Excited-state Electron-transfer Reactions Research Articles

Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene Conjugates: Dark vs. Light-Induced Electron Transfer.

Using the popular metal-ligand axial coordination self-assembly approach, donor-acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im-TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded-tetracyanobutadiene (Im-DCNQ) as electron acceptors. The newly formed donor-acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 104 -105 M-1 in o-dichlorobenzene. Free-energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im-DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im-DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump-probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 1011 s-1 suggesting efficient processes. These findings show that the present self-assembly approach could be utilized to build donor-acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices.

Photoredox catalysis via consecutive 2LMCT- and 3MLCT-excitation of an Fe(iii/ii)-N-heterocyclic carbene complex.

Fe–N-heterocyclic carbene (NHC) complexes attract increasing attention as photosensitisers and photoredox catalysts. Such applications generally rely on sufficiently long excited state lifetimes and efficient bimolecular quenching, which leads to there being few examples of successful usage of Fe–NHC complexes to date. Here, we have employed [Fe(iii)(btz)3]3+ (btz = (3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene))) in the addition of alkyl halides to alkenes and alkynes via visible light-mediated atom transfer radical addition (ATRA). Unlike other Fe–NHC complexes, [Fe(iii/ii)(btz)3]3+/2+ benefits from sizable charge transfer excited state lifetimes ≥0.1 ns in both oxidation states, and the Fe(iii) 2LMCT and Fe(ii) 3MLCT states are strong oxidants and reductants, respectively. The combined reactivity of both excited states enables efficient one-electron reduction of the alkyl halide substrate under green light irradiation. The two-photon mechanism proceeds via reductive quenching of the Fe(iii) 2LMCT state by a sacrificial electron donor and subsequent excitation of the Fe(ii) product to its highly reducing 3MLCT state. This route is shown to be more efficient than the alternative, where oxidative quenching of the less reducing Fe(iii) 2LMCT state by the alkyl halide drives the reaction, in the absence of a sacrificial electron donor.

Theoretical Investigation of the 4,5-Dibromorodamine Methyl Ester (TH9402) Photosensitizer Used in Photodynamic Therapy: Photophysics, Reactions in the Excited State, and Interactions with DNA.

Photosensitizer (PS) molecules play a critical role in photodynamic therapy of cancer and the understanding of the molecular mechanism involved in the photophysics of these compounds, and their reactions in the excited state are, therefore, of great interest for the development of this technique. In this article, the photophysics of the cationic PS 4,5-dibromorodamine methyl ester (TH9402), its electron- and energy-transfer reactions in the excited triplet state, with molecular oxygen, nitric oxide, guanosine-5'-monophosphate (GMP), and guanine, and the interaction with DNA were evaluated. Time-dependent density functional theory calculations at the TPSSh/Def2-TZVP//B3LYP/Def2-TZVP level of theory in water solution reveals that the PS has a bright S1 state 2.33 eV above the ground state that produces a fluorescent rate constant of 5.40 × 107 s-1, calculated using Fermi's golden rule within a path integral formalism. Once excited to the bright state, the main intersystem crossing (ISC) channel involves the coupling with the T2 state just below S1 (S1 → T2 → T1) with an overall ISC rate constant of 10.1 × 107 s-1, in good agreement with the experimental data. Excited-state reaction thermodynamics, computed at the M06-2X/Def2-TZVP//B3LYP/Def2-TZVP level of theory in water, showed that from all the excited-state electron-transfer reactions studied, only the transfer from GMP to the PS is thermodynamically favorable, independent of the protonation state of guanosine, which indicates a possible DNA photo-oxidation mechanism for the PS. Triplet-triplet energy-transfer reactions from TH9402 to molecular oxygen, producing reactive singlet oxygen, and to the deprotonated guanosine, producing 3GMP2-, are also thermodynamically favorable, with ΔG = -2.0 and -24.0 kcal//mol, respectively. However, the energy transfer to the monoprotonated guanosine is not favorable, (ΔG = 36.1), suggesting that in the DNA double-strand environment, this energy-transfer process may not be observed. The results show that the PS can act through electron transfer and triplet-triplet energy-transfer reactions involved in mechanism types I and II in photodynamic therapy. Interactions of TH9402 with the d(AGACGTCT)2 octanucleotide revealed that the PS can intercalate between the d(GpC)-d(CpG) base pairs in three different orientations and, upon intercalation, the π → π* transition of the PS shows a bathochromic shift up to 90 nm and up to 60% decrease in intensity. Interactions through groove binding showed a smaller bathochromic shift of 52.2 nm and a 56% decrease in intensity of the main transition band.

