A series of 5-substituted (X) 2,2-dimethoxyadamantanes (4, X ) F, Cl, Br, COOCH 3 , OCH 3 , CH 3 , C 6 H 5 , p-NO 2 C 6 H 4 , p-BrC 6 H 4 , SiMe 3 , and SnMe 3 ) as well as 2-methylene-5-tert-butyladamantane (6, Y ) CH 2 ; X ) C(CH 3 ) 3 ) have been synthesized and characterized. Reduction of the ketals 4 under ionic conditions with triethylsilane and phenylsilane provide π-facial diastereoselectivities for hydride trapping of 5-substituted (X) 2-methoxyadamant-2-yl cations (1, R ) OCH 3 T 5). A comparison of this data with known diastereoselectivities for nucleophilic capture of tertiary and secondary 5-substituted (X) 2-adamantyl cations highlights that stereoselectivity in these systems is a function of electron demand. Diastereoselectivities for the hydrochlorination of 6 (Y ) CH 2 , X ) C(CH 3 ) 3 ) in CH 2 Cl 2 and NO 2 CH 3 have been determined and compared with the corresponding data for the silicon analogue (6, Y ) CH 2 ; X ) SiMe 3 ). Low electron demand coupled with the stereoelectronic requirement of double hyperconjugation appears the most likely explanation, rather than a long-range steric factor, for the total lack of stereoselectivity in the Clcapture of the 2-methyl-5-(trimethylsilyl)adamant-2-yl cation (1, Y ) CH 2 ; X ) SiMe 3 ) in NO 2 CH 3 as solvent.

The rate of photoinduced charge separation is measured as a function of solvent for four donor (D)−bridge−acceptor (A) systems:  DMN[10]DCV, DMN[10nb]DCV, DMN[10cy]DCV, and DMAN[10cy]DCV. In the first three members of this series, the D/A pair is kept constant and contains the strong dimethoxynaphthalene (DMN) donor which enables detection of electron transfer over a wide range of solvent polarity. In the fourth member, DMN is substituted by a dimethoxyanthracene (DMAN) unit, which decreases the driving force for photoinduced charge separation by about 0.58 eV and thereby limits the occurrence of electron transfer to polar solvents. In all systems the bridge is held at a length of 10 σ bonds. The configuration of the bridge is, however, varied to increase its bending in the series, which leads to center-to-center D/A distances decreasing from 13.4 Å in the first system to 9.54 Å in the second, and 7.50 Å in the latter two. In DMN[10nb]DCV, the rate of intramolecular charge separation over 9.54 Å is always smaller than that over 13.4 Å in DMN[10]DCV, which is in line with a dominant through-bond mechanism that is more efficient via an extended array of σ bonds. However, in DMN[10cy]DCV, the rate is as high as or even higher than that in DMN[10]DCV. Although changes in the driving force are also important, as shown, for example, by the dramatic rate decrease in DMAN[10cy]DCV as compared to that of DMN[10cy]DCV, the high rates observed for DMN[10cy]DCV in polar aromatic solvents as well as in acetonitrile strongly indicate an important contribution of through-solvent interaction across the 7.5 Å D/A distance, which in principle allows the intercalation of a single solvent molecule in close contact with both D and A. At the longer distance of 9.54 Å in DMN[10nb]DCV, a smaller contribution of through-solvent interaction can still be detected for polar aromatic solvents but not for acetonitrile. The inherently discontinuous distance dependence of through-solvent interaction and its possible interesting dependence on molecular structure and temperature are discussed.

The behavior is reported of three fluorescent D-bridge-A systems that display a fascinating temperature dependence in glass forming solvents over the temperature range between 77 and 293 K. In two of these systems, a rigid, saturated alkane bridge maintains an extended conformation, and as a result, the chargetransfer (CT) state is of giant dipolar nature. This causes the position of the CT fluorescence to be an extremely sensitive probe for the reorientation polarization of the surrounding medium. As a result, the thermochromism of the continuous CT fluorescence maximum in 2-methyltetrahydrofuran (MTHF) covers the full visible region. In the higher temperature domain (above ca. 145 K), this thermochromism can be quantitatively described via the Lippert-Mataga relation. At lower temperatures, solvent relaxation slows down sufficiently to detect exceptionally large dynamic Stokes shifts of the fluorescence maximum on time scales up to g40 ns. The third D-bridge-A system studied features a U-shaped ground state conformation. Remarkably, this system displays a significant thermochromic shift over a narrow temperature region around 175 K in the nonpolar methylcyclohexane (MCH) in which the other systems display only very minor thermochromism. In this U-shaped system therefore, one monitors the temperature dependence of an internal reorganization instead of a medium relaxation. Extensive ab initio calculations indicate that this internal reorganization must be related to an electrostatically driven conformational collapse of the U-shaped system in the CT state. † Part of the special issue "Noboru Mataga Festschrift".