Convenient synthesis of a highly soluble and stable phosphorescent platinum porphyrin dye.

A new highly soluble platinum porphyrin derivative 10 with suppressed photobleaching is prepared on a multigram scale from inexpensive starting materials. 10 possesses intense absorption bands at λ = 463 nm (log ε = 5.39) and 633 nm (log ε = 5.20) with near-IR emission at 755 nm. Efficient NIR phosphorescence (PLQY = 0.45) and a large Stokes shift (eliminating self-absorption) make it an attractive and readily available material for a number of applications.

A detailed NMR- and DFT-based study of the Sakurai-Hosomi-Yamamoto asymmetric allylation reaction.

A Lewis acid complex between benzaldehyde and the silver catalyst was detected by (31)P NMR and shown to be the direct precursor to allylation within the Sakurai-Hosomi-Yamamoto reaction. Structural and thermochemical hybrid-DFT calculations indicated that benzaldehyde predominantly formed an η(1)-σ-complex with the catalyst; however, two other competing conformers involving different coordination modes were found, including an activated µ(2)-bound complex. The differences in (31)P NMR shifts upon complexation were calculated by the gauge-independent atomic orbital (GIAO-DFT) method for each conformer. The minimum energy conformer was found to correlate well with chemical shift trends observed experimentally, and an analysis of Mullikan charge populations revealed that the carbonyl carbon of the highest-energy conformer was the most electron-deficient. Furthermore, one minor and three major silicon intermediates were detected by (29)Si NMR and, with the aid of (1)H-(29)Si HSQC, were assigned by comparison with parent compounds and GIAO-DFT calculations. Finally, a tentative mechanism was proposed based on these findings.

Long-lived charge separation in novel axial donor-porphyrin-acceptor triads based on tetrathiafulvalene, aluminum(III) porphyrin and naphthalenediimide.

Two self-assembled supramolecular donor-acceptor triads consisting of Al(III) porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al(III) on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4-(n-pentyl)-4'-cyanobiphenyl (5CB), a spin-polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF(.+)NDI(.-) is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet-triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF(.+)NDI(.-) is estimated to be 200-300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X-band EPR spectrometer is about 10 µs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.

Maximizing field-effect mobility and solid-state luminescence in organic semiconductors.

Conductive and emissive: organic transistors made from a simple styrylanthracene derivative have high charge mobility and high luminescence quantum yields. These properties are attributed to the lack of singlet fission, and challenge the idea that the efficient π interactions required for high mobility always lead to quenching of emission. The transistors emit blue electroluminescence and are stable during operation and storage.

Photoinduced electron transfer in zinc naphthalocyanine-naphthalenediimide supramolecular dyads.

Photoinduced electron transfer was studied in self-assembled donor-acceptor dyads, formed by axial coordination of pyridine appended with naphthalenediimide (NDI) to zinc naphthalocyanine (ZnNc). The NDI-py:ZnNc (1) and NDI(CH(2))(2)-py:ZnNc (2) self-assembled dyads absorb light over a wide region of the UV/Vis/near infrared (NIR) spectrum. The formation constants of the dyads 1 and 2 in toluene were found to be 2.5×10(4) and 2.2×10(4) M(-1), respectively, from the steady-state absorption and emission measurements, suggesting moderately stable complex formation. Fluorescence quenching was observed upon the coordination of the pyridine-appended NDI to ZnNc in toluene. The energy-level diagram derived from electrochemical and optical data suggests that exergonic charge separation through the singlet state of ZnNc ((1)ZnNc*) provides the main quenching pathway. Clear evidence for charge separation from the singlet state of ZnNc to NDI was provided by femtosecond laser photolysis measurements of the characteristic absorption bands of the ZnNc radical cation in the NIR region at 960 nm and the NDI radical anion in the visible region. The rates of charge-separation of 1 and 2 were found to be 2.2×10(10) and 4.4×10(9) s(-1), respectively, indicating fast and efficient charge separation (CS). The rates of charge recombination (CR) and the lifetimes of the charge-separated states were found to be 8.50×10(8) s(-1) (1.2 ns) for 1 and 1.90×10(8) s(-1) (5.3 ns) for 2. These values indicate that the rates of the CS and CR processes decrease as the length of the spacer increases. Their absorption over a wide portion of the solar spectrum and the high ratio of the CS/CR rates suggests that the self-assembled NDI-py:ZnNc and NDI(CH(2))(2)-py:ZnNc dyads are useful as photosynthetic models.

