Subphthalocyanines as Light-Harvesting Electron Donor and Electron Acceptor in Artificial Photosynthetic Systems

El-Khouly, Mohamed E.,Kim, Jong-Hyung,Kim, Jung-Hoon,Kay, Kwang-Yol,Fukuzumi, Shunichi American Chemical Society 2012 The Journal of Physical Chemistry Part C Vol.116 No.37

<P>Light-harvesting subphthalocyanine–ferrocenophane (SubPc–Fc; <B>1</B>) and subphthalocyanine–naphthalenediimide (SubPc–NDI; <B>2</B>) dyads have been synthesized, characterized, and probed by femtosecond laser photolysis. In dyads <B>1</B> and <B>2</B>, both the electron-donating ferrocenophane and the electron-accepting naphthalenediimide are axially linked with the functional O–Ph groups (at the para position) in the axial positions of SubPc. Electrochemical data show that the SubPcs can act as both electron donors and electron acceptors. The geometric and electronic structures of dyads <B>1</B> and <B>2</B> were calculated by ab initio B3LYP/6-311G methods. The optimized structures showed that the Fc and NDI entities are separated from SubPc by 8.32 Å (for dyad <B>1</B>) and 8.85 Å (For dyad <B>2</B>). The distribution of HOMOs and LUMOs suggests the formation of SubPc<SUP>•–</SUP>–Fc<SUP>+</SUP> and SubPc<SUP>•+</SUP>–NDI<SUP>•–</SUP> as charge-separated states for dyads <B>1</B> and <B>2</B>, respectively. Upon photoexcitation of the subphthalocyanine unit, these arrays undergo photoinduced electron transfer to form the corresponding charge-separated species, SubPc<SUP>•–</SUP>–Fc<SUP>+</SUP> and SubPc<SUP>•+</SUP> −NDI<SUP>•–</SUP>, in which SubPc acts as an electron acceptor and an electron donor, respectively, as expected from their redox potentials determined by cyclic voltammetry. Femtosecond transient spectroscopic studies have revealed that a fast charge separation (10<SUP>11</SUP>∼10<SUP>12</SUP> s<SUP>–1</SUP>) occurs for dyads <B>1</B> and <B>2</B>. From the kinetic studies, the rate of charge recombination and the lifetime of the charge-separated state (SubPc<SUP>•+</SUP>–NDI<SUP>•–</SUP>) were found to be 2.9 × 10<SUP>9</SUP> s<SUP>–1</SUP> and 345 ps, respectively.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2012/jpccck.2012.116.issue-37/jp3066103/production/images/medium/jp-2012-066103_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp3066103'>ACS Electronic Supporting Info</A></P>

Supramolecular Tetrad of Subphthalocyanine–Triphenylamine–Zinc Porphyrin Coordinated to Fullerene as an “Antenna-Reaction-Center” Mimic: Formation of a Long-Lived Charge-Separated State in Nonpolar Solvent

El-Khouly, Mohamed E.,Ju, Dong Kyu,Kay, Kwang-Yol,D'Souza, Francis,Fukuzumi, Shunichi WILEY-VCH Verlag 2010 Chemistry Vol.16 No.21

<P>We report here the formation of a long-lived charge-separated state of a self-assembled donor–acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C<SUB>60</SUB>Im) to the zinc center of a subphthalocyanine–triphenylamine–zinc porphyrin (SubPc–TPA–ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0×10<SUP>4</SUP> M<SUP>−1</SUP>, suggesting a moderately stable complex formation. The geometric and electronic structures of the covalently linked SubPc–TPA–ZnP triad and self-assembled SubPc–TPA–ZnP:C<SUB>60</SUB>Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc–TPA–ZnP<SUP>.+</SUP>:C<SUB>60</SUB>Im<SUP>.−</SUP>). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C<SUB>60</SUB>Im with a lifetime of 1.1 ns. Interestingly, slow charge recombination (1.6×10<SUP>5</SUP> s<SUP>−1</SUP>) and the long lifetime of the charge-separated state (6.6 μs) were obtained in toluene by utilizing the nanosecond transient measurements.</P> <B>Graphic Abstract</B> <P>Long live the tetrad! Efficient charge separation and a long-lived charge-separated state, in nonpolar toluene, of a self-assembled tetrad formed by axial coordination of a fulleropyrrolidine appended with an imidazole ligand coordinating to the zinc center of subphthalocyanine–triphenylamine–zinc porphyrin was achieved (see picture). <img src='wiley_img_2010/09476539-2010-16-21-CHEM201000045-content.gif' alt='wiley_img_2010/09476539-2010-16-21-CHEM201000045-content'> </P>