Laser-induced linear and non-linear optical features in novel benzoxazole-based on donor-acceptor chromophores

The photophysical properties of donor-acceptor based benzoxazoles were investigated by means of steady-state and time-resolved fluorescence techniques. Additionally, laser stimulated second-order optical effects (preliminary second harmonic generation (SHG) were studied. The studies show that the nitro-benzoxazole derivatives decorated with different alkylo/arylo amino groups dissolved in medium polar aprotic solvents: tetrahydrofuran and butyl chloride, exhibit dual fluorescence from the locally excited state (1LE) and a highly polar charge transfer (CT) state. Ambient-temperature time-resolved fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction 1LE→1CT. The lack of any CT emissions in cyclohexane and highly polar acetonitrile is presumably due to an efficient inter-system crossing process or fast deactivation of a low-lying CT state. In addition, strong CT properties inspired us to check a possibility to vary the absorption of the first UV–Vis maxima under influence of the external bicolour coherent laser light and the related optical second harmonic generation (SHG). The two coherent beams of the infrared Er:glass laser at 1540 nm/770 nm formed gratings in the photopolymer solidified chromophore. The performed experiments are compared with the theoretically calculated electronic parameters within the B3LYP DFT approach. The presented data open novel type of materials, allowing to operate its transparency using external IR laser light in the near IR spectral range.

The Driving Force of Photoinduced Charge Separation in Metal-Cluster-Encapsulated Triphenylamine-[80]fullerenes.

Understanding photoinduced charge separation in fullerene-based dye-sensitized solar cells is crucial for the development of photovoltaic devices. We investigate here how the driving force of the charge separation process in conjugates of M@C80 (M=Sc3 N, Sc3 CH, Sc3 NC, Sc4 O2 , and Sc4 O3 ) with triphenylamine (TPA) depends on the nature of the metal cluster. Both singlet and triplet excited-state electron-transfer reactions are considered. These results based on TD-DFT calculations demonstrate that the driving force of charge separation in TPA-M@C80 can be tuned well by varying the structure of the metal cluster encapsulated inside the fullerene cage.

Ground and excited-state electron-transfer reactions of a few (anthracen-9-yl)methanamines: a comparative study

Comparative study of chemical and light induced ele ctron-transfer reactions of some (anthracen- 9-yl)methanamines is reported. Ceric ammonium nitrate was used to initiate chemically-induced electron-transfer reactions of anthracenemethanamin es. Nitration at the 2-position followed by oxidation was the major pathway observed in the rea ction with CAN while electron-transfer mediated C-N bond cleavage followed by dimerization was the major pathway in photochemical reactions.

Photoinduced electron transfer reactions of ruthenium(II) phenanthroline complexes with dimethylaniline in aqueous and micellar media.

Four [Ru(NN)(3)](2+) complexes (NN = polypyridine) with ligands of varying hydrophobicity with different charges +2, 0 and -4 were synthesized. The photophysics and photoinduced electron transfer reactions of these Ru(II)-complexes with dimethylaniline (DMA) as the quencher have been studied in aqueous medium and ionic and non-ionic micellar medium. The extent of binding of the complexes with the surfactant interface is evident from the calculated binding constant values (K). Dimethylaniline (DMA) being a neutral quencher, the hydrophobic and electrostatic interactions competing with one another and their combined effect with the surfactants were reported by observing the quenching rate constant (k(q)) values. The formation of anilinium cation radical in transient absorption spectrum confirms the excited state electron transfer reactions of ruthenium(II) complexes with dimethylaniline. The calculated rate constant values (k(q)) are in good agreement with the experimental k(q) values giving quantitative evidence for the bimolecular reductive quenching rate constant for the complexes with DMA. Pseudophase ion exchange model is successfully applied to analyse the quenching data.

