A two-state model of twisted intramolecular chargetransfer in monomethine dyes

A two-state model Hamiltonian is proposed to model the coupling of twisting displacements to charge-transfer behavior in the ground and excited states of a general monomethine dye molecule. This coupling may be relevant to the molecular mechanism of environment-dependent fluorescence yield enhancement. The model is parameterized against quantum chemical calculations on different protonation states of the green fluorescent protein chromophore (GFP), which are chosen to sample different regimes of detuning from the cyanine (resonant) limit. The model provides a simple yet realistic description of the charge transfer character along two possible excited state twisting channels associated with the methine bridge. It describes qualitatively different behavior in three regions that can be classified by their relationship to the resonant (cyanine) limit. The regimes differ by the presence or absence of twist-dependent polarization reversal and the occurrence of conical intersections. We find that selective biasing of one twisting channel over another by an applied diabatic biasing potential can only be achieved in a finite range of parameters near the cyanine limit.Comment: 45 pages, 9 Figures (incl. 2 chemical schemes). Accepted for publication by the Journal of Chemical Physics. Changes include 2 additional figures to and expanded discussion of key points felt to be important, and condensed discussion of some points felt to be less importan

Study of the Excited-State Absorption Properties of Polymethine Molecules

This dissertation investigates excited-state nonlinearities in a series of polymethine dyes for the application of nanosecond optical limiting. Optical limiters are devices that for low intensity light exhibit a high linear transmittance, but for high intensity light strongly attenuate the incident radiation. These devices would serve to protect optical sensors from intense laser radiation by clamping the maximum energy allowed through an optical system below the damage threshold of the sensor. The search is ongoing for optical materials that are both broadband and have high damage thresholds to be effective materials for limiting applications. Polymethine dyes are promising compounds due to a strong and broad excited-state absorption (ESA) band in the visible region. However, the effectiveness of polymethine molecules as applied to optical limiting is hindered by a saturation of the ESA process at high fluences. Experiments and theoretical modeling are performed to determine the root causes of this saturation effect in both the picosecond and nanosecond time regime. The polymethine molecules studied have chromophore lengths from di- to pentacarbocyanine (2 to 5 -CH=CHgroups) with various bridge structures. This allows us to develop relationships between the molecular parameters of the polymethine molecules and overall nonlinear absorption performance. The experiments conducted included femtosecond white light continuum pumpprobe experiments to measure ESA spectra, picosecond two-color polarization-resolved pumpprobe to measure excited-state dynamics and the orientation of transition dipole moments, and picosecond and nanosecond optical limiting and z-scans. From these experiments we are able to develop energy level models that describe the nonlinear absorption processes in polymethines from the picosecond to nanosecond time regime. This work, along with the quantum chemical modeling performed at the Institute of Physics and National Academy of Sciences of Ukraine, has resulted in the creation of dyes that have improved photochemical stability with larger nonlinearities. These are useful not only for optical limiting but also for a wide variety of nonlinear optical applications

Molecular Structure-nonlinear Optical Property Relationships For A Series Of Polymethine And Squaraine Molecules

This dissertation reports on the investigation of the relationships between molecular structure and two-photon absorption (2PA) properties for a series of polymethine and squaraine molecules. Current and emerging applications exploiting the quadratic dependence upon laser intensity, such as two-photon fluorescence imaging, three-dimensional microfabrication, optical data storage and optical limiting, have motivated researchers to find novel materials exhibiting strong 2PA. Organic materials are promising candidates because their linear and nonlinear optical properties can be optimized for applications by changing their structures through molecular engineering. Polymethine and squaraine dyes are particularly interesting because they are fluorescent and showing large 2PA. We used three independent nonlinear spectroscopic techniques (Z-scan, two-photon fluorescence and white-light continuum pump-probe spectroscopy) to obtain the 2PA spectra revealing 2PA bands, and we confirm the experimental data by comparing the results from the different methods mentioned. By systematically altering the structure of polyemthines and squaraines, we studied the effects of molecular symmetry, strength of donor terminal groups, conjugation length of the chromophore chain, polarity of solvents, and the effects of placing bridge molecules inside the chromophore chain on the 2PA properties. We also compared polymethine, squaraine, croconium and tetraon dyes with the same terminal groups to study the effects of the different additions inserted within the chromophore chain on their optical properties. Near IR absorbing squaraine dyes were experimentally observed to show extremely large 2PA cross sections ([approximately equal to] 30000GM). A simplified three-level model was used to fit the measured 2PA spectra and detailed quantum chemical calculations revealed the reasons for the squaraine to exhibit strong 2PA. In addition, two-photon excitation fluorescence anisotropy spectra were measured through multiple 2PA transitions. A theoretical model based on four-levels with two intermediate states was derived and used for analysis of the experimental data

