Steroids as Photonic Wires. Z → E Olefin Photoisomerization Involving Ketone Singlet → Triplet Switches by Through-Bond Energy Transfer1,2

Catalyzed and Promoted Aliphatic Fluorination

In the last six years, the direct functionalization of aliphatic C-H (and C-C) bonds through user-friendly, radical-based fluorination reactions has emerged as an exciting research area in fluorine chemistry. Considering the historical narratives about the challenges of developing practical radical fluorination in organic frameworks, notable advancements in controlling both reactivity and selectivity have been achieved during this time. As one of the participants in the field, herein, we a provide brief account of research efforts in our laboratory from the initial discovery of radical monofluorination on unactivated C-H bonds in 2012 to more useful strategies to install fluorine on biologically relevant molecules through directed fluorination methods. In addition, accompanying mechanistic studies that have helped guide reaction design are highlighted in context.

Ketones as directing groups in photocatalytic sp3 C-H fluorination

The ubiquitous ketone carbonyl group generally deactivates substrates toward radical-based fluorinations, especially sites closest to it. Herein, ketones are used instead to direct aliphatic fluorination using Selectfluor, catalytic benzil, and visible light. Selective β- and γ-fluorination are demonstrated on rigid mono-, di-, tri-, and tetracyclic (steroidal) substrates employing both cyclic and exocyclic aliphatic ketones as directing groups.

On the Mechanism of Intramolecular Sensitization of Photocleavage of the 2-(2-Nitrophenyl)propoxycarbonyl (NPPOC) Protecting Group

A spectroscopic study of a variety of covalently linked thioxanthone(TX)-linker-2-(2-nitrophenyl)propoxycarbonyl(NPPOC)-substrate conjugates is presented. Herein, the TX chromophore functions as an intramolecular sensitizer to the NPPOC moiety, a photolabile protecting group used in photolithographic DNA chip synthesis. The rate of electronic energy transfer between TX and NPPOC was quantified by means of stationary fluorescence as well as nanosecond and femtosecond time-resolved laser spectroscopy. A dual mechanism of triplet-triplet energy transfer has been observed comprising a slower mechanism involving the T1(pipi*) state of TX with linker-length-dependent time constants longer than 20 ns and a fast mechanism with linker-length-dependent time constants shorter than 3 ns. Evidence is provided that the latter mechanism is due to energy transfer from the T2(npi*) state which is in fast equilibrium with the fluorescent S1(pipi*) state. In the case of direct linkage between the aromatic rings of TX and NPPOC, the spectroscopic properties are indicative of one united chromophore which, however, still shows the typical NPPOC cleavage reaction triggered by intramolecular hydrogen atom transfer to the nitro group.

Benzil-tethered precipitons for controlling solubility: a round-trip energy-transfer mechanism in the isomerization of extended stilbene analogues

We are investigating photoresponsive molecules called "precipitons" that undergo a solubility change co-incident with isomerization. Isomerization can be induced by light or by catalytic reagents. Previous work demonstrated that covalent attachment of a metal complex, Ru(II)(bpy)3, greatly accelerates photoisomerization and influences the photostationary state. In this paper, we describe precipitons (1,2-biphenylethenes; analogous to stilbenes) that are activated by a covalently attached organic sensitizer (benzil). We find that isomerization of these stilbene analogues is little effected by the presence of benzil in solution but that the intramolecular benzil effect is to increase the rate of isomerization and to significantly change the photostationary state. What is most interesting about these observations is that the precipiton is the primary chromophore in this bichromophoric system (precipiton absorbance is many times greater than benzil absorbance in the 300-400 nm range), yet the neighboring benzil has a significant effect on the rate and the photostationary state. The effect of unattached benzil on the rate was small, about a 24% increase in rate as compared with 4-6-fold changes for an attached benzil. We speculate that the isomerization process occurs by a "round-trip" energy-transfer mechanism. Initial excitation of the precipiton chromophore initiates a sequence that includes (1) formation of the precipiton singlet state, (2) singlet excitation transfer from the precipiton unit to the benzil, (3) benzil-centered intersystem crossing to the localized benzil triplet state, (4) triplet energy transfer from the benzil moiety back to the precipiton, and (5) isomerization.

Intramolecular Triplet Energy Transfer in Donor–Acceptor Molecules Linked by a Crown Ether Bridge

Bichromophoric compounds BP-C-NP and BP-C-NBD were synthesized with benzophenone chromophore (BP) as the donor, and 2-naphthyl (NP) and norbornadiene group (NBD) as the acceptor, respectively. Their intramolecular triplet energy transfer was examined. The bridges linking the donor and acceptors in these molecules involve a crown ether moiety complexing a sodium ion. Phosphorescence quenching, flash photolysis and photosensitized isomerization experiments indicate that intramolecular triplet energy transfer occurs with rate constants of about 3.3 x 10(5) and 4.8 x 10(5) s(-1) and efficiencies of about 33 and 42 % for BP-C-NP and BP-C-NBD, respectively. Theoretical calculations indicate that these molecules adopt conformations below room temperature which allow their two-end chromophores conducive to through-space energy transfer.

