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ABSTRACT: This work describes a comprehensive assignment of the vibrational spectra of the platinum(II) diimine bisthiolate and chloride complexes as a prototype structure for a diversity of Pt(II) diimine chromophores. The dynamics and energy dissipation pathways in excited states of light harvesting molecules relies largely on the coupling between the high frequency and the low frequency modes. As such, the assignment of the vibrational spectrum of the chromophore is of utmost importance, especially in the low-frequency region, below 500 cm(-1), where the key metal-ligand framework modes occur. This region is experimentally difficult to access with infrared spectroscopy and hence frequently remains elusive. However, this region is easily accessible with Raman and inelastic neutron scattering (INS) spectroscopies. Accordingly, a combination of inelastic neutron scattering and Raman spectroscopy with the aid of computational results from periodic-DFT and the mode visualizations, as well as isotopic substitution, allowed for an identification of the modes that contain significant contributions from Pt-Cl, Pt-S, and Pt-N stretch modes. The results also demonstrate that it is not possible to assign transition energies to "pure", localized modes in the low frequency region, as a consequence of the anticipated severe coupling that occurs among the skeletal modes. The use of INS has proved invaluable in identifying and assigning the modes in the lowest frequency region, and overall the results will be of assistance in analyzing the structure of the electronic excited state in the families of chromophores containing a Pt(diimine) core.
Inorganic Chemistry 08/2012; 51(18):9748-56. · 4.60 Impact Factor
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Jonathan Best,
Igor V Sazanovich,
Harry Adams,
Robert D Bennett,
E Stephen Davies,
Anthony J H M Meijer,
Michael Towrie,
Sergei A Tikhomirov,
Oleg V Bouganov,
Michael D Ward,
Julia A Weinstein
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ABSTRACT: A series of mononuclear complexes of the type [Pt(Bu(2)cat)(4,4'-R(2)-bipy)] [where Bu(2)cat is the dianion of 3,5-(t)Bu(2)-catechol and R = H, (t)Bu, or C(O)NEt(2)] and analogous dinuclear complexes based on the "back-to-back" bis-catechol ligand 3,3',4,4'-tetrahydroxybiphenyl have been studied in detail in both their ground and excited states by a range of physical methods including electrochemistry, UV/vis/near-IR, IR, and electron paramagnetic resonance spectroelectrochemistry, and time-resolved IR (TRIR) and transient absorption (TA) spectroscopy. Density functional theory calculations have been performed to support these studies, which provide a detailed picture of the ground- and excited-state electronic structures, and excited-state dynamics, of these complexes. Notable observations include the following: (i) for the first time, the lowest-energy catecholate → bipyridine (bpy) ligand-to-ligand charge-transfer (LL'CT) excited states of these chromophores have been studied by TRIR spectroscopy, showing a range of transient bands associated with the bpy radical anion and semiquinone species, and back-electron-transfer occurring in hundreds of picoseconds; (ii) strong electronic coupling between the two catecholate units in the bridging ligand of the dinuclear complexes results in a delocalized, planar (class 3) "mixed-valence" catecholate(2-)/semiquinone(•-) state formed by one-electron oxidation of the bridging ligand; (iii) in the LL'CT excited state of the dinuclear complexes, the bridging ligand is symmetrical and delocalized, whereas the bpy radical anion is localized at one terminus of the complex. This study is the first example of an investigation of excited-state behavior in platinum(II) catecholate complexes, performed with the use of picosecond TRIR and femtosecond TA spectroscopy.
Inorganic Chemistry 11/2010; 49(21):10041-56. · 4.60 Impact Factor
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Renske M van der Veen,
Chris J Milne,
Amal El Nahhas,
Frederico A Lima,
Van-Thai Pham, Jonathan Best,
Julia A Weinstein,
Camelia N Borca,
Rafael Abela,
Christian Bressler,
Majed Chergui
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ABSTRACT: Metallica: A large contraction of the Pt-Pt bond in the triplet excited state of the photoreactive [Pt(2)(P(2)O(5)H(2))(4)](4-) ion is determined by time-resolved X-ray absorption spectroscopy (see picture). The strengthening of the Pt-Pt interaction is accompanied by a weakening of the ligand coordination bonds, resulting in an elongation of the platinum-ligand bond that is determined for the first time.
