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Dos Santos CHD, Zucolotto Cocca LH, Pelosi AG, Batista VF, Pinto DCGA, Faustino MAF, Vivas MG, de Paula Siqueira J, Mendonça CR, De Boni L. Observation of the two-photon transition enhanced first hyperpolarizability spectra in cinnamaldehyde derivatives: A femtosecond regime study. J Chem Phys 2023; 158:2893715. [PMID: 37260001 DOI: 10.1063/5.0151622] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2023] [Accepted: 05/11/2023] [Indexed: 06/02/2023] Open
Abstract
The application of nonlinear optical effects in optoelectronic devices is still scarce because the irradiance threshold necessary to induce a specific effect is very high. In this context, knowing the frequency-resolved first order molecular hyperpolarizability (β) is essential to identifying regions where this response is intense enough to allow for applications in commercial devices. Thus, herein, we have determined the β spectral dependence of five new push-pull cinnamylidene acetophenone derivatives using femtosecond laser-induced Hyper-Rayleigh Scattering (HRS). A considerable increase in β values was observed in molecules. We found remarkable β values in regions near the two-photon resonance, which are mediated by electron withdrawing and donating groups. This effect was mapped using wavelength-tunable femtosecond Z-scan technique. Furthermore, it was modeled in light of the sum-over-states approach for the second- and third-order nonlinearities. Finally, our outcomes suggest a strategy to obtain large β values mediated by the 2PA transition.
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Affiliation(s)
- Carlos H D Dos Santos
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, Sao Paulo, Brazil
| | - Leandro H Zucolotto Cocca
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, Sao Paulo, Brazil
| | - André Gasparotto Pelosi
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, Sao Paulo, Brazil
| | - Vasco F Batista
- LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
| | - Diana C G A Pinto
- LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
| | - M Amparo F Faustino
- LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
| | - Marcelo G Vivas
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas, Poços de Caldas, Minas Gerais, Brazil
| | | | - Cleber R Mendonça
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, Sao Paulo, Brazil
| | - Leonardo De Boni
- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, Sao Paulo, Brazil
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2
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Zucolotto Cocca LH, Pelosi AG, Abegão LMG, de Q Garcia R, Mulatier JC, Pitrat D, Barsu C, Andraud C, Mendonça CR, Vivas MG, De Boni L. Unveiling the molecular structure and two-photon absorption properties relationship of branched oligofluorenes. Phys Chem Chem Phys 2023; 25:5021-5028. [PMID: 36722878 DOI: 10.1039/d2cp05189c] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Organic molecules have been intensively studied during the last few decades because of their photonics and biological applications. In this material class, the fluorene molecules present outstanding optical features, for example, high values of two-photon absorption (2PA) cross-sections, visible transparency, and high fluorescence quantum yield. Also, it is possible to improve the nonlinear optical response by modifying the fluorene molecular structure. In this context, herein, we have synthesized V and Y-shaped branching oligofluorenes containing two and three fluorene moieties in each branch. Such a molecular strategy may exponentially enhance the nonlinear optical response due to the coherent coupling among the molecular arms. Thus, we combined the use of femtosecond Z-scan spectroscopy and white light transient absorption spectroscopy (TAS) to understand the molecular structure and 2PA property relationship of branching oligofluorenes. The results show that there is a universal relationship between the 2PA cross-section and the effective π-electron number (Neff) given by σ2PA(GM) = (079 ± 0.03)Neff2, which is independent of the molecular shape (linear, V or Y-shaped). Therefore, the intramolecular charge transfer responsible for the cooperative effect among the branches does not occur. This statement is corroborated by the results of the femtosecond TAS technique.
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Affiliation(s)
- Leandro H Zucolotto Cocca
- Photonics Group, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil.
| | - André Gasparotto Pelosi
- Photonics Group, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil.
| | - Luis M G Abegão
- Grupo de Fotônica, Instituto de Física, Universidade Federal de Goiás, Goiânia, 74690-900, GO, Brazil
| | - Rafael de Q Garcia
- Photonics Group, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil.
| | | | - Delphine Pitrat
- ENSL, CNRS, Laboratoire de Chimie UMR 5182, 46 allée d'Italie, 69364 Lyon, France
| | - Cyrille Barsu
- ENSL, CNRS, Laboratoire de Chimie UMR 5182, 46 allée d'Italie, 69364 Lyon, France
| | - Chantal Andraud
- ENSL, CNRS, Laboratoire de Chimie UMR 5182, 46 allée d'Italie, 69364 Lyon, France
| | - Cleber R Mendonça
- Photonics Group, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil.
| | - Marcelo G Vivas
- Laboratório de Espectroscopia Ótica e Fotônica, Universidade Federal de Alfenas, 37715-400 Pocos de Caldas, MG, Brazil
| | - Leonardo De Boni
- Photonics Group, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil.
