1,720,965 research outputs found
Triplet Pathways in Diarylethene Photochromism. A combined ab initio and density functional study.
No full textA combined ab initio and DFT computational study has been carried out in the attempt to provide a working model for the recently discovered triplet reactivity of photochromic diarylethenes. The investigation of 1,2-bis(2-methylbenzothiophene-3-yl)maleimide type (DAE) by DFT methods provides a detailed description of the potential energy surfaces (PES) for ground state (S0) and lowest triplet state (T1) along the reaction coordinate for photocyclization/cycloreversion. The transition-state structures are alike on S0 and T1 and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S0 and T1 PES are almost degenerate. On S0, a large isomerization barrier (ca. 45 kcal/mol) separates the open- and closed-ring minima, whereas on T1 the activation barriers are much smaller, that for cycloreversion (ca. 15-20 kcal/mol) being larger than that for cyclization (ca. 8 kcal/mol). These features account for the behaviour observed in Ru-DAE dyads, where the diarylethene is coupled to a Ru(II) polypyridine photosensitizer moiety: efficient sensitized photocyclization, in a microsecond time scale, inefficient sensitized cycloreversion. Triplet cyclization is viewed as a non-adiabatic process originating on T1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S0 at closed-ring geometry. Calculations on the model system 1,2-di(3-thienyl)ethene (mod-DAE) have been used to (i) validate the DFT results against ab initio (CASSCF and CASPT2) results and (ii) demonstrate the general value of the main topological features of the S0 and T1 PES obtained for DAE
Iridium cyclometalated complexes with axial symmetry. Synthesis and photophysical properties of a trans-biscyclometalated complex containing the terdentate ligand 2,6-diphenylpyridine
No full textThe first example of an iridium biscyclometalated complex with a Cboolean ANDNboolean ANDC 2,6-diphenylpyridine (dppy)-type ligand, [(4'-(4-bromophenyl)-2:2',6':2"-terpyridine)lr(2,6-diphenyl-4-(4-tolyl)pyridine)](NO3) (1), has been synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, H-1 and C-13 NMR, cyclic voltammetry, and both steady-state and time-resolved emission and absorption studies. Preliminary density functional theory calculations have also been conducted. 1 crystallizes in the monoclinic space group P2(1)/n. The crystallographic data are as follows: C(45)H(31)BrN(4)lrO(3).2H(2)O, a = 17.4308(4) Angstrom, b = 9.0312(2) Angstrom, c = 26.7601(7) Angstrom, beta = 104.496(1)degrees, V = 4078.5(2) Angstrom,, Z = 4. The relatively long Ir-C distances (2.122 and 2.094 Angstrom) reflect the strong mutual trans effect of the cyclometalating carbons. The complex exhibits strong visible absorption and long-lived (1.7,mus) emission ( lambda(max), 690 nm) in room temperature solution. The inherent asymmetry of the coordination environment offers a unique directional character to the emitting excited state, which is predominately ligand-to-ligand charge transfer (dppy --> 2,2':6',2"-terpyridine) in nature
Long Range Charge Separation in a Ferrocene-(Zinc Porphyrin)-Naphthanenediimide Triad with 1,2,3-Triazole Linkers
No full textNew dyad and triad systems based on a zinc porphyrin (ZnP), a naphthalenediimide (NDI), and a ferrocene (Fc) as molecular components, linked by 1,2,3-triazole bridges, ZnP-NDI (3) and Fc-ZnP-NDI (4), have been synthesized. Their photophysical behavior has been investigated by both visible excitation of the ZnP chromophore and UV excitation of the NDI unit. Dyad 3 exhibits relatively inefficient quenching of the ZnP singlet excited state, slow charge separation and fast charge recombination processes. Excitation of the NDI chromophore, on the other hand, leads to charge separation by both singlet and triplet quenching pathways, with the singlet charge-separated (CS) state recombining in a sub-ns time scale, and the triplet CS state decaying in ca. 90 ns. In the triad system 4, primary formation of Fc-ZnP+-NDI− charge separated state is followed by a secondary hole shift process from ZnP to Fc. The product of the stepwise charge-separation, Fc+-ZnP-NDI−, undergoes recombination to the ground state, as expected for a long-range process, in a much longer time scale, 1.9 s. The charge separated states are always formed more efficiently upon NDI excitation than upon ZnP excitation. DFT calculations on a bridge-acceptor fragment show that the bridge is expected to mediate a fast donor-to-bridge-to-acceptor electron cascade following excitation of the acceptor. More generally, triazole bridges may behave asymmetrically with respect to photoinduced electron transfer in dyads, kinetically favoring hole-transfer pathways triggered by excitation of the acceptor over electron-transfer pathways promoted by excitation of the donor
ELECTRONIC EFFECTS IN ENERGY/ELECTRON TRANSFER PROCESSES OF INORGANIC DYADS
No full textEnergy and electron transfer are the subject of continuing attention, as key processes in molecular devices for light energy conversion and various electronic/photonic applications. The simplest systems for the study of such processes are D-B-A “dyads”, where donor (D) and acceptor (A) molecular components are connected by an appropriate bridge (B). A large number of inorganic dyads have been studied, where A and D are transition metal complexes, often of the polypyridine family [1,2]. Two features of these inorganic systems make their energy/electron transfer behavior more complex than that of common organic dyads: (i) the complex nature A and D, each comprising metal-localized and ligand-localized electronic subsystems, (ii) the presence of a bridging ligand, that may complicate the picture of B, D, and A as electronically localized subsystems. Because of these complex localization aspects, electronic factors play often very important roles in the behavior of inorganic dyads. Some examples from recent work in the authors’ laboratory will be presented, including (i) energy/electron transfer in dyads with aromatic polyquinoxaline bridges, (ii) energy transfer in isomeric cyclometalated dyads and (ii) long-range electron transfer across oligophenylene bridges.
