Photoactive Units Research Articles

DFT study of five naphthalimide derivatives: Structures and redox properties

Geometries for five naphthalimide (NI) derivatives proposed as photoactive redox units in cancer treatment, have been optimized in the ground state, reduced and oxidized states using density functional theory (DFT) with the B3LYP hybrid functional and the 6-31G (d,p) basis set. The bond lengths of the N-[3-(dimethylamino)propyl]-1,8-naphthalimide derivative showed the most variation from the other four NI derivatives due to the nature of its substituent. An examination of atomic charges and spin densities shows in general that the ring carbons are most significant, except in the N-(propyl)-1,8-naphthalimide,3′-phosphoric acid derivative, where charges and densities are localized in the phosphate group. Single point energy calculations, both in vacuum and solution were performed for each NI derivative. The adiabatic and vertical electron affinities and ionization potentials are given for each of the NI derivatives in the ground state. The results suggest that none of the five NI derivatives studied can oxidize DNA nucleobases via direct electron transfer from the ground states, and that the dark toxicity of the compounds is thus most likely very low.

Phase behavior and photo‐responsive studies of photoactive liquid crystalline hyperbranched polyethers containing benzylidene moiety

AbstractTwo sets of hyperbranched polyether epoxies were synthesized to study the effect of substituent, rigidity, and nature of photoactive unit on the thermal and photoresponsive properties. Each set was comprised of one molecule with an acyclic moiety in the repeating unit, and two molecules with a cyclic moiety of varying rigidity (cycle size) in the repeating unit. Two substituents on aromatic rings in the repeating unit were present in one set, and other set was without a substituent. The mesogenic and photoresponsive properties were studied and correlated to the varied structural parameters. The effects of varied molecular structural parameters on phase behavior and photoresponsive properties were very prominent. Out of six monomeric diols, only four have exhibited liquid crystalline phase while the polymers corresponding to all monomeric diols revealed mesophase. The findings in photoresponsive properties were further supported by molecular modeling studies. The changes in refractive index, photoviscosity, and fluorescence intensity with irradiation time substantiated the spectral pattern observed in UV‐Vis spectroscopy. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2774–2786, 2009

Solvent-free microwave-assisted Bingel reaction in carbon nanohorns

In this paper we exploit the benefits of microwave-assisted functionalization of carbon nanohorns (CNHs). Using Bingel reaction conditions we have successfully incorporated malonyl moieties in CNHs. The resulting hybrid materials were characterized by diverse and complementary analytical techniques, including Raman, UV-Vis and photoluminescence spectroscopy, transmission electron microscopy, thermogravimetric analysis, while electrochemistry was employed to determine possible variations in the energy levels when photoactive units were grafted onto the skeleton of CNHs.

Multistate Photo-Induced Relaxation and Photoisomerization Ability of Fumaramide Threads: A Computational and Experimental Study

Fumaric and maleic amides are the photoactive units of an important and widely investigated class of photocontrollable rotaxanes as they trigger ring shuttling via a cis-trans photoisomerization. Here, ultrafast decay and photoinduced isomerization in isolated fumaramide and solvated nitrogen-substituted fumaramides (that are employed as threads in those rotaxanes) have been investigated by means of CASPT2//CASSCF computational and time-resolved spectroscopic techniques, respectively. A complex multistate network of competitive deactivation channels, involving both internal conversion and intersystem crossing (ISC) processes, has been detected and characterized that accounts for the picosecond decay and photochemical/photophysical properties observed in the singlet as well as triplet (photosensitized) photochemistry of fumaramides threads. Interestingly, singlet photochemistry appears to follow a non-Kasha rule model, where nonequilibrium dynamical factors control the outcome of the photochemical process: accessible high energy portions of extended crossing seams turn out to drive the deactivation process and ground-state recovery. Concurrently, extended singlet/triplet degenerate regions of twisted molecular structures with significant spin-orbit-coupling values account for ultrafast (picosecond time scale) ISC processes that lead to higher photoisomerization efficiencies. This model discloses the principles behind the intrinsic photochemical reactivity of fumaramide and its control.