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3
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Dos Santos CHD, Ferreira DL, Zucolotto Cocca LH, Mourão RS, Schiavon MA, Mendonça CR, De Boni L, Vivas MG. Size-dependent photoinduced transparency in colloidal CdTe quantum dots in the strong confinement regime: an inverse linear relationship. Phys Chem Chem Phys 2022; 25:359-365. [PMID: 36477139 DOI: 10.1039/d2cp05006d] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022]
Abstract
Nanomaterials have been investigated as saturable absorbers for ultrafast lasers because of their large photoinduced transparency related to ground-state bleaching. However, the quantum dot size effect on the photoinduced transparency in the strong confinement regime has not been explored due to the challenge of accurately measuring the ground state and the excited-state absorption cross-sections. At the same time, these optical properties are essential to calculate several chemical and physical quantities at the nanoscale. In this context, we have employed the photoluminescence saturation method to determine the ground-state absorption cross-section and the femtosecond open-aperture Z-scan technique to investigate the size-dependent ground-state bleaching of glutathione-capped CdTe QDs synthesized in an aqueous medium. The results were modeled using rate equations within the three-energy levels approach. Our results pointed out that the photoinduced transparency rate at the 1S3/2(h) → 1S(e) transition peak presents an inverse linear relationship with the QD diameter (from 2.2 nm up to 3 nm). Otherwise, the larger QDs have a higher ground-state cross-section, which is directly proportional to the ground-state bleaching. To explain this apparent contradiction, we calculate the effective absorption coefficient αeff = σ/V (σ is the absorption cross section and V is the QD volume) for the QDs and observed that the smaller QDs have a higher absorption from the ground to the first excited state, corroborating our results. Finally, our results showed that the saturable absorption effect in CdTe-QDs is slightly higher than that obtained for graphene and other 2D materials and smaller than the black phosphorus in the visible region.
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Affiliation(s)
- Carlos H D Dos Santos
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas, Poços de Caldas, MG, Brazil. .,Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil.
| | - Diego L Ferreira
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas, Poços de Caldas, MG, Brazil.
| | - Leandro H Zucolotto Cocca
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil.
| | - Rafael S Mourão
- Grupo de Pesquisa em Química de Materiais, Universidade Federal de São João del-Rei, São João del-Rei, MG, Brazil
| | - Marco A Schiavon
- Grupo de Pesquisa em Química de Materiais, Universidade Federal de São João del-Rei, São João del-Rei, MG, Brazil
| | - Cleber R Mendonça
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil.
| | - Leonardo De Boni
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil.
| | - Marcelo G Vivas
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas, Poços de Caldas, MG, Brazil.
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4
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Abegão LMG, Cocca LHZ, Mulatier JC, Pitrat D, Andraud C, Misoguti L, Mendonça CR, Vivas MG, De Boni L. Effective π-electron number and symmetry perturbation effect on the two-photon absorption of oligofluorenes. Phys Chem Chem Phys 2021; 23:18602-18609. [PMID: 34612397 DOI: 10.1039/d1cp02553h] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Fluorene-based molecules exhibit significant nonlinear optical responses and multiphoton absorption in the visible region, which, combined with the high fluorescence quantum yield in organic solvents, could make this class of materials potentially engaging in diverse photonics applications. Thus, herein, we have determined the two-photon absorption (2PA) of oligofluorenes containing three, five, and seven repetitive units by employing the wavelength-tunable femtosecond Z-scan technique. Our outcomes have shown that the 2PA cross-section in oligofluorenes presents an enhanced value of around 18 GM per Neff, in which Neff is the effective number of π-electrons, for the pure 2PA allowed transition (11Ag-like → 21Ag-like). Furthermore, a weak 2PA transition was observed in the same spectral region strongly allowed by one-photon absorption (11Ag-like → 11Bu-like). This last result suggests a molecular symmetry perturbation, probably induced by the molecular disorder triggered by the increase of moieties in the oligofluorene structure. We have calculated the permanent dipole moment difference related to the lowest-energy transition using the Lippert-Matagaformalism and the 2PA sum-over-states approach to confirm this assumption. Moreover, we have estimated the fundamental limits for the 2PA cross-section in oligofluorenes.
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Affiliation(s)
- Luis M G Abegão
- Photonics Group, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos, SP, Brazil.