Angewandte Chemie International Edition 02/2009; 48(15):2711-4. · 13.45 Impact Factor
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Igor V Sazanovich,
Mohammed A H Alamiry, Jonathan Best,
Robert D Bennett,
Oleg V Bouganov,
E Stephen Davies,
Vyacheslav P Grivin,
Anthony J H M Meijer,
Victor F Plyusnin,
Kate L Ronayne,
Alexander H Shelton,
Sergei A Tikhomirov,
Michael Towrie,
Julia A Weinstein
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ABSTRACT: A combination of picosecond time-resolved infrared spectroscopy, picosecond transient absorption spectroscopy, and nanosecond flash photolysis was used to elucidate the nature and dynamics of a manifold of the lowest excited states in Pt(phen-NDI)Cl 2 ( 1), where NDI = strongly electron accepting 1,4,5,8-naphthalene-diimide group. 1 is the first example of a Pt (II)-diimine-diimide dyad. UV/vis/IR spectroelectrochemistry and EPR studies of electrochemically generated anions confirmed that the lowest unoccupied molecular orbital (LUMO) in this system is localized on the NDI acceptor group. The lowest allowed electronic transition in Pt(phen-NDI)Cl 2 is charge-transfer-to-diimine of a largely Pt-->phen metal-to-ligand charge-transfer (MLCT) character. Excitation of 1 in the 355-395 nm range initiates a series of processes which involve excited states with the lifetimes of 0.9 ps ( (1)NDI*), 3 ps ( (3)MLCT), 19 ps (vibrational cooling of "hot" (3)NDI and of "hot" NDI ground state), and 520 mus ( (3)NDI). Excitation of 1 with 395 nm femtosecond laser pulses populates independently the (1)MLCT and the (1)NDI* excited states. A thermodynamically possible decay of the initially populated (1)MLCT to the charge-transfer-to-NDI excited state, [Pt (III)(phen-NDI (-*))Cl 2], is not observed. This finding could be explained by an ultrafast ISC of the (1)MLCT to the (3)MLCT state which lies about 0.4 eV lower in energy than [Pt (III)(phen-NDI (-*))Cl 2]. The predominant decay pathway of the (3)MLCT is a back electron transfer process with approximately 3 ps lifetime, which also causes partial population of the vibrationally hot ground state of the NDI fragment. The decay of the (1)NDI* state in 1 populates vibrationally hot ground state of the NDI, as well as vibrationally hot (3)NDI. The latter relaxes to form (3)NDI state, that is, [Pt(phen- (3)NDI)Cl 2]*, which possesses a remarkably long lifetime for a Pt (II) complex in fluid solution of 520 mus. The IR signature of this excited state includes the nu(CO) bands at 1607 and 1647 cm (-1), which are shifted considerably to lower energies if compared to their ground-state counterparts. The assignment of the vibrational bands is supported by the density-functional theory calculations in CH 2Cl 2. Pt(phen-NDI)Cl 2 acts as a modest photosensitizer of singlet oxygen.
Inorganic Chemistry 10/2008; 47(22):10432-45. · 4.60 Impact Factor
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ABSTRACT: The essential thiol of the enzyme papain has been caged by linking to an aromatic thiol. The resulting caged protein is inactive but enzymatic activity is fully restored upon chemical cleavage of the protective disulfide bond. We have exploited the chemistry of this disulfide bond to uncage papain by pulse radiolysis. We have shown that up to 10% of the enzyme activity can be restored by reductive pulse radiolysis. This approach has been tested on a small-molecule model system, and experiments on this model compound show that pulse radiolysis of the mixed cysteine-aromatic disulfide results in selective reduction of the disulfide bond to generate a thiol in 10-20% yield, consistent with the radiolytically restored activity of the caged papain quantified by the biochemical assay.
