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The photo-induced oxidation of NADH and its model compound, 1-benzyl-1,4–dihydronicotinamide (BNAH), by N,N′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen: MV++) was studied in homogeneous and in micellar solution. In the overall, slightly endoergonic, reaction: NADH + 2 MV+ + + hv NAD+ + 2 MV +• + H+ the dihydropyridines lose a hydride equivalent via a stepwise e,H+,e sequence in which only the initial one-electron transfer requires photostimulation. It is shown that the singlet-excited state of the dihydropyridine is the photoreactive intermediate in this initial electron transfer process and that the overall quantum yield depends critically on the ratio of the rates for ionic dissociation and back-electron transfer in the encounter complex between this excited state and MV+ + molecules. This ratio can be influenced dramatically by changing the solvent medium. In aqueous solution (for NADH) and especially in anionic micellar solution (for BNAH) strong ground state association between MV++ and the dihydropyridines occurs. Evidence for this association is provided by the appearance of a long-wavelength charge transfer absorption, as well as by the large deviations from a Stern-Volmer behaviour observed in the quenching of the dihydropyridine fluorescence by MV++. The ground state complexation appears to hamper ionic dissociation of the excited complex. This is evident from the low quantum yield for photooxidation of such systems, upon excitation of the dihydropyridine chromophore (365 nm), and the complete absence of such oxidation upon their excitation within the charge transfer absorption (546 nm). The data obtained are shown to allow for a complete thermodynamic analysis of the Gibbs free-energy changes involved in loss of a hydride equivalent from a 1,4-dihydropyridine via a stepwise e,H+,e mechanism. The consequences of this analysis for the mechanism of reactions in which 1,4-dihydropyridines transfer a hydride equivalent to various substrates under thermal conditions are discussed.

We report a two-step synthesis for N-aryl- and N-alkyl-substituted 4-piperidones, in which the N substituent can easily be varied. A number of intramolecular donor-acceptor systems was synthesized from these piperidones by conversion of the carbonyl functionality. The influence of the N-aryl donor on the electronic absorption and fluorescence spectra was investigated systematically. It was concluded that some systems can be used as efficient fluorescent probes with a high sensitivity for solvent polarity.

The mechanism of exciplex formation of two donor-bridge–acceptor compounds, both containing a strong donor - acceptor pair interconnected by a piperidine ring, was studied in nonpolar solvents as a function of temperature. Fluorescence spectra at low temperature in methylcyclohexane show that in both compounds the exciplex is formed via the harpooning mechanism, i.e. long-range charge separation leading to an extended charge-transfer state, followed by Coulomb-induced folding to the exciplex state. The kinetic link between these two emissive states could be established by studying their transient fluorescence curves. Furthermore, the activation energy of the folding process was estimated from the decay rates of the extended charge-transfer state in various solvents. An interpretation of the observed activation energies in terms of Kramers theory is presented, which gives an indication of the relative importance of internal steric effects and solvent viscosity on the dynamics of the folding process.

The incorporation of fullerene-C60 and of C60 adducts into Triton X-100 (TX100) micelles and into liposomes made of phosphatidyl-ethanolamine from Escherichia coli has been studied. Incorporation curves and UV-Vis absorption spectra for the liposomes are presented. Flash photolysis experiments in TX100 micelles in the presence of the electron donors N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and cytochrome c resulted in the observation of oxidized transient species.

The condensation of amines of the general type H2N-(CH2)n-D-(CH2)m-NH2 (D is an aromatic system with electron donor properties) with 1,2:4,5-benzene-tetracarboxylic dianhydride or with 1,8:4,5-naphthalene-tetracarboxylic dianhydride under high dilution conditions is shown to lead to the formation of electron donor(D)-acceptor(A) cyclophanes of the type containing the 1,2:4,5-benzene-tetracarboxylic diimide or the 1,8:4,5-naphthalene-tetracarboxylic diimide group as an electron acceptor. The fragmentation pattern of the cyclophanes upon electron impact is studied and found to agree completely with the proposed structures.

Synthesis of enamines 2a-f, derived from α-tetralones 1a, 1b, is described. Steric effects in the formation of these enamines and in the spectral properties of the corresponding 2-substituted derivatives are discussed.