Self-Organization of Phthalocyanine−[60]Fullerene Dyads in Liquid Crystals

The use of blends in which a mesogen induces mesomorphism into a non-mesogenic compound has made possible the self-organization of phthalocyanine--[60]fullerene (Pc-C60) dyads into liquid crystals. Pc-C60 dyads 1, 2, or 3, in which two photoactive units are brought together by a phenylenevinylene spacer, have been synthesized through a Heck reaction that links 4-vinylbenzaldehyde to a monoiodophthalocyanine precursor, followed by standard cycloaddition of azomethine ylides--generated from the formylPc derivative and N-methylglycine--to one of the double bonds of C60. The mesomorphic and thermal properties of different mixtures formed by the liquid-crystalline phthalocyanine 4 and dyads 1, 2, or 3 were examined using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). DSC diagrams of the blends show clear transitions from the crystalline state to a mesophase, and the measured structural parameters obtained from the powder diffraction experiments are consistent with a discotic hexagonal columnar (Col h) structure. Considering that segregation in domains of separated molecules of Pc-C60 dyad and phthalocyanine 4 would preclude mesomorphism due to the mismatch in the column diameter and to the lack of mesogenic character of the pure dyads, a predominance of alternating stacking is proposed. Additionally, the observed decrease in the calculated density of the blend mesophases relative to the mesophase of pure compound 4 is important evidence in this direction.

New and Efficient Arrays for Photoinduced Charge Separation Based on Perylene Bisimide and Corroles

The bichromophoric systems C2-PI, C3-PI, and C3-PPI consisting of corrole and perylene bisimide units and representing one of the rare cases of elaborate structures based on corrole, have been synthesized. Corroles C2 and C3 are, respectively, meso-substituted corroles with 2,6-dichlorophenyl and pentafluorophenyl substituents at the 5 and 15 positions. The three dyads were prepared by divergent strategy with the corrole-forming reaction as the last step of the sequence. C2-PI and C3-PI differ in the nature of the corroles, whereas C3-PI differs from C3-PPI in the presence of a further phenyl unit in the linker between photoactive units. The dyads display spectroscopic properties which are the superposition of the component spectra, indicating a very weak electronic coupling. Excitation of the corrole unit leads to charge separation with a rate which decreases from 2.4 x 10(10), to 5.0 x 10(9), and to 4.9 x 10(7) s(-1) for C2-PI, C3-PI, and C3-PPI, respectively, where the reaction is characterized by a delta G degrees >0. Excitation of the perylene bisimide unit is followed by competing reactions of: 1) energy transfer to the corrole unit, which subsequently deactivates to the charge-separated state and; 2) electron transfer to directly form the charge-separated state. The ratio of electron-to-energy-transfer rates is 9:1 and 1:1 for C3-PI and C3-PPI, respectively. The yield of charge separation is essentially 100 % for C2-PI and C3-PI, and approximately 50 % (excitation of peryleneimide) or 15 % (excitation of the corrole) for C3-PPI. The lifetime of the charge-separated state, observed for the first time in corrole-based structures, is 540 ps for C2-PI, 2.5 ns for C3-PI, and 24 ns for C3-PPI, respectively. This is in agreement with an inverted behavior, according to Marcus theory.

Characterization of photoactive polymer thin films using transmission spectrum

There are many different methods applied to obtain the optical constants of thin films using transmission spectrum, none of them applied to photoactive polymer thin films. The aim of this work is to find the suitable method for routine measurements of photoactive polymer thin films. In order to achieve this goal, different methods are applied to simulated thin films of known optical constants to compare their accuracies, hence choosing the most suitable method for studying the photoactive polymer thin films. It is found that the fitting methods are not suitable for photoactive polymer thin films while numerical solution of two independent transmission measurements is suitable for such films, so poly methy1 methacrylate (PMMA) films doped with different concentrations of photoactive unit Disperse Red 1 (DSR1) are characterized using this method. The refractive index of PMMA doped with 0.2% of DSR1 (1 g PMMA, 2 mg DSR1) is found to be 1.534 with error 0.656% at λ=630 nm. It is also found that the refractive index of photoactive polymer thin films increases linearly with increasing photoactive unit up to concentration 0.25%.