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5
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Vivas MG, Barboza CA, Germino JC, Fonseca RD, Silva DL, Vazquez PAM, Atvars TDZ, Mendonça CR, De Boni L. Molecular Structure-Optical Property Relationship of Salicylidene Derivatives: A Study on the First-Order Hyperpolarizability. J Phys Chem A 2021; 125:99-105. [PMID: 33372791 DOI: 10.1021/acs.jpca.0c08530] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The first-order hyperpolarizability of π-conjugated organic molecules is of particular interest for the fabrication of electro-optical modulators. Thus, we investigated the relationship between the molecular structure and the incoherent second-order nonlinear optical response (βHRS) of four salicylidene derivatives (salophen, [Zn(salophen)(OH2)], 3,4-benzophen, [Zn(3,4-benzophen)(OH2)]) dissolved in DMSO. For that, we employed the Hyper-Rayleigh Scattering technique with picosecond pulse trains. Our experimental results pointed out dynamic βHRS values between 32.0 ± 4.8 × 10-30 cm5/esu and 58.5 ± 8.0 × 10-30 cm5/esu at 1064 nm, depending on the molecular geometry of the salicylidene molecules. More specifically, the outcomes indicate a considerable increase of βHRS magnitude (∼30%) when in the ligands are incorporated the Zn(II) ion. We ascribed such results to the rise of the planarity of the π-conjugated backbone of the chromophores caused by the Zn(II). Furthermore, we observed an increase of ∼50% in dynamic βHRS when there is a replacement of one hydrogen atom (salophen molecule) by an acetophenone group (3,4-benzophen). This result is related to the increase of the effective π-electron number and the higher charge transfer induced at the excited state. All these findings were interpreted and supported in the light of time-dependent density functional theory (DFT) calculations. Solvent effects were considered in the quantum chemical calculations using the integral equation formalism variant of the polarizable continuum model.
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Affiliation(s)
- Marcelo G Vivas
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas, Poços de Caldas, Minas Gerais 37130-001, Brazil
| | - Cristina A Barboza
- Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland
| | - Jose C Germino
- Instituto de Química, Universidade Estadual de Campinas, Campinas, São Paulo 13083-861, Brazil
| | - Ruben D Fonseca
- Departamento de Fisica, Universidad Popular del Cesar, Barrio Sabana, 2000004 Valledupar, Cesar, Colombia
| | - Daniel L Silva
- Departamento de Ciências da Natureza, Matemática e Educação, Universidade Federal de São Carlos, Rod. Anhanguera-Km 174, Araras, São Paulo 13600-970, Brazil
| | - Pedro A M Vazquez
- Instituto de Química, Universidade Estadual de Campinas, Campinas, São Paulo 13083-861, Brazil
| | - Teresa D Z Atvars
- Instituto de Química, Universidade Estadual de Campinas, Campinas, São Paulo 13083-861, Brazil
| | - Cleber R Mendonça
- Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, São Paulo 13566-590, Brazil
| | - Leonardo De Boni
- Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, São Paulo 13566-590, Brazil
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6
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Silva LM, Silva DL, Boas MV, Bretonniere Y, Andraud C, Vivas MG. Probing the high performance of photoinduced birefringence in V-shaped azo/PMMA guest-host films. RSC Adv 2020; 10:40806-40814. [PMID: 35519213 PMCID: PMC9057713 DOI: 10.1039/d0ra08379h] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2020] [Accepted: 10/29/2020] [Indexed: 12/23/2022] Open
Abstract
Optical birefringence in polymeric films containing azo-chromophores is an important feature related to the development of several technologies such as electro-optic modulators, optical switching, and optical gates, to cite a few. Therefore, it is essential to understand the main underlying mechanisms describing dynamic switching. In this context, we have investigated the optical birefringence performance of a guest-host film produced from a poly(methyl methacrylate) (PMMA) matrix containing a V-shaped azo-chromophore, which exhibited a larger optical response in comparison to the linear chromophores. The optical birefringence was induced by a linearly polarized diode laser (532 nm, writing laser), while a low-intensity HeNe (632.8 nm) laser and a tungsten-halogen lamp are employed, respectively, to monitor the optical storage and the absorption change during the photoinduced birefringence. Our results pointed out that the guest-host film presents maximum residual optical memory at around 50% and local optical birefringence at around 3.3 × 10-4 in the low concentration and intensity regimes. The high optical birefringence obtained in guest-host films was attributed to the considerable photoisomerization quantum yield in the solid-state (0.15 ± 0.02 for 532 nm). Besides, we have shown that the switching mechanism is driven by angular hole-burning during the first seconds after excitation, and, subsequently, molecular reorientation quickly rises, dominating the photochemical process. The latter mechanism is highly efficient in converting cis to trans molecules (100%), which is responsible for the high residual optical memory obtained. In order to better understand the isomerization mechanism of the azo-chromophore/PMMA film, we performed quantum chemical calculations within the DFT framework. The electronic transitions of the azo-chromophore isomers were determined using the TD-DFT method and potential energy curves (PECs) were constructed to investigate the possibility of the thermal-isomerization process of the V-shaped azo-chromophore through both rotation and inversion mechanisms. For both mechanisms, the amplitude of the energy barrier and activation energy for thermal isomerization are determined and the results are discussed.
