Vyacheslav V. Diev

Efficient dipyrrin-centered phosphorescence at room temperature from bis-cyclometalated iridium(III) dipyrrinato complexes.

A series of seven dipyrrin-based bis-cyclometalated Ir(III) complexes have been synthesized and characterized. All complexes display a single, irreversible oxidation wave and at least one reversible reduction wave. The electrochemical properties were found to be dominated by dipyrrin centered processes. The complexes were found to display room temperature luminescence with emission maxima ranging from 658 to 685 nm. Through systematic variation of the cyclometalating ligand and the meso substituent of the dipyrrin moiety, it was found that the observed room temperature emission was due to phosphorescence from a dipyrrin-centered triplet state with quantum efficiencies up to 11.5%. Bis-cyclometalated Ir(III) dipyrrin based organic light emitting diodes (OLEDs) display emission at 682 nm with maximum external quantum efficiencies up to 1.0%.

Independent Control of Bulk and Interfacial Morphologies of Small Molecular Weight Organic Heterojunction Solar Cells

We demonstrate that solvent vapor annealing of small molecular weight organic heterojunctions can be used to independently control the interface and bulk thin-film morphologies, thereby modifying charge transport and exciton dissociation in these structures. As an example, we anneal diphenyl-functionalized squaraine (DPSQ)/C(60) heterojunctions before or after the deposition of C(60). Solvent vapor annealing of DPSQ before C(60) deposition results in molecular order at the heterointerface. Organic photovoltaics based on this process have reduced open circuit voltages and power conversion efficiencies relative to as-cast devices. In contrast, annealing following C(60) deposition locks in interface disorder found in unannealed junctions while improving order in the thin-film bulk. This results in an increase in short circuit current by >30% while maintaining the open circuit voltage of the as-cast heterojunction device. These results are analyzed in terms of recombination dynamics at excitonic heterojunctions and demonstrate that the optimal organic photovoltaic morphology is characterized by interfacial disorder to minimize polaron-pair recombination, while improved crystallinity in the bulk increases exciton and charge transport efficiency in the active region.

Porphyrin-Tape/C60 Organic Photodetectors with 6.5% External Quantum Efficiency in the Near Infrared

/1 0 4 While few examples have been demonstrated, near-infrared (NIR) organic photodetectors with response at wavelengths ( λ ) beyond the cutoff of Si (i.e., λ > 1100 nm) are interesting for use in imaging and other detection applications. [ 1 ] In previous work, polymer photodetectors with response at λ > 1000 nm have been demonstrated, but the optical sensitivity is generally due to a long absorption tail having an external quantum effi ciency (EQE) less than a few percent. [ 2 – 4 ] Organic materials systems with a large NIR photoresponse are rare for several reasons. A type-II (staggered) heterojunction must be formed between the donor and acceptor materials with a suffi cient energy offset to dissociate photogenerated excitons; as the energy gap is decreased, fi nding molecular combinations with suitable energy alignments becomes increasingly diffi cult. In addition, exciton lifetimes generally decrease with energy gap due to exciton–phononinduced recombination (i.e., internal conversion). [ 5 , 6 ] These diffi culties have motivated the development of hybrid organic– inorganic devices using polymeric and small-molecule materials in conjunction with II–VI quantum dots (with EQE <1% at λ > 1000 nm) [ 7 ] or single-walled carbon nanotubes (EQE ≈ 2% at λ = 1150 and 1300 nm). [ 8 ] Here, we demonstrate an NIR EQE = 6.5% at λ = 1350 nm using photodetectors based on triply linked porphyrin-tape dimers. These porphyrin tapes are representative of a promising new class of materials that can be modifi ed to exhibit even longer wavelength response by spatially extending the conjugation of the π-electron system. [ 5 ]

Fused Pyrene–Diporphyrins: Shifting Near‐Infrared Absorption to 1.5 μm and Beyond