A kinetic study of the reduction of the 10-methyl-9-phenylacridinium ions (AcPh⊕) by 1-benzyl-1,4-dihydronicotinamide (PyH2) is presented. The primary kinetic isotope effect (kH/kD = 4.5) of this reaction equals the product isotope partitioning observed upon reduction with mono C-4 deuterated 1-benzyl-1,4-dihydronicotinamide (PyHD). Furthermore, the rate constant for the reduction decreases upon increasing the solvent polarity. This behaviour contrasts with that reported for the reduction of the 10-methylacridinium ion (AcH⊕) where acceleration due to increased solvent polarity and significant discrepancies between the kinetic isotope effect and the product isotope partitioning have been observed. Reinvestigation of the latter process by means of kinetic measurements and via a competition experiment, involving reduction with equal amounts of PyH2 and the dideuterated species (PyD2), shows that the kinetic data and especially the kinetic isotope effect data are easily subject to experimental error. Thus it is shown that an intolerable inaccuracy is introduced if the value of kH/kD is determined by comparing the rate of reduction with PyH2 with that of PyHD rather than by comparing those of PyH2 and PyD2. Furthermore, the kinetic data in protic solvents for AcH⊕, but not for AcPh⊕, are found to be disturbed by a reversible side reaction. This reaction is thought to involve formation of a covalent adduct by attack of PyH2 at the electrophilic C-9 position of AcH⊕. The less electrophilic nature of this position in AcPh⊕ prevents such adduct formation and leads to uncomplicated kinetic behaviour in all the solvent systems investigated.

Purpose: Radiographic parameters of mandibular trabecular bone structure between 67 subjects having hypodontia and those without were studied on digital panoramic radiographs. Materials and Methods: Three regions of interest (ROI) were defined: the ascending ramus, apical of the mandibular molar and mesial of the first mandibular molar. The effects of the presence of hypodontia and the ROI on the mandibular trabecular bone structure were tested for statistical significance by means of multivariate analysis. Results: Radiographic parameters of trabecular bone architecture were found to differ between various regions of the mandible (p = 0.000), but not between the group of hypodontia subjects and their controls (p = 0.23). There was no interaction effect between the ROIs and the two groups (p = 0.79). For people having hypodontia, some directional parameters of trabecular bone have a reverse correlation with the number of missing teeth. The fractal dimension and the number and perimeter of white segments in the binarized image correlate positively with the number of congenitally missing teeth. Conclusions: A limited number of parameters of radiographic mandibular trabecular bone structure correlate with the number of missing teeth. However, a markable difference in radiographic parameters of mandibular trabecular bone structure between hypodontia and non- hypodontia subjects could not be demonstrated.

Photoinduced electron transfer is a widely applied method to convert photon energy into a useful (electro)chemical potential, both in nature and in artificial devices. There is a continuing effort to develop molecular systems in which the charge-transfer state, populated by photoinduced electron transfer, survives sufficiently long to tap the energy stored in it. In general this has been found to require the construction of rather complex molecular systems, but more recently a few approaches have been reported that allow the use of much more simple and relatively small electron donor-acceptor dyads for this purpose. The most successful examples of such systems seem to be those that apply "electron spin control" to slow down the spontaneous decay of the charge-transfer state, and these are reviewed in this minireview, with a discussion of the underlying principles and a critical evaluation of some of the claims made with regard to using a pronounced "inverted-region effect" as an alternative method to prolong the lifetime of charge-transfer states.

Investigation of the photophysical and photochemical processes that occur in single molecules by means of fluorescence microscopy has rapidly developed over the last decade.[1,2] We have focused on the photophysics of synthetic multichromophore systems and have shown evidence for intramolecular (excitation) energy migration, that is, energy hopping[3,4] and directional F%rster energy transfer.[5] Photoinduced electron transfer is another fundamental physical process, but it is more difficult to monitor by single-molecule spectroscopy. Electron transfer is usually an efficient mechanism for fluorescence quenching which leaves no signal to be detected at the single-molecule level. In some special cases, chargetransfer fluorescence may be observed after photoinduced electron transfer,[6] but its quantum yield is usually quite low and therefore less suited for single-molecule detection. Despite this difficulty some reports on forward electron transfer in single molecules exist in the literature. Interfacial electron transfer from a single molecule of a cresyl violet dye to the indium–tin oxide surface on which it was physisorbed was shown by Lu and Xie.[7] Weaker fluorescence and a decreased fluorescence lifetime with respect to the same molecule on a glass surface demonstrated the occurrence of electron transfer. In a similar way electron transfer between a guanine base and a dye linked to a DNA molecule could also be monitored, and conformational changes of DNA molecules were observed.[8–10] Intramolecular electron transfer in a synthetic system was recently demonstrated by Zang et al.[11] in a compound in which a perylenediimide dye that serves as the electron acceptor upon excitation is connected to an aniline donor. On a sufficiently polar substrate, no fluorescence from single molecules could be observed, but after protonation of the amine by treatment of the sample with hydrogen chloride, the molecules could be visualized. This experiment nicely dem