[1] F. Scandola, C. Chiorboli, M. T. Indelli, M. A. Rampi, in: Electron Transfer in Chemistry, V. Balzani, Ed.; Wiley-VCH, Weinheim, 2001. Volume III, Chapter 2.3, p. 337-408.
[2] F. Scandola, C. Chiorboli, M. T. Indelli, M. A. Rampi, “Covalently Linked Systems Containing Metal Complexes” in: Electron Transfer in Chemistry, V. Balzani, Ed.; Wiley-VCH, Weinheim, 2001. Volume III, Chapter 2.3, p. 337-408. C. Chiorboli, M. T. Indelli, F. Scandola Top. Curr. Chem. 2005, 257, 63-10
Can Diarylethene Photochromism Be Explained by a Reaction Path Alone? A CASSCF Study with Model MMVB Dynamics
No full textThe origin of the photochromic properties of diarylethenes is a conical intersection (which we have located computationally), but we show that dynamics calculations are necessary to explain why the conical intersection is accessible, because the excited-state reaction path is not contained in the branching space defining the intersection. Four different systems have been studied: 1,2-di(3-furyl)ethene, 1,2-di(3-thienyl)ethene, 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, and a model hydrocarbon system. Critical points on the ground- and excited-state potential energy surfaces were calculated using complete active space self-consistent field (CASSCF) theory; dynamics calculations were carried out using the molecular mechanics-valence bond (MMVB) method. The main experimental observations (i.e., picosecond time domain, quantum yield, temperature dependence, and fluorescence) can be interpreted on the basis of our results
PHOTOINDUCED PROCESSES IN AN ALUMINUM-PORPHYRIN / NAPHTHALENEDIIMIDE AXIAL DYAD.
No full textPorphyrins are widely studied cromphorores, well-known as constituents of both natural and artificial supramolecular systems which show interesting photophysical features such as photoinduced electron and/or energy transfer. Recently, attention has been paid to the possibility of exploiting Aluminium(III) porphyrins as supramolecular building blocks1,2, due to the stability of the axial coordination between the Al-porphyrin and ligands carrying oxygen-containing groups. Here we present an example of such a supramolecular system, a dyad based on an Al(III)-tetra-phenyl-porphyrin moiety linked by axial coordination to a naphtalene-diimide unit carrying a carboxylate group. The photophysical properties of the dyad system and of appropriate model systems have been studied by means of both stationary and time-resolved spectroscopic techniques, including single-photon counting, nanosecond laser flash photolysis and ultrafast transient absorption spectroscopy. The experimental data indicate the occurrence of photoinduced electron transfer in the dyad, with clear evidence given by solvent effects on excited-state kinetics. Density functional theory (DFT) calculations have been performed to gain a better insight on the electronic structure and provide a reliable geometry for the system in order to remedy the lack of any pentacoordinated Al-porphyrin x-ray structure in literature
PHOTOINDUCED ELECTRON TRANSFER THROUGH OLIGOPHENYLENE BRIDGES IN [Ru(bpy)3–(ph)n–DQ]4+ (n=1-5) COMPLEXES.