Porphyrins have been explored for a number of potential optoelectronic applications that require strong absorption in the near-infrared (NIR) spectral region; these applications include organic electronics, nonlinear optics, and telecommunication technologies. Porphyrins have also been investigated as active materials in photovoltaic cells because of their high efficiency of charge separation and transport, strong absorbance in the visible region, high chemical stability, and the ease with which their optoelectronic properties can be tuned with chemical modification. The absorption bands of porphyrins are not readily shifted into the deepred and NIR spectral regions, and also tend to be narrow, thus minimizing their overlap with the solar spectrum. Triply bridged, (b,meso,b), porphyrin tapes (Figure 1a, n= 0–22) show marked red-shifts in the porphyrin absorption bands, which extend deep into the NIR region. Triply fused porphyrins with n= 1,2 give absorbance in the mid-NIR region (i.e., conventional wavelengths for telecommunications, ca. 1.5 mm), however, these porphyrins are difficult to synthesize, have low solubility, and are isolated only in small quantities. Triply connected porphyrin dimers (Figure 1a, n= 0) have a strong absorbance at l= 1050 nm, are photoand chemically stable, have a high solubility, and can be easily prepared from monoporphyrins. Development of new organic dyes based on these accessible porphyrin dimers with absorption at the wavelengths for telecommunications (l= 1.5 mm) still remains a challenge. Extending the size of p conjugation in porphyrin systems results in most cases in a bathochromic (red) shift of the absorption. The conjugation of porphyrin dimers can be extended through several modes of substitution involving the meso, (b,b), (b,meso) and (b,meso,b) positions. For diporphyrins substituted with two alkyne groups at the terminal meso positions, the Q band is red-shifted by 130 nm (l= 1181 nm) relative to the parent dimer. In contrast, extending the conjugation in porphyrin dimers by benzannulating b,bpyrrolic positions red-shifts the Q band by only 18 nm, and the resulting compounds have poor solubility. Recently, it has been shown that anthracene rings can be fused to porphyrin dimers through the (b,meso,b) mode, which leads to a red-shift of the Q band to 1495 nm. However, the anthracene-fused diporphyrin exhibits the same undesirable difficulties found with higher porphyrin tapes, for example, synthetic difficulty, low yields and low solubility. Moreover, fusion of anthracene rings is limited only to alkoxy-substituted derivatives. The effects of aromatic ring fusion to porphyrin tapes in a (meso,b) mode have not been explored. We have analyzed the structures of the diporphyin core (Figure 1b), a (b,meso,b) triply fused aromatic system (Figure 1c), and a (b,meso) doubly fused molecule (Figure 1d) using standard DFT methods. Significant bathochromic shifts of the lowestenergy transition are expected in all cases. Unlike the case of anthracene-fused porphyrins and porphyrin tapes, in which the planarity causes aggregation and low solubility, the pyrene–(b,meso)-fused diporphyrin displays out-of-plane distortion that is known to improve solubility and processibility in conjugated aromatics. By taking into account the predicted bathochromic shift, distortion from planarity, and ease of synthesis, the (b,meso)-fused pyrene diporphyrin from Figure 1. a) General structure of triply fused porphyrins. b)–d) Structures of diporphyrin hybrids calculated at B3LYP/6-31G with calculated red-shifts of the lowest-energy transitions compared to the parent diporphyrin.

Porphyrins Fused with Unactivated Polycyclic Aromatic Hydrocarbons

A systematic study of the preparation of porphyrins with extended conjugation by meso,β-fusion with polycyclic aromatic hydrocarbons (PAHs) is reported. The meso-positions of 5,15-unsubstituted porphyrins were readily functionalized with PAHs. Ring fusion using standard Scholl reaction conditions (FeCl(3), dichloromethane) occurs for perylene-substituted porphyrins to give a porphyrin β,meso annulated with perylene rings (0.7:1 ratio of syn and anti isomers). The naphthalene, pyrene, and coronene derivatives do not react under Scholl conditions but are fused using thermal cyclodehydrogenation at high temperatures, giving mixtures of syn and anti isomers of the meso,β-fused porphyrins. For pyrenyl-substituted porphyrins, a thermal method gives synthetically acceptable yields (>30%). Absorption spectra of the fused porphyrins undergo a progressive bathochromic shift in a series of naphthyl (λ(max) = 730 nm), coronenyl (λ(max) = 780 nm), pyrenyl (λ(max) = 815 nm), and perylenyl (λ(max) = 900 nm) annulated porphyrins. Despite being conjugated with unsubstituted fused PAHs, the β,meso-fused porphyrins are more soluble and processable than the parent nonfused precursors. Pyrenyl-fused porphyrins exhibit strong fluorescence in the near-infrared (NIR) spectral region, with a progressive improvement in luminescent efficiency (up to 13% with λ(max) = 829 nm) with increasing degree of fusion. Fused pyrenyl-porphyrins have been used as broadband absorption donor materials in photovoltaic cells, leading to devices that show comparatively high photovoltaic efficiencies.

