Photophysical properties and electronic structure of bacteriochlorin-chalcones with extended near-infrared absorption

Synthetic bacteriochlorins enable systematic tailoring of substituents about the bacteriochlorin chromophore and thereby provide insights concerning the native bacteriochlorophylls of bacterial photosynthesis. Nine free-base bacteriochlorins (eight prepared previously and one prepared here) have been examined that bear diverse substituents at the 13- or 3,13-positions. The substituents include chalcone (3-phenylprop-2-en-1-onyl) derivatives with groups attached to the phenyl moiety, a "reverse chalcone" (3-phenyl-3-oxo-1-enyl), and extended chalcones (5-phenylpenta-2,4-dien-1-onyl, retinylidenonyl). The spectral and photophysical properties (τs, Φf, Φ(ic), Φ(isc), τT, k(f), k(ic), k(isc)) of the bacteriochlorins have been characterized. The bacteriochlorins absorb strongly in the 780-800 nm region and have fluorescence quantum yields (Φf) in the range 0.05-0.11 in toluene and dimethylsulfoxide. Light-induced electron promotions between orbitals with predominantly substituent or macrocycle character or both may give rise to some net macrocycle ↔ substituent charge-transfer character in the lowest and higher singlet excited states as indicated by density functional theory (DFT) and time-dependent DFT calculations. Such calculations indicated significant participation of molecular orbitals beyond those (HOMO - 1 to LUMO + 1) in the Gouterman four-orbital model. Taken together, the studies provide insight into the fundamental properties of bacteriochlorins and illustrate designs for tuning the spectral and photophysical features of these near-infrared-absorbing tetrapyrrole chromophores.

Molecular electronic tuning of photosensitizers to enhance photodynamic therapy: synthetic dicyanobacteriochlorins as a case study

Photophysical, photostability, electrochemical and molecular-orbital characteristics are analyzed for a set of stable dicyanobacteriochlorins that are promising photosensitizers for photodynamic therapy (PDT). The bacteriochlorins are the parent compound (BC), dicyano derivative (NC)2BC and corresponding zinc (NC)2BC-Zn and palladium chelate (NC)2BC-Pd. The order of PDT activity against HeLa human cancer cells in vitro is (NC)2BC-Pd > (NC)2BC > (NC)2BC-Zn ≈ BC. The near-infrared absorption feature of each dicyanobacteriochlorin is bathochromically shifted 35-50 nm (748-763 nm) from that for BC (713 nm). Intersystem crossing to the PDT-active triplet excited state is essentially quantitative for (NC)2BC-Pd. Phosphorescence from (NC)2BC-Pd occurs at 1122 nm (1.1 eV). This value and the measured ground-state redox potentials fix the triplet excited-state redox properties, which underpin PDT activity via Type-1 (electron transfer) pathways. A perhaps counterintuitive (but readily explicable) result is that of the three dicyanobacteriochlorins, the photosensitizer with the shortest triplet lifetime (7 μs), (NC)2BC-Pd has the highest activity. Photostabilities of the dicyanobacteriochlorins and other bacteriochlorins studied recently are investigated and discussed in terms of four phenomena: aggregation, reduction, oxidation and chemical reaction. Collectively, the results and analysis provide fundamental insights concerning the molecular design of PDT agents.

Stable synthetic bacteriochlorins for photodynamic therapy: role of dicyano peripheral groups, central metal substitution (2H, Zn, Pd), and Cremophor EL delivery

A series of four stable synthetic bacteriochlorins was tested in vitro in HeLa cells for their potential in photodynamic therapy (PDT). The parent bacteriochlorin (BC), dicyano derivative (NC)(2)BC and corresponding zinc chelate (NC)(2)BC-Zn and palladium chelate (NC)(2)BC-Pd were studied. Direct dilution of a solution of bacteriochlorin in an organic solvent (N,N-dimethylacetamide) into serum-containing medium was compared with the dilution of bacteriochlorin in Cremophor EL (CrEL; polyoxyethylene glycerol triricinoleate) micelles into the same medium. CrEL generally reduced aggregation (as indicated by absorption and fluorescence) and increased activity up to tenfold (depending on bacteriochlorin), although it decreased cellular uptake. The order of PDT activity against HeLa human cancer cells after 24 h incubation and illumination with 10 J cm(-2) of near-infrared (NIR) light is (NC)(2)BC-Pd (LD(50)=25 nM) > (NC)(2)BC > (NC)(2)BC-Zn ≈ BC. Subcellular localization was determined to be in the endoplasmic reticulum, mitochondria and lysosomes, depending on the bacteriochlorin. (NC)(2)BC-Pd showed PDT-mediated damage to mitochondria and lysosomes, and the greatest production of hydroxyl radicals as determined using a hydroxyphenylfluorescein probe. The incorporation of cyano substituents provides an excellent motif for the enhancement of the photoactivity and photostability of bacteriochlorins as PDT photosensitizers.