Electrochemically formed fullerene-based polymeric films

The recent results of investigations involving the electrochemical formation of polymers containing fullerenes and studies of their properties and applications are critically reviewed. From a structural point of view, these polymers can be divided into four main categories including (1) polymers with fullerenes physically incorporated into the foreign polymeric network without forming covalent bonds, (2) fullerene homopolymers formed via [2+2] cycloaddition, (3) “pearl necklace” polymers with fullerenes mutually linked covalently to form polymer chains, and (4) “charm bracelet” polymers containing pendant fullerene substituents. The methods of electrochemical polymerization of these systems are described and assessed. The structural features and properties of the electrochemically prepared polymers and their chemically synthesized analogs are compared. Polymer films containing fullerenes are electroactive in the negative potential range due to electroreduction of the fullerene moieties. Related films made with fullerenes derivatized with electron-donating moieties as building blocks are electroactive in both the negative and positive potential range. These can be regarded as “double cables” as they exhibit both p- and n-doping properties. Fullerene-based polymers may find numerous applications. For instance, they can be used as charge-storage and energy-converting materials for batteries and photoactive units of photovoltaic cell devices, respectively. They can be also used as substrates for electrochemical sensors and biosensors. Films of the C60/Pt and C60/Pd polymers containing metallic nano-particles of platinum and palladium, respectively, effectively catalyze the hydrogenation of olefins and acetylenes. Laser ablation of electrochemically formed C60/M and C70/M polymer films (M=Pt or Ir) results in fragmentation of the fullerenes leading to the formation of hetero-fullerenes, such as [C59M]+ and [C69M]+.

Stabilization of Charge-Separated States in Phthalocyanine−Fullerene Ensembles through Supramolecular Donor−Acceptor Interactions

A novel ZnPc-C60 dyad (3), in which two photoactive units are brought together by a phenylenevinylene spacer has been synthesized. The synthetic strategy en route toward 3 involves a Heck reaction to attach 4-vinylbenzaldehyde to a monoiodophthalocyanine precursor, followed by standard cycloaddition of azomethine ylides (generated from the formylPc derivative and N-methylglycine) to one of the double bonds of C60. Electrochemical studies reveal that in 3 the ZnPc is about 39 mV more difficult to oxidize than in the corresponding ZnPc reference, which points to appreciable electronic communication between ZnPc and C60 in the ground state. In the excited state, photoexcitation leads to the formation of a charge-separated ZnPc*+-C60*- state, for which a lifetime of 130 ns was determined in THF. Hetero-association between complementary Pcs (1 and 2 or 3 and 2), which carry different peripheral functionalities (i.e., either electron-donating alkoxy groups or electron-deficient alkylsulfonyl chains) was assessed by different techniques. They provided evidence for donor-acceptor 1:1 complex formation with a stability constant of ca. 10(5) M(-1) in CHCl3. Interestingly, hetero-association of ZnPc-C60 dyad 3 with an electron-deficient PdPc (2) allowed the construction of supramolecular triads, in which a substantial stabilization of the radical pair is seen relative to that of the covalently linked dyad ZnPc-C60 (3).

Bio-inspired, side-on attachment of a ruthenium photosensitizer to an iron hydrogenase active site model

The first ruthenium-diiron complex [(mu-pdt)Fe2(CO)5{PPh2(C6H4CCbpy)}Ru(bpy)2]2+ 1 (pdt = propyldithiolate, bpy = 2,2'-bipyridine) is described in which the photoactive ruthenium trisbipyridyl unit is linked to a model of the iron hydrogenase active site by a ligand directly attached to one of the iron centers. Electrochemical and photophysical studies show that the light-induced MLCT excited state of the title complex is localized towards the potential diiron acceptor unit. However, the relatively mild potential required for the reduction of the acetylenic bipyridine together with the easily oxidized diiron portion leads to a reductive quenching of the excited state, instead. This process results in a transiently oxidized diiron unit which may explain the surprisingly high light sensitivity of complex 1.

