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Hyperpolarizabilities of end-capped triply branched dendrimers: a theoretical study
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
KTH, School of Biotechnology (BIO), Theoretical Chemistry (closed 20110512).
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(English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690Article in journal (Other academic) Submitted
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-7958OAI: oai:DiVA.org:kth-7958DiVA: diva2:13147
Note
QS 20120328Available from: 2008-02-05 Created: 2008-02-05 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Theoretical Characterization of Optical Processes in Modecular Complexes
Open this publication in new window or tab >>Theoretical Characterization of Optical Processes in Modecular Complexes
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The main theme of this thesis is to study effects of different environments on geometric and electronic structures, as well as optical responses, of molecules using time-(in)dependent density functional theory. Theoretical calculations have been carried out for properties that can be measured by conventional and advanced experimental techniques, including one-photon absorption (OPA), two-photon absorption (TPA), surface-enhanced Raman scattering (SERS) and second order nonlinear optical (NLO) response. The obtained good agreement between the theory and the experiment allows to further extract useful information about inter- and intra-molecular interactions that are not accessible experimentally.

By comparing calculated one-photon absorption spectra of aluminum phthalocyanine chloride (AlPcCl) and AlPcCl -water complexes with the corresponding experiments, detailed information about the interaction between water molecules and AlPcCl, and geometric changes of AlPcCl molecule has been obtained. Effects of aggregation on two-photon absorption spectra of octupolar molecules have been examined. It is shown that the formation of clusters through inter-molecular hydrogen bonding can drastically change profiles of TPA spectra. It has also demonstrated that a well designed molecular aggregate/cluster, dendrimer, can enhance the second order nonlinear optical response of the molecules. In collaboration with experimentalists, a series of end-capped triply branched dendritic chromophores have been characterized, which can lead to large enhancement of the second order NLO property when the dipoles of the three branches in the dendrimers are highly parallelized.

Surface-enhanced Raman scattering has made the detection of single molecules on metal surface become possible. Chemically bonded molecule-metal systems have been extensively studied. We have shown in a joint experimental and theoretical work that stable Raman spectra of a non-bonding molecule, perylene, physically adsorbed on Ag nano-particles can also be observed at low temperature. It is found that the local enhanced field has a tendency to drive molecule toward a gap of two closely lying nano-particles. The trapped molecule can thus provide a stable Raman spectrum with high resolution when its thermal motion is reduced at low temperature.

For the ever growing size of molecular complexes, there is always the need to develop new computational methods. A conceptually simple but computationally efficient method, named as central insertion scheme (CIS), is proposed that allows to calculate electronic structure of quasi-periodic system containing more than 100,000 electrons at density functional theory levels. It enables to monitor the evolution of electronic structure with respect to the size of the system.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. 60 p.
Keyword
one-photon absorption, two-photon absorption, surface-enhanced Raman scattering
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-4629 (URN)978-91-7178-857-3 (ISBN)
Public defence
2008-02-25, FA32, Roslagstullsbacken 21, AlbaNova University Center, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20100823Available from: 2008-02-05 Created: 2008-02-05 Last updated: 2010-08-23Bibliographically approved
2. Theoretical Design of Molecular Photonic Materials
Open this publication in new window or tab >>Theoretical Design of Molecular Photonic Materials
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This thesis presents a theoretical study on optical properties of molecular materials. Special emphasis has been put on the influence of solvent environment, nuclear vibrations, and aggregation effects on molecular properties like linear and nonlinear polarizabilities, one- and two-photon absorption probabilities. All calculations have been performed by means of time independent and dependent quantum chemical methods at the Hartree-Fock and density functional theory levels. Solvation models that include both long range and short range interactions have been employed for calculations of optical properties of molecules in solutions. Pure vibrational and zero-point vibrationally averaged contributions have been taken into account for linear and nonlinear polarizabilities. The linear coupling model is applied to simulate vibronic profiles of optical absorption spectra. The computational strategies described in this thesis are very useful for the design of efficient molecular photonic materials.

More specifically, the nonmonotonic behavior of the solvatochromic shifts and the first hyperpolarizability of para-nitroaniline (pNA) with respect to the polarity of the solvents have been theoretically confirmed for the first time. The significant contributions of the hydrogen bonding on the electronic structures of pNA are revealed. Vibrational contributions to the linear and nonlinear polarizabilities of methanol, ethanol and propanol have been calculated both at the static limit and in dynamic optical processes. The importance of vibrational contributions to certain nonlinear optical processes have been demonstrated. A series of end-capped triply branched dendritic chromophores have been studied with the result that their second order nonlinear optical properties are found strongly dependent on the mutual orientations of the three chromophores, numbers of caps and the conjugation length of the chromophores. Several possible mechanisms for the origin of the Q-band splitting of aluminum phthalocyanine chloride have been examined. Calculated vibronic one-photon absorption profiles of two molecular systems are found to be in very good agreement with the corresponding experiments, allowing to provide proper assignments for different spectral features. Furthermore, effects of vibronic coupling in the nonradiative decay processes have been considered which helps to understand the aggregation enhanced luminescence of silole molecules. The study of molecular aggregation effects on two-photon absorption cross sections of octupolar molecules has highlighted the need to use a hybrid method that combines density functional response theory and molecular dynamics simulations for the design of molecular materials.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. 52 p.
Series
Trita-TEC-PHD, ISSN 1653-4468
Keyword
Optical property, Molecular material, Solvent environment, Nuclear vibrations, Aggregation effect
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-4333 (URN)978-91-7178-619-7 (ISBN)
Public defence
2007-04-27, FA32, Roslagstullsbacken 21, AlbaNova University Center, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20100820Available from: 2007-04-16 Created: 2007-04-16 Last updated: 2010-08-20Bibliographically approved

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Ågren, HansLuo, Yi

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