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Affiliation(s)
- Lidiana M Silva
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas Poços de Caldas MG Brazil
| | - Daniel L Silva
- Departamento de Ciências da Natureza, Matemática e Educação, Universidade Federal de São Carlos Rod. Anhanguera - Km 174 13600-970 Araras SP Brazil
| | - Mariana V Boas
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas Poços de Caldas MG Brazil
| | - Yann Bretonniere
- Univ. Lyon, ENS de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon 1, Laboratoire de Chimie F69342 Lyon France
| | - Chantal Andraud
- Univ. Lyon, ENS de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon 1, Laboratoire de Chimie F69342 Lyon France
| | - Marcelo G Vivas
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas Poços de Caldas MG Brazil
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7
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Dipold J, Vivas MG, Koeckelberghs G, Siqueira JP, De Boni L, Mendonca CR. Probing the Strong Near-IR Two-Photon Transition in Supramolecular Triphenylamine-based Polymers by Nonlinear Absorption Spectroscopy. J Phys Chem B 2020; 124:6147-6153. [PMID: 32539394 DOI: 10.1021/acs.jpcb.0c04127] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Due to their capability of film formation and remarkable optical features, semiconductor polymers with high two-photon absorption (2PA) have been studied as potential candidates for the development of organic photonic platforms. Furthermore, there is a high demand for photonic devices operating in the near-infrared (IR) region. However, the magnitude of the nonlinear optical response of random coil polymers in the IR region is weak due to the loss of molecular structure caused by increasing the π-conjugated backbone. Thus, herein we aim to investigate the molecular structure and 2PA features relationship for four polymers with supramolecular (helical) rodlike structure. Such polymers have a rigid core based on triphenylamine groups connected to the chiral binaphthalene units and a strong electron-withdrawing group (EWG). This kind of structure allows a very high chromophore density, which was responsible for generating 2PA cross-section between 305 GM and 565 GM in the near-IR (900-1300 nm), depending on the EWG strength. in light of the two-level model within the sum-overstates approach, we estimated the degree of intramolecular charge transfer induced by 2PA in the IR region, and values as high as 50-70% were found. Such a critical outcome allows the 2PA cross-section in the IR region to remain high even though the ratio between the visible/IR-band 2PA cross-section increases as a function of EWG strength.
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Affiliation(s)
- Jessica Dipold
- Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, São Paulo, Brazil
| | - Marcelo G Vivas
- Laboratório de Espectroscopia Óptica e Fotônica, Universidade Federal de Alfenas, Poços de Caldas, Minas Gerais 37715-400, Brazil
| | - Guy Koeckelberghs
- Laboratory for Polymer Synthesis, Katholieke Universiteit Leuven, Celestijnenlaan 200 F, B-3001 Leuven, Belgium
| | - Jonathas P Siqueira
- Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, São Paulo, Brazil.,Instituto de Fı́sica "Gleb Wataghin", Universidade Estadual de Campinas, Campinas, São Paulo 13083-859, Brazil
| | - Leonardo De Boni
- Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, São Paulo, Brazil
| | - Cleber R Mendonca
- Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, São Paulo, Brazil
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8
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Fonseca RD, Vivas MG, Silva DL, Eucat G, Bretonnière Y, Andraud C, Mendonca CR, De Boni L. Intramolecular Cooperative and Anti-Cooperative Effect on the Two-Photon Absorption Cross Section in Triphenylamine Derivatives. J Phys Chem Lett 2019; 10:2214-2219. [PMID: 30958006 DOI: 10.1021/acs.jpclett.9b00518] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
The intramolecular cooperative effect in branched molecules is a consequence of the interaction and extent of electronic coupling among the different axes of charge transfer. Such an effect is the key to obtain remarkable nonlinear optical response in molecular systems. Here we show that triphenylamine derivative molecules containing only two branches present the strongest electronic interaction between them at the excited state, generating exponential enhancement of the 2PA cross section. The primary factor for such behavior was ascribed to the substantial extent and interaction of the π-electron delocalization promoted by the strong electron-donating and acceptor antisymmetrical groups present in each branch. However, for the three-branch molecules we observed an anticooperative effect, i.e., the 2PA cross section decreases as compared to the one-branch structure as we normalized the signal by the effective π-electron number in each molecule.
