Electronic excitation of a molecule (D-B-A) consisting of an
electron donating group D and an electron accepting group A that are
separated by a (partly) saturated hydrocarbon bridge B, can give rise
to electron transfer (ET) from D to A. Along with this, a
charge-separated state D·+-B-A·- is
formed that, in the absence of other reactions, returns to the initial
state (D-B-A). When excitation is performed with light (hn) one speaks of photoinduced ET. Both a locally excited donor,
(D*-B-A), and a locally excited acceptor,
(D-B-A*), can initiate ET, although in the latter case it
is more appropriate to speak of hole transfer.
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This Thesis treats some aspects of photoinduced electron transfer in organic D-B-A compounds with fully or partly saturated, semirigid hydrocarbon bridges in solution and in the solid state. It consists of two parts. The aim of Part I (Chapters 2 to 4) is to determine how and to which extent non-conjugated double bonds in an otherwise saturated hydrocarbon bridge affect the rate of forward and backward electron transfer and other photophysical processes. The type of D-B-A compound central to Part I has an N ,N -dialkylanilino electron donor and a 1,1-dicyanoethene acceptor. The bridge is of the oligo(cyclohex-1,4-diylidene) type 1(n), of the oligo(cyclohex-1,4-diyl) type 2(n),1 or of a mixed type (3 en 4); see Figure 1.
In Chapter 2 the influence of a double bond in the saturated hydrocarbon bridge on the photophysical properties of the N ,N -dialkylanilino group is investigated. Using nanosecond transient absorption spectroscopy and time-resolved microwave conductivity, efficient intramolecular triplet-triplet energy transfer from the N ,N -dialkylanilino chromophore to the isolated double bond was observed in the model electron donor compounds 5 and 7 in solution. This enables the unequivocal assignment of the absorption spectrum of the first singlet excited state of the N ,N -dialkylaniline chromophore in 5, 6, 7 and 8. It has a maximum positioned between 600 and 635 nm.
Chapter 3 is devoted to the ground state s-p and p-p* interactions in 1(1) and 2(1). Both the ionization potentials of these compounds measured in the gas phase, and ab initio RHF/6-31G calculations in combination with a natural bond orbital analysis show that the ground state through-bond interaction (TBI) between the 1-phenylpiperidine electron donor and the 1,1-dicyanoethene electron acceptor in 1(1) and 2(1) is distinct but small. The isolated double bond in 1(1) enhances the interaction between the electron donor and the electron acceptor as compared to 2(1). The TBI between the N ,N -dialkylaniline donor and the isolated double bond in 1(1) can be modulated by rotation of the phenyl group around the C-N bond. In addition, the solid state structures of 1(1) and 2(1) were determined by single crystal X-ray diffraction. In the solid state intermolecular electron-donor-acceptor complexes are formed, which give rise to an intermolecular charge-transfer absorption in the solid state.
Chapter 4 treats the charge separation and charge recombination kinetics of 1(n) (n = 1, 2, 3), 2(n) (n = 1,2), 3 and 4 in solution. The replacement of an exocyclic C-C single bond by a double bond increases the rate of charge separation with a factor of 3.0 ±0.8 per replaced bond. For all D-B-A compounds the extended, fully charge-separated conformer folds to a more compact charge-separated conformer due to the electrostatic attraction between D·+ and A·-. The rate of charge recombination in this conformer increases with solvent polarity for those compounds having an olefinic bond located at three s bonds from the acceptor. In cyclohexane, for example, the charge recombination rate is equal for all compounds. In benzene, however, it is 10 times larger for compounds with an olefinic bond near the acceptor than in compounds with a single bond at that position. It is believed that a (virtual) charge-separated state involving the radical cation of the olefinic bond and the radical anion of the acceptor (D-B·+-A·-) is responsible for the enhanced charge recombination process.
The aim of Part II is to incorporate D-B-A moieties in materials (a polymer, monolayers) and to study the effect thereof on the electron transfer process. The D-B-A compounds and moieties central to Part II consist of an N ,N -dialkylaniline electron donor and a 1-R2-1-R3-functionalized ethene acceptor, which are separated by a four s bond saturated hydrocarbon bridge. The functionalities R2 and R3 are cyano- and/or alkoxycarbonyl groups. The common structural element and compounds derived thereof are given in Figure 2.
In Chapter 5 the syntheses and properties of five of such semirigid D-B-A compounds are described (9-13). The electronic properties of these compounds were studied by means of photoelectron spectroscopy in combination with ab initio MO calculations, cyclic voltammetry, UV-vis absorption spectroscopy, (time-resolved) fluorescence spectroscopy and time-resolved microwave conductivity (TRMC). It is shown that these D-B-A compounds exhibit a weak intramolecular charge-transfer absorption band and, upon photoexcitation, give a charge-separated state in a near quantitative yield. For two of the D-B-A compounds, 9 and 13, this (extended) state appears to convert into a folded charge-separated state in apolar solvents. The absence of folding in compounds 10-12 is believed to be due to a very short lifetime of the charge-separated state. This in turn is probably caused by the presence of an additional decay process involving rotation around the ethenyl-carbonyl bond in the acceptors of these compounds.
In Chapter 6 the preparation of two high molecular weight polyesters (Figure 3) via solution polymerization at room temperature is described. One polymer (14) has an N ,N -dialkylaniline electron donor in the main chain. In the other (15) the N ,N -dialkylaniline group is part of a D-B-A moiety with a 1,1-dicyanoethene acceptor. Polymer 14 is soluble in some common solvents, whereas 15 is not soluble. Polymer 14 displays dual fluorescence emission. One band is ascribed to local emission while the other is attributed to excimer emission. For 15 charge-transfer absorption and emission are observed. It is argued that charge transfer between different D-B-A moieties (whether inter- or intrachain) rather than charge transfer between D and A within single D-B-A moieties occurs in the solid state.
In Chapter 7 the synthesis of two D-B-A adsorbates for gold surfaces, 16 and 17, is reported. It is known that dialkyl sulfide groups, such as present in these compounds have a large affinity for gold. It is shown that these adsorbates give self-assembled monolayers (SAMs) on a Au(111) surface. The ability to block the electrical current between a SAM covered gold electrode and an external redox couple was tested with cyclic voltammetry and was found to be better for SAMs of 16 than for SAMs of 17. On both SAMs a dense monolayer of spherical gold nanoparticles (16 nm diameter) could be deposited. It appears that 16 and possibly 17 adsorb preferentially via their dialkyl sulfide moieties to the flat Au(111) surface, while the Au nanoparticles in turn attach to the N ,N -dimethylaniline moieties pendent in solution. This leads to the formation of a Au|D-B-A|Au structure.
1Oligo(cyclohex-1,4-diyliden)s and
oligo(cyclohex-1,4-diyl)s are frequently referred to as
oligo(cyclohexylidene)s and oligo(cyclohexyl)s, respectively.