Fused Porphyrin–Single-Walled Carbon Nanotube Hybrids: Efficient Formation and Photophysical Characterization

A systematic study of the interaction between π-extended porphyrins and single-walled carbon nanotubes (SWNTs) is reported here. Zinc porphyrins with 1-pyrenyl groups in the 5,15-meso positions, 1, as well as compounds where one or both of the pyrene groups have been fused at the meso and β positions of the porphyrin core, 2 and 3, respectively, have been examined. The strongest binding to SWNTs is observed for porphyrin 3, leading to debundling of the nanotubes and formation of stable suspensions of 3-SWNT hybrids in a range of common organic solvents. Absorption spectra of 3-SWNT suspensions are broad and continuous (λ=400-1400 nm), and the Q-band of 3 displays a significant bathochromic shift of 33 nm. The surface coverage of the SWNTs in the nanohybrids was estimated by spectroscopic and analytical methods and found to reach 64% for (7,6) nanotubes. The size and shape of π-conjugated porphyrins were found to be important factors in determining the strength of the π-π interactions, as the linear anti-3 isomer displays more than 90% binding selectivity compared to the bent syn-3 isomer. Steady-state photoluminescence measurements show quenching of porphyrin emission from the nanohybrids. Femtosecond transient absorption spectroscopy reveals that this quenching results from ultrafast electron transfer from the photoexcited porphyrin to the SWNT (1/kCT=260 fs) followed by rapid charge recombination on a picosecond time scale. Overall, our data demonstrate that direct π-π interaction between fused porphyrins and SWNTs leads to electronically coupled stable nanohybrids.

Corrigendum to “An efficient method of synthesis of isoxazolidine-fused β-lactams via base-promoted cyclization-ring opening of carbamoyl-spirocyclopropane isoxazolidines” [Tetrahedron 69 (2013) 5173–5177]

Corrigendum to “An efficient method of synthesis of isoxazolidinefused b-lactams via base-promoted cyclization-ring opening of carbamoyl-spirocyclopropane isoxazolidines” [Tetrahedron 69 (2013) 5173e5177] Tung Q. Tran , Ruslan S. Savinkov , Vyacheslav V. Diev , Galina L. Starova , Alexander P. Molchanov b,* a School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet Road, Ha Noi, Viet Nam Department of Chemistry, Saint Petersburg State University, Universitetsky pr. 26, Saint Petersburg 198504, Russian Federation Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA

Squaraine donors for high efficiency small molecule solar cells

It has been a challenge to achieve high efficiency organic photovoltaics (OPV) that absorb long wavelength solar radiation without incurring unacceptable reductions in open circuit voltage (Voc) or charge separation efficiency. Based on the parent structure of the 2, 4-bis[4-(N, N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ), we have increased Voc using a family of highly near-infrared absorbing SQs, achieving values as high as 0.94 V. These SQ donors are: 2, 4-bis[4-(N-Phenyl-1-naphthylamino)-2,6-dihydroxyphenyl] squaraine (1-NPSQ),2,4-bis[4-(N, N-diphenylamino)-2,6 dihydroxyphenyl] squaraine (DPSQ), 2,4-bis[4-(N, N-diphenylamino)-2,6-dihydroxyphenyl] asymmetric squaraine (DPASQ). The spin-cast SQ, 1-NPSQ, DPSQ and DPASQ donors are then coated with the acceptor C60 to form bulk heterojunction (BHJ) solar cells that take advantage of their exceptionally high absorption coefficient and nanocrystalline morphology to overcome the short diffusion length characteristic of these materials. Combined with a high short-circuit current density (Jsc=10.6 mA/cm2) and high fill factor (FF=0.64), the optimized 1-NPSQ/C60 photovoltaic cells with 1-NPSQ annealed at elevated temperature have a power conversion efficiency of ηp as high as 6.0% (correcting for solar mismatch) at 1 sun (AM 1.5G) simulated solar illumination, which to our knowledge is the highest efficiency reported to date for small molecule OPVs.