 2-ethynyl-6-octyloxynaphthalene (8). 5a and 5b showed a monotropic smectic C phase. 9a/c displayed a enantiotropic nematic (N) mesophase, whereas 9b/d showed a monotropic nematic mesophase. No mesophase was observed for 7a–d. An odd–even effect was observed for 5a–d and 9a–d associated with the crystal to isotropic phase transition and crystal to nematic phase, respectively, as the length of the spacer was increased from 1 to 4 carbon atoms. The transitional properties were higher for odd-numbered members (n = 1 and 3) for all of the series studied. The X-ray data of compounds 5a and 5b are in agreement with polarizing optical microscopy observations with the assignment of an SmC mesophase. Density functional theory calculations using the B3LYP hybrid functional with the level 6-311G(d,p) basis set were performed for molecules 5a–d to correlate the conformation of the flexible spacer and the transitional properties. The conformational analysis showed that the most stable conformation for 5a–d is one where all of the carbon atoms of the flexible spacer are orientated at 180° (antiperiplanar orientation) except for 5a because the spacer is too short. The odd-numbered members have a more bent shape and are less elongated molecules than the even-numbered members. Thus, mesomorphic behavior is dictated by the conformational constraint imposed by the flexible spacer on the mesogenic groups

The 2:1 cycloadducts from [3+2] 1,3-dipolar cycloaddition of nitrile oxide and vinylacetic acid. Synthesis and liquid crystal behaviour

Four liquid crystals (LC) 3,7a-bis(4-alkyloxyphenyl)-7,7a-dihydro-6H-isoxazolo[2,3-d][1,2,4]oxadiazol-6-yl)acetic acid (7a-d) were synthesised and the mesomorphic behaviour reported. The LCs were characterised as 2: 1 bisadducts, which were obtained from a double [3+2] 1,3-dipolar cycloaddition. In the first step, the cycloaddition of 4-alkyloxyphenylnitrile oxide (4a-d) and vinylacetic acid (5) gave the initial unobserved 1:1 cycloadducts 2-[3-(4-alkyloxyphenyl)-4,5-dihydroisoxazol-5-yl]acetic acid (6a-d). In the second step, the addition of a second equivalent of 4 to 6 yielded the 2: 1 bisadducts 7a-d without any traces of 6. All compounds 7a-d were unstable during the transition from the mesophase to the isotropic state upon first heating as evidenced by the large peaks in the differential scanning calorimetry traces. Due to the chemical instability of the compounds upon heating, the transition temperature related to the smectic C to smectic A transitions was acquired by means of an image processing method. X-Ray diffraction experiments were also used to analyse the liquid-crystalline phases. A theoretical calculation was performed using density functional theory (DFT) methods at the PBE1PBE/6-311+G(2d,p) level (with solvent effect) in order to get information about the energetic profile of the 2: 1 cycloaddition. DFT studies revealed that the cycloaddition process is controlled by the HOMO(dipolarophile) - LUMO(1,3-dipole), and that the double [3+2] 1,3-dipolar cycloaddition reaction is quite possible.CNPq [471194-2008-5]CAPESINCT-Catalise[PROCAD-2007/CAPES

Direct structural observation of ultrafast photoisomerization dynamics in sinapate esters

Sinapate esters have been extensively studied for their potential application in ‘nature-inspired’ photoprotection. There is general consensus that the relaxation mechanism of sinapate esters following photoexcitation with ultraviolet radiation is mediated by geometric isomerization. This has been largely inferred through indirect studies involving transient electronic absorption spectroscopy in conjunction with steady-state spectroscopies. However, to-date, there is no direct experimental evidence tracking the formation of the photoisomer in real-time. Using transient vibrational absorption spectroscopy, we report on the direct structural changes that occur upon photoexcitation, resulting in the photoisomer formation. Our mechanistic analysis predicts that, from the photoprepared ππ* state, internal conversion takes place through a conical intersection (CI) near the geometry of the initial isomer. Our calculations suggest that different CI topographies at relevant points on the seam of intersection may influence the isomerization yield. Altogether, we provide compelling evidence suggesting that a sinapate ester’s geometric isomerization can be a more complex dynamical process than originally thought

New generation UV-A filters : understanding their photodynamics on a human skin mimic