No full textThe photophysical properties of an extended series of donor-bridge-acceptor systems (D-B-A) based on modular polyphenylene spacers has been investigated. In these D-B-A systems, the metal complex Ru(bpy)32+ acts as a photoexcitable donor and the quaternarized bipyridinium unit (DQ2+) is the electron acceptor.
The intense emission of metal-to-ligand charge-transfer (MLCT) nature, exhibited by the parent [Ru-phn-bpy]2+, is strongly quenched in all the quaternarized systems. Evidences from both stationary and time-resolved spectroscopic techniques, including single-photon counting and ultrafast transient absorption spectroscopy, indicate donor-to-acceptor photoinduced electron transfer (PET) as the quenching mechanism for n = 1-3, whereas a different quenching pathway involving bridge-to-acceptor CT states takes place for n = 4-5. Although there is no accumulation of the D+-B-A- charge-separated state, likely due to charge-recombination faster than charge-separation, validation for the PET occurrence in compounds with n = 1-3 is attained by solvent effect investigation. With regard to the quenching mechanism for compounds with n = 4-5, new solvent-sensitive bands in the UV-visible spectra, increasing in intensity and red-shifting with increasing number of phenylene units, suggest the presence of new low-lying excited states of bridge-to-acceptor CT in nature.
Support for this hypothesis has been obtained from DFT (Density Functional Theory) calculations performed on systems with n = 2-4. The calculations predict the lowest excited state to be a D+-B-A- charge-separated state for n = 2 and a D-B+-A- CT state for n = 4. The presence of these low-lying CT states also accounts for the fast charge-recombination taking place in the systems with n = 1-3.
In conclusion, this experimental and computational investigation shows that oligophenylene bridges are efficient mediators of photoinduced electron transfer within inorganic dyads. Nonetheless, as the oligophenylene bridges are lengthened, CT excited states of the D-B+-A- type become sufficiently low in energy to introduce new quenching pathways
Primary Photoinduced Processes in Bimetallic Dyads with Extended Aromatic Bridges. Tetraazatetrapyridopentacene Complexes of Ruthenium(II) and Osmium(II)
No full textThe photophysics of the binuclear complexes [(phen)2M(tatpp)M(phen)2]4+, where M ) Ru or Os, phen ) 1,10-
phenanthroline, and tatpp ) 9,11,20,22-tetraazatetrapyrido[3,2-a:2¢3¢-c:3¢¢,2¢¢-l:2¢¢¢,3¢¢¢]pentacene, has been studied
in acetonitrile and dichloromethane by femtosecond and nanosecond time-resolved techniques. The results
demonstrate that complexes of different metals have different types of lowest excited state: a tatpp ligand-centered
(LC) triplet in the case of Ru(II); a metal-to-ligand charge-transfer (MLCT) triplet state in the case of Os(II). The
excited-state kinetics is strongly solvent-dependent. In the Ru(II) system, the formation and decay of the LC state
take place, respectively, in 25 ps and ca. 5 ns in CH3CN and in 0.5 ps and 1.3 ís in CH2Cl2. These solvent effects
can be rationalized on the basis of a thermally activated decay of the LC state through the upper MLCT state. In
the Os(II) system, the formation and decay of the MLCT state take place, respectively, in 3.8 and 60 ps in CH3CN
and in 0.5 and 4 ps in CH2Cl2. These effects are consistent with the solvent sensitivity of the MLCT energy, in
terms of driving force and energy-gap law arguments. The relevance of these results for the use of ladder-type
aromatic bridges as potential molecular wires is discussed
Iridium Cyclometalated Complexes with Axial Symmetry: Time-Dependent Density Functional Theory Investigation of trans-Bis-Cyclometalated Complexes Containing the Tridentate Ligand 2,6-Diphenylpyridine
No full textA new series of iridium cyclometalated complexes with a C^N^C dppy-type ligand and a N^N^N tpy-type ligand have been synthesized and characterized by various techniques such as mass spectrometry, 1H and 13C NMR, cyclic voltammetry, both steady-state and time-resolved emission and absorption studies, and time-dependent DFT (TDDFT) calculations. The complexes exhibit strong visible absorptions and long-lived (1.6-2.0 ís) emissions (ìmax, ca. 680 nm) in room-temperature solution. DFT calculations on the ground-state geometry match that of an X-ray crystal structure. TDDFT calculations give accurate predictions of the electronic absorption energies and intensities, while geometry optimizations on the lowest energy triplet state give accurate energies for the emission. Examination of the relevant molecular orbitals shows that the inherent asymmetry of the coordination environment offers a unique directional character to the emitting excited state, which is predominately LLCT (dppy f tpy) in nature
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