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Affiliation(s)
- Ruben D Fonseca
- Instituto de Física de São Carlos , Universidade de São Paulo , 13560-970 São Carlos , SP , Brazil
- Departamento de Fisica , Universidad popular del Cesar , Barrio Sabana , 2000004 Valledupar , Cesar , Colombia
| | - Marcelo G Vivas
- Laboratório de Espectroscopia Óptica e Fotônica , Universidade Federal de Alfenas , 37715-400 Poços de Caldas , MG , Brazil
| | - Daniel Luiz Silva
- Departamento de Ciências da Natureza, Matemática e Educação , Universidade Federal de São Carlos , Rod. Anhanguera - Km 174 , 13600-970 Araras , SP , Brazil
| | - Gwenaelle Eucat
- Laboratoire de Chimie , Univ Lyon, Ens de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon 1 , F69342 Lyon , France
| | - Yann Bretonnière
- Laboratoire de Chimie , Univ Lyon, Ens de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon 1 , F69342 Lyon , France
| | - Chantal Andraud
- Laboratoire de Chimie , Univ Lyon, Ens de Lyon, CNRS UMR 5182, Université Claude Bernard Lyon 1 , F69342 Lyon , France
| | - Cleber R Mendonca
- Instituto de Física de São Carlos , Universidade de São Paulo , 13560-970 São Carlos , SP , Brazil
| | - Leonardo De Boni
- Instituto de Física de São Carlos , Universidade de São Paulo , 13560-970 São Carlos , SP , Brazil
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9
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Sousa JCL, Vivas MG, Vale BRC, Ferrari JL, Schiavon DMA. Effect of Mercaptosuccinic Acid Stabilizer Agent on the Optical Properties of Colloidal CdTe Quantum Dots. J Nanosci Nanotechnol 2018; 18:651-658. [PMID: 29768891 DOI: 10.1166/jnn.2018.13932] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
In the present work, we report on the positive effect of mercaptosuccinic acid (MSA) stabilizer agent on the optical features of colloidal CdTe quantum dots (QDs). With this aim, we performed some spectroscopic measurements such as steady-state absorption and fluorescence, fluorescence quantum yield and time-resolved photoluminescence for five MSA-capped CdTe QD samples with different synthesis times. The first general aspect to highlight is that the QDs' average size increased with synthesis time (from 30 to 150 min) while the size dispersion decreased due to the Ostwald ripening mechanism. Second, comparing the optical properties of CdTe QDs obtained from the same synthesis route, we show that MSA stabilizer agent enhanced the optical properties of CdTe QDs as compared with other widely used stabilizer agents such as GSH and TGA. We ascribe this outcome to reduction of the number of surface defects of the CdTe QDs because the MSA stabilizer agent decreases the growth rate of nanocrystals, causing an improvement in their surface quality. In the light of Fermi's golden rule, we observed that for longer synthesis time the optical properties of CdTe QDs increases due to the enhancement of the direct radiative recombination rate of electrons and holes and decrease in the decay rate for core states. Finally, we investigated the pH-dependent fluorescence and demonstrated the similar behaviour in acidic range between MSA-capped CdTe and mercaptocarboxylates-capped CdTe.