Steady-State and Ultrafast Fluorescence Depolarisation in Rigid-Rod Conjugated Polymers

Polarised spectroscopic techniques were used to investigate the underlying physics of steady-state and ultrafast fluorescence depolarisation in conjugated polymers. Depolarisation is due to fluorescence anisotropy: the angular difference between the absorption and emission transition dipole moments of a molecule. Polarised spectroscopy results from a polymer with a flexible backbone, poly (9,9-di(ethylhexyl)fluorene) were compared with those from two rigid backbone polymers: methyl-substituted ladder-type poly (para-phenylene) and the newly synthesised naphthylene ladder-type polymer (2,6-NLP). This revealed that there is an intrinsic anisotropy directly associated with the molecular backbone. This work is the first reported on 2,6-NLP. Fluorescence anisotropy was shown to be dependent upon the conjugation length; the transition dipole moments show larger angles for short lengths, tending to a minimum as the length increases. For rigid-rod polymers, this behaviour is replicated at each vibronic position. In the flexible polymer, planarisation of the backbone elongates the excited state over more conjugated bonds, changing the angle between the transition dipole moments, whereas in rigid-rod polymers, such elongation can only be electronic. Linear dichroism results obtained for all the polymers has shown the angle between the absorption transition dipole moment and the molecular backbone is large and that the emission transition dipole moment is aligned with the backbone. “Off-chain” to “on-chain” transition dipole moments arise from transitions from localised to delocalised states suggesting that the excited state in conjugated polymers is delocalised. Time-dependent measurements show that the main fluorescence depolarisation mechanism occurs in under 5 ps for both flexible and rigid polymers. The ultrafast timescale and the similarity of the two systems requires the process to be electronic in origin and not linked to a physical deformation. This work proposes that ultrafast fluorescence depolarisation is a result of the delocalisation of the electronic state as the conjugation length extends over more of the polymer

Modelling environment effects on spectroscopic molecular responses with hybrid QM/classical methods

The objective of this work is the accurate modelling of environment effects, i.e. the modifications in the properties and the processes of molecular systems when interacting with an environment, using hybrid Quantum Mechanical/classical methods. In particular, the focus is on the changes induced by the environment on electronic transitions (both in absorption and in the emission processes), vibrational transitions and vibrational couplings. The hybrid QM/classical methods used introduce either an atomistic or a continuum description for the classical part of the system but in both cases mutual polarization effects between the QM and the classical subsystems are included. These methods have been applied to environments of increasing complexity going from isotropic solvents to lipid bilayers and DNA fragments

Theoretical prediction of solubility, reactivity and degradation pathways of selected azo disperse dyes