Diverse porphyrin dimers as candidates for high-density charge-storage molecules

Porphyrinic molecules have been shown to be viable candidates for a molecular-based information storage medium on the basis of redox activity. An optimal redox-based information storage medium requires a large charge density in the molecular footprint on the anchoring substrate. The use of dimeric versus monomeric architectures affords one route to achieving increased charge density without sacrificing surface cross sectional area. Towards this goal, a series of zinc and cobalt containing porphyrin dimers has been prepared and characterized. The interporphyrin linkages in the dimers include p-phenylene, ethynyl, 1,4-butadiynyl, and ethynylphenylethynyl joining porphyrin meso-positions; Crossley-type fusion bridging porphyrin β-positions, and Osuka-type triple fusions bridging one meso- and two β-positions. The electrochemical features of each dimer have been evaluated.

Encoding isotopic watermarks in molecular electronic materials as an anti-counterfeiting strategy: Application to porphyrins for information storage

An approach for information storage employs tetrapyrrole macrocycles as charge-storage elements attached to a (semi)conductor in hybrid chips. Anti-counterfeiting measures must cohere with the tiny amounts of such electroactive material and strict constraints on composition in chips; accordingly, the incorporation of typical anti-counterfeiting taggants or microcarriers is precluded. The provenance of the tetrapyrroles can be established through the use of isotopic substitution integral to the macrocycle. The isotopic substitution can be achieved by rational site-specific incorporation or by combinatorial procedures. The formation of a mixture of such macrocycles with various isotopic composition (isotopically unmodified, isotopologues, isotopomers) provides the molecular equivalent of an indelible printed watermark. Resonance Raman spectroscopic examination can reveal the watermark, but not the underlying molecular and isotopic composition; imaging mass spectrometry can reveal the presence of isotopologues but cannot discriminate among isotopomers. Hence, deciphering the code that encrypts the watermark in an attempt at forgery is expected to be prohibitive. A brief overview is provided of strategies for incorporating isotopes in meso-substituted tetrapyrrole macrocycles.

Multistate molecular information storage using S-acetylthio-derivatized dyads of triple-decker sandwich coordination compounds

An approach toward molecular information storage employs redox-active molecules attached to an electroactive surface. The chief advantages of such molecular capacitors include higher charge density and more versatile synthetic design than is afforded by typical semiconductor charge-storage materials. An architecture containing two triple-decker sandwich coordination complexes and an S-acetylthiomethyl-terminated tether has been designed for multibit storage. Each triple decker is composed of two phthalocyanines, one porphyrin, and two europium atoms. The oxidation potentials of each triple decker are tuned through the use of different substituents on the phthalocyanines (t-butyl, methyl, H ) and porphyrins (pentyl, p-tolyl). Interleaving of the four cationic oxidation states of each triple decker potentially affords eight distinct oxidation states. Two dyads were examined in solution and in self-assembled monolayers (SAMs) on a Au surface. One dyad exhibited eight distinct states in solution and in the SAM, thus constituting a molecular octal counter. The potentials ranged from −0.1-+1.3 V in solution and +0.1-+1.6 V in the SAM. Taken together, this approach provides a viable means of achieving multibit information storage at relatively low potential.

Synthesis of porphyrins for metal deposition studies in molecular information storage applications

Porphyrins may serve as the active charge-storage medium in memory chips. A series of 14 porphyrins (trans-A2B2, trans-AB2C, or A4 type; free base, copper or zinc chelate) has been synthesized for investigation of metal deposition on porphyrin monolayers. Each trans-AB2C porphyrin is equipped with a surface attachment group and a distal functional group (or flanking functional groups). The surface attachment groups include S-acetylthiomethyl, hydroxymethyl, TMS-ethynyl, allyl, and triallylmethyl; the functional groups include amino, cyano, dipyrrin-5-yl, formyl, and nitro; all of which are attached to the p-position of a porphyrin meso-phenyl group. Two non-redox-active triallylmethyl-substituted arenes were also prepared for use as control compounds.

Solution STM images of porphyrins on HOPG reveal that subtle differences in molecular structure dramatically alter packing geometry

Solution STM images are reported for free-base octaethylporphyrin ( H 2 OEP ) and the Cu ( II ) and Zn ( II ) chelates ( CuOEP and ZnOEP ) on HOPG in 1,2-dichlorobenzene solution under ambient conditions. H 2 OEP and CuOEP each form a quasi-square lattice, whereas ZnOEP forms a quasi-hexagonal lattice on the surface. The binary mixture of any two of the porphyrins forms a well-ordered two-layer structure on the HOPG surface, with one species occupying the bridge sites of the array of the other species. All of the mixed adlayers exhibit a quasi-hexagonal lattice with a higher packing density than the single-component adlayers. Collectively, these observations indicate that the structure of the adlayers is controlled by a complex interplay of substrate-molecule and molecule-molecule interactions.