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Affiliation(s)
- J C L Sousa
- Departamento de Ciências Naturais, Universidade Federal de São João Del-Rei, Campus Dom Bosco, Praça Dom Helvécio, 74, 36301-160 São João Del-Rei, MG, Brazil
| | - M G Vivas
- Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas, Cidade Universitária, BR 267 Km 533, 37715-400, Poços de Caldas, MG, Brazil
| | - B R C Vale
- Departamento de Ciências Naturais, Universidade Federal de São João Del-Rei, Campus Dom Bosco, Praça Dom Helvécio, 74, 36301-160 São João Del-Rei, MG, Brazil
| | - J L Ferrari
- Departamento de Ciências Naturais, Universidade Federal de São João Del-Rei, Campus Dom Bosco, Praça Dom Helvécio, 74, 36301-160 São João Del-Rei, MG, Brazil
| | - D M A Schiavon
- Departamento de Ciências Naturais, Universidade Federal de São João Del-Rei, Campus Dom Bosco, Praça Dom Helvécio, 74, 36301-160 São João Del-Rei, MG, Brazil
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10
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Silva DL, Fonseca RD, Vivas MG, Ishow E, Canuto S, Mendonca CR, De Boni L. Experimental and theoretical investigation of the first-order hyperpolarizability of a class of triarylamine derivatives. J Chem Phys 2015; 142:064312. [DOI: 10.1063/1.4906893] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Affiliation(s)
- Daniel L. Silva
- Departamento de Ciências da Natureza, Matemática e Educação, Universidade Federal de São Carlos, Rod. Anhanguera–Km 174, 13600-970 Araras, SP, Brazil
- Instituto de Física, Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil
| | - Ruben D. Fonseca
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Marcelo G. Vivas
- Instituto de Ciência de Tecnologia, Universidade Federal de Alfenas, Cidade Universitária - BR 267 Km 533, 37715-400 Poços de Caldas, MG, Brazil
| | - E. Ishow
- CEISAM–UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, 44322 Nantes, France
| | - Sylvio Canuto
- Instituto de Física, Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil
| | - Cleber R. Mendonca
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Leonardo De Boni
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
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Vivas MG, De Boni L, Cooper TM, Mendonca CR. Understanding the Two-Photon Absorption Spectrum of PE2 Platinum Acetylide Complex. J Phys Chem A 2014; 118:5608-13. [DOI: 10.1021/jp503318u] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Affiliation(s)
- Marcelo G. Vivas
- Grupo
de Fotônica, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos, SP Brazil
- Instituto
de Ciência de Tecnologia, Universidade Federal de Alfenas, Cidade
Universitária—BR 267 Km 533, 37715-400 Poços de Caldas, MG Brazil
| | - Leonardo De Boni
- Grupo
de Fotônica, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos, SP Brazil
| | - Thomas M. Cooper
- Materials and Manufacturing
Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States
| | - Cleber R. Mendonca
- Grupo
de Fotônica, Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos, SP Brazil
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12
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Vivas MG, Silva DL, Malinge J, Boujtita M, Zaleśny R, Bartkowiak W, Ågren H, Canuto S, De Boni L, Ishow E, Mendonca CR. Molecular structure-optical property relationships for a series of non-centrosymmetric two-photon absorbing push-pull triarylamine molecules. Sci Rep 2014; 4:4447. [PMID: 24658327 PMCID: PMC3963076 DOI: 10.1038/srep04447] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/21/2014] [Accepted: 03/07/2014] [Indexed: 12/31/2022] Open
Abstract
This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.
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Affiliation(s)
- Marcelo G Vivas
- 1] Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil [2] Instituto de Ciência e Tecnologia, Universidade Federal de Alfenas, Cidade Universitária - BR 267 Km 533, 37715-400 Poços de Caldas, MG, Brazil
| | - Daniel L Silva
- 1] Instituto de Física, Universidade de São Paulo, CP 66318, 05314-970, São Paulo, SP, Brazil [2] Departamento de Ciências da Natureza, Matemática e Educação, Universidade Federal de São Carlos, Rod. Anhanguera - Km 174, 13600-970 Araras, SP, Brazil
| | - Jérémy Malinge
- PPSM-UMR CNRS 8531, ENS Cachan, 61 avenue du Président Wilson, 94235 Cachan, France
| | - Mohammed Boujtita
- CEISAM-UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, 44322 Nantes, France
| | - Robert Zaleśny
- Theoretical Chemistry Group, Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
| | - Wojciech Bartkowiak
- Theoretical Chemistry Group, Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
| | - Hans Ågren
- Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, SE-106 91 Stockholm, Sweden
| | - Sylvio Canuto
- Instituto de Física, Universidade de São Paulo, CP 66318, 05314-970, São Paulo, SP, Brazil
| | - Leonardo De Boni
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Eléna Ishow
- 1] PPSM-UMR CNRS 8531, ENS Cachan, 61 avenue du Président Wilson, 94235 Cachan, France [2] CEISAM-UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, 44322 Nantes, France
| | - Cleber R Mendonca
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
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Vivas MG, Silva DL, De Boni L, Bretonniere Y, Andraud C, Laibe-Darbour F, Mulatier JC, Zaleśny R, Bartkowiak W, Canuto S, Mendonca CR. Revealing the Electronic and Molecular Structure of Randomly Oriented Molecules by Polarized Two-Photon Spectroscopy. J Phys Chem Lett 2013; 4:1753-1759. [PMID: 26282990 DOI: 10.1021/jz4007004] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/04/2023]
Abstract
In this Letter, we explored the use of polarized two-photon absorption (2PA) spectroscopy, which brings additional information when compared to methods that do not use polarization control, to investigate the electronic and molecular structure of two chromophores (FD43 and FD48) based on phenylacetylene moieties. The results were analyzed using quantum chemical calculations of the two-photon transition strengths for circularly and linearly polarized light, provided by the response function formalism. On the basis of these data, it was possible to distinguish and identify the excited electronic states responsible for the lowest-energy 2PA-allowed band in both chromophores. By modeling the 2PA circular-linear dichroism, within the sum-over-essential states approach, we obtained the relative orientation between the dipole moments that are associated with the molecular structure of the chromophores in solution. This result allowed to correlate the V-shape structure of the FD48 chromophore and the quantum-interference-modulated 2PA strength.