Abstract :Ph.D. (Chemistry

Molecular Sizing using Fluorescence Correlation Spectroscopy

Die Größe ist eine grundlegende Eigenschaft eines jeden Objektes, speziell von Molekülen. Sie ist direkt verbunden mit fundamentalen Phänomenen, wie z.B. der Diffusion, und zwar unabhängig von anderen Eigenschaften. Die Größe von Molekülen kann sich ändern, wenn sie mit anderen Molekülen wechselwirken (z. B. wenn sie Ionen binden), oder wenn die Temperatur, der pH-Wert oder die chemische Zusammensetzung des umgebenden Mediums sich ändert. Somit kann die Größe ein sehr sensitiver Reporter für den Zustand eines Moleküls sein. Daher findet die Größenbestimmung von Molekülen breite Anwendung in der Physik, der Chemie und der Biologie. In den meisten Fällen erfordern diese Anwendungen eine Größenbestimmung mit einer Genauigkeit von wenigen Angström. In dieser Arbeit untersuche ich die Größe von Molekülen bei pico- und nanomolaren Konzentrationen mittels hoch-genauer Fluoreszenz-Korrelations-Spektroskopie (FCS). Eine spezielle Abwandlung dieser Methode, die zwei-Focus FCS (2fFCS) erlaubt die Bestimmung des absoluten Diffusionskoeffizienten und somit auch der absoluten Größe eines Moleküls. Die Genauigkeit dieser Methodik wird unter anderen anhand weit verbreiteter globulärer Proteine bestimmt. Weiterhin wird die gemessene quantitative Beziehung zwischen dem molekularen Gewicht und dem gemessenen Diffusionskoeffizienten diskutiert. Ausgehend von der Fluoreszenz-Korrelations-Spektroskopie habe ich eine neue Methode entwickelt, die Rotations-Diffusions-Konstanten von Makromolekülen bestimmt. Diese Methode ist geeignet, um Rotations-Diffusions-Konstanten zwischen zehn und einhundert Nanosekunden zu bestimmen, also einem Bereich in dem Fluoreszenz-Anisotropie-Messungen nicht gut angewendet werden können. Mittels des gemessenen Rotations-Diffusions-Koeffizienten wurden die hydrodynamischen Radien einiger weitverbreiteter, globulärer Proteine bestimmt. Die erhaltenen Werte werden mit den Resultaten aus den Diffusionsmessungen verglichen

Heterojunction Structures for Photon Detector Applications

The work presented here report findings in (1) infrared detectors based on p-GaAs/AlGaAs heterojunctions, (2) J and H aggregate sensitized heterojunctions for solar cell and photon detection applications, (3) heterojunctions sensitized with quantum dots as low cost solar energy conversion devices and near infrared photodetectors. (1)A GaAs/AlGaAs based structure with a graded AlGaAs barrier is found to demonstrate a photovoltaic responsivity of ~ 30mA/W (~ 450mV/W) at the wavelength of 1.8 mm at 300K. Additionally the graded barrier has enhanced the photoconductive response at 78 K, showing a responsivity of ~ 80mA/W with a D*=1.4×108 Jones under 1V bias at 2.7 mm wavelength. This is an approximately 25 times improvement compared to the flat barrier detector structure, probably due to the improved carrier transport, and low recapture rate in the graded barrier structure. However, these graded barrier devices did not indicate a photoresponse with photoconductive mode at 300K due to high shot noise. Additionally, two generation-recombination noise components and a 1/f noise component were identified. A series of GaAs/AlGaAs multilayer hetero-junction structures were tested as thermal detectors. A superlattice structure containing 57% Al fraction in the barrier and 3 × 1018 cm-3 p-doped GaAs emitter showed the highest responsivity as a thermal detector with a TCR of ~ 4% K-1, at 300K. (2)The photovoltaic properties of heterojunctions with J-/ H- aggregated dye films sandwiched between n– and p-type semiconductors were investigated for potential application as solar cells and IR detectors. Films of cationic dye Rhodamine-B-thiocyanate adsorbed on Cu2O substrate are found to form organized dye layers by self-assembled J- aggregation, resulting in large red-shifts in the photo -response. Additionally, cells sensitized with a pentamethine cyanine dye exhibited a broad spectral response originating from J- and H-aggregates. The photocurrent is produced by exciton transport over relatively long distances with significant hole-mobility as well as direct sensitized injection at the first interface. (3) A ZnO/PbS-QD/Dye heterostructure had enhanced efficiency compared to ZnO/Dye heterostructure as a solar cell. Furthermore, a ZnO/PbS-QD structure has demonstrated UV and NIR responses with 4×105V/W (390 nm) and 5.5×105 V/W (750 nm) under 1V bias at 300K

Molecular Dynamics of Bioinspired Nanotubes

No matter how complex a model or a theory is, in the end, it is a simpler representation of reality based on our interpretations of how reality works. Consequently, the models we use might not always be either correct or accurate. However, time has shown that employing models and computations together with experimental data can provide useful information on fundamental mechanisms of the systems under study and validate or disprove proposed theories, when experiments or logic alone fail. The aim of this thesis was to study different nanomaterials and chemical compounds and obtain high resolution computational results on their structural and thermodynamic properties