Energy Transfer Switching in a Bistable Molecular Machine.

A novel [2]rotaxane, containing pyrene and perylene bisimide as both stoppers and photoactive units, has been prepared. The shuttling of the protonated macrocycle switched the energy transfer (EN) from a pyrene moiety to a perylene moiety, which resulted in changes of fluorescence of the perylene moiety.

Energy Transfer Switching in a Bistable Molecular Machine

[reaction: see text] A novel [2]rotaxane, containing pyrene and perylene bisimide as both stoppers and photoactive units, has been prepared. The shuttling of the protonated macrocycle switched the energy transfer (EN) from a pyrene moiety to a perylene moiety, which resulted in changes of fluorescence of the perylene moiety.

Tuning Photoinduced Energy‐ and Electron‐Transfer Events in Subphthalocyanine–Phthalocyanine Dyads

A series of subphthalocyanine-phthalocyanine dyads has been prepared by means of palladium-catalyzed cross-coupling reactions between a monoalkynylphthalocyanine and different monoiodosubphthalocyanines. Electronic coupling between the two photoactive units is ensured by a rigid and pi-conjugated alkynyl spacer. In addition, the electronic characteristics of the subphthalocyanine moiety were modulated by the introduction of different peripheral substituents. Cyclic and Osteryoung square-wave voltammetry experiments revealed that the reduction potential of this subunit can be decreased by about 400 mV on going from thioether or no substituents to nitro groups. As a consequence, the energy level of the charge-transfer state could be fine-tuned so as to gain control over the fate of the photoexcitation energy in each subunit. The diverse steady-state and time-resolved photophysical techniques employed demonstrated that, when the charge-transfer state lies high in energy, a quantitative singlet-singlet energy-transfer mechanism from the excited subphthalocyanine to the phthalocyanine takes place. On the contrary, stabilization of the radical pair by lowering the redox gap between electron donor and acceptor results in a highly efficient photoinduced electron-transfer process, even in solvents of low polarity such as toluene (Phi(ET) approximately 0.9). These features, together with the extraordinary absorptive cross section that these molecular ensembles display across the whole UV/Vis spectrum, make them model candidates for application in situations where broadband light sources are needed.

Correlating electrical properties and molecular structure of SAMs organized between two metal surfaces

We describe the results obtained on a series of junctions that are formed by mercury electrodes. The junctions comprise self-assembled monolayers (SAMs) sandwiched between two metal electrodes, i.e., metal (mercury)–SAM//SAM–metal (mercury, gold or silver) junctions. We describe three different variations on this type of Hg-based junction. The first junction, formed by bringing into contact two mercury drops covered by the same type of SAM, is a prototype system that provided useful information on the structure and electrical properties of the Hg-based junctions. The second junction consists of a Hg drop covered by one SAM (Hg–SAM(1)) in contact with a second SAM supported on a silver film (Ag–SAM(2))—i.e., a Hg–SAM(1)//SAM(2)–Ag junction. This junction (for constant SAM(1)) allowed systematic measurements of the current that flowed across SAM(2) as a function of its chemical structure. The same type of junction, when comprising a transparent solid metal electrode, allows to irradiate through the transparent surface photoactive units organized in a SAM and to measure the current photoresponse. The third type of junction, Hg–SAM//R//SAM–Hg (or Hg–SAM//SAM–Hg for redox-active SAMs), is an electrochemical junction that can (i) trap redox-active molecules (R) in the interfacial region between the SAMs and (ii) control the potential of the electrodes with respect to the redox potential of R using an external reference electrode. This junction becomes conductive when the electrode potentials are adjusted to the formal potential of the redox centers, and it shows diode- and transistor-like characteristics analogous to those of solid-state devices.

Forward (singlet-singlet) and backward (triplet-triplet) energy transfer in a dendrimer with peripheral naphthalene units and a benzophenone core.