Excited state electron transfer reactions of ProtoporphyrinIX with fullerene

Excited-state dynamics between ProtoporphyrinIX (PPIX) and fullerene (C60) have been studied by using steady state and time resolved absorption and fluorescence spectroscopy. The emission intensity of PPIX is quenched by fullerene. Time-resolved fluorescence study implies that the quenching of PPIX by fullerene is static in nature. The dominant process for quenching is attributed to the process of photoinduced electron transfer from excited PPIX to fullerene. The obtained results signify that the formation of complex between PPIX and fullerene plays an important role in improvement of the light energy conversion.

Ground-State and Excited-State Structures of Tungsten–Benzylidyne Complexes

The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.

Transient Exciplex Formation Electron Transfer Mechanism

Transient exciplex formation mechanism of excited‐state electron transfer reactions is analyzed in terms of experimental data on thermodynamics and kinetics of exciplex formation and decay. Experimental profiles of free energy, enthalpy, and entropy for transient exciplex formation and decay are considered for several electron transfer reactions in various solvents. Strong electronic coupling in contact pairs of reactants causes substantial decrease of activation energy relative to that for conventional long‐range ET mechanism, especially for endergonic reactions, and provides the possibility for medium reorganization concatenated to gradual charge shift in contrast to conventional preliminary medium and reactants reorganization. Experimental criteria for transient exciplex formation (concatenated) mechanism of excited‐state electron transfer are considered. Available experimental data show that this mechanism dominates for endergonic ET reactions and provides a natural explanation for a lot of known paradoxes of ET reactions.

Modulation of the Excited State Intramolecular Electron Transfer Reaction and Dual Fluorescence of Crystal Violet Lactone in Room Temperature Ionic Liquids

The influence of polarity, viscosity, and hydrogen bond donating ability of the medium on the fluorescence behavior of crystal violet lactone (CVL), which undergoes excited state electron transfer reaction and exhibits dual fluorescence from two different electronic states, termed as CT(A) and CT(B), has been studied in six different room temperature ionic liquids (ILs) using steady state and time-resolved emission techniques. It is shown that the excited state CT(A) --> CT(B) transformation and dual fluorescence of CVL can be controlled by appropriate choice of the ILs. While dual fluorescence of CVL is clearly observed in pyrrolidinium IL, the molecule exhibits a single fluorescence band in ammonium IL. While the second emission from the CT(B) state can barely be seen in 1,3-dialkylimidazolium ILs, dual fluorescence is quite prominent in 1-butyl-2,3-dimethylimidazolium IL, [bmMim][Tf(2)N]. These contrasting results have been explained taking into account the hydrogen bonding interactions of the 1,3-dialkylimidazolium ions (mediated through the C(2)-hydrogen) with CVL and the viscosity of the ILs. The excited state CT(A) --> CT(B) reaction kinetics has been studied in IL by monitoring the time-evolution of the CT(B) emission in [bmMim][Tf(2)N]. The solvation dynamics in this IL has been studied by following the dynamic fluorescence Stokes shift of C153, which is used as a probe molecule. A comparison of the excited state reaction time and solvation time suggests that the rate of the CT(A) --> CT(B) reaction in moderately viscous ILs is primarily dictated by the rate of solvation. Very little or negligible excitation wavelength dependence of the emission behavior of CVL can be observed in these ILs.

Poly(Pyridinium Phenylene)s: Water-Soluble N-Type Polymers

Poly(pyridinium phenylene) conjugated polymers are synthesized by a cross-coupling and cyclization sequence. These polyelectrolytes are freely soluble in water and display high degrees of electroactivity. When reduced (n-doped) these materials display in situ conductivities as high as 160 S/cm. The high conductivity is attributed to the planar structure that is enforced by the cyclic structures of the polymer. The electron affinities are compared to PCBM, a C(60) based n-type material. We find that these polymers undergo excited state electron transfer reactions with other donor conjugated polymers and hence may find utility in photovoltaic devices.

