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Electron-vibration coupling and its effects on optical and electronic properties of single molecules
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0002-3915-300X
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis is devoted to theoretical investigations of electron-vibration coupling and its effects on optical and electronic properties of single molecules, especially for molecules confined between metallic electrodes.

A density-matrix approach has been developed to describe the photon emission of single molecules confined in the scanning tunneling microscope (STM). With this new method electronic excitations induced by both the tunneling electron and the localized surface plasmon (LSP) can be treated on an equal footing. Model calculations for porphyrin derivatives have successfully reproduced and explained the experimentally observed unusual variation of the photon emission spectra. The method has also been extended to study the STM induced fluorescence and phosphorescence of C60 molecules in combination with the first principles calculations. In particularly, the non-Condon vibronic couplings have been exclusively included in the calculations. The experimental spectra have been nicely reproduced by our calculations, which also enable us to identify the unique spectral fingerprint and origin of the measured spectra. The observed rich spectral features have been finally correctly assigned.

The electron transport properties of molecular junctions with bipyridine isomers have been studied in the sequential tunneling (SET) regime by assuming that the molecules are weakly coupled to metallic electrodes. It is shown that the strong electron-vibration coupling in the 2, 2’-bipyridine molecule and the 4,4’-bipyridine molecule can lead to observable Franck-Condon blockade. Taking advantage of such novel effect, a gate-controlled conductance switch with ideal on-off ratio has been proposed for a molecular junction with the 4, 4’-bipyridine molecule.

The effect of the electron-vibration coupling on one-photon and two-photon absorption spectra of green fluorescent protein (GFP) has been systematically examined. The hydroxybenzylidene-2, 3-dimethylimidazolinone molecule in the deprotonated anion state (HBDI) is used to model the fluorescence chromophore of the GFP. Both Condon and non-Condon vibronic couplings have been considered in the calculations. The calculated spectra are in good agreement with the available experimental spectra. It confirms the notion that the observed blue-shift of the two-photon absorption spectrum with respect to its one-photon absorption counterpart is caused by the non-Condon vibronic coupling.

All the calculations are carried out with our own software package, DynaVib. It is capable of modeling a variety of vibrational-resolved spectroscopies, such as absorption, emission, and resonant Raman scattering (RRS) spectra. In our package, the Duschinsky rotation and non-Condon effect have been fully taken into account. Both time-independent and time-dependent approaches have been implemented, allowing to simulate the spectra of very large molecules.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , xiv, 66 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2013:10
National Category
Theoretical Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-122180ISBN: 978-91-7501-773-0 (print)OAI: oai:DiVA.org:kth-122180DiVA: diva2:621266
Public defence
2013-06-07, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20130520

Available from: 2013-05-20 Created: 2013-05-14 Last updated: 2013-05-20Bibliographically approved
List of papers
1. Density-Matrix Approach for the Electroluminescence of Molecules in a Scanning Tunneling Microscope
Open this publication in new window or tab >>Density-Matrix Approach for the Electroluminescence of Molecules in a Scanning Tunneling Microscope
2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 106, no 17, 177401- p.Article in journal (Refereed) Published
Abstract [en]

The electroluminescence (EL) of molecules confined inside a nanocavity in the scanning tunneling microscope possesses many intriguing but unexplained features. We present here a general theoretical approach based on the density-matrix formalism to describe the EL from molecules near a metal surface induced by both electron tunneling and localized surface plasmon excitations simultaneously. It reveals the underlying physical mechanism for the external bias dependent EL. The important role played by the localized surface plasmon on the EL is highlighted. Calculations for porphyrin derivatives have reproduced corresponding experimental spectra and nicely explained the observed unusual large variation of emission spectral profiles. This general theoretical approach can find many applications in the design of molecular electronic and photonic devices.

Keyword
LIGHT-EMISSION; METAL-SURFACES; FLUORESCENCE
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-33953 (URN)10.1103/PhysRevLett.106.177401 (DOI)000290098200005 ()2-s2.0-79960647801 (Scopus ID)
Note
QC 20110609Available from: 2011-06-09 Created: 2011-05-23 Last updated: 2017-12-11Bibliographically approved
2. Electroluminescence of molecules in a scanning tunneling microscope: Role of tunneling electrons and surface plasmons
Open this publication in new window or tab >>Electroluminescence of molecules in a scanning tunneling microscope: Role of tunneling electrons and surface plasmons
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 20, 205419- p.Article in journal (Refereed) Published
Abstract [en]