The photochemical and photophysical behaviour of two dendrimers consisting of a benzophenone core and branches that contain four (4) and eight (5) naphthalene units at the periphery has been investigated in CH(2)Cl(2) solution (298 K) and in CH(2)Cl(2)/CHCl(3) 1:1 v/v rigid matrix (77 K). For comparison purposes, the photophysical properties of dimethoxybenzophenone (1), 2-methylnaphthalene (2) and of a dendron containing four naphthalene units (3) have also been studied. In both dendrimers 4 and 5, excitation of the peripheral naphthalene units is followed by fast (1.1 x 10(9) s(-1) at 298 K, > 2.5 x 10(9) s(-1) at 77 K for 5; 2.9 x 10(8) s(-1) at 298 K, 7 x 10(5) s(-1) at 77 K for 5) singlet-singlet energy transfer to the benzophenone core. On a longer time scale (>1 x 10(6) s(-1) at 298 K, >6 x 10(3) s(-1) at 77 K for 4; 3.1 x 10(7) s(-1) at 298 K, ca. 3 x 10(2) s(-1) at 77 K for 5) a back energy transfer process takes place from the triplet state of the benzophenone core to the triplet state of the peripheral naphthalene units. Selective excitation of the benzophenone unit is followed by intersystem crossing and triplet-triplet energy transfer to the peripheral naphthalene units. In hydrogen donating solvents, the benzophenone core is protected from degradation by the presence of the naphthalene units. In solutions containing Tb(CF(3)SO(3))(3), sensitization of the green Tb(3+) luminescence is observed on excitation of both the peripheral naphthalene units and the benzophenone core of 5. Upon excitation of the naphthalene absorption band (266 nm) with a laser source, intradendrimer triplet-triplet annihilation of naphthalene excited states leads to delayed naphthalene fluorescence (lambda(max)= 335 nm), that can also be obtained upon excitation at 355 nm (benzophenone absorption band). The results obtained show that preorganization of photoactive units in a dendritic structure can be exploited for a variety of useful functions, including photosensitized emission, protection from undesired photoreactions, and energy up-conversion.

Ground and excited-state electronic interactions in poly(propylene amine) dendrimers functionalized with naphthyl units: effect of protonation and metal complexation.

We report the absorption spectra and the photophysical properties (fluorescence spectrum, quantum yield, and lifetime) of four dendrimers of the poly(propylene amine) family (POPAMs) functionalized at the periphery with naphthylsulfonamide (hereafter called naphthyl) units. Each dendrimer Gn, where n = 1 to 4 is the generation number, comprises 2n + 1 (i.e., 32 for G4) naphthyl functions in the periphery and 2n + 1--2 (i.e., 30 for G4) tertiary amine units in the branches. All the experiments have been carried out in acetonitrile solutions. Comparison with two reference compounds (N-methyl-naphthalene-2-sulfonamide, A, and N-(3-dimethylamino-propyl)-2-naphthalene-1-sulfonamide, B) has shown that the absorption spectra of the dendrimers are significantly different from those expected from the component units. Furthermore, the intense fluorescence band of the naphthyl unit (lambda max = 343 nm; phi = 0.15, tau = 8.5 ns) is strongly quenched in the dendrimers. The quenching effect increases with increasing generation and is accompanied by the appearance of a weak and broad emission tail at lower energy. Protonation of the amine units of the dendrimers by addition of CF3SO3H (triflic) acid causes a strong increase in the intensity of the naphthyl luminescence and a change in the form of the emission tail. The shapes of the titration curves depend on dendrimer generation, but in any case, the effect of the acid can be fully reversed by successive addition of a base (tributylamine). The results obtained show that in the dendrimers there are interactions (both in the ground and excited states) between naphthyl units as well as between naphthyl units and amine units of the branches; this gives rise to dimer/excimer and charge-transfer/exciplex excited states. Titration with Zn(CF3SO3)2 has the same effect as acid titration, as far as the final emission spectrum is concerned, but a much higher concentration of Zn(CF3SO3)2 has to be used and the shapes of the titration plots are very different. Titration with Co(NO3)2.6H2O causes a much smaller increase in the intensity of the naphthyl fluorescence compared with Zn(CF3SO3)2. The results obtained have shown that protonation and metal coordination can reveal the presence of ground and excited state electronic interactions in functionalized poly(propylene amine) dendrimers, and that the presence of photo-active units in the dendrimers can be useful to reveal some peculiar aspects of the protonation and metal coordination processes.