Stabilization of the Charge-Separated States of Covalently Linked Zinc Porphyrin–Triphenylamine–[60]Fullerene

El-Khouly, Mohamed E.,Han, Ki-Jong,Kay, Kwang-Yol,Fukuzumi, Shunichi WILEY-VCH Verlag 2010 CHEMPHYSCHEM -WEINHEIM- Vol.11 No.8

<P>Spectroscopic, redox, computational, and electron transfer reactions of the covalently linked zinc porphyrin–triphenylamine–fulleropyrrolidine system are investigated in solvents of varying polarity. An appreciable interaction between triphenylamine and the porphyrin π system is revealed by steady-state absorption and emission, redox, and computational studies. Free-energy calculations suggest that the light-induced processes via the singlet-excited porphyrin are exothermic in benzonitrile, dichlorobenzene, toluene, and benzene. The occurrence of fast and efficient charge-separation processes (≈10<SUP>12</SUP> s<SUP>−1</SUP>) via the singlet-excited porphyrin is confirmed by femtosecond transient absorption measurements in solvents with dielectric constants ranging from 25.2 (benzonitrile) to 2.2 (benzene). The rates of the charge separation processes are much less solvent-dependent, which suggests that the charge-separation processes occur at the top region of the Marcus parabola. The lifetimes of the singlet radical-ion pair (70–3000 ps at room temperature) decrease substantially in more polar solvents, which suggests that the charge-recombination process is occurring in the Marcus inverted region. Interestingly, by utilizing the nanosecond transient absorption spectral technique we can obtain clear evidence about the existence of triplet radical-ion pairs with relatively long lifetimes of 0.71 μs (in benzonitrile) and 2.2 μs (in o-dichlorobenzene), but not in toluene and benzene due to energetic considerations. From the point of view of mechanistic information, the synthesized zinc porphyrin–triphenylamine–fulleropyrrolidine system has the advantage that both the lifetimes of the singlet and triplet radical-ion pair can be determined.</P> <B>Graphic Abstract</B> <P>Photoinduced charge separation: A stabilization of the charge-separated states of the zinc porphyrin–triphenylamine–[60]fullerene system (see figure) is observed in solvents of varying polarity by using femto- and nanosecond transient absorption spectroscopy. The observed high ratio of charge separation (picoseconds) versus charge recombination (microseconds) is a desirable property of molecular systems designed for photoinduced charge separation. <img src='wiley_img_2010/14394235-2010-11-8-CPHC200900885-content.gif' alt='wiley_img_2010/14394235-2010-11-8-CPHC200900885-content'> </P>

Light harvesting zinc naphthalocyanine–perylenediimide supramolecular dyads: long-lived charge-separated states in nonpolar media

El-Khouly, Mohamed E.,Gutié,rrez, Ana M.,Sastre-Santos, Á,ngela,Ferná,ndez-Lá,zaro, Fernando,Fukuzumi, Shunichi The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.10

