Edwin Haselbach

Dual fluorescence and fast intramolecular charge transfer with 4-(diisopropylamino)benzonitrile in alkane solvents

Abstract Dual fluorescence and fast intramolecular charge transfer (ICT) is observed with 4-(diisopropylamino)benzonitrile (DIABN) in alkane solvents. The rate constant ka for the reaction from the locally excited (LE) to the ICT state has a value of 3.4×1011 s−1 in n-hexane at 25°C, with an activation energy Ea of 6 kJ mol−1. Efficient intersystem crossing with a yield of 0.94 takes place from the ICT state. With 4-(dimethylamino)benzonitrile, in contrast, dual fluorescence is not observed in alkanes. The charge transfer reaction of DIABN is mainly favoured by its small energy gap ΔE(S1,S2), in accordance with the PICT model for ICT in aminobenzonitriles.

Quenching of triplet benzophenone by methyl and methoxy benzenes: are triplet exciplexes involved?

The quenching of triplet benzophenone by methyl and methoxy substituted benzenes in acetonitrile was investigated. For both series of donors the quenching rate constants are considerably higher than expected from the Rehm-Weller relationship, which could be explained by a mechanism involving a (triplet) exciplex intermediate. Two corresponding models from the literature were chosen for simulating the experimental data. The results are in accord with the exciplex proposal. For a given driving force the quenching rate constants differ substantially for the two kinds of donor employed. Together with the free ion yields, obtained by transient photoconductivity measurements, this confirms the important role of the chemical structure of the donors which determines the stability of the exciplexes.

Photoreduction of triplet benzophenone by amines: role of their structure

Abstract The reaction triplet benzophenone with the tertiary amines triisopropylamine and diisopropyl-3-pentylamine does not lead to photoreduction. An explanation of this unusual behaviour is given based on the particular structure of these amines. The initial charge transfer events are compared with those exhibited by 1,4-diazabicyclo[2.2.2.]octane and triethylamine.

MARCUS INVERTED REGION : NATURE OF DONOR-ACCEPTOR PAIRS AND FREE ION YIELDS

The free ion yield (ΦR) resulting from the fluorescence quenching of 9,10-dicyanoanthracene (DCA) by various electron donors in acetonitrile has been studied using ns laser photoconductivity. The influence of the chemical nature of the doors is established in a general manner. For a given oxidation potential Eox(D/D+), the rate constant of geminate ion recombination, kbac, decreases significantly as the electronic delocalization of the donor increases. As a consequence multiple Marcus plots are observed in the inverted region. These plots show decreasing curvature when going from stilbenes to amines as donors. This ‘fan effect’ is tentatively explained by considering the detailed roles of the parameters V, λ and hν in the Marcus model.

Light induced processes in organic molecular ions

Of the many different chemical processes where organic molecular ions (in particular cations Nt) are known to engage in, most are thermally activated ones. Recently, however, a growing body of evidence has begun to emerge which indicates that Mt also undergo a variety of photochemical transformations. They are quite distinct from those undergone by their parent neutrals (M) in that they can usually be induced with visible light and often follow different courses. This short review intends to summarize the present state of knowledge about Mt-photoprocesses and highlights some pertinent photophysical parameters of Mt used to rationalize these reactions. A general comparison between gas— and condensed phase results is made and the example of trans-stilbenet and cis_stilbenet interconversion which appears at present the best studied Mt-photoreaction is discussed in some detail.

Comparative electron-transfer quenching rates of 9,10-dicyanoanthracene by various donors in solvents of different polarities

Multiple Rehm–Weller plots of the rate constants of quenching of 9,10-dicyanoanthracene fluorescence by electron transfer with a variety of donors of different adiabatic ionization potentials have been observed in polar solvents (1-fluoropentane and acetonitrile). The donors fall into two classes which can be described as ‘n’(localised positive charge in the cation) and ‘π’(delocalised charge), the former being the more efficient quenchers. Possible reasons for this difference are discussed: electrostatic and solvation energies, internal reorganisation energy. Current evidence favours the former explanation.

Comparison of photoinduced electron transfer reactions of aromatic carbonyl vs. cyano compounds with electron donors in condensed phase:the importance

Photoinduced electron transfer reactions in acetonitrile with bensopheneone, anthraquinone, 9-cyanoanthracene and 9,10-dicyanoanthracene as electron acceptors, and with 1,4-diasabicyclo[2,2,2]octane and N,N-dimethylaniline as electron donors have been studied with ns-laser flash photolysis and fluorescence quenching measurements. For these systems the resulting free ion yield depends on the spin state of the geminate ion pair: its separation is very efficient if formed in a triplet state (carbonyl compounds/donors), while it is very inefficient if formed in a singlet state (cyanoanthracenes/donors). In the triplet systems, geminate back electron transfer is limited by the rate of spin flip.

The reliability of free ion yield in photoinduced electron transfer reactions. The model system 9,10-dicyanoanthracene/biphenyl in acetonitrile

A detailed study of the separation efficiency in the photoinduced electron transfer reaction between 9,10-dicyanoanthracene and biphenyl in acetonitrile is presented. Both transient absorption and photoconductivity indicate a separation efficiency of about 0.4. This value is in discrepancy with two of three previously reported efficiencies. The problems arising with too large donor concentrations and with the use of a secondary donor to determine the separation efficiency are discussed.

TRIPLET ENERGIES AND ELECTRON AFFINITIES OF METHYL-ACRYLATE AND METHYL-METHACRYLATE

Abstract— The gas phase electron affinities and triplet and singlet excitation energies of the title compounds have been measured by electron transmission and electron‐energy‐loss spectroscopies. The implications of the results to understanding the mechanism of electron‐beam curing of materials containing the acrylate and methacrylate moieties are discussed.

Tuning the ion formation processes from triplet-triplet annihilation to triplet-mediated photoionization

Free ion formation in acetonitrile is examined through transient photoconductivity for a set of ketones excited at different wavelengths. According to the photophysical parameters of the ketones and the incident photon energy, two mechanisms can be operative: triplet–triplet annihilation (bimolecular process) and/or photoionization (monomolecular biphotonic process). By using a tunable laser, excited state mediated photoionization was studied. From the threshold energy (Ethr) required for this process to occur, ionization potentials in solution (IS) were deduced and compared to the corresponding values in gas phase (IG). A simple energetic model enables the determination of the oxidation potential (Eox) of the ketones that are compared to the corresponding values obtained through electrochemical measurements. 2003 Elsevier B.V. All rights reserved.