A highly flexible molecular tweezer, (aqua)(2,6-bis[(dimethylamino)methyl]-4-{3-[(1-pyrenyloxy)methyl]-5-(trimethylsilyl)benzyloxy}phenyl)palladium(II) tetrafluoroborate (1a), constructed from a para-hydroxy-functionalized NCN-pincer palladium(II) complex {NCN−OH = [2,6-(Me2NCH2)2C6H2-4-OH]}, a pyrenoxy unit and a xylyl spacer, was synthesized. X-ray analysis of the structure of 1-[3-({3,5-bis[(dimethylamino)methyl]-4-(trimethylsilyl)phenyloxy}methyl)-5bromobenzyloxy]pyrene (10), a precursor ligand of 1a, showed a completely flattened conformation in the solid state, imposed by favorable intramolecular π-stacking interactions. The molecular structure of 1a in solution proved to be fluxional on the NMR timescale. UV/Vis and (time-resolved) fluorescence spectroscopy revealed that, in spite of the high flexibility, the catalyst and pyrenoxy units are in close proximity because of cation−π interactions. This was further supported by conformer searches using molecular mechanics (MM2). The binding affinity of picric acid towards the pyrenoxy moiety was studied by 1H NMR titrations, revealing association constants Ka in the order of 101 to 102M−1. Tweezer 1a shows small rate enhancements in aldol condensations of aromatic aldehydes with methyl isocyanoacetate compared to (aqua)(2,6-bis[(dimethylamino)methyl]4-{[3-(trimethylsilyl)benzyloxy}phenyl)palladium(II) tetrafluoroborate (2), which lacks the pyrenoxy unit. The tweezer was immobilized on a G2-carbosilane dendrimer, generating the dendritic (aqua)(G2-carbosilane)palladium complex 1b, showing a decreased catalytic activity compared to 1a. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The light generating mechanism of a series of light emitting diodes with electron donor-bridge-acceptor systems (D-b-A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly-N-vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge-transfer (CT) fluorescence of the donor-bridge-acceptor systems occurred readily in OLED devices with a D-b-A system as the emissive layer. In diodes with PS as the host matrix, hole-injection and electron-injection occurred directly in the D-b-A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole-injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D-b-A molecule, whereas electron-injection at the cathode side occurred directly into the D-b-A molecules. Charge-hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence.

The crystal structures of five N-arylpiperidin-4-one derivatives 2P2, 3P2, 5P2, 1P3, and 2P3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1P2 and 4P2 published previously. In all but one structure, 1P2, the aryl group is in an equatorial position. The piperidine ring adopts a normal chair conformation. In 1P2, the piperidine ring central CC bonds are significantly elongated, which is consistent with the idea that through-bond interaction is more pronounced in the axial conformation. Through-bond interaction also influences the pyramidalization at the piperidine C(4)-atom in such a way that a strong interaction is directing the ethylene C-atom C(9) into the axial direction.

The crystal structures of seven N-aryltropan-3-one (=8-aryl-8-azabicyclo[3.2.1]octan-3-one) derivatives 1T1, 2T1, 2T2, 3T2, 5T2, 2T3, and 3T3 are presented (Fig. 2 and Tables 1–5) and discussed together with the derivatives 1T2 and 4T2 published previously. The piperidine ring adopts a chair conformation. In all structures, the aryl group is in the axial position, with the plane through the aryl C-atoms nearly perpendicular to the mirror plane of the piperidine ring. The through-bond interaction between the piperidine ring N-atom (one-electron donor) and the substituted exocyclic CC bond (acceptor) not only elongates the central CC bonds of the piperidine ring but also increases the pyrimidalization at C(4) of the piperidine ring. Flattening of the C(2)–C(6) part of the piperidine ring decreases the through-bond interaction.

