Solid-state irradiation of the crystalline inclusion complex of (E)-stilbene in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetraphenylcyclobutane stereoselectively in high yield. In contrast, the photodimerization of stilbene in solution is very inefficient and unselective, and no photodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, viz. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond time-resolved diffuse-reflectance spectroscopy) indicate that the stilbene guests are located in at least two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest molecule and an isomerization site where excited stilbene does not find a close neighbor for dimerization and thus undergoes trans --> cis isomerization only.

Nitric oxide in solitary confinement: Conformationally mobile p‐tert‐butylcalix[4]arene methyl ether captures nitric oxide (NO) with unprecedented efficiency (K>5×108 M−1) upon oxidative activation. X‐ray crystallographic and NMR spectroscopic studies confirm that NO penetrates deep into the interior of the quasi‐cylindrical cavity of the 1,3‐alternate conformer (see picture).

: The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of PET promoted reductive activation of selenosilane 1a to its radical anion 1a •-. PET activation of 1a is achieved through a photosystem comprising a light-absorbing electron-rich aromatic (ERA), such as DMN or DMA, as an electron donor and ascorbic acid as a co-oxidant. The evidence for the ET from excited singlet states of DMN as well as DMA to 1a is suggested by estimating negative ΔG et (−51 and −43.46 kcal mol-1, respectively) values and nearly diffusion-controlled fluorescence quenching rate constants (k qTR) 0.36 × 1010 M-1 s-1 and 0.28 × 1010 M-1 s-1, respectively, from time-resolved fluorescence quenching study. The transient absorption spectra of DMN•+, DMA•+, and 1a •- are obtained initially by pulse radiolysis in order to correlate the time-resolved absorption spectral data. Laser flash photolysis studies in the nanosecond time domain have confirmed the generation of 1a •-, DMN•+, and DMA•+, supporting the participation of the triplet state of DMN or DMA in the ET reaction. Mesolytic cleavage of 1a •- produced a silyl radical and a phenyl selenide anion. The preparative PET activation of 1a in acetonitrile in the presence of DMN or DMA leads to the formation of 5 and 6, confirming the fragmentation pattern of 1a •-. The overall ET rate constants (k r(DMN) = 0.99 × 1010 M-1 s-1 and k r(DMA) = 1.62 × 1010 M-1 s-1) and limiting quantum yields (φlim(DMN) = 0.034 and φlim(DMA) = 0.12) are estimated from the inverse plot (1/[1a] vs 1/φdis) obtained by measuring the dependence of photodissociation quantum yields of 1a at its maximum concentration in the presence of DMN or DMA. Silicon-centered radical species generated from the mesolysis of 1a •- are utilized for initiating a radical reaction by the abstraction of halogen atom from −C−X (X = Cl, Br) bonds, while PhSe- terminates the radical sequences via PhSeSePh. This concept is successfully applied for the bimolecular group transfer (BMGT) radical reactions and intermolecular radical chain addition reactions.

A novel design for initiating radical-based chemistry in a catalytic fashion is described. The design of the concept is based on the phenylselenyl group transfer reaction from alkyl phenyl selenides by utilizing PhSeSiR(3) (1) as a catalytic reagent. The reaction is initiated by the homolytic cleavage of -C-Se- bond of an alkyl phenyl selenide by the in situ generated alkylsilyl radical (R(3)Si(*)), obtained by the mesolysis of PhSeSiR(3)](*)(-)( )()(1(*)(-)). The oxidative dimerization of counteranion PhSe(-) to PhSeSePh functions as radical terminator. The generation of 1(*)(-) is achieved by the photoinduced electron transfer (PET) promoted reductive activation of 1 through a photosystem comprising of a visible-light (410 nm)-absorbing electron rich DMA as an electron donor and ascorbic acid as a co-oxidant (Figure 1). The optimum mole ratio between the catalyst 1 and alkyl phenyl selenides for successful reaction is established to be 1:10. The generality of the concept is demonstrated by carrying out variety of radical reactions such as cyclization (10, 15-18), intermolecular addition (25), and tandem annulations (32).

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of photosensitized electron transfer (PET) promoted reductive activation of organoselenium substrates. PET activation of substrates 1-5 is achieved through a photosystem comprised of light-absorbing 1,5-dimethoxynaphthalene (DMN) as electron donor and ascorbic acid as co-oxidant. The fluorescence quenching of (1)DMN by organoselenium compounds 1-5, correlation of fluorescence quenching rate constant with the reduction potentials of 1-5, and the dependence of photodissociation quantum yields of 1-5 on their concentration suggests the occurrence of electron-transfer (ET) processes between (1)DMN and 1-5. Steady state photolysis of organoselenium substrates (R(2)CHSePh) in the presence of (1)DMN and ascorbic acid leads to the cleavage of the -C-Se- bond to produce a carbon-centered radical and PhSe(-) species via the intermediacy of R(2)CH-SePh&uprhbr;(-)(*). The mechanistic interpretation for the reductive activation of -C-Se- bonds and the synthetic utility of observed cleavage pattern is extended for the unimolecular group transfer radical sequences.