Systematic Manipulation of the Light-Harvesting Properties for Tridentate Cyclometalated Ruthenium(II) Complexes

The response of the metal-to-ligand charge-transfer (MLCT) band to variability in terminal substituents within a related set of tridentate polypyridyl and cyclometalated Ru(II) complexes is reported. These complexes are formulated as [Ru(tpy-R(1))(tpy-R(2))](PF(6))(2) (1-6; tpy = 2,2':6',2''-terpyridine; R(1) = -H, -2-furyl, or -OMe; R(2) = -H, -2-furyl, or -CO(2)H) and [Ru(tpy-R(2))(dpb-R(1))]PF(6) (7-10; Hdpb = 1,3-di(pyridin-2-yl)benzene; R(2) = -H or -2-furyl; R(1) = -H or -OMe). Absorption spectra for the [Ru(tpy-R(1))(tpy-R(2))](2+) series highlight the sensitivity of the MLCT band to the indicated substituents at the 4' position of one or both tpy ligands (e.g., a bathochromic shift up to 24 nm coupled with a 2-fold increase in absorption intensity). Similar observations are made for the [Ru(tpy-R(2))(dpb-R(1))](+) series, where a single Ru-N dative bond is replaced by a Ru-C sigma-bond to form a cyclometalated complex. The reduced symmetry at the metal center within this series results in a broadening of the lowest-energy MLCT band, while an additional set of transitions at higher energies emerges that involves an excited state localized on the cyclometalating ligand. These MLCT transitions collectively render a broad absorption envelope of substantial intensity at wavelengths longer than ca. 525 nm. Optimal results are obtained for compound 10 (R(1) = -OMe; R(2) = -2-furyl), where a strong electron-donating group is situated para to the Ru-C bond (lambda(max) = 523 nm; epsilon = 2.6 x 10(4) M(-1) cm(-1)). This approach imparts substantial polarization within the molecule, which should benefit excited-state electron-transfer reactions for photosensitizing applications (e.g., dye-sensitized solar cells). Spectroscopic data are corroborated by electrochemical and TD-DFT measurements for all compounds.

Stark Realities

Electric fields affect any process or transition that involves the movement of charge. Stark spectroscopy is a general term describing the study of spectral changes in the presence of electric fields, and it has proven to be a broadly useful approach for characterizing the change in dipole moment and polarizability for electronic and vibrational transitions. This article focuses primarily on the evolution of the approach and interconnected applications in diverse fields from our laboratory and prospects for the future. Our work began with studies of chromophores in photosynthetic reaction centers whose function is light-driven charge separation, so perturbations by an electric field were a natural approach. The same methods have been applied to many other biological and nonbiological chromophores. A common theme has been understanding the mechanism(s) of symmetry breaking in molecules or organized assemblies of high symmetry. Spectral shifts in organized systems due to mutations, conformational changes, and ligand binding can, in some cases, be interpreted as Stark shifts. In this case, Stark spectroscopy in a well-defined electric field provides a calibration of the probe transition's sensitivity to an electric field, and the Stark shifts of suitable probes can be used to measure the magnitude and direction of electric fields in proteins, nucleic acids, and membranes. Electric fields can also perturb the populations or reaction dynamics of processes where charge separation occurs. When detected by spectroscopic methods, we call these nonclassical Stark effects. Nonclassical Stark effects arise in the spectroscopy of intervalence charge transfer transitions and both ground- and excited-state electron transfer reactions. Because the movement of charge is ubiquitous in chemistry, biology, and materials science and because electric fields directly affect the energetics of charge-separated species, many phenomena can be viewed as generalizations of the Stark effect.