<P>Photoinduced electron-transfer dynamics of self-assembled donor–acceptor dyads formed by axial coordination of zinc naphthalocyanine, ZnNc, and perylenediimide (PDI) bearing either pyridine (py) or imidazole (im) coordinating ligands were investigated. The PDIim unit was functionalized with <I>tert</I>-octylphenoxy groups at the bay positions, which avoid aggregation providing solubility, to examine the effect of the bulky substituents at the bay positions on the rates of electron-transfer reactions. The combination between zinc naphthalocyanine and perylenediimide entities absorbs light over a wide region of the visible and near infrared (NIR) spectrum. The binding constants of the self-assembled ZnNc:PDIpy (<B>1</B>) and ZnNc:PDIim (<B>2</B>) in toluene were found to be 2.40 × 10<SUP>4</SUP> and 1.10 × 10<SUP>5</SUP> M<SUP>−1</SUP>, respectively, from the steady-state absorption and emission measurements, indicating formation of moderately stable complexes. The geometric and electronic calculations by using an <I>ab initio</I> B3LYP/6-311G method showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the perylenediimide entities, suggesting that the charge-separated states of the supramolecular dyads are ZnNc&z.rad;<SUP>+</SUP>:PDI&z.rad;<SUP>−</SUP>. The electrochemical results suggest the exothermic charge-separation process <I>via</I> the singlet states of both ZnNc and PDI entities in nonpolar toluene. Upon coordination of perylenediimide to ZnNc, the main quenching pathway involved charge separation <I>via</I> the singlet-excited states of ZnNc and PDIs. Clear evidence of the intramolecular electron transfer from the singlet-excited state of ZnNc to PDI within the supramolecular dyads in toluene was monitored by the femtosecond laser photolysis by observing the characteristic absorption band of the PDI radical anion (PDI&z.rad;<SUP>−</SUP>) and the ZnNc radical cation (ZnNc&z.rad;<SUP>+</SUP>) in the visible and NIR regions. The rate constants of charge-separation (<I>k</I><SUB>CS</SUB>) processes of the self-assembled dyads <B>1</B> and <B>2</B> were determined to be 4.05 × 10<SUP>10</SUP> and 1.20 × 10<SUP>9</SUP> s<SUP>−1</SUP>, respectively. The rate constant of charge recombination (<I>k</I><SUB>CR</SUB>) and the lifetime of charge-separated states (<I>τ</I><SUB>CS</SUB>) of dyad <B>1</B> were determined to be 2.34 × 10<SUP>8</SUP> s<SUP>−1</SUP> and 4.30 ns, respectively. Interestingly, a slower charge recombination (2.20 × 10<SUP>7</SUP> s<SUP>−1</SUP>) and a longer lifetime of the charge separated state (45 ns) were observed in dyad <B>2</B> in nonpolar toluene by utilizing the nanosecond transient measurements. The absorption in a wide section of the solar spectrum and the high charge-separation/charge-recombination ratio suggest the usefulness of the self-assembled zinc naphthalocyanine–perylenediimide dyads as good photosynthetic models.</P> <P>Graphic Abstract</P><P>Good combination! Efficient electron transfer and relatively long-lived charge-separated states are achieved of light harvesting zinc naphthalocyanine:perylenediimide supramolecular dyads using the femtosecond transient absorption technique in nonpolar toluene. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp23285e'> </P>

Fabrication and characterization of graphene oxide–titanium dioxide nanocomposite for degradation of some toxic insecticides

Nagi M. El-Shafai,Mohamed E. El-Khouly,Maged El-Kemary,Mohamed S. Ramadan,Aly S. Derbalah,Mamdouh S. Masoud 한국공업화학회 2019 Journal of Industrial and Engineering Chemistry Vol.69 No.-

Graphene oxide–titanium dioxide (GO@TiO2) nanocomposite with mean diameter size of 14 nm has been fabricated, characterized, and used as photocatalyst for degradation of two highly toxic insecticides, namely carbaryl and imidacloprid. Characterizations of GO@TiO2 have been carried out using various analytical and spectroscopic techniques. Compared to TiO2NPs, the fabricated GO@TiO2 nanocomposite has advantage of its smaller band gap. The finding that the photocatalytic degradation of the examined insecticides by the fabricated GO@TiO2 is more efficient compared to that of TiO2NPs suggests the superiority for using the fabricated GO@TiO2 nanocomposite for degradation of the examined toxic insecticides.