Unexpected reactivity difference between iodo-alkene moieties of steroids possessing remote lactame or cycloalkane structural units: a theoretical approach

The steroidal ring A possessing cyclohexane vs. delta-valerolactame structures shows a 'long-range effect' on the reactivity of the 'iodovinyl' functionality of the ring D. The strikingly different reactivity of 17-iodo-16-ene functionality of steroids is explained on the basis of the redistribution of kinetic energy of the system. The vibrational energy localized on the 'iodovinyl' moiety proved to be determinant in the CI bond-breaking process. The appearance of the lactame moiety in the ring A supports a redistribution of the kinetic energy on the molecule. As a result, a drastic decrease in C-I bond stretching has been obtained, which could diminish the C-I bond breaking, and therefore, results in decreased reactivity of the iodovinyl moiety in agreement with experimental findings.

Structure and photophysical properties of porphyrin-modified metal nanoclusters with different chain lengths

Three-dimensional porphyrin-monolayer-protected gold clusters with different chain lengths (MPCs) have been prepared to examine the structure and photophysical properties, in comparison with self-assembled monolayers (SAMs) of the porphyrins on a flat gold surface. The three-dimensional porphyrin MPCs exhibit electrochemical and photophysical properties that are much closer to those of a porphyrin reference compound in solution than those of two-dimensional porphyrin SAMs on the flat gold surface. The three-dimensional architectures of porphyrin MPCs with large surface area have improved the light-harvesting efficiency relative to the corresponding porphyrin SAM on the two-dimensional flat gold surface. Time-resolved single photon counting fluorescence and transient absorption spectroscopic studies have demonstrated that undesirable quenching of the porphyrin excited singlet state via energy transfer to the gold surface of the three-dimensional MPCs is much suppressed, as compared to the quenching of the porphyrin SAMs on the two-dimensional flat gold surface. Both the quenching rate constants of the porphyrin excited singlet state by the surfaces of bulk gold and gold nanoclusters reveal weak chain length dependence of the energy transfer quenching.

Intramolecular singlet–singlet energy transfer in antenna-substituted azoalkanes

Two novel azoalkane bichromophores and related model compounds have been synthesised and photophysically characterised. Dimethylphenylsiloxy (DPSO) or dimethylnaphthylsiloxy (DNSO) serve as aromatic donor groups (antenna) and the azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as the acceptor. The UV spectral window of DBO (250-300 nm) allows selective excitation of the donor. Intramolecular singlet-singlet energy transfer to DBO is highly efficient and proceeds with quantum yields of 0.76 with DPSO and 0.99 with DNSO. The photophysical and spectral properties of the bichromophoric systems suggest that energy transfer occurs through diffusional approach of the donor and acceptor within a van der Waals contact at which the exchange mechanism is presumed to dominate. Furthermore, akin to the behaviour of electron-transfer systems in the Marcus inverted region, a rate of energy transfer 2.5 times slower was observed for the system with the more favourable energetics, i.e. singlet-singlet energy transfer from DPSO proceeded slower than from DNSO, although the process is more exergonic for DPSO (-142 kJ mol(-1) for DPSO versus-67 kJ mol(-1) for DNSO).

Intramolecular sensitization within steroids: Excited-state interaction of C-17 α and β carbon–bromine bonds with a C-6 carbonyl group

The synthesis, photochemistry, and photophysics of 17α-bromo-3α-(triphenylsilyloxy)-5α-androstan-6-one (1) and 17β-bromo-3β-(triphenylsilyloxy)-5α-androstan-6-one (2) have been studied in aqueous tetrahydrofuran. The 17α-bromo isomer gives evidence (reduced ϕf, τ1, and ϕisc for the ketone) for interaction between the ketone and C–Br moieties in the excited singlet state. Some photodehalogenation is also observed upon excitation of the ketone chromophore. This interaction seems to be absent or minimal for the 17β-bromo isomer.Key words: photodehalogenation, bromosteroid, ketosteroid, intramolecular singlet–singlet energy transfer (ISSET).

Long-range through-bond photoactivated sigma bond cleavage in steroids. Intramolecular sensitized debromination

[structure: see text] The photolysis of 17alpha-bromo-3alpha-(triphenylsilyloxy)-5alpha-androstane (2; 3alphaTPSO/17alphaBr) and 17alpha-bromo-3alpha-(triphenysilyloxy)-5-androstan-6-one (3; 3alphaTPSO/6ketone/17alphaBr) is described. Irradiation of 2 with 266 nm light leads to debromination via intramolecular transfer of triplet excitation energy with a quantum efficiency of 0.0011. Photolysis of 3 with both 266 and 308 nm light leads to debromination with quantum efficiencies of ca. 0.0066. The debromination of 3 is attributed to activation via the ketone excited singlet state, with singlet energy transfer from C6 to C17 ca. 35% efficient and occurring with a rate constant of 1.4 x 10(8) s(-1).