The sparsity of efficient commercial ultraviolet-A (UV-A) filters is a major challenge toward developing effective broadband sunscreens with minimal human- and eco-toxicity. To combat this, we have designed a new class of Meldrum-based phenolic UV-A filters. We explore the ultrafast photodynamics of coumaryl Meldrum, CMe, and sinapyl Meldrum (SMe), both in an industry-standard emollient and on a synthetic skin mimic, using femtosecond transient electronic and vibrational absorption spectroscopies and computational simulations. Upon photoexcitation to the lowest excited singlet state (S1), these Meldrum-based phenolics undergo fast and efficient nonradiative decay to repopulate the electronic ground state (S0). We propose an initial ultrafast twisted intramolecular charge-transfer mechanism as these systems evolve out of the Franck–Condon region toward an S1/S0 conical intersection, followed by internal conversion to S0 and subsequent vibrational cooling. Importantly, we correlate these findings to their long-term photostability upon irradiation with a solar simulator and conclude that these molecules surpass the basic requirements of an industry-standard UV filter

Towards developing novel and sustainable molecular light-to-heat converters

Light-to-heat conversion materials generate great interest due to their widespread applications, notable exemplars being solar energy harvesting and photoprotection. Another more recently identified potential application for such materials is in molecular heaters for agriculture, whose function is to protect crops from extreme cold weather and extend both the growing season and the geographic areas capable of supporting growth, all of which could help reduce food security challenges. To address this demand, a new series of phenolic-based barbituric absorbers of ultraviolet (UV) radiation has been designed and synthesised in a sustainable manner. The photophysics of these molecules has been studied in solution using femtosecond transient electronic and vibrational absorption spectroscopies, allied with computational simulations and their potential toxicity assessed by in silico studies. Following photoexcitation to the lowest singlet excited state, these barbituric absorbers repopulate the electronic ground state with high fidelity on an ultrafast time scale (within a few picoseconds). The energy relaxation pathway includes a twisted intramolecular charge-transfer state as the system evolves out of the Franck–Condon region, internal conversion to the ground electronic state, and subsequent vibrational cooling. These barbituric absorbers display promising light-to-heat conversion capabilities, are predicted to be non-toxic, and demand further study within neighbouring application-based fields

A semiótica da mídia e sua relação com a epistemologia da ciência : uma leitura junto à educação de jovens e adultos

O emprego de imagens e ilustrações é frequentemente utilizado na química para facilitar a apropriação de conceitos e termos muitas vezes abstratos, que essa ciência apresenta. A mídia é uma das principais produtoras de representações sociais e imagéticas, hoje em dia e, quando se refere à ciência pode corroborar ou não com a apropriação dos referidos conceitos abstratos. Nesse sentido, procurou-se refletir sobre os entendimentos gerados nos processos de significação e representações químicas, sob a perspectiva dos obstáculos epistemológicos de Bachelard e da teoria dos signos proposta por Peirce. Para tanto, foi escolhido um recorte midiático, com recursos centrados na imagem, exibido em um programa de televisão e apresentado a uma turma de EJA, na qual foi realizado um grupo focal. Pode-se analisar junto aos alunos que o excesso de imagens em detrimento da explicação dificulta a aprendizagem por parte destes e a ocorrência de vários obstáculos epistemológicos na relação entre o objeto de estudo, sua representação e interpretação bem como a utilização de vários elementos de senso comum levam a refletir acerca do papel e poder da mídia como veículo difusor da ciência.The use of images and ilustrations in chemistry is often due to facilitate the appropriation of usually abstract concepts and terms that this science presents. Accordingly, sought to reflect on the understandings generated in meaning and representations of chemical, in the light epistemiological obstacles aspects of Bachelard and fluctuating theory proposed by Peirce. The media is one of the main producers of social and imagetic representations on these days and, when related to science, it might or might not validate the appropriation of those abstract concepts. Thereby, the present work sought to make a reflection upon the understandings generated in the process of chemical signification and representations, based upon the epistemiological obstacles aspects of Bachelard and the signs theory proposed by Peirce, and, thereto, it was made a midiatic selection, with resources focusing on the image, displayed on a television programme and presented to students from EJA, institute in which a focal group was formed. It was verified that images in excess instead of explanation makes difficult for the students to learn and that the occurrence of multiple epistemological obstacles aspects in the relation between the object of study, their representation and interpretation as well as the use of various elements of common sense lead to reflect upon the role and power of the media as a vehicle that difuses science

Fotoquímica de carbamatos aromáticos e derivados por métodos de estrutura eletrônica : do rearranjo foto-fries à fotodegradação de pesticidas