Self-Assembled via Metal–Ligand Coordination AzaBODIPY–Zinc Phthalocyanine and AzaBODIPY–Zinc Naphthalocyanine Conjugates: Synthesis, Structure, and Photoinduced Electron Transfer

Bandi, Venugopal,El-Khouly, Mohamed E.,Nesterov, Vladimir N.,Karr, Paul A.,Fukuzumi, Shunichi,D’Souza, Francis American Chemical Society 2013 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.117 No.11

<P>Using near-IR emitting photosensitizers, viz., zinc phthalocyanine (ZnPc), zinc naphthalocyanine (ZnNc), and BF<SUB>2</SUB>-chelated azadipyrromethene (azaBODIPY), donor–acceptor conjugates have been newly formed using the well-known metal–ligand axial coordination approach. To accomplish this task, the electron-deficient azaBODIPY was functionalized to possess either two pyridine or imidazole ligating entities. The X-ray crystal structure of one such derivative revealed the presence of axial binding capable pyridine entities on the azaBODIPY macrocycle. The structural integrity of the newly formed conjugates was established from optical absorption, emission, <SUP>1</SUP>H NMR, electrochemical, and computational methods. The experimentally determined binding constants suggested moderately stable conjugates. The absence of excitation energy transfer from singlet excited azaBODIPY to either ZnPc or ZnNc in the conjugates was confirmed from the steady-state emission measurements. However, free-energy calculations suggested photoinduced electron transfer to be exothermic from singlet excited ZnPc or ZnNc to azaBODIPY. Femtosecond transient studies confirmed such predictions from where occurrence of ultrafast photoinduced electron transfer was observed. The measured rates of charge separation and charge recombination (<I>k</I><SUB>CS</SUB> and <I>k</I><SUB>CR</SUB>) revealed charge stabilization to some extent in these conjugates.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2013/jpccck.2013.117.issue-11/jp400046b/production/images/medium/jp-2013-00046b_0015.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp400046b'>ACS Electronic Supporting Info</A></P>

Elongation of lifetime of the charge-separated state of ferrocene-naphthalenediimide-[60]fullerene triad via stepwise electron transfer.

Supur, Mustafa,El-Khouly, Mohamed E,Seok, Jai Han,Kay, Kwang-Yol,Fukuzumi, Shunichi American Chemical Society 2011 The journal of physical chemistry. A, Molecules, s Vol.115 No.50

<P>Photoinduced electron-transfer processes of a newly synthesized rodlike covalently linked ferrocene-naphthalenediimide-[60]fullerene (Fc-NDI-C(60)) triad in which Fc is an electron donor and NDI and C(60) are electron acceptors with similar first one-electron reduction potentials have been studied in benzonitrile. In the examined Fc-NDI-C(60) triad, NDI with high molar absorptivity is considered to be the chromophore unit for photoexcitation. Although the free-energy calculations verify that photoinduced charge-separation processes via singlet- and triplet-excited states of NDI are feasible, transient absorption spectra observed upon femtosecond laser excitation of NDI at 390 nm revealed fast and efficient electron transfer from Fc to the singlet-excited state of NDI ((1)NDI*) to produce Fc(+)-NDI(?-)-C(60). Interestingly, this initial charge-separated state is followed by a stepwise electron transfer yielding Fc(+)-NDI-C(60)(?-). As a result of this sequential electron-transfer process, the lifetime of the charge-separated state (τ(CS)) is elongated to 935 ps, while Fc(+)-NDI(?-) has a lifetime of only 11 ps.</P>

Synthesis and Photodynamicsof Fluorescent Blue BODIPY-PorphyrinTweezers Linked by Triazole Rings

Eggenspiller, Antoine,Takai, Atsuro,El-Khouly, Mohamed E.,Ohkubo, Kei,Gros, ClaudeP.,Bernhard, Claire,Goze, Christine,Denat, Franck,Barbe, Jean-Michel,Fukuzumi, Shunichi AmericanChemical Society 2012 The Journal of physical chemistry A Vol.116 No.15

