Kinetics of Intramolecular Charge Transfer withN-Phenylpyrrole in Alkyl Cyanides

Abstract

For the electron acceptor/donor molecule N-phenylpyrrole (PP), the fast intramolecular charge transfer (ICT) reaction accompanied by dual fluorescence from a locally excited (LE) and an ICT state is investigated in alkyl cyanide solvents as a function of temperature. After a comparison of the X-ray crystal structure of PP with calculations from the literature, absorption and fluorescence spectra of PP in a series of solvents over a wide polarity range are discussed. ICT with PP strongly depends on solvent polarity and starts to appear in solvents more polar than diethyl ether. From an analysis of the ICT/LE fluorescence quantum yield ratio Phi'(ICT)/Phi(LE), approximate data for the change in enthalpy -DeltaH of the ICT reaction of PP are obtained, ranging from 9 kJ/mol in acetonitrile (MeCN) to 4 kJ/mol in n-butyl cyanide (BuCN). From ICT and LE fluorescence decays of PP measured as a function of temperature, the forward (Ea = 9 kJ/mol in ethyl cyanide (EtCN) and 6 kJ/mol in MeCN) and backward (Ed = 16 kJ/mol in EtCN and MeCN) ICT reaction barriers are determined. From these data, -Delta H (7 kJ/mol (EtCN); 10 kJ/mol (MeCN)) is calculated, in good agreement with the results coming from Phi'(ICT)/Phi(LE). The data for Ea show that the forward ICT barrier becomes smaller with increasing solvent polarity, whereas the absence of change for Ed comes from the compensating increase of -DeltaH. Both observations are indicative of a late transition state for the LE --> ICT reaction. For PP in EtCN and MeCN, the ICT radiative rate constant k'(f)(ICT) increases with temperature. This is caused by the ICT low transition dipole moment and hence does not contain information on the molecular structure (twisted or planar) of the ICT state. The fast ICT observed with PP supports our previous conclusion, based on a comparison of PP with its planarized derivative fluorazene, that the pyrrole and phenyl moieties in the ICT state of PP are coplanar and possess substantial electronic coupling.

An anti-quinoid structure in dual fluorescence of fluozazene molecule and solvent effect of intramolecular charge transfer

“…2, the twist angle, \C2-N1-C6-C11, is negligibly small from both methods. The small twist angle implies that the first excited state of PP has almost planar geometry, which is consistent to the reported results by other authors [46,48,49]. The calculated structural parameters show that PP has also a Q-like geometry (listed in Fig.…”

Ultrafast Intramolecular Charge Transfer and Internal Conversion with Tetrafluoro‐aminobenzonitriles

“…As a consequence, the reciprocal of the shortest decay time t 2 is approximately equal to the LE!ICT rate constant k a (Scheme 1), see Equation (10). Although the apparent times t 2 obtained from the global analysis of the LE and ICT decays in Figures 6 and 7 are definitely too short (0.5-1.0 ps, i.e., the LE decay is hardly different from the excitation pulse) to be by themselves meaningful in the SPC experiments, [6,24,31] the absence of an LE decay time longer than 1 ps is of importance in the analysis of the LE and ICT femtosecond transient absorption measurements of the XABN4Fs in the following section.…”

“…The excitation wavelength was 308 nm (XeCl, Lambda Physik EMG 101 excimer laser) with perylene as quencher, [29,30] or 266 nm from a frequency-quadrupled Nd:YAG laser (Continuum Surelight) when the energy acceptor was anthracene. [31] In view of the relatively short triplet lifetime of the XABN4Fs (11-19 ms), a 3-9 % correction was necessary, to compensate for the loss of transient absorption intensity caused by the competition of the first-order triplet decay and the energy transfer process.…”

“…The picosecond laser system (excitation wavelength l exc : 276 nm) consists of a mode-locked titanium-sapphire laser (Coherent, MIRA 900F) pumped by an argon ion laser (Coherent, Innova 415). [31] The instrument response function of the laser SPC system has a full width at half maximum (FWHM) of 19 ps. The time range was 300 ps (0.5 ps per channel in 1200 effective channels).…”

Hydrogen Bond Basicity in the Excited State: Concept and Applications

“…The triplet yields (F ISC ) of the DMAP and its complexed forms were measured by laser flash photolysis using the energy transfer method with anthracene as energy acceptor [5]. The excitation wavelength was 266 nm, from a frequency-quadrupled Nd:YAG laser (Continuum Surelight).…”