The photophysical properties of a bicyclohexylidene (1DA) and a bicyclohexyl (2DA) substituted with an anilino electron donor and a dicyanoethylene electron acceptor have been studied. Quenching of local donor emission is observed for these compounds as well as quenching of the "pseudo-local" acceptor emission. Transient absorption spectra show dialkylanilino-type radical-cation and dicyanoethylene-type radical-anion absorptions. These results show that intramolecular charge separation takes place in 1DA and 2DA. This was corroborated by time-resolved microwave conductivity measurements from which large excited-state dipole moments were found for both 1DA and 2DA. Time-resolved fluorescence spectroscopy revealed that in the charge-separated state in cyclohexane for 2DA, molecular folding takes place on a nanosecond timescale. For 1DA in cyclohexane, either charge separation takes place in a (fully) folded conformation or very rapid (subnanosecond timescale) folding takes place subsequent to charge separation. In addition to this difference in conformational behavior, the presence of the exocyclic double bond between the cyclohexyl-type rings results in efficient quenching of the anilino donor triplet state and acceleration of the charge recombination rate by a factor of 20.

The highly solvatochromic probe “Fluoroprobe”, which fluoresces in a wide range of the visible spectrum, was covalently attached to silica and alkylated silica particles to study solvent accessibility and surface micropolarity. A two step attachment method was used which was found to lead to a quite homogeneous distribution of the probes without extensive local clustering. On silica the probe experiences a local dielectric constant of ϵ = 6, while its solvent accessibility is ca. 30%. On alkylated silica this reduces to ϵ = 3.4 and ca. 23% respectively.

Six new donor-bridge-acceptor compounds have been synthesized which contain a long n-tetradecyl chain attached to the donor or acceptor moiety, or to both of them. Systems 1, 2, and 3 are analogs of the fluorescent probe molecule Fluoroprobe (4). They contain a rigidly extended 4-methylenepiperidine bridge and show relatively strong charge transfer fluorescence in solvents of low and medium polarity. Systems 1a, 2a, and 3a contain a semiflexible 4-methylpiperidine bridge, obtained after hydrogenation of the exocyclic double bond of 1, 2 and 3, respectively. These systems undergo a conformational change following photoinduced charge separation (harpooning) in nonpolar solvents and probably also in solvents of medium polarity. Both the steady state fluorescence spectra and the fluorescence decay times of the extended charge transfer (ECT) species show that the photoinduced folding process is effectively slowed down by the introduction of the long alkyl tails. This is most pronounced for 1a which has an n-tetradecyl group attached to both donor and acceptor. In solution a small difference in the rate of folding is observed between 2a and 3a, which have a single n-tetradecyl chain attached to the acceptor only and to the donor only, respectively.

The process of intramolecular charge separation was studied in three nearly identical donor-bridge-acceptor compounds (1-3) under jet-cooled conditions. Each consists of a vinylcyanonaphthalene group as a powerful electron acceptor and an anilino derivative as an electron donor, separated by a rigid hydrocarbon bridge. The only difference between the three compounds is the substituent at the para position of the anilino group, which directly influences the ionization potential of the donor. For compound 1, which has the weakest donor of the three, it is found that excitation at (or very near) the electronic origin of the acceptor chromophore results exclusively in local emission, while excitations with an excess energy >60 cm" lead to a distinct red shift. Comparison with 2 and 3 shows that the red-shifted band can be attributed to a charge-separated state with an extended conformation, in which an electron has been transferred across three sigma bonds. Interestingly, this charge separation process appears to be more efficient in 2 and 3 than in 1, resulting in complete quenching of the local emission even at the lowest energy excitations. These observations are interpreted in terms of intramolecular radiationless transition theory and a comparison is made with the behaviour of (semi)flexibly bridged systems in which charge separation was shown earlier to be linked to a major, electrostatically driven reorganization.

A probe molecule, which shows an unusually large change in fluorescent properties in response to changes in its environment, was introduced into a monodisperse polystyrene latex. Upon addition of low-molecular-weight compounds, the ensuing swelling process of the latex could be followed via the changes in the fluorescence spectrum of the probe molecule. Polymerization reactions of monomers, introduced into the particles, could also be monitored by means of this approach. Using glycidylmethacrylate as the monomer led to the formation of particles in which two distinctly different surroundings could be detected after such a polymerization. © 1995 John Wiley & Sons, Inc.