Free Radical Biology and Medicine 09/2008; 45(9):1271-8. · 5.42 Impact Factor
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ABSTRACT: A series of catechols with attached imide functionality (imide = phthalimide PHT, 1,8-naphthalimide NAP, 1,4,5,8-naphthalenediimide NDI, and NAP-NDI) has been synthesized and coordinated to the Pt (II)(bpy*) moiety, yielding Pt(bpy*)(cat-imide) complexes (bpy* = 4,4'-di- tert-butyl-2,2'-bipyridine). X-ray crystal structures of PHT and NAP complexes show a distorted square-planar arrangement of ligands around the Pt center. Both complexes form "head-to-tail" dimers in the solid state through remarkably short unsupported Pt...Pt contacts of 3.208 (PHT) and 3.378 A (NAP). The Pt(bpy*)(cat-imide) complexes are shown to combine optical (absorption) and electrochemical properties of the catecholate (electron-donor) and imide (electron-acceptor) groups. The complexes show a series of reversible reduction processes in the range from -0.5 to -1.9 V vs Fc (+)/Fc, which are centered on either bpy* or imide groups, and a reversible oxidation process at +0.07 to +0.14 V, which is centered on the catecholate moiety. A combination of UV-vis absorption spectroscopy, cyclic voltammetry, UV-vis spectroelectrochemistry, and EPR spectroscopy has allowed assignment of the nature of frontier orbitals in Pt(bpy*)(cat-imide) complexes. The HOMO in Pt(bpy*)(cat-imide) is centered on the catechol ligand, while the LUMO is localized either on bpy* or on the imide group, depending on the nature of the imide group involved. Despite the variations in the nature of the LUMO, the lowest-detectable electronic transition in all Pt(bpy*)(cat-imide) complexes has predominantly ligand-to-ligand (catechol-to-diimine) charge-transfer nature (LLCT) and involves a bpy*-based unoccupied molecular orbital in all cases. The LLCT transition in all Pt(bpy*)(cat-imide) complexes appears at 530 nm in CH2Cl2 and is strongly negatively solvatochromic. The energy of this transition is remarkably insensitive to the imide group present, indicating lack of electronic communication between the imide and the catechol moieties within the cat-imide ligand. The high extinction coefficient, approximately 6 x 10(3) L mol(-1) cm(-1) of this predominantly LLCT transition is the result of the Pt orbital contribution, as revealed by EPR spectroscopy of the complexes in various redox states. The CV profile of the oxidation process of Pt(bpy*)(cat-imide) in CH2Cl2 and DMF is concentration dependent, as was shown for NDI and PHT complexes as typical examples. Oxidation appears as a simple diffusion-limited process at low concentrations, with an increasing anodic-to-cathodic peak separation eventually resolving as two independent consecutive waves as the concentration of the complex increases. It is suggested that aggregation of the complexes in the diffusion layer in the course of oxidation is responsible for the observed concentration dependence. Overall, the Pt(bpy*)(cat-imide) complexes are electrochromic compounds in which a series of stepwise reversible redox processes in the potential range from 0.2 to -2 V (vs Fc (+)/Fc) leads to tuneable absorbencies between 300 and 850 nm.
Inorganic Chemistry 04/2008; 47(5):1532-47. · 4.60 Impact Factor
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ABSTRACT: We present the first structural detn. of the Pt2(P2O5H2)44- anion in soln. by analyzing the extended X-ray absorption fine structure (EXAFS) spectrum of the Pt LIII edge. The data could be fit with a simple model involving single and multiple scattering paths to near and far P-atoms, bridging O-atoms, and the other Pt-atom in the binuclear complex. A Pt-Pt distance of 2.876(28) .ANG. and a Pt-P bond length of 2.32(4) .ANG. are obtained. These values are in line with distances found in previous X-ray diffraction studies. The assignment of the EXAFS spectrum of the Pt2(P2O5H2)44- anion in its ground state is required for future time-resolved X-ray absorption measurements with the goal of detg. the structure and dynamics of the complex in the 1,3A2u excited states.
CHIMIA International Journal for Chemistry 01/2008; 62(4):287. · 1.21 Impact Factor
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ABSTRACT: The synthesis and photophysical study of (C/\N)Pt(II)Q complexes, where C/\N is a bidentate cyclometalating ligand and Q is 8-hydroxyquinoline or quinoline-8-thiol, are presented. The compounds were obtained as a single isomer with N atoms of the C/\N and Q ligands trans-coordinated to the Pt(II) center as shown by X-ray crystallography. These chromophores absorb intensely in the visible region and emit in the deep-red spectral region from a quinolate-centered triplet intraligand charge-transfer excited state. The emission maxima are in the range 675-740 nm, with the quantum yields and lifetimes of up to 0.82% and 5.3 mus, respectively, in deoxygenated organic solvents at room temperature. These complexes are efficient photosensitizers of singlet oxygen in air-saturated solutions, with yields up to 90%.
Inorganic Chemistry 12/2006; 45(23):9410-5. · 4.60 Impact Factor