Photocapture of dynamic hydrogen‐bonded assemblies

Recent advances in the continuing study of [2+2] photodimerization reactions in supramolecular, non‐covalent systems are presented. The covalent photocapture of small dynamic assemblies which are formed using weak hydrogen‐bonding interactions between two different complementary units, barbiturates and melamines, is discussed. One unit serves as a photo‐inert supramolecular template capable of bringing two photoactive units together using multiple hydrogen‐bonds. The second type of unit unites the corresponding, complementary hydrogen‐bonding motif with a photoactive unit. Irradiation of the supramolecular assemblies leads to photodimerization of adjacent units and generation of an imprinted site for the template. Moieties which are adapted to participate in photodimerization reactions are styrene, cinnamate, stilbene and fullerene units. The results are interpreted on the basis of topochemical reaction control.

Hydrogen bond-assembled fullerene molecular shuttle.

A novel [2]rotaxane has been prepared in which fullerene C(60) behaves as both a stopper and a photoactive unit. The amphiphilic nature of the rotaxane thread can be used to shuttle the macrocycle from close to the fullerene spheroid (in nonpolar solvents) to far away (in polar solvents). The differing location of the macrocycle in dichloromethane and dimethyl sulfoxide gives rise to effects detectable by (1)H NMR and time-resolved spectroscopy.

Metallocyclodextrins as building blocks in noncovalent assemblies of photoactive units for the study of photoinduced intercomponent processes.

Cyclodextrin cups have been employed to build supramolecular systems consisting of metal and organic photoactive/redox-active components; the photoinduced communication between redox-active units assembled in water via noncovalent interactions is established. The functionalization of a beta-cyclodextrin with a terpyridine unit, ttp-beta-CD, is achieved by protection of all but one of the hydroxyl groups by methylation and attachment of the ttp unit on the free primary hydroxyl group. The metalloreceptors [(beta-CD-ttp)Ru(ttp)][PF(6)](2), [(beta-CD-ttp)Ru(tpy)][PF(6)](2), and [Ru(beta-CD-ttp)(2)][PF(6)](2) are synthesized and fully characterized. The [(beta-CD-ttp)Ru(ttp)][PF(6)](2) metalloreceptor exhibits luminescence in water, centered at 640 nm, from the (3)MLCT state with a lifetime of 1.9 ns and a quantum yield of Phi = 4.1 x 10(-)(5). Addition of redox-active quinone guests AQS, AQC, and BQ to an aqueous solution of [(beta-CD-ttp)Ru(ttp)](2+) results in quenching of the luminescence up to 40%, 20%, and 25%, respectively. Measurement of the binding strength indicates that, in saturation conditions, 85% for AQS and 77% for AQC are bound. The luminescence quenching is attributed to an intercomponent electron transfer from the appended ruthenium center to the quinone guest inside the cavity. Control experiments demonstrate no bimolecular quenching at these conditions. A photoactive osmium metalloguest, [Os(biptpy)(tpy)][PF(6)], is designed with a biphenyl hydrophobic tail for insertion in the cyclodextrin cavity. The complex is luminescent at room temperature with an emission band maximum at 730 nm and a lifetime of 116 ns. The osmium(III) species are formed for the study of photoinduced electron transfer upon their assembly with the ruthenium cyclodextrin, [(beta-CD-ttp)Ru(ttp)](2+). Time-resolved spectroscopy studies show a short component of 10 ps, attributed to electron transfer from Ru(II) to Os(III) giving an electron transfer rate 9.5 x 10(9) s(-)(1).

Structure-property relationships in dendritic encapsulation.

Several molecular structure-property relationships are presented and compared to illustrate our current understanding of macromolecular encapsulation using dendrimers. Specifically, the effect that dendrimer architectures have on encapsulating photoactive and redox-active units fixed at the molecular core is considered.