Evidence for Iodine Atoms as Intermediates in the Dye Sensitized Formation of I−I Bonds

Visible light excitation of [Ru(bpz)(2)(deeb)](PF(6))(2), where bpz is 2,2'-bipyrazine and deeb is 4,4'-(CO(2)Et)(2)-2,2'-bipyridine, in acetonitrile solutions with iodide is shown to initiate excited-state electron transfer reactions that yield iodine atoms. The iodine atoms subsequently react with iodide to form the I-I bond in I(2)(-*). The resultant Ru(bpz(-))(bpz)(deeb)(+), I(2)(-*) stores approximately 1.64 eV of free energy and returns cleanly to ground-state products with k(cr) = (2.1 +/- 0.3) x 10(10) M(-1) s(-1).

On the parallel formation of long-lived excited states of dipyridil[3,2-a:2′3′-c]phenazine, dppz: A contrast between the electrochemically and photochemically induced reduction of dppz

The photophysical and photochemical properties of dipyridil[3,2-a:2′3′-c]phenazine, dppz, were investigated in homogeneous solution by time-resolved and steady-state photochemical techniques. Irradiation of dppz in CH 3OH, CH 3CN or CH 2Cl 2 induced short-lived fluorescence with a lifetime, τ ≤20 ns. Different emission quantum yields were determined when dppz was irradiated at 350 nm, ϕ LUM ≤2.5 × 10 −4, and when it was irradiated at ∼400 nm, 3.2 × 10 −3 ≤ ϕ LUM ≤ 2 × 10 −2. Reactions of the luminescent excited states with electron donors, TEOA or TEA, and incapacity for an inefficient H atom abstraction from 2-propanol suggested that they were ππ* excited states. Long-lived transient absorption spectra with lifetimes in the microsecond time domain were associated with adducts of the ground and excited states. The dppz radical, product of the excited state electron transfer reactions, and the product of the reduction of dppz by pulse radiolytically generated e sol − have the same UV–vis spectrum. Their spectrum differs, however, with the spectrum of the radicals from the electrochemically reduced dppz. The photophysical and photochemical results are attributed to low-lying ππ* excited states of dppz.

Picosecond kinetics of excited-state charge separation in 4-(dimethylamino)pyridine

4-(Dimethylamino)pyridine (DMAP) shows solvent-dependent dual fluorescence from the initially excited state B* and a highly polar TICT state A*. Room-temperature time-resolved picosecond fluorescence investigations prove the bimodal kinetics of the excited-state electron transfer reaction B*-->A* in polar aprotic media. In medium polarity solvents (such as ethyl acetate) two emitting states of DMAP are shown to reach equilibrium within 50 ps. Both emitting states originate from the same ground state. The rate of excited-state charge separation depends on polarity and proton donating ability of the surrounding medium. The effects of temperature on the quantum yields of both fluorescences of DMAP in polar aprotic media indicate the transition from the kinetic regime (at low temperatures) to the equilibrium regime (at high temperatures). The kinetic behaviour of the dual luminescence of DMAP in protic solvents is more complex than in aprotic ones. In alcohols an efficient nonradiative channel competes with excited-state charge separation.

Molecular structures of electron-transfer active complexes [Re(XQ +)(CO) 3(NN)] 2+ (XQ += N-Me-4,4 ′-bipyridinium or N-Ph-4,4 ′-bipyridinium; NN=bpy, 4,4 ′-Me 2-2,2 ′-bpy or N, N′-bis-isopropyl-1,4-diazabutadiene) in the solid state and solution: an X-ray and NOESY NMR study