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Affiliation(s)
- Marcelo G Vivas
- †Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Daniel L Silva
- ‡Instituto de Fı́sica, Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil
| | - Leonardo De Boni
- †Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Yann Bretonniere
- §CNRS, Université Lyon I, ENS-Lyon, 46 allée d'Itale, 69364 Lyon, France
| | - Chantal Andraud
- §CNRS, Université Lyon I, ENS-Lyon, 46 allée d'Itale, 69364 Lyon, France
| | - Florence Laibe-Darbour
- ⊥Laboratoire de Chimie, CNRS UR 5182, ENS de Lyon, Université Lyon I, 46 allée d'Italie, 69364 Lyon cedex 07, France
| | - J-C Mulatier
- ⊥Laboratoire de Chimie, CNRS UR 5182, ENS de Lyon, Université Lyon I, 46 allée d'Italie, 69364 Lyon cedex 07, France
| | - Robert Zaleśny
- #Theoretical Chemistry Group, Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
| | - Wojciech Bartkowiak
- #Theoretical Chemistry Group, Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland
| | - Sylvio Canuto
- ‡Instituto de Fı́sica, Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil
| | - Cleber R Mendonca
- †Instituto de Fı́sica de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
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Vivas MG, Diaz C, Echevarria L, Mendonca CR, Hernández FE, De Boni L. Two-Photon Circular–Linear Dichroism of Perylene in Solution: A Theoretical–Experimental Study. J Phys Chem B 2013; 117:2742-7. [DOI: 10.1021/jp311065n] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Affiliation(s)
- Marcelo G. Vivas
- Instituto de Física de
São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Carlos Diaz
- Department of Chemistry, University of Central Florida, P.O. Box 162366, Orlando,
Florida 32816-2366, United States
| | - Lorenzo Echevarria
- Department of Chemistry, University of Central Florida, P.O. Box 162366, Orlando,
Florida 32816-2366, United States
- Departamento de Química, Universidad Simón Bolivar, Caracas 1020A, Venezuela
| | - Cleber R. Mendonca
- Instituto de Física de
São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
| | - Florencio E. Hernández
- Department of Chemistry, University of Central Florida, P.O. Box 162366, Orlando,
Florida 32816-2366, United States
- CREOL/The College of Optics
and Photonics, University of Central Florida, P.O. Box 162366, Orlando, Florida 32816-2366, United States
| | - Leonardo De Boni
- Instituto de Física de
São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
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Vivas MG, Koeckelberghs G, Mendonca CR. Effect of solvent-induced coil to helix conformational change on the two-photon absorption spectrum of poly(3,6-phenanthrene). J Phys Chem B 2012; 116:14708-14. [PMID: 23176606 DOI: 10.1021/jp308161x] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
This paper investigates the effect of solvent-induced conformational changes of poly(3,6-phenanthrene) on their two-photon absorption (2PA). Such effect was studied employing the wavelength-tunable femtosecond Z-scan technique and modeled using the sum-over-essential states approach. We observed a strong reduction of the 2PA cross-section when the sample was prepared in hexane (poor solvent) in comparison to chloroform (good solvent), which is related to the conformation adopted by the polymer in each case. In chloroform it adopts a random coil conformation, as opposed to the one-handed helix conformation in hexane. Our results pointed out that the coil to helix conformation change decreases the degree of molecular planarity of the polymer π-conjugated backbone, which is primarily responsible for their optical nonlinearity, contributing to diminishing the effective transition dipole moments and, consequently, the 2PA cross-section. Moreover, by studying the nonlinear response with different light polarization, we showed that, although the solvent-induced conformational change does not alter the molecular symmetry of the polymer, it modifies considerably the direction of the transition dipole moments between the excited states.
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Affiliation(s)
- M G Vivas
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP.
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Vivas MG, Silva DL, De Boni L, Bretonniere Y, Andraud C, Laibe-Darbour F, Mulatier JC, Zaleśny R, Bartkowiak W, Canuto S, Mendonca CR. Experimental and Theoretical Study on the One- and Two-Photon Absorption Properties of Novel Organic Molecules Based on Phenylacetylene and Azoaromatic Moieties. J Phys Chem B 2012; 116:14677-88. [DOI: 10.1021/jp310731t] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Marcelo G Vivas
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil.