Photophysical properties and electronic structure of stable, tunable synthetic bacteriochlorins: extending the features of native photosynthetic pigments

Bacteriochlorins, which are tetrapyrrole macrocycles with two reduced pyrrole rings, are Nature's near-infrared (NIR) absorbers (700-900 nm). The strong absorption in the NIR region renders bacteriochlorins excellent candidates for a variety of applications including solar light harvesting, flow cytometry, molecular imaging, and photodynamic therapy. Natural bacteriochlorins are inherently unstable due to oxidative conversion to the chlorin (one reduced pyrrole ring) or the porphyrin. The natural pigments are also only modestly amenable to synthetic manipulation, owing to a nearly full complement of substituents on the macrocycle. Recently, a new synthetic methodology has afforded access to stable synthetic bacteriochlorins wherein a wide variety of substituents can be appended to the macrocycle at preselected locations. Herein, the spectroscopic and photophysical properties of 33 synthetic bacteriochlorins are investigated. The NIR absorption bands of the chromophores range from ∼700 to ∼820 nm; the lifetimes of the lowest excited singlet state range from ∼2 to ∼6 ns; the fluorescence quantum yields range from ∼0.05 to ∼0.25; and the yield of the lowest triplet excited state is ∼0.5. The spectroscopic/photophysical studies of the bacteriochlorins are accompanied by density functional theory (DFT) calculations that probe the characteristics of the frontier molecular orbitals. The DFT calculations indicate that the impact of substituents on the spectral properties of the molecules derives primarily from effects on the lowest unoccupied molecular orbital. Collectively, the studies show how the palette of synthetic bacteriochlorins extends the properties of the native photosynthetic pigments (bacteriochlorophylls). The studies have also elucidated design principles for tuning the spectral and photophysical characteristics as required for a wide variety of photochemical applications.

Molecules for charge-based information storage

The inexorable drive to miniaturize information storage and processing devices has fueled the dreams of scientists pursuing molecular electronics: researchers in the field envisage exquisitely tailored molecular materials fulfilling the functions now carried out by semiconductors. A bottom-up assembly of such all-molecular devices would complement, if not supplant, the present top-down lithographic procedures of modern semiconductor fabrication. Short of these grand aspirations, a more near-term objective is to construct hybrid architectures wherein molecules are incorporated in semiconductor-based devices. Such a combined approach exploits the advantages of molecules for selected device functions while retaining the well-developed lithographic approaches for fabrication of the overall chip. In this Account, we survey more than a decade of results from our research programs to employ porphyrin molecules as charge-storage elements in hybrid semiconductor-molecular dynamic random access memory. Porphyrins are attractive for a variety of reasons: they meet the stability criteria for use in real-world applications, they are readily prepared and tailored synthetically, they undergo read-write processes at low potential, and they store charge for extended periods (up to minutes) in the absence of applied potential. Porphyrins typically exhibit two cationic redox states. Molecular architectures with greater than two cationic redox states are achieved by combinations of porphyrins in a variety of structures (for example, dyads, wherein the porphyrins have distinct potentials, triple deckers, and dyads of triple deckers). The incorporation of porphyrins in hybrid architectures has also required diverse tethers (alkyl, alkenyl, alkynyl, aryl, and combinations thereof) and attachment groups (alcohol, thiol, selenol, phosphonate, and hydrocarbon) for linkage to a variety of surfaces (Au, Si, SiO(2), TiN, Ge, and so forth). The porphyrins as monolayers exhibit high charge density and are robust to high-temperature excursions (400 °C for 30 min) under inert atmosphere conditions. Even higher charge densities, which are invaluable for device applications, were achieved by in situ formation of porphyrin polymers or by stepwise growth of porphyrin-imide oligomers. The various molecular architectures have been investigated by diverse surface characterization methods, including ellipsometry, atomic force microscopy, FTIR spectroscopy, and X-ray photoelectron spectroscopy, as well as a variety of electrochemical methods. These studies have further revealed that the porphyrin layers are robust under conditions of deposition of a top metal contact. The results to date indicate the superior features of selected molecular architectures for molecular electronics applications. The near-term utilization of such materials depends on further work for appropriate integration in semiconductor-based devices, whereas ultimate adoption may depend on advances that remain far afield, such as the development of fully bottom-up assembly processes.