Experimental Section

Synthesis of the Investigated Compounds

DMABN – 4-(N,N-dimethylamino)benzonitrile: It was obtained from Aldrich. Compounds p-PBN and m-PBN were synthesized in our group according to the procedure [29, 30]. The absence of possible traces of impurities was checked by thin layer chromatography (TLC).
DMABN-F4 – 2,3,5,6-tetrafluoro-4-(N,N-dimethylamino)benzonitrile: It was synthesized by the reaction of pentafluorobenzonitrile with dimethylformamide using the procedure as described in ref. [31] The compounds was a gift of Prof. Alexei I. Tolmachev, Institute for organic chemistry of the national academy of sciences of Ukraine, Kiew, Ukaraine. The other fluorinated analogues ABN-F4, A-F5 and PBN-F4 were also a gift of the latter group.
p-DPBN - and m-DPBN: p-DPBN was synthesized by the condensation of 2,5-hexanedione with 4-aminobenzonitrile and taken from a previous publication [32]. m-DPBN was synthesized by the cyclocondensation of 2,5-hexanedione with 3-aminobenzonitrile [33]. The latter synthesis was kindly done by Mr. Sascha Jautze. The absence of impurities has been confirmed by NMR method and elemental analysis.
The other pyrrole derivatives such as MP2BN and MP2-B25CN were a gift of Prof. René Lapouyade, Ecole nationale supérieure de chimie et de physique de Bordeaux, Bordeaux, France.

Solvents Used

The solvents used in this work range from non-polar (alkane) to polar solvents (acetonitrile) and are of spectroscopic grade Merck (Uvasol) except n-butlychloride (Lichrosolv) and butyronitrile. The absorption and fluorescence spectra of all solvents were checked to make sure that they are devoid of impurities. The following table gives a list of the solvents and their parameters used in this work.

Table 3.1: Solvents used in this work and their parameters

Purification of Butyronitrile

To 500 ml of butyronitrile were added 2.5 spoons of charcoal and left to stand for one day. After filtration 2.5 g of potassium carbonate (K₂CO₃) were added. The mixture was allowed to stand for another day and then filtered. 20 g of phosphorus pentoxide were added to the filtrate and after allowing to stand for one day, a distillation was carried out. The purity of butyronitrile was tested by both absorption and fluorescence measurements.

Absorption and Fluorescence Measurements

Absorption spectra were measured on ATI UNICAM UV Series Spectrometer UV4-21113. Before measuring the absorption spectrum of the sample, the base line correction was done by placing solvent in both sample and reference Quartz cuvettes of 1 cm. The true absorption spectrum was measured by maintaining optical density of a solution between 0.1 and 0.2.

Fluorescence spectra were measured by using an AMINCO-Bowmann series 2 Luminescence spectrometer in which the excitation source is a 150 W Xenon lamp. The emission parameters in the set up are adjusted by keeping the optimum voltage of the photomultiplier between 600 and 800 V, slit widths of both excitation and emission monochromators at 4 nm and setting a scan rate of 2 or 3 nm per second.

Correction of the Emission Spectra

The emission spectrum recorded directly from a fluorescence spectrometer, when the emission monochromator is scanned at constant slit width and constant photomultiplier sensitivity, is an uncorrected spectrum. To determine the true spectrum, the observed spectrum has to be corrected with the wavelength dependent factors, namely the quantum efficiency of the photomultiplier, the band width of the monochromator and the transmission factor of the monochromator. These factors were determined by the manufacturer using a calibrated tungsten lamp. A reflector made of freshly prepared magnesium oxide is introduced into the sample holder and set at 45°, and is illuminated by the lamp externally positioned at right angles. The spectral response of the detection system is recorded and the correction factors are obtained by dividing this spectral response by the spectral output data provided with the lamp. For wavelengths shorter than about 320 nm, where the intensity of tungsten lamps is too low to get reliable correction factors, a hydrogen or deuterium lamp can be used.Here in this work, all the uncorrected fluorescence and excitation spectra have been corrected with the help of a correction file, determined in this way.