Photochemistry of cyclic trisilanes: "spring-loaded" precursors to methylphenylsilylene.

Two new cyclic trisilanes containing a SiMePh moiety in the central position have been synthesized, and their photochemistries studied by steady-state and laser flash photolysis methods. The yield of the transient silylene SiMePh increases at the expense of that of the corresponding cyclic silene derivative (formed by photochemical 1,3-silyl migration into the phenyl ring) as the ring size is decreased from 7 to 6, facilitating the direct detection of the silylene by laser flash photolysis. The UV/Vis spectrum of SiMePh in hexanes solution (lambda(max)=270, 505 nm) and absolute rate constants for its reaction with methanol, triethylsilane, a terminal alkene, alkyne, and two aliphatic dienes at 25 degrees C are reported.

First direct detection of 2,3-dimethyl-2,3-diphenylcyclopropanone.

3,5-dihydro-3,5-dialkyl-3,5-diaryl-4H-pyrazol-4-ones stimulate interest as potential precursors for 2,3-diarylcyclopropanones. Photoreactions of trans-3,5-dihydro-3,5-dimethyl-3,5-diphenyl-4H-pyrazol-4-one were studied by continuous-wave (CW) and pulsed laser UV photolysis revealing an intermediate that undergoes rearrangement to form cis- and trans-1,3-dimethyl-1-phenyl-2-indanones with the yield of ca. 60%. Steady-state photolysis (254 and 350 nm excitation) in different solvents produced an intermediate cyclohexadiene as evidenced by UV/vis, IR, and 1H NMR spectra. In contrast, the nanosecond laser pulsed photolysis at 355 nm produced 2,3-dimethyl-2,3-diphenylcyclopropanone along with two products of retro-1,3-dipolar addition phenylmethylketene and 1-phenyldiazoethane. These can be observed by time-resolved IR (TRIR) spectroscopy as characteristic absorption bands at 1814, 2101, and 2038 cm-1, respectively. Similar retro-1,3-dipolar addition showed 1-phenyldiazoethane formed following flash photolysis of 1-pyrazoline (trans-4,5-dihydro-3,5-dimethyl-3,5-diphenyl-3H-pyrazol-4-ol). The formation of the corresponding cyclopropanone as well the products of retro-1,3-dipolar addition during photoreaction of starting pyrazol-4-one is directly confirmed by the nanosecond TRIR spectroscopy for the first time. On the basis of the CW and pulsed laser UV photolysis, a dynamic equilibrium between cyclopropanone and intermediate 2,4-diphenyl-3-pentanone-2,4-diyl (dimethyldiphenyloxyallyl) was proposed.

Diphenylsilylene.

Lamp or pulsed laser photolysis of 1,1,3,3-tetramethyl-2,2-diphenyl-1,2,3-trisilacycloheptane affords diphenylsilylene (SiPh2) with significantly higher selectivity than that reported previously from other SiPh2 precursors, allowing the direct detection of SiPh2 in solution for the first time. The UV/vis spectrum of the silylene in anhydrous hexane at 25 degrees C, determined by laser flash photolysis methods, agrees well with that obtained in frozen hydrocarbon matrixes, and that of the dimerization product, tetraphenyldisilene, is tentatively assigned. Absolute rate constants measured for the reaction of SiPh2 with a number of characteristic silylene scavengers correspond closely to those reported previously for SiMe2 with the same or similar substrates.