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Vivas MG, De Boni L, Bretonniere Y, Andraud C, Mendonca CR. Polarization effect on the two-photon absorption of a chiral compound. Opt Express 2012; 20:18600-18608. [PMID: 23038499 DOI: 10.1364/oe.20.018600] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
In this report, we investigate the polarization effect (linear, elliptical and circular) on the two-photon absorption (2PA) properties of a chiral compound based in azoaromatic moieties using the femtosecond Z-scan technique with low repetition rate and low pulse energy. We observed a strong 2PA modulation between 800 nm and 960 nm as a function the polarization changes from linear through elliptical to circular. Such results were interpreted employing the sum-over-essential states approach, which allowed us to model the 2PA circular-linear dichroism effect and to identifier the overlapping of the excited electronic states responsible by the 2PA allowed band.
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Affiliation(s)
- M G Vivas
- Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, SP, Brazil.
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Vivas MG, Fernandes EG, Rodríguez-Méndez ML, Mendonca CR. Study of singlet excited state absorption spectrum of lutetium bisphthalocyanine using the femtosecond Z-scan technique. Chem Phys Lett 2012. [DOI: 10.1016/j.cplett.2012.02.028] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Ferreira PHD, Vivas MG, De Boni L, dos Santos DS, Balogh DT, Misoguti L, Mendonca CR. Femtosecond laser induced synthesis of Au nanoparticles mediated by chitosan. Opt Express 2012; 20:518-523. [PMID: 22274373 DOI: 10.1364/oe.20.000518] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
This paper reports the synthesis of Au nanoparticles by 30-fs pulses irradiation of a sample containing HAuCl4 and chitosan, a biopolymer used as reducing agent and stabilizer. We observed that it is a multi-photon induced process, with a threshold irradiance of 3.8 × 10(11) W/cm2 at 790 nm. By transmission electron microscopy we observed nanoparticles from 8 to 50 nm with distinct shapes. Infrared spectroscopy indicated that the reduction of gold and consequent production of nanoparticles is related to the fs-pulse induced oxidation of hydroxyl to carbonyl groups in chitosan.
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Affiliation(s)
- P H D Ferreira
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, Sao Paulo, Brazil
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Vivas MG, Silva DL, Misoguti L, Zaleśny R, Bartkowiak W, Mendonca CR. Degenerate two-photon absorption in all-trans retinal: nonlinear spectrum and theoretical calculations. J Phys Chem A 2010; 114:3466-70. [PMID: 20155894 DOI: 10.1021/jp910010g] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
In this work we investigate the degenerate two-photon absorption spectrum of all-trans retinal in ethanol employing the Z-scan technique with femtosecond pulses. The two-photon absorption (2PA) spectrum presents a monotonous increase as the excitation wavelength approaches the one-photon absorption band and a peak at 790 nm. We attribute the 2PA band to the mixing of states (1)B(u)(+)-like and |S(1)>, which are strongly allowed by one- and two-photon, respectively. We modeled the 2PA spectrum by using the sum-over-states approach and obtained spectroscopic parameters of the electronic transitions to |S(1)>, |S(2)> ("(1)B(u)(+)"), |S(3)>, and |S(4)> singlet-excited states. The results were compared with theoretical predictions of one- and two-photon transition calculations using the response functions formalism within the density functional theory framework with the aid of the CAM-B3LYP functional.
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Affiliation(s)
- M G Vivas
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP
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Vivas MG, Shih T, Voss T, Mazur E, Mendonca CR. Nonlinear spectra of ZnO: reverse saturable, two- and three-photon absorption. Opt Express 2010; 18:9628-9633. [PMID: 20588810 DOI: 10.1364/oe.18.009628] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
We present a broadband (460 - 980 nm) analysis of the nonlinear absorption processes in bulk ZnO, a large-bandgap material with potential blue-to-UV photonic device applications. Using an optical parametric amplifier we generated tunable 1-kHz repetition rate laser pulses and employed the Z-scan technique to investigate the nonlinear absorption spectrum of ZnO. For excitation wavelengths below 500 nm, we observed reverse saturable absorption due to one-photon excitation of the sample, agreeing with rate-equation modeling. Two- and three-photon absorption were observed from 540 to 980 nm. We also determined the spectral regions exhibiting mixture of nonlinear absorption mechanisms, which were confirmed by photoluminescence measurements.
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Affiliation(s)
- M G Vivas
- Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970 São Carlos, SP, Brazil
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