| [page 7↓] |
Electron transfer (ET) processes play a major role in the photophysics of donor-acceptor aromatic systems. They can occur between separate molecules (intermolecular) or among the distinct regions within the same molecule (intramolecular). The resulting product, which occurs in these latter processes, is called intramolecular charge transfer (CT) state. The photoinitiated ET processes and CT states are of paramount importance in elucidating the photosynthesis in plants, and in the application of molecular device technology. Among the intramolecular CT processes, twisted intramolecular charge transfer (TICT) states have been the focus for many years due to the burgeoning area of physical, physical-organic and organic chemistry connected with a rationalization of the excited state behaviour of many dye systems. The innumerable applications are growing in various fields such as tailor-made fluorescent dyes [1, 2], sensing of free volume in polymers [3, 4], fluorescent pH or ion indicators [5, 6], fluorescent solar collectors [2], and electron transfer photochemistry [7]. The aim of this thesis lies in the investigation of the CT characteristics of fluorinated derivatives of N,N-dimethylamino benzonitrile (DMABN) and N-phenyl pyrroles. The central part of this dissertation deals with the nature of the excited states and dipole moment changes of different donor-acceptor systems. Furthermore, the modification of either the donor or acceptor strength in these types of molecules resulted in changes of photophysical properties that have been well studied in this work. Apart from that, also the change of the position of the acceptor or donor part influences the excited state properties of these systems. In order to gain further insight into the excited state of these investigated molecules, quantum chemical calculations were also done to support the experimental findings. The main families of the molecules investigated in this thesis are as follows:
| [page 8↓] |
This introductory chapter presents the general motivation of this thesis, and serves as a guide to other chapter’s work. Each following chapter contains its own introduction to the molecules investigated and its corresponding references.
In Chapter 2, the necessary theoretical background for the understanding of the work is clearly described. It also gives the general introduction of the concepts involved in this work.
Chapter 3 give details about the solvents used and their purification, and about the experimental methods like absorption spectroscopy, transient absorption spectroscopy, steady state and time resolved-fluorescence at different temperature used in this work.
In chapter 4, absorption and emission properties of the tetrafluoro analogue of DMABN, DMABN-F4 have been investigated and compared with the parent compound. The formation of a CT state of DMABN-F4 is a quasi-barrierless process in both polar and non polar solvents when compared to DMABN, and the TICT character has been confirmed by the combination of time resolved absorption and emission spectroscopy. The assumptions of Onsager radii by different methods on the excited state dipole moments values has been discussed. Electronic structure and conformational analysis of the investigated molecules were studied by quantum chemical calculations using semiempirical method.
Chapter 5 discusses mainly steady state absorption and emission characteristics of fluorinated derivatives of anilines and p-PBN such as such as ABN-F4, A-F5 and PBN-F4. Their non-fluorescent properties have been compared with DMABN-F4. The main reason for their non-fluorescent behaviour has been rationalized in terms of ‘F’ atom substitution, which acts as a new non-radiative funnel in all these type of compounds.
In Chapter 6, the photophysical properties of meta- and para-cyano N-phenyl pyrrole (m- and p-PBN) are compared. It has been found that both compounds show highly red shifted and strongly forbidden emission in polar solvents, assigned to a twisted intramolecular charge transfer state (TICT). Comparison to quantum chemical calculations [page 9↓]indicates that the twisted structure possesses an antiquinoid distortion of the benzonitrile group. It has been concluded that m-PBN differs from p-PBN by a less exergonic formation of the TICT state from the LE/ICT quinoid state, and it therefore shows only single LE/ICT fluorescence in nonpolar alkane solvents, whereas p-PBN shows dual fluorescence (LE/ICT and TICT).
Chapter 7 covers the photophysical studies on the meta-positioning effect on the dimethyl derivative of N-phenylpyrrole such as p-DPBN and m-DPBN. The results indicate that the CT state arises due to the twisting of the acceptor group. The twisting was enhanced by the positioning of the cyano group in the acceptor moiety that would lead to the higher rate of non-radiative decay in m-DPBN. The conclusion was drawn from the fact that irrespective of the meta positioning effect, these molecules possess similar excited state properties.
Chapter 8 presents photophysical studies on donor-acceptor compounds with a different linkage position on the donor such as MP2BN and additionally by changing the orientation of the acceptor part by increasing its strength such as MP2-B25CN. The mesomeric interaction between donor and the different acceptor units has been investigated, and it was found that the behaviour could switch between ICT states with large mesomeric interaction (MICT -Mesomeric Intramolecular Charge Transfer) such as in MP2BN, and with the minimum mesomeric interaction (TICT – Twisted Intramolecular Charge Transfer) states such as in p-PBN and MP2-B25CN. The factors are i) the relative energies of LE/ICT (MICT) and TICT state ii) the strength of the mesomeric interaction in the MICT state.
© Die inhaltliche Zusammenstellung und Aufmachung dieser Publikation sowie die elektronische Verarbeitung sind urheberrechtlich geschützt. Jede Verwertung, die nicht ausdrücklich vom Urheberrechtsgesetz zugelassen ist, bedarf der vorherigen Zustimmung. Das gilt insbesondere für die Vervielfältigung, die Bearbeitung und Einspeicherung und Verarbeitung in elektronische Systeme. | ||
DiML DTD Version 4.0 | Zertifizierter Dokumentenserver der Humboldt-Universität zu Berlin | HTML generated: 07.07.2005 |