Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 10 de 10
Filtrar
Mais filtros










Tipo de estudo
Intervalo de ano de publicação
1.
J Phys Chem A ; 2020 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-32567862

RESUMO

Triplet-excited-state energies of perylene-monoimides (PMIs) lie in the range 1.12 eV ± 2 meV when compared to singlet-excited-state energies of about 2.39 eV ± 2 meV; therefore, the corresponding naphthalene-linked PMI-Dimer was investigated as a novel singlet-fission (SF) material. Ultrafast transient absorption measurements demonstrated the (S1S0)-to-1(T1T1) transformation and the involvement of a mediating step in the overall 1(T1T1) formation. The intermediate is a charge-transfer state that links the initial (S1S0) with the final 1(T1T1), and imposes charge-transfer character on both, which are thus denoted (S1S0)CT and 1(T1T1)CT. At room temperature, the decorrelation and stability of 1(T1T1)CT is affected by the geminate triplet-triplet recombination (G-TTR) of the two triplets. Independent confirmation for G-TTR to afford up-converted (S1S0)UC in fsTA and nsTA measurements with PMI-Dimer, came from probing PMI-Monomer (T1)s in triplet-triplet annihilation up-conversion (TTA-UC). The G-TTR channel, active in the PMI-Dimer at room temperature, is suppressed by working at either low temperatures (∼140 K) or in polar solvents (benzonitrile): Both scenarios assist in stabilizing (T1T1)CT. As a consequence, the triplet quantum yields are 4.2% and 14.9% at room temperature and 140 K, respectively, in 2-methyltetrahydrofuran.

2.
Angew Chem Int Ed Engl ; 57(33): 10742-10747, 2018 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-29863297

RESUMO

A novel pentacene dimer (P2) and a structurally analogous monomer (P1) were synthesized for use in n-type dye-sensitized solar cells. In P2, the triplet excited states formed by the rapid, spin-allowed process singlet fission were expected to enable carrier multiplication in comparison to the slow, spin-forbidden intersystem crossing seen in P1. A meta-positioning of the two pentacenes and the carboxylate anchor were chosen in P2 to balance the intramolecular dynamics of singlet fission and electron injection. Electron injection from energetically low-lying triplet excited states of pentacene units necessitated the intrinsic and extrinsic lowering of the Fermi level of the semiconductor. Indium-zinc oxide in the presence of Li+ was found to be the optimum choice for the photoelectrodes. Efficient electron injection from the triplet excited states of P1 and P2 was found, with a carrier multiplication of nearly 130 %.

3.
Nanoscale ; 10(18): 8515-8525, 2018 May 10.
Artigo em Inglês | MEDLINE | ID: mdl-29693096

RESUMO

We report on the synthesis, as well as photophysical and electrochemical characterization of a new family of pentacene derivatives, which are applied in n-type dye-sensitized solar cells (DSSCs). As far as the molecular structure of the pentacene is concerned, the synthetic design focuses on cyano acrylic tethered at the 13-position of the pentacene chromophore. The electrolyte composition features increasing amounts of Li+ ions as an additive. In general, the increase of Li+ concentrations extrinsically reduces the quasi Fermi level of the photoanode and as such facilitates the electron injection process. We demonstrate that pentacene derivatives give rise to a unique charge injection process, which is controlled by the positioning of the quasi Fermi level energies as a function of the Li+ concentration. As a result of the enhanced charge injection, device efficiencies as high as 1.5% are achieved, representing a 3-fold increase from previously reported efficiencies in pentacene-based DSSCs. These findings are supported by device analysis in combination with transient absorption and electrochemical impedance spectroscopy assays.

4.
Nanoscale ; 8(19): 10113-23, 2016 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-27122097

RESUMO

We show unambiguous and compelling evidence by means of pump-probe experiments, which are complemented by calculations using ab initio multireference perturbation theory, for intramolecular singlet fission (SF) within two synthetically tailored pentacene dimers with cross-conjugation, namely XC1 and XC2. The two pentacene dimers differ in terms of electronic interactions as evidenced by perturbation of the ground state absorption spectra stemming from stronger through-bond contributions in XC1 as confirmed by theory. Multiwavelength analysis, on one hand, and global analysis, on the other hand, confirm that the rapid singlet excited state decay and triplet excited state growth relate to SF. SF rate constants and quantum yields increase with solvent polarity. For example, XC2 reveals triplet quantum yields and rate constants as high as 162 ± 10% and (0.7 ± 0.1) × 10(12) s(-1), respectively, in room temperature solutions.

5.
Proc Natl Acad Sci U S A ; 112(17): 5325-30, 2015 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-25858954

RESUMO

Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley-Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.