We study electroluminescence from molecules confined in a scanning tunneling microscope based on a recently proposed density-matrix approach. The molecule is treated by a two-state model with each state consisting of a set of vibrational energy levels. The interband transition probabilities are described by Franck-Condon factors as calculated from a harmonic model. The role played by the tunneling electron as well as by the surface plasmon resonance is investigated. The dependence of the electron-tunneling-induced electroluminescence on the temperature, the bias voltage, the coupling strength between the molecule and the electrodes, and the radiative decay rate of the excited state has been systematically studied. It is found that under high temperature, due to the thermally assisted electron tunneling, photon emissions can still be detected when the bias voltage is less than the excitation energy of the molecule. We also find that when the molecule is asymmetrically coupled to the electrodes, electroluminescence does not strictly follow the Franck-Condon distribution. Our simulations also show that the increase of the radiative decay rate of the excited state cannot lead to the hot luminescence from higher vibrational levels. The involvement of the surface plasmon can drastically alter the spectral profiles, resulting in hot luminescence from molecules, when the surface plasmon in a scanning tunneling microscope is of high strength and ultrashort duration. The influence of the strength and the duration of the plasmon pulse on the electroluminescence spectra has been discussed.

Keyword
2-PHOTON ABSORPTION, METAL-SURFACES, LIGHT-EMISSION, FLUORESCENCE
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-52548 (URN)10.1103/PhysRevB.84.205419 (DOI)000297104700019 ()2-s2.0-82755177359 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20111219Available from: 2011-12-19 Created: 2011-12-19 Last updated: 2017-12-08Bibliographically approved
3. Fluorescence and Phosphorescence of Single C60 Molecules as Stimulated by a Scanning Tunneling Microscope
Open this publication in new window or tab >>Fluorescence and Phosphorescence of Single C60 Molecules as Stimulated by a Scanning Tunneling Microscope
2013 (English)In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 52, no 18, 4814-4817 p.Article in journal (Refereed) Published
Abstract [en]

Glow-in-the-dark buckyballs: Reproduction of the fluorescence and phosphorescence spectra of single C60 molecules in a scanning tunneling microscope by simulations according to first principles (see picture) revealed the underlying mechanisms and enabled the correct assignment of the spectra.

Keyword
electroluminescence, fullerenes, scanning tunneling microscopy, single-molecule studies, vibrational spectroscopy
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-122164 (URN)10.1002/anie.201301209 (DOI)000318107400012 ()2-s2.0-84876521602 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130520

Available from: 2013-05-14 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved
4. Isomeric dependent Franck-Condon blockade in weakly coupled bipyridine molecular junctions
Open this publication in new window or tab >>Isomeric dependent Franck-Condon blockade in weakly coupled bipyridine molecular junctions
(English)Article in journal (Other academic) Submitted
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-122175 (URN)
Note

QS 2013

Available from: 2013-05-14 Created: 2013-05-14 Last updated: 2013-05-20Bibliographically approved
5. Role of non-Condon vibronic coupling and conformation change on two-photon absorption spectra of green fluorescent protein
Open this publication in new window or tab >>Role of non-Condon vibronic coupling and conformation change on two-photon absorption spectra of green fluorescent protein
2013 (English)In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 111, no 9-11, 1316-1321 p.Article in journal (Refereed) Published
Abstract [en]

Two-photon absorption spectra of green fluorescent proteins (GFPs) often show a blue-shift band compared to their conventional one-photon absorption spectra, which is an intriguing feature that has not been well understood. We present here a systematic study on one- and two-photon spectra of GFP chromophore by means of the density functional response theory and complete active space self-consistent field (CASSCF) methods. It shows that the popular density functional fails to provide correct vibrational progression for the spectra. The non-Condon vibronic coupling, through the localised intrinsic vibrational modes of the chromophore, is responsible for the blue-shift in the TPA spectra. The cis to trans isomerisation can be identified in high-resolution TPA spectra. Our calculations demonstrate that the high level ab initio multiconfigurational CASSCF method, rather than the conventional density functional theory is required for investigating the essential excited-state properties of the GFP chromophore.

Keyword
non-Condon vibronic coupling, two-photon absorption spectra, green fluorescent proteins (GFP)
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-122173 (URN)10.1080/00268976.2013.771800 (DOI)000323409600026 ()2-s2.0-84882453867 (Scopus ID)
Note

QC 20130930

Available from: 2013-05-14 Created: 2013-05-14 Last updated: 2017-12-06Bibliographically approved

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