<P>Novel zinc porphyrin tweezers in which two zinc porphyrins were connected with pi-conjugated boron dipyrromethenes (BDP meso-Por(2) and BDP beta-Por(2)) through triazole rings were synthesized to investigate the photoinduced energy transfer and electron transfer. The UV-vis spectrum of BDP beta-Por(2) which has less bulky substituents than BDP meso-Por(2) exhibits splitting of the Soret band as a result of the interaction between porphyrins of BDP beta-Por(2) in the excited state. Such interaction between porphyrins of both BDP beta-Por(2) and BDP meso-Por(2) is dominant at room temperature, while the coordination of the nitrogen atoms of the triazole rings to the zinc ions of the porphyrins occurs at low temperature. The conformational change of the BDP-porphyrin composites was confirmed by the changes in UV-vis and fluorescence spectra depending on temperature. Photodynamics of BDP meso-Por2 and BDP beta-Por(2) has also been investigated by laser flash photolysis. Efficient singlet-singlet energy transfer from the ZnP to the pi-conjugated BDP moiety of both BDP meso-Por2 and BDP beta-Por(2) occurred in opposite direction as compared to energy transfer from conventional BDP to ZnP due to the pi-conjugation in nonpolar toluene. In polar benzonitrile, however, additional electron transfer occurred along with energy transfer.</P>

Electron Delocalization in One-Dimensional Perylenediimide Nanobelts through Photoinduced Electron Transfer

Supur, Mustafa,Yamada, Yusuke,El-Khouly, Mohamed E.,Honda, Tatsuhiko,Fukuzumi, Shunichi American Chemical Society 2011 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.115 No.30

<P>Photoinduced electron transfer (PET) of a hybrid system comprising <I>N</I>,<I>N</I>′-ditridecylperylenediimide (LPDI), which forms nanobelt structures of the form (LPDI)<SUB><I>n</I></SUB>, and soluble zinc (tetra-<I>tert</I>-butyl)phthalocyanine (ZnTBPc) has been investigated in polar benzonitrile. The PET of a mixture system comprising <I>N</I>,<I>N</I>′-diheptadecan-9-ylperylene- diimide (BPDI) dissolved thoroughly in benzonitrile and ZnTBPc was also examined for comparison. LPDI nanobelt structures were identified using steady-state absorption and emission spectroscopies, as well as dynamic light scattering (DLS), in suspension and detected using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in the solid state. The electron paramagnetic resonance (EPR) spectrum of the radical anion of LPDI nanobelts [(LPDI)<SUB><I>n</I></SUB><SUP>•–</SUP>] was quite different from that of BPDI (BPDI<SUP>•–</SUP>) because of enhanced electron delocalization within the one-dimensional LPDI aggregates. Polar benzonitrile enables intermolecular light-induced electron transfer from the low-lying triplet state of ZnTBPc to the LPDI nanobelts through its stabilization effect on the electron-transfer species, as indicated by free energy calculations. Nanosecond transient absorption spectra displayed marked broadening of the radical anion peak of LPDI nanobelts in the near-infrared (NIR) region upon excitation, confirming the delocalization of the transferred electron within the nanostructure. Whereas both the hybrid and mixture systems have nearly the same rate constants (<I>k</I><SUB>et</SUB>) of PET from the PDIs to ZnTBPc, the rate of back electron transfer (<I>k</I><SUB>bet</SUB>) of (LPDI)<SUB><I>n</I></SUB><SUP>•–</SUP>/ZnTBPc<SUP>•+</SUP> is lower than that of BPDI<SUP>•–</SUP>/ZnTBPc<SUP>•+</SUP>, which might result from the effect of electron delocalization within the nanobelt structure.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2011/jpccck.2011.115.issue-30/jp204417v/production/images/medium/jp-2011-04417v_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp204417v'>ACS Electronic Supporting Info</A></P>

Photoinduced Processes of Subphthalocyanine–Diazobenzene–Fullerene Triad as an Efficient Excited Energy Transfer System

Kim, Jong-Hyung,El-Khouly, Mohamed E.,Araki, Yasuyuki,Ito, Osamu,Kay, Kwang-Yol The Chemical Society of Japan 2008 Chemistry letters Vol.37 No.5

<P>Photoinduced processes of a newly synthesized subphthalocyanine–diazobenzene–fullerene triad have been studied by the time-resolved spectroscopic techniques. On photo-excitation of subphthalocyanine (SubPc) moiety, the fluorescence quenching of SubPc was observed, suggesting the energy-transfer process from singlet excited energy of the light-harvesting SubPc to C<SUB>60</SUB> through diazobenzene. This finding is confirmed by the nanosecond transient absorption of the triplet excited state of C<SUB>60</SUB>.</P>