Crystal and molecular structures of a series of complexes [Re(XQ +)(CO) 3(NN)] 2+ (XQ= N-methyl-4,4 ′-bipyridinium (MQ +) and N-phenyl-4,4 ′-bipyridinium (PQ +) and NN=bpy, 4,4 ′-dimethyl-bpy (dmb) or N, N ′-bis-isopropyl-1,4-diazabutadiene ( iPr-DAB)) in the solid state have been determined by X-ray diffraction. Aromatic rings within the XQ + ligand were found to be highly staggered. The dihedral angles between the pyridine and pyridinium rings were found in the range 39°–45° for MQ + and 28°–46° for PQ +. The exceptionally low dihedral angle of 8° in [Re(MQ +)(CO) 3(dmb)] 2+ is due to crystal-packing effects. The pyridinium and phenyl rings of the PQ + ligand are even more staggered, with dihedral angles in the range 40°–55°. The pyridine ring of the XQ + ligand is oriented relative to the equatorial ligands in such a way that it bisects the angles between the equatorial Re–N and Re–C bonds in all complexes, except for [Re(PQ +)(CO) 3( iPr-DAB)](PF 6) 2, where it bisects the DAB ligand. Structures of the complexes [Re(XQ +)(CO) 3(NN)] 2+ (NN=bpy, dmb) were also studied in solution using NOESY NMR. It was found that the orientation of the XQ + ligand relative to the equatorial ligands is the same as in the solid state. The XQ + ligands become even more staggered on going to the solution where pyridine–pyridinium dihedral angles range from 42° to 45°. A value of ∼69° was found for the pyridinium-phenyl dihedral angle in PQ + complexes. The structural data obtained are related to electron-transfer activity of XQ + complexes. It follows that any ground- or excited-state electron transfer reactions or optical charge transfer excitation have to be coupled with a major reorganization of the XQ + ligand, namely twisting of its aromatic rings and shortening of the interring C–C bond. This conclusion has important implications for estimates of Marcus inner reorganization energy and electron transfer dynamics.

Synthesis and photophysical properties of neutral luminescent rhenium-based molecular rectangles.

A series of neutral luminescent molecular rectangles [[Re(CO)(3)(mu-bpy)Br][Re(CO)(3)(mu-L)Br]](2) (1-4) having fac-Re(CO)(3)Br as corners and 4,4'-bipyridine (bpy) as the bridging ligand on one side and other bipyridyl ligands of varying length (L) on the other side have been synthesized and characterized. The crystal structure of 1 shows a rectangular cavity with the dimensions of 11.44 x 7.21 A. When the cavity size is tuned from 1 to 4, a dimension of 11.4 x 20.8 A could be achieved, as revealed by the molecular modeling. These rectangles exhibit luminescence in solution at room temperature. In particular, compound 4 containing 1,4-bis(4'-pyridylethynyl)benzene (bpeb) as bridging ligand shows the excited-state lifetime of 495 ns. Fine-tuning of the cavity size of the rectangles improves their excited-state properties. These properties facilitate the study of excited-state electron-transfer reactions with electron acceptors and donors and host-guest binding. Crystallographic information: 1.6CH(3)COCH(3) is monoclinic, P2(1)/c, with a = 12.0890(2), b = 24.2982(2), and c = 12.8721(2) A, beta = 107.923(1) degrees, and Z = 2.

Self-assembled supramolecular triad composed of fulleropyrrolidine bearing two pyridine moieties axially coordinated to two zinc porphyrins

Spectroscopic, redox, and photochemical behavior of a self-assembled donor-acceptor triad formed by axial coordination of zinc meso-tetraphenylporphyrin, Zn ( TPP ) with fulleropyrrolidine bearing two pyridine moieties ( C 60( Py )2) was investigated. The UV-visible and 1 H NMR spectral studies revealed supramolecular 1:2 triad formation between C 60( Py )2 and Zn ( TPP ). The determined overall formation constant, K was as large as 1.45 × 104 mol -2 in o-dichlorobenzene (o-DCB). The 1 H NMR studies revealed axial coordination of both the pyridine entities of C 60( Py )2 to the central metal ion of the zinc porphyrins. Electrochemical studies of the isolated self-assembled triad in o-DCB established the molecular stoichiometry of the supramolecular complex possessing one C 60( Py )2 and two Zn ( TPP ) entities. The excited state electron transfer reactions were monitored by both steady-state emission as well as transient absorption techniques. In o-DCB, upon coordination of the pyridine entities of C 60( Py )2 to two Zn ( TPP ), the main quenching pathway involved charge-separation from the singlet excited Zn ( TPP ) to the C 60 moiety. However, in a coordinating solvent like PhCN , intermolecular electron-transfer takes place from the triplet excited state of Zn ( TPP ) to the C 60 moiety.