Making Hydrogen from Water Using a Homogeneous System Without Noble Metals

Making Hydrogen from Water Using a Homogeneous System Without Noble Metal

Sensitizing the Sensitizer: The Synthesis and Photophysical Study of Bodipy−Pt(II)(diimine)(dithiolate) Conjugates

The dyads 3, 4, and 6, combining the Bodipy chromophore with a Pt(bpy)(bdt) (bpy = 2,2′-bipyridine, bdt = 1,2-benzenedithiolate, 3 and 6) or a Pt(bpy)(mnt) (mnt = maleonitriledithiolate, 4) moiety, have been synthesized and studied by UV−vis steady-state absorption, transient absorption, and emission spectroscopies and cyclic voltammetry. Comparison of the absorption spectra and cyclic voltammograms of dyads 3, 4, and 6 and those of their model compounds 1a, 2, 5, and 7 shows that the spectroscopic and electrochemical properties of the dyads are essentially the sum of their constituent chromophores, indicating negligible interaction of the constituent chromophores in the ground state. However, emission studies on 3 and 6 show a complete absence of both Bodipy-based fluorescence and the characteristic luminescence of the Pt(bpy)(bdt) unit. Dyad 4 shows a weak Pt(mnt)-based emission. Transient absorption studies show that excitation of the dyads into the Bodipy-based 1ππ* excited state is followed by singlet energy transfer (SEnT) to the Pt(dithiolate)-based 1MMLL′CT (mixed metal-ligand to ligand charge transfer) excited state (τSEnT3 = 0.6 ps, τSEnT4 = 0.5 ps, and τSEnT6 = 1.6 ps), which undergoes rapid intersystem crossing to the 3MMLL′CT state due to the heavy Pt(II) ion. The 3MMLL′CT state is then depopulated by triplet energy transfer (TEnT) to the low-lying Bodipy-based 3ππ* excited state (τTEnT3 = 8.2 ps, τTEnT4 = 5 ps, and τTEnT6 = 160 ps). The transition assignments are supported by TD-DFT calculations. Both energy-transfer processes are shown to proceed via a Dexter electron exchange mechanism. The much longer time constants for dyad 6 relative to 3 are attributed to the significantly poorer coupling and resonance of charge-separated species that are intermediates in the electron exchange process

Electron vs Energy Transfer in Arrays Featuring Two Bodipy Chromophores Axially Bound to a Sn(IV) Porphyrin via a Phenolate or Benzoate Bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn­(IV) porphyrin center either via a phenolate (3) or via a carboxylate (6) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of 3 are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in 3 results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on 6 show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in 3 is suppressed. Instead, Bodipy excitation in 3 and 6 results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state

Sensitizing the Sensitizer: The Synthesis and Photophysical Study of Bodipy−Pt(II)(diimine)(dithiolate) Conjugates

The dyads 3, 4, and 6, combining the Bodipy chromophore with a Pt(bpy)(bdt) (bpy = 2,2′-bipyridine, bdt = 1,2-benzenedithiolate, 3 and 6) or a Pt(