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

    • 3.4.1  BESSY II

    • 3.4.2 ps-Laser

  • 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

    • 4.2.1 Materials

    • 4.2.2 Apparatus and Methods

    • 4.2.3 Calculational Details

  • 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

    • 6.4.1 AM1 Calculations

    • 6.4.2 CASSCF Calculations

  • 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

    • 6.6.1 AM1 Calculations

    • 6.6.2 CASSCF Calculations

  • 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

• 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 τ_f, radiative k_r and non-radiative k_nr rate constants, the CT Transition Dipole Moment, M_f) 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 μ_eq, Equilibrium Twist Angle α_eq, Equilibrium Pyramidalization Angle β_eq, 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 Energy^a (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 I_e 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, μ_g(D) and derived μ_e(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 (μ_g) and excited-state (μ_e)_{dipole moments derived for p-PBN, MP2BN and MP2-B25N from the Mataga plot [ref.21,22] (Fig 4a and Fig 4b).}

• 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, n_α is the bisector vector of the two N-methyl bonds of the dimethylamino-group, n_β 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 (ε_homo) and LUMO [(ε_lumo) 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 k_nr 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 k_nr 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 C_2v 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; E_mes- Mesomeric energy – zero for the perpendicular, of varying size for the planar geometry resulting in the TICT (perpendicular) and the MICT states (planar geometry; ΔE_M-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.