Low Temperature Measurements

Temperature dependent fluorescence spectra were measured with a homemade cooling apparatus that allows to simultaneously freeze and control the temperature of four samples in quartz cuvettes by pumping cold nitrogen gas through the cryostat. The temperature in the cuvettes was monitored using PT 100 resistor. The lowest temperature achieved with this set-up was 100 K.

For the 77 K measurements, a dewar flask with an optical access was filled with liquid nitrogen in which a quartz tube was filled with the sample solution was inserted. The solvents that form a glassy matrix are used for this kind of low temperature measurement. e.g. the MCH:IP alkane mixture, EOE and BCl.

Determination of Fluorescence Quantum Yields

For the determination of the fluorescence quantum yields of the probe, the optical densities of the solutions were determined at the excitation wavelengths in a 1cm absorption quartz cell and were adjusted to a value in the range 0.1-0.2 with a precision of 0.001. Fluorescence quantum yields of any substance can be determined by comparing with a fluorescence standard whose quantum yield value is already known. For that purpose, the fluorescence standard, quinine bisulfate was used. The latter can be prepared as a solution in 0.05M in H₂SO₄, and the reference value is = 0.515 [34]. While calculating the quantum yield of a sample, the value has to be corrected for the refractive index of the solvents using [35] equation (3.1).

where n_p and n_s are refractive indices of the solvents , OD_p and OD_s are the optical densities, and are the quantum yields, and A_p and A_s denote the computed area of the corrected fluorescence bands, each parameter for the sample solution and standard (reference), respectively.

The temperature dependent relative fluorescent intensities I_f(T) are corrected for the linear increase of the refractive index n(T) [36] and density [36] of the solvent relative to room temperature conditions using equation (3.2).

where the terms in the above equation have their usual meanings. The error of the low temperature fluorescence quantum yields determined from the integrated intensity area relative to the values at room temperature is estimated to be 10%.

Time Resolved Fluorescence

The fluorescence decay measurements were performed by using time correlated single photon counting (TCSPC) [37]. They have been done either with Synchrotron radiation from the Berlin Storage Ring for Synchrotron radiation (BESSY) or with a ps-laser source. Both the methods are explained in the following sub sections.

BESSY II

Synchrotron radiation from the Berlin Synchrotron facility BESSY II was used as light source in conjunction with an excitation monochromator (Jobin Yvon, II, 10 UV). It delivers a 1.25 MHz pulse train with characteristic pulse widths of 30-50 ps. The fluorescence decays were detected by a microchannel plate photomultiplier (MCP, Hamamatsu R 1564-U-01) cooled to –30 ^oC, coupled to an emission monochromator (Jobin Yvon II, 10 VIR) by means of quartz fiber optics. The signal from a constant fraction discriminator (CFD, Tennelec 454) was used as the start pulse for the time-to-amplitude converter (TAC, Tennelec TC864) operating in the reverse mode. The BESSY II synchronisation pulse was used as the stop pulse. The MCP pulses were amplified by an amplifier (INA 10386) and coupled into the CFD. A multichannel analyser (Fast Comtec MCDLAP) was used for data accumulation. The decays were analysed by the “least squares” iterative reconvolution method on the basis of the Marquardt-Levenberg algorithm, which is implemented in the commercial global analysis program [38]. The instrument response function was obtained by the detection of Rayleigh scattered light in a scattering solution and had a width of 120 ps. The quality of the exponential fits was evaluated on the basis of the reduced χ² values.

Figure 3.1: Construction of the Single Photon Counting (SPC) set up

M

:

Monochromators

S

:

Sample holder

MCP-PM

:

Microchannel plate photomultiplier ( Hamamatsu R 1564-U-01)

GPA

:

Gigahertz pre amplifier (INA 10386)

CFD

:

Constant fraction discriminator

TAC

:

Time to amplitude converter

ADC

:

Analog to Digital converter

MCA

:

Multi channel analyser

PC

:

personal computer

SR

:

Synchrotron radiation

ps-Laser

The measurement has been done with a conventional setup using an argon ion laser-pumped, passively mode locked Ti:sapphire laser as the excitation source. The pulse duration is about 80 fs, and the repetition rate is 82 MHz. The excitation wavelength was obtained by frequency doubling or tripling of the fundamental wavelength of about 800 nm. The fluorescence and scatter lightwere detected as described in the method above. The instrument response function was obtained by detection of Rayleigh scattered light in pure solvents and had a width of 50-60 ps at the excitation wavelength and is dominated by the optical path difference in the monochromator. Detection without the monochromator yielded a pulse width of 28 ps. The entire operation of the equipment is also described in detail elsewhere [39, 40].