Photoinduced charge separation as well as subsequent charge recombination is studied in bichromophoric molecules containing a quinone unit (Q) and either a porphyrin (P) or a zinc porphyrin (ZnP) unit that are interconnected by a rigid saturated hydrocarbon bridge that unequivocally determines both the separation and the relative orientation of the chromophores. Across a bridge comprising a separation equivalent to two saturated carbon-carbon bonds (i.e., in P(2)Q), extensive direct overlap between the {pi}-systems of the chromophores is still possible and accordingly very fast photoinduced charge separation (typically on a 30-40-ps time scale) is observed. However, even across a bridge comprising an extended array of six saturated bonds, charge separation times in the order of 100 ps can still be realized if the driving force for this process is optimized by modification of the porphyrin and of the solvent (a minimum charge-separation time of 65 ps was observed for ZnP(6)Q in chloroform). This implies a rate of charge separation comparable to or somewhat higher than that of the charge transfer from pheophytin to quinone in natural photosynthesis in spite of the fact that the interchromophore distance in the latter process is slightly smaller. The time-resolved microwave conductivity method was employed tomore » confirm the occurrence of photoinduced charge separation as well as to measure the rate of charge recombination in ZnP(6)Q.« less

We report a two-step synthesis for N-aryl- and N-alkyl-substituted 4-piperidones, in which the N substituent can easily be varied. A number of intramolecular donor-acceptor systems was synthesized from these piperidones by conversion of the carbonyl functionality. The influence of the N-aryl donor on the electronic absorption and fluorescence spectra was investigated systematically. It was concluded that some systems can be used as efficient fluorescent probes with a high sensitivity for solvent polarity.

The chromoionophoric behaviour of compound 1 in solution was studied in relation to the X-ray structure of its complexes with Na+, Ba2+ and Ag+. Interaction between the metal ion and the “soft” nitrogen atom, which forms part of the aza crown ether of 1, is concluded to be of crucial importance. It is shown that in solution a soft cation such as Ag+ induces a greater special spectral shift than do hard cations of equal charge and almost the same size, such as Na+ and K+. Solid-state structures of the Ag+ and Na+ complexes of 1 show a relatively short Ag+ -N distance and a relatively long Na+ - N distance. Netherlands Organization for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO).

Photoinduced electron transfer is a widely applied method to convert photon energy into a useful (electro)chemical potential, both in nature and in artificial devices. There is a continuing effort to develop molecular systems in which the charge-transfer state, populated by photoinduced electron transfer, survives sufficiently long to tap the energy stored in it. In general this has been found to require the construction of rather complex molecular systems, but more recently a few approaches have been reported that allow the use of much more simple and relatively small electron donor-acceptor dyads for this purpose. The most successful examples of such systems seem to be those that apply "electron spin control" to slow down the spontaneous decay of the charge-transfer state, and these are reviewed in this minireview, with a discussion of the underlying principles and a critical evaluation of some of the claims made with regard to using a pronounced "inverted-region effect" as an alternative method to prolong the lifetime of charge-transfer states.

Two bichromophoric systems (3 and 4) are studied that contain an electron donor (D) and an electron acceptor (A) moiety interconnected by a fully rigid, saturated hydrocarbon-bridge. In both the shortest through-bond pathway from D to A has a length of 8 sigma-bonds, but whereas this path has an all-trans configuration in 3, two cis elements have been introduced in 4. Photoinduced, intramolecular electron transfer from D to A occurs more than an order of magnitude faster across the stretched all-trans bridge in 3 than across the bent bridge in 4 notwithstanding the smaller spatial distance in the latter.

The reaction-coordinate for degenerate hydride-transfer between 1,4-dihydro-pyridine and the pyridinium-ion was searched by MNDO calculations. A linear transition-state (TS) structure is found to be strongly (> 16 kcal/mole) preferred over a bent TS. In the former, rotation about the C-hydride-C axis occurs unimpeded, thus creating a continuum of linear TS-structures with ring to ring orientations varying between exo and endo.

The fluorescence of Fluoroprobe, a molecule with an extremely polar excited state, is studied in matrices consisting of (macro)molecules which contain repeating units of different polarity. In matrices of low molecular weight Fluoroprobe displays its typical bathochromic shift with increasing contribution of polar groups, but an hypsochromic shift is observed in viscous or solid matrices of high molecular weight. This remarkable phenomenon is attributed to loss of mobility, which allows fluorescence of the probe to occur prior to full relaxation of the surrounding matrix sites.

It is proposed that in certain enzymes transfer of hydrogen nuclei occurs by tunnelling in a symmetrical double minimum potential.

The jet-cooled fluorescence properties of two bichromophoric molecules (1 and 2), differing only in the acceptor chromophore, are compared. The barrier to exciplex formation in 1, containing a powerful cyanonaphthalene acceptor chromophore, is much lower than that in 2 which contains a less powerful naphthalene acceptor chromophore. It is thus proposed that the barrier is not only of a conformational nature, but also related to the energy required for intramolecular charge separation in the Franck-Condon excited conformation which may then be followed by a fast, electrostatically-driven conformational change.