6.
J Am Chem Soc ; 137(2): 857-63, 2015 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-25478639

RESUMO

Understanding the fundamental spin dynamics of photoexcited pentacene derivatives is important in order to maximize their potential for optoelectronic applications. Herein, we report on the synthesis of two pentacene derivatives that are functionalized with the [(2,2,6,6-tetramethylpiperidin-1-yl)oxy] (TEMPO) stable free radical. The presence of TEMPO does not quench the pentacene singlet excited state, but does quench the photoexcited triplet excited state as a function of TEMPO-to-pentacene distance. Time-resolved electron paramagnetic resonance experiments confirm that triplet quenching is accompanied by electron spin polarization transfer from the pentacene excited state to the TEMPO doublet state in the weak coupling regime.


Assuntos
Óxidos N-Cíclicos/química , Fenômenos Magnéticos , Naftacenos/química , Processos Fotoquímicos , Espectroscopia de Ressonância de Spin Eletrônica , Radicais Livres/química , Modelos Moleculares , Conformação Molecular , Naftacenos/síntese química
7.
Chem Commun (Camb) ; 50(77): 11339-42, 2014 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-25119111

RESUMO

In the current work, we have explored a novel synthetic route towards metalated porphycenes and their use in p-type NiO-based dye-sensitized solar cells. Particular emphasis is placed on the influence that the relative positioning of the anchoring group exerts on the DSSC performance.


Assuntos
Metaloporfirinas/química , Energia Solar , Metais/química , Modelos Moleculares , Níquel/química , Oxirredução , Semicondutores
8.
Rev. ADM ; 52(2): 79-84, mar.-abr. 1995. ilus, tab
Artigo em Espanhol | LILACS | ID: lil-162972

RESUMO

Se analizaron 1600 expedientes odontopediátricos de niños mexicanos, con el objeto de determinar la relación existentes entre algunos factores biopsicosociales y los malos hábitos bucales, manifestándose una predisposición significativa en los pacientes hijos únicos, niños llamados con algún nombre extranjero y niños con el mismo nombre que su padre o madre, niños con padre ausente, hijos de padres que rehusan a describir las características de conducta del pacientes, e hijos de padres incongruentes en sus descripciones sobre salud (mental y/o física) y su realidad. Asimismo se detectó una correlación significativa con los niños en mal estado de salud presente, especialmente alergias, así como niños con antecedentes de problemas de salud pasada, especialmente las de tipo crónico. Existe valiosa información en los cuestionarios de admisión odontopediátricos, la cual demuestra asociación significativa entre factores biopsicosociales y los malos hábitos bucales, los cuales una vez reconocidos como problema de conducta que afecta los tejidos bucales y parte de los principales problemas de salud bucal, requieren para su prevención y tratamiento, de un abordaje multidisciplinario orientado a atacar directamente la causa del problema


Assuntos
Humanos , Masculino , Feminino , Pré-Escolar , Adolescente , Hábitos , Bruxismo/epidemiologia , Comportamento Infantil/psicologia , Hábito de Roer Unhas/psicologia , Hipersensibilidade/epidemiologia , Respiração Bucal/epidemiologia , Problemas Sociais , Sucção de Dedo/psicologia , Transtornos Psicóticos/etiologia
9.
Rev. ADM ; 52(1): 22-6, ene.-feb. 1995. ilus
Artigo em Espanhol | LILACS | ID: lil-156447

RESUMO

Se analizaron 1600 expedientes odontopediátricos con el objeto de determinar la incidencia, prevalencia y distribución de los malos hábitos bucales. Un 56 por ciento de los niños demostraron algún hábito bucal pernicioso, manifestándose una predisposición significativa en los pacientes de sexo femenino. Los hábitos de succión no nutritiva se presentaron con mayor frecuencia en los pacientes del grupo de niñez temprana, mientras que los hábitos de morder se manifestaron predominantemente en preadolescentes y adolescentes. La onicofagia y el bruxismo se presentaron en un 23 por ciento, succión digital y respiración bucal en un 11 por ciento, succión del labio y/o lengua en un 7 por ciento. Un 2 por ciento de los pacientes mostraron aglún otro tipo de mal hábito bucal


Assuntos
Humanos , Masculino , Feminino , Pré-Escolar , Adolescente , Hábitos , Bruxismo/epidemiologia , Sucção de Dedo , Hábito de Roer Unhas , Respiração Bucal/epidemiologia , Hábitos Linguais
10.
Rev. ADM ; 49(4): 231-4, jul.-ago. 1992. ilus
Artigo em Espanhol | LILACS | ID: lil-120960

RESUMO

Se presenta el caso de un paciente infantil con fibromatosis gingival y su seguimiento por 7 años, describiéndose sus manifestaciones bucales, clínicas, radiográficas e histológicas, así como sus características generales. Se analiza la evolución del caso y el efecto de los factores que estimulan la hiperplasia gingival, señalándose los procedimientos empleados para evitar las retenciones dentarias y favorecer el periodonto y la oclusión


Assuntos
Humanos , Feminino , Pré-Escolar , Fibromatose Gengival/terapia , Gengivectomia/métodos , Hirsutismo/diagnóstico , Higiene Bucal
SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA