Murali Sukumaran
: Photophysical Kinetics in TICT-forming Compounds – Derivatives of DMABN |
|
Photophysical Kinetics in TICT-forming Compounds – Derivatives of DMABN
DISSERTATION
zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)
im Fach Chemie
eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
der Humboldt-Universität zu Berlin
von
M. Phil. Murali
Sukumaran
geb. am 05.06.1978 in Cheyyar, Indien
Präsident der Humboldt-Universität zu Berlin
Prof. Dr. J. Mlynek
Dekan:
Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I
Prof. T. Buckhout, Ph.D.
Gutachter:
1. Prof. Dr. Wolfgang Rettig
2. Prof. Dr. W. Abraham
3. Prof. Dr. Hans-Gerd Löhmannsröben
Tag der mündlichen Prüfung: 10.6.2005
Zusammenfassung
Die photophysikalische Kinetik von TICT-Zustände bildenden Verbindungen –Derivate von DMABN
Das Hauptaugenmerk der vorliegenden Arbeit richtet sich auf die Untersuchung der photophysikalischen Eigenschaften von Derivaten von N,N-Dimethylaminobenzonitril (DMABN) und N-Phenyl-pyrrolobenzonitril (PBN) als Donor-Akzeptor Verbindungen. Die untersuchten Verbindungen zeichnen sich durch Einführung von Fluor-Substituenten durch eine erhöhte Akzeptorstärke aus, wodurch neue Erkenntnisse bzgl. der intramolekularen Charge-Transfer-Zustände (ICT) gewonnen werden konnten. Hierbei wurden die Ergebnisse zum Verhalten der untersuchten Moleküle im angeregten Zustand mit denen der entsprechenden Basisverbindungen verglichen.
Die spektroskopischen und photophysikalischen Eigenschaften wurden sowohl durch die Anwendung der stationären und zeitaufgelösten Fluoreszenzspektroskopie bei Raum- und Tieftemperatur als auch durch Nutzung der transienten Absorptionsspektroskopie in Kombination mit quantenchemischen Berechnungen untersucht.
Im Unterschied zu den Basisverbindungen DMABN und PBN zeigen die Spektren der fluorierten Derivate nur eine einzige stark rotverschobene Fluoreszenzbande, die dem ICT-Zustand zugeordnet werden kann. Die extrem kleinen Quantenausbeuten, die typisch für alle fluorierten Derivate sind, können auf die Existenz eines weiteren strahlungslosen Deaktivierungskanals zurückgeführt werden. Der beobachtete ICT kann mit dem TICT-Modell (Twisted intramolecular Charge Transfer), bei dem von einer gegenseitigen Verdrillung der Donor- und Akzeptoreinheiten ausgegangen wird, erklärt werden. Weiterhin wurden die Variation der Verknüpfungsposition zwischen Donor- und Akzeptoreinheit sowie der Einfluss zusätzlicher Akzeptor-Substituenten auf die Eigenschaften der ICT-Zustände untersucht.
Durch die Ergebnisse dieser Arbeit konnte ein vertieftes Verständnis über die Ladungstrennungsprozesse in Donor-Akzeptor-Systemen, die sich durch eine starke Solvatochromie und die Existenz von strahlungslosen Deaktivierungskanälen auszeichnen, entwickelt werden. Es konnte die Möglichkeit der Besetzung von zwei verschiedenen ICT-Zuständen (TICT – verboten, mesomerer ICT – erlaubt) gezeigt werden.
Eigene Schlagworte:
DMABN,
Charge transfer,
dual fluorescence,
TICT
Abstract
The focus of this work is mainly concerned with the investigation of photophysical properties of electron donor-acceptor compounds, namely, derivatives of N,N-dimethylamino benzonitrile (DMABN) and N-phenyl-pyrrolobenzonitrile (PBN). New insights into the intramolecular charge transfer (ICT) states were obtained while dealing with an acceptor moiety of increased strength in the form of fluorinated analogues of both these compounds. The molecules studied in this work have been compared with their corresponding parent compounds to get more useful information on the excited state behaviour.
The spectroscopic and photophysical properties were studied using steady-state and time-resolved fluorescence at room and low temperature as well as with transient absorption spectroscopy in conjunction with quantum chemical calculations.
Unlike in the parent compounds DMABN and PBN, their fluorinated derivatives show only a single strongly red-shifted fluorescence emitted from the ICT state, and possess low quantum yields. The nearly non-fluorescent behaviour for all of these fluorinated derivatives investigated is due to the presence of a photochemical mechanism additional to that of ICT, which acts as a new non-radiative funnel. The ICT observed in these compounds can be explained by twisting motion taking place between the donor and acceptor moieties. Thus, twisted intramolecular charge transfer (TICT) model supports the observations. Apart from the changes in the strength of the acceptor moieties, the ICT nature has been further explored by changing their linking positions as well as with additional acceptor substituents.
From the findings obtained in this work, a deeper understanding of the charge separation processes occurring in donor-acceptor systems with high solvatochromism and non-radiative decay properties was obtained. The possibility for populating two different ICT states (of forbidden nature – TICT, and allowed nature – mesomeric ICT) has been exemplified.
Keywords:
DMABN,
Charge transfer,
dual fluorescence,
TICT
Table of contents
-
1
Introduction
-
1.1 Background and Motivation
-
1.2 Outline of other Chapters
-
2
Theoretical Background
-
2.1 Mechanism of Dual Fluorescence
-
2.2 Photoinduced Charge Transfer
-
2.2.1 Intramolecular Electron Transfer
-
2.3
Intermolecular Electron Transfer
-
2.4 TICT Model Compounds
-
3
Experimental Section
-
3.1 Synthesis of the Investigated Compounds
-
3.2 Solvents Used
-
3.2.1 Purification of Butyronitrile
-
3.3 Absorption and Fluorescence Measurements
-
3.3.1 Correction of the Emission Spectra
-
3.3.2 Low Temperature Measurements
-
3.3.3 Determination of Fluorescence Quantum Yields
-
3.4 Time Resolved Fluorescence
-
3.5 Transient Absorption Spectroscopy
-
3.6 Quantum chemical Calculations
-
4
The Tetrafluoro Analogue of DMABN: Anomalous Fluorescence and Mechanistic Considerations
-
4.1 Introduction
-
4.2 Experimental Section
-
4.3 Results and Discussion
-
4.3.1 Absorption Spectra
-
4.3.2 Fluorescence at Room Temperature
-
4.3.3 Fluorescence at Low Temperatures
-
4.3.4 Geometry of the Ground State
-
4.3.5 Dipole moments at Room Temperature
-
4.4 Theoretical Results
-
4.4.1 Electronic Property of the Acceptor Fragments
-
4.4.2 Electronic Transitions
-
4.4.3
The Discrepancy of Experimental and Calculated CT Dipole moments
-
4.4.4 Competing Photochemical Reaction paths
-
4.5
Conclusion
-
5
Excited State Properties of Fluorinated Analogues of DMABN and PBN
-
5.1 Introduction
-
5.2 Experimental
-
5.2.1 Synthesis of the Compounds used in this Study
-
5.3
Results and Discussion
-
5.3.1 Absorption and Emission Spectroscopy
-
5.4 Conclusion
-
6
TICT Formation and Antiquinoid Distortion in para- and meta-Derivatives of N-Phenyl Pyrrole
-
6.1 Introduction
-
6.2 Experimental
-
6.2.1 Materials
-
6.2.2 Quantum Chemical Calculations
-
6.3
Results
-
6.3.1 Room Temperature Spectroscopy
-
6.3.2 Solvatochromic Measurements
-
6.3.3 Spectroscopic Measurements at Low Temperatures
-
6.4 Computational Results
-
6.5 Discussion
-
6.5.1 Absorption
-
6.5.2 Dual Fluorescence at Room Temperature
-
6.5.3
Radiative rates and Dipole Moments
-
6.6 Theoretical Investigations
-
6.7 Conclusion
-
7
Meta- positioning effect in DPBN: a photophysical study
-
7.1 Introduction
-
7.2 Experimental Section
-
7.3
Results and Discussion
-
7.3.1 Absorption and Emission Spectra
-
7.3.2 Fluorescence Quantum Yields and Rate Constants
-
7.3.3 Low Temperature Studies
-
7.3.4 Excited State Dipole Moments
-
7.4 Discussion
-
7.5 Conclusion
-
8
Photophysical Properties of Pyrrolobenzenes with Different Linking Pattern: The Transition Between Large (MICT) and Small (TICT) Charge Transfer Interaction behaviour
-
8.1 Introduction
-
8.2 Experimental Section
-
8.3
Results and Discussion
-
8.3.1 Absorption and Fluorescence Spectra
-
8.3.2
Potential Energy Surfaces
-
8.3.3
Dipole Moments and Radiative and Nonradiative Rate Constants
-
8.4
Strength and Position of the Acceptor Part
-
8.5 Transient Absorption Studies
-
8.6 Conclusion
-
9
Final Conclusion
-
References and Notes
-
List of Abbreviations and Symbols
-
Acknowledgement
-
Eidesstattliche Erklärung
-
Publications
-
Lebenslauf
Tables
-
Table 3.1: Solvents used in this work and their parameters
-
Table 4.1a: Photophysical parameters of DMABN-F4 in Various Solvents at Room Temperature
-
Table 4.1b: Photophysical parameters of DMABN in Various Solvents at Room Temperature
-
Table 4.1c: Photophysical Parameters (radiative life time τf, radiative kr and non-radiative knr rate constants, the CT Transition Dipole Moment, Mf) of DMABN-F4 and DMABN in Various Solvents at Room Temperature
-
Table 4.2: Temperature Dependence of the Photophysical Data of DMABN-F4 in BCl
-
Table 4.3: Ground State Characteristics of the molecules studied: Dipole Moment μeq, Equilibrium Twist Angle αeq, Equilibrium Pyramidalization Angle βeq, Activation Barrier of the Intramolecular Fragment Rotation to the Planar (ΔH(00)) and the Perpendicular (ΔH(90)) Geometry for DMABN-F4 and DMABN as Calculated by Different ab initio and Semiempirical Methods and different basis sets.
-
Table 4.4: Relative Energya (from DFT-B3LYP/6-31G(d) calculations) in kcal/mol, (number of negative Eigenvalues χ of the Hessian matrix) and the Symmetry Point Group for Planar Arrangements of the Dimethylamino Group (figure 4.8) of the Compounds DMABN and DMABN-F4
-
Table 4.5a:Onsagar radius, Solvatochromic Slope of the Mataga plot, Calculated Ground and Derived Excited State Dipole Moments of DMABN and DMABN-F4
-
Table 4.5b: Densities and Onsager radius, compared for benzene, fluorinated benzenes, DMABN and DMABN-F4.
-
Table 4.6: The energies ε of the four frontier orbitals (the orbital symmetries within point group C2), calculated for optimised geometries of benzene with different substituents (fluorine atom F and CN group) and comparison to the negative experimental values of ionisation potential Ie and the electron affinity EA. (F0 – Benzene, F1 – Fluorobenzene, CN - Cyanobenzene, F 1,4 - 1,4-difluorobenzene, F 1,3,5- 1,3,5-trifluorobenzene, F 1,2,4,5 - 1,2,4,5-tetrafluorobenzene, F 1,2,3,4,5 - 1,2,3,4,5-pentafluorobenzene, F 1,2,4,5,CN - 1,2,4,5-tetrafluorobenzonitrile). The symmetry designations A and B mean that orbitals with A symmetry have a zero orbital coefficient for the atoms lying on the C2 twist axis, those with B symmetry have a nonzero coefficient for these orbitals and are symmetric with respect to a plane perpendicular to the molecular plane and containing the C2 axis.
-
Table 4.7: The energy difference Δε (eV) = ε(B) - ε(A), of the molecular orbitals of benzene with different substituents and with different symmetry as calculated by the following methods: HF/6-31G(d), AM1, DFT (B3LYP/6-31G(d)) and compared to experimental values as far as available. Upper rows: difference of the first two LUMOs, lower rows: difference of the two highest occupied orbitals.
-
Table 4.8:Comparison of the transition energy ΔE, oscillator strength f, dipole moments and configuration interaction analysis for the long wavelength absorption transitions for DMABN and DMABN-F4 as calculated by ZINDO/s for the optimized ground state equilibrium geometry and two further twist angles. (full optimization at the different fixed twist angles using DFT (B3LYP/6-311++G(d)).
-
Table 5.1: Photophysical parameters of fluorinated analogues of DMABN and aniline in various solvents at room temperature and comparison to nonfluorinated derivatives
-
Table 6.1: Photophysical Characteristics of m-PBN and p-PBN in Solvents of Different Polarity at Room Temperature
-
Table 6.2: Results of the Solvatochromic Measurements at Room Temperature and Low Temperature (Solvatochromic slopes, assumed Onsager factor a, μg(D) and derived μe(D) for the 2 methods)
-
Table 6.3: Equilibrium Twist angles, Rotational barriers to each the Planar and the Perpendicular Geometry and Dipole moments in the S0 state as Calculated by AM1.
-
Table 6.4: Results of Semiempirical AM1-CI Calculations for the BN radical anion, and for p-PBN and m-PBN in the Planar and Perpendicular Geometry.
-
Table 7.1: Spectral and photophysical data of p-DPBN and m-DPBN in various solvents at room temperature.
-
Table 7.2: Temperature dependence of the photophysical data of m-DPBN in the solvent mixture Methylcyclohexane:Isopentane (1:4) and in diethylether.
-
Table 7.3: Dipole moments for the ground and excited states derived for p-DPBN and m-DPBN from the Mataga plot (see fig. 7.4).
-
Table 8.1: Spectral and photophysical data of p-PBN, MP2BN and MP2-B25CN in various solvents at room temperature.
-
Table 8.2: Onsager radius (a), ground state equilibrium twist angles (α), solvatochromic slopes, ground (μg) and excited-state (μe)dipole moments derived for p-PBN, MP2BN and MP2-B25N from the Mataga plot [ref.21,22] (Fig 4a and Fig 4b).
Images
-
Figure 2.1: Kinetic scheme for the dual fluorescence of DMABN. Straight arrows represent radiative channels and dotted arrows represent non-radiative channels from the respective states.
-
Figure 2.2: Potential energy diagrams of adiabatic and non adiabatic intersecting curves during electron transfer.
-
Figure 2.3: The TICT model involves a twisted product species with charge transfer or charge shift properties (A* state) formed through an adiabatic photoreaction from the precursor (B* state) with a nearly coplanar conformation.
-
Figure 2.4:Scheme of model compounds
-
Figure 3.1: Construction of the Single Photon Counting (SPC) set up
-
Figure 3.2: Block diagram for the time resolved fluorescence measurements with the ps laser
-
Figure 3.3:Pump-Probe set-up of Trasient Absorption Spectroscopy.
-
Figure 4.1: Structure of the molecules investigated
-
Figure 4.2: Absorption and normalised fluorescence spectra at room temperature of DMABN-F4 (a) and DMABN (b) in various solvents of different polarity. Hex = n-hexane; BCl = n-butyl chloride ; EOE =diethyl ether; ACN = acetonitrile.
-
Figure 4.3: Low temperature effects on the fluorescence spectra of DMABN-F4 in n-butyl chloride. Down head arrow indicates the decreasing of temperature. The data points in the range 600-630 were omitted (second order of excitation wavelength).
-
Figure 4.4: Fluorescence spectra of a) DMABN-F4 in BCl and BCN at 77 K and b) DMABN in BCl at room temperature and at 77 K.
-
Figure 4.5: The planar transition, twisted equilibrium and perpendicular transition structures of DMABN-F4 and the planar equilibrium geometry of DMABN with some geometrical characteristics calculated by DFT (B3LYP/6-311++G(d)).
-
Figure 4.6: The ground state potentials of DMABN-F4 (1) and of DMABN (2) calculated by DFT (B3LYP/6-31G(d)) method. The torsion angle is determined according to figure 4.7. The inflection in the potential for DMABN at around 15° is due to different methyl group conformations being the most stable ones (see Scheme 4.8).
-
Figure 4.7: Planar, equilibrium and perpendicular structures of the dimethylamino group for DMABN-F4 in the ground state (top). Determination of the dimethylamino-group twist angle α and the dimethylamino group pyramidalization angle β (bottom): n is a vector perpendicular to the plane of the aromatic ring, nα is the bisector vector of the two N-methyl bonds of the dimethylamino-group, nβ is a vector perpendicular to the CNC plane of the dimethylamino group.
-
Figure 4.8: conformation of methyl-group of dimethylamino-group for DMABN-F4 and DMABN in the ground state.
-
Figure 4.9: Correlation diagram of the energies of the occupied and unoccupied orbitals of fluorinated benzenes and benzonitrile as calculated by HF (values see table 6)
-
Figure 4.10: The comparison of the orbital energy for HOMO (εhomo) and LUMO [(εlumo) calculated by different methods (HF/6-31G(d) and DFT (B3LYP/6-31G(d))] with the negative experimental values of electron affinity EA and ionisation potential IP for different compounds. The compounds are defined in table 6.
-
Figure 4.11: Schematic diagram showing the state energies of DMABN and DMABN-F4 in the gas phase, as calculated by ZINDO/s.
-
Figure 5.1:
Structures of the molecules
-
Figure 5.2: Normalised Absorption and fluorescence spectra of ABN-F4, A-F5 and PBN-F4 in various solvents of different polarity (Hex – n-hexane, EOE – diethylether, ACN - acetonitrile). Abs. spectra are superimposed in the case of PBN-F4.
-
Figure 5.3: Plot of log knr of DMABN against the number of fluorine atoms. The numbers in the brackets represent the position of the fluorine atoms with respect to the cyano group. The knr values for the monofluorinated DMABN-derivatives have been extrapolated for labels F1(2) and F1(3) from the compounds (ref.7) P4CF2 and P4CF3 in Fig. 5.1.
-
Figure 6.1: Structures of the molecules
-
Figure 6.2:
Normalised Absorption and fluorescence spectra of p-PBN and m-PBN in various solvents of different polarity (HEX- n-hexane, EOE – diethylether, ACN - Acetonitrile).
-
Figure 6.3: Solvatochromic fluorescence plot of a) m-PBN and b) p-PBN derived from differently polar solvents at room temperature. (HEX- n-hexane, BOB- dibutyl ether, EOE – diethyl ether, THF- tetrahydrofuran, DCM- dichloromethane, ACN - acetonitrile).
-
Figure 6.4: Solvatochromic fluorescence plot of a) m-PBN and b) p-PBN derived from measurements in diethyl ether at variable temperature
-
Figure 6.5: Low temperature fluorescence spectra of a) m-PBN and b) p-PBN in diethyl ether (EOE). For Fig. 4b, the second order Rayleigh scattering in the spectral region 560-585 nm has been omitted.
-
Figure 6.6: Low temperature fluorescence spectra of a) m-PBN and b) p-PBN in the non-polar solvent mixture methylcyclohexane:isopentane, MCH/IP (1:4)
-
Figure 6.7:
Equilibrium structures of benzonitrile (BN), BN radical anion, p-PBN and m-PBN in the S0 state.
-
Figure 6.8: The highest four occupied and lowest two unoccupied molecular orbitals for p-PBN and m-PBN in the equilibrium S0 geometry as calculated by AM1. The corresponding molecular orbitals for benzonitrile are also shown and arranged such that the coupling pattern with the orbitals of the pyrrole group becomes visible. The lower indices a and b denote subgroup orbitals transforming as symmetry species a and b in the symmetry point group C2. As can be seen, only the subgroup orbitals of b-symmetry can couple leading to the a
+ (bonding) and a
– (antibonding) combination. The position of the cyano group is not important here: Even for m-PBN, the orbitals correspond approximately to the a and b symmetry species.
-
Figure 6.9: The lowest unoccupied MOs of benzonitrile, labelled according to C2v symmetry. The corresponding symmetry species in the C2 point group is given in square brackets (see also the lower indices of the orbitals shown in Fig. 7).
-
Figure 6.10: Optimized structure with bond lengths given, and Mulliken charges (italic numbers in square brackets) of three states of the benzonitrile radical anion with different orbital occupation patterns. The 12B1 state is the global minimum on the PES of the anion radical. The relative energies in kcal/mol are given in round brackets.
-
Figure 6.11: Energy differences between the LE State (broken line) and the CT State (full line) of p-PBN and m-PBN in the Gas Phase as calculated by AM1 and Schematic Energy Ordering of these States in Alkane Solvents.
-
Figure 7.1: Structures of the molecules investigated
-
Figure 7.2: Normalised Absorption and fluorescence spectra of p- DPBN and m-DPBN at room temperature in various solvents of different polarity (HEX = n-hexane;BCl = n-butyl chloride; THF = tetrahydrofuran).
-
Figure 7.3: Temperature effects on the fluorescence spectra of m-DPBN in a) diethyl ether and in b) methylcyclohexane and isopentane mixture (1:4)
-
Figure 7.4:Mataga plot of p-DPBN and m-DPBN in various solvents of different polarity.
-
Figure 8.1: Molecular structures of the investigated compounds and their abbreviated formulas.
-
Figure 8.2: Normalised Absorption and fluorescence spectra of p-PBN, MP2BN and MP2-B25CN in solvents of different polarity (Hex – n-hexane, BOB – dibutylether, EOE – diethylether, THF – tetrahydrofuran, ACN – acetonitrile.
-
Figure 8.3:. Schematic representation of the excited state hypersurfaces for p-PBN, MP2BN and MP2-B25CN in the planar and perpendicular geometry. (LE0 - locally excited state; CT0 - charge transfer state, both without mesomeric interaction; Emes- Mesomeric energy – zero for the perpendicular, of varying size for the planar geometry resulting in the TICT (perpendicular) and the MICT states (planar geometry; ΔEM-T - energy difference between MICT and TICT states; ΔE0- energy difference LE0 and CT0 states)
-
Figure 8.4a: Fluorescence maxima of p-PBN, MP2BN and MP2-B25CN at room temperature versus the solvent polarity parameter Δf’ (see text).
-
Figure 8.4b: Fluorescence maxima of p-PBN in diethylether versus Δf’at different temperatures, indicated in Kelvin on the curve.
-
Figure 8.5 : Transient absorption spectra of MP2BN in acetonitrile (red) and in THF (black) measured 50 ps after excitation with a subpicosecond laser pulse. The stimulated emission of the MICT state is observed in the red-wing of the fluorescence spectra, (the maxima of which are indicated by the vertical arrows) due to the overlap with the transient absorption band.
© 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 |