Fast excited-state electron transfer is observed between two chromophores held apart by a rigid saturated steroid moiety.

The fluorescence of the C70 fullerene chromophore in three solvents is reported. Quenching of this fluorescence by organic electron donors occurs as a result of photoinduced electron transfer. With aniline-type donors ground-state charge transfer complex formation is evident from curved Stern-Volmer plots as well as from additional long wavelength absorption.

A calix[8]arene derivative has been found to form a water soluble complex with fullerene-C60, but not with C70, which allows extraction of C60 from e.g. toluene into the aqueous phase. Charge transfer character of the complex is evident from strong charge transfer absorption around 440 nm measured in water. Molecular modeling calculations indicate a core-shell structure for the complex.

A simple perturbational frontier molecular orbital description with inclusion of sigma electrons is presented for bifunctional intramolecular reactivity. This description readily accounts for the preferred formation of odd membered rings in (di) radical cyclization and of even membered transition states in intramolecular hydrogen or hydride migration.

Intramolecular singlet energy transfer can be detected in a series of rigid bichromophoric molecules (1(n)) where a dimethoxynaphthalene chromophore and a carbonyl chromophore are separated by extended all-trans arrays of up to eight C-C {sigma} bonds (1(8)). In the series of compounds 2(n) kinks are introduced in the array of {sigma} bonds of the saturated hydrocarbon system, which bridges the chromophores. Singlet energy transfer is then much less efficient (i.e. in 2(6)) or even absent (i.e. in 2(8)), which supports the earlier interpretation of the energy transfer mechanism in 1(n) as being mainly mediated by through-bond exchange interaction and furthermore explains the virtual absence of such interaction in more flexible systems where the chromophores are linked by polymethylene bridges.

Two rigidly bridged electron donor-acceptor systems are reported that display discrete, intramolecular charge-transfer (CT) absorption and emission spectra in solution at room temperature. For both the CT-absorption is highly structured irrespective of solvent, for one also the CT-fluorescence in a nonpolar solvent is structured.

Frontier molecular orbital theory has been applied to the [4 + 2]-cycloaddition of singlet oxygen, to mono- and dimethylnaphthalenes and to a number of other polycyclic aromatic compounds. Semilogarithmic plots of rate constants versus second order perturbation energies are found to be linear. The good correlations are due partly to the fact that singlet oxygen is such a small dienophile.

Cyclic voltammetry and preparative controlled potential electrolysis show that the 9-phenyl-10-methyl-acridinium/acridan (AcPh+/AcPhH) redox couple can be cycled electrochemically between the oxidized (AcPh+) and reduced states (AcPhH) without any apparent side-reaction. The 9-phenyl-10-methyl-acridanyl radical (AcPh•) was identified as a first intermediate in the electrochemical reduction of AcPh+ by cyclic voltammetry as well as by electronic absorption and ESR spectroscopy. In contrast to other acridanyl and related dihydropyridyl radicals, AcPh• shows no tendency to undergo dimerization. In aprotic media, AcPh• is shown to undergo a second reversible one-electron reduction to yield AcPh−, which is, even in these media readily protonated to give AcPhH. The stability of these intermediates seems to be the major factor responsible for the clean electrochemical interconversion of the AcPh+/AcPhH redox couple. The implementation of this redox couple as part of photo-electrochemical energy conversion systems is discussed.

Compounds D-(CH2)n-A (n = 1-5) are studied, in which D = an aromatic group with low ionization potential (e.g. an electron donor) and A is mainly the N-3,6-dinitro-1,8-naphthalimide group. Especially for n = 1 and n = 3, interaction between D and A leads to the observation of discrete intramolecular C(harge) T(ransfer) transitions in the electronic absorption spectra. For n = 1 this is explained by “homoconjugative” interaction, while for n = 3 the molecules probably adopt a folded conformation, a suggestion which is suppported by NMR data. For n = 2 close approach of D and A seems to be sterically excluded, while for n > 3 the interaction between D and A rapidly diminishes. For the compound with D = 9-anthryl and n = 3 steric interaction between the peri-hydrogens of the anthryl group and the trimethylene chain seems to inhibit the molecule from adopting a folded conformation. For the compounds with D = p-dimethylaminophenyl and n = 3 or 4 a double intramolecular CT transition is observed; a tentative explanation for this phenomenon is given.