Figure 3.2: Block diagram for the time resolved fluorescence measurements with the ps laser

MHG

:

Multi harmonic generation

S

:

Sample

MC

:

Monochromator

MCP

:

Micro channel plate photomultiplier

AMP

:

Amplifier

PD

:

Photodiode

SPC

:

Single photon counting setup

PC

:

Personal computer

Transient Absorption Spectroscopy
Time-resolved transient absorption and gain experiments were performed at the Ecole Normale Supérieure, Paris in collaboration with Dr. Monique Martin with the pump-probe technique using a home-made dye laser described in details elsewhere [41]. Subpicosecond pulses were generated at 610 nm and frequency doubled in order to obtain excitation pulses at 305 nm. The probe was a white-light continuum produced by focusing the residual 610-nm into a 1-cm water cell. The differential absorbance spectra were recorded in the 340-700 nm range through a polychromator by a CCD camera.

Figure 3.3:Pump-Probe set-up of Trasient Absorption Spectroscopy.

The energy of the pump beam was determined found to be 55 micro joule. The spectra were averaged over 500 laser shots and corrected for the group velocity dispersion in the probe beam. The experimental time resolution was estimated to be about 1.5 ps.

Quantum chemical Calculations
The electronic properties of a molecule can be calculated by solving the Schrödinger equation,

There are various methods available to calculate the structural and electronic properties of a molecule, such as ab initio, semiempirical and density functional theory (DFT) methods. The latter method has been gaining popularity over the recent years because of the intermediacy between ab initio and semiempirical methods. The Hartree-Fock procedure or self-consistent field (SCF) model plays a crucial role in electronic structure calculations. The SCF model uses the idea of particles moving in an average electrostatic field and therefore cannot accurately treat the instantaneous interaction between electrons (electron correlation). The SCF model for the calculation of orbitals makes use of the variational principle to minimize the energy of the system iteratively until it is self-consistent. Ab initio methods are characterized by the introduction of chosen basis set for expanding the molecular orbitals and then the explicit calculation of all required integrals involving this basis set. The same is valid for Density Functional Theory (DFT) calculations. DFT calculations have a different effective Hamiltonian than Hartree-Fock calculations but the SCF procedure used to solve for the molecular orbitals (Kohn-Sham orbitals in one case and Hartree-Fock orbitals in the other case) is very similar.

Ab initio calculations can be extremely demanding in terms of the computational resources. But nevertheless, improvements in the computer hardware have made it possible that ab initio methods are a widely used computational tool nowadays. The approximate quantum chemical methods require significantly less computational resources. Especially, semi-empirical methods, which satisfy the latter criteria by incorporating the parameter, derived from the experimental data can calculate some electronic properties more accurately than even very high levels of ab initio calculations. There are number of ways in which correlation effects can be incorporated into the molecular orbital (MO) calculation. One popular approach is configuration interaction, in which various excited configurations are included in the description of an electronic state.The electron correlation problem is meticulously handled on the basis of configuration interaction by both ab initio and semi-empirical method. In this work, mainly the semiempirical method AM1 (Austin model 1) [42, 43], which has an increased improvement over the other semiempirical method like MNDO, was used together with multiexcited configuration interaction.

The treatment of the compounds studied in this work included full geometry optimization in the ground state without configuration interaction using the AM1 method [42] contained in the AMPAC program package [44] running under the Linux operating system. Single point calculations (1SCF) for the Franck-Condon excited states were performed by taking the fixed optimized ground state geometry and using configuration interaction including 300-400 singly and multiply excited configurations constructed on the basis of the central sixteen molecular orbitals.