A fotodegradação de pesticidas, como os derivados de carbamatos orgânicos, possui um papel importante do ponto de vista ambiental, pois estes são extensivamente utilizados e comumente encontrados como contaminantes da água e do solo. O Rearranjo Foto-Fries (PFR) - a conversão fotoquímica de aril ésteres para orto- e para-hidroxifenonas - desempenha um importante papel na fotodegradação de drogas e pesticidas carbamatos, além de ser um passo chave para a síntese de um grande número de compostos e polímeros funcionais. Embora exista um grande número de estudos experimentais a respeito desse rearranjo, alguns pontos dessa reação ainda estão sob debate. O objetivo deste trabalho é estudar o processo de fotodissociação em carbamatos aromáticos e derivados e, particularmente, fornecer uma visão geral do mecanismo do Rearranjo Foto-Fries, utilizando metodologias computacionais. Entre essas metodologias estão TDDFT, cálculos multiconfiguracionais e Surface Hopping. A superfície de energia potencial para o PFR foi estudada no vácuo gasosa e utilizando solvatação implícita e explícita. Nessa última, a distribuição do solvente foi obtida por simulações de Monte Carlo. Um modelo envolvendo três estados é proposto para o PFR, baseado em cálculos CASSCF(14,12)/CASPT2(14,12). Este modelo fornece uma visão global de todos os passos envolvidos na reação, da fotodissociação até a tautomerização final. O papel do solvente ainda não foi esclarecido, pois a sua adição (implícita ou explícita) não alterou significativamente o comportamento do caminho dissociativo.The photodegradation of pesticides, such as carbamate derivatives, has an environmentally important role, since they are extensively used and commonly found as contaminants in water and soils. The Photo-Fries rearrangement (PFR) - a photochemical conversion of aryl esters to ortho- and para-hydroxyphenones - plays an important role in the photodegradation of drugs and carbamate pesticides, besides being a key step in the synthesis of a large number of compounds and functional polymers. Although there is a large number of experimental studies about the mechanism of PFR, some points of this reaction are still under debate. The goal of this work is to study the photodissociation process in aromatic carbamates and derivatives and, particularly, is to provide a comprehensive picture of PFR, based on computational methods. Several methodologies were used, such as TDDFT, multiconfigurational methods and Trajectory Surface Hopping simulations. Comprehensive explorations of the potential energy surfaces were done in the gas phase and with implicit and explicit solvent, whose distribution was obtained from sequential Monte Carlo sampling. A three-state model for the Photo-Fries Rearrangement is proposed based on CASSCF(14,12)/CASPT2(14,12) calculations. It provides a comprehensive mechanistic picture of all steps of the reaction, from the photoabsorption to the final tautomerization. The role of the solvent is still not understood since the addition of solvent (implicit or explicit) do not change the dissociative pathway significantly

Estudo fotofísico computacional da transferência protônica intramolecular no estado excitado de derivados de benzazóis

A transferência protônica intramolecular no estado excitado (ESIPT) caracteriza-se pela transferência de um próton, presente na estrutura do estado excitado, de um grupo doador para um grupo aceptor de elétrons, a partir de uma ligação de hidrogênio intramolecular. O aumento do interesse no estudo de moléculas que apresentam ESIPT é um reflexo de suas propriedades fotoquímicas e fotofísicas únicas, que as tornam atrativas do ponto de vista sintético e tecnológico. Nesse trabalho, a Teoria do Funcional da Densidade Dependente do Tempo foi utilizada para investigar o processo de ESIPT e as propriedades fotofísicas de dois derivados de benzazóis substituídos por grupos aceptores e doadores de elétrons, denominados NAHBI e NAHBO. Os funcionais utilizados foram o CAM-B3LYP e o PBE1PBE, juntamente com o conjunto de base cc-pVDZ para as otimizações das geometrias e jun-cc-pVTZ para os cálculos de energias e propriedades moleculares. Os efeitos do solvente foram incluídos através do modelo PCM com os solventes 1,4-dioxano, diclorometano, etanol e acetonitrila. Os resultados obtidos com os dois funcionais foram comparados à dados experimentais e à cálculos RI-CC2. Observou-se que no estado fundamental não há a possibilidade de transferência do próton devido à grande instabilidade do tautômero ceto frente ao enol. No primeiro estado excitado os dois tautômeros são estáveis, porém o ESIPT nem sempre é favorecido O funcional PBE1PBE aponta para a forma ceto como sendo a mais estável somente para o NAHBI em 1,4-dioxano e diclorometano e para o NAHBO em dioxano. Já com o funcional CAM-B3LYP, a forma enol é sempre observada como a mais estável. Porém, os valores de comprimentos de onda de máxima absorção e emissão calculados com esse funcional são comparáveis aos experimentais e aos calculados com RI-CC2. Ambos, o solvente e os grupos substituintes influenciam no processo de transferência do próton. A inclusão dos substituintes nas posições específicas implica em uma grande separação de cargas e em um significativo aumento no momento do dipolo no estado excitado. Como resultado, o processo de transferência de carga é favorecido em relação à transferência do próton e deve ocorrer antes dos ESIPT. Isso implica em uma grande influência da polarização causada pelo solvente, refletida no desfavorecimento do ESIPT em solventes polares

A three-state model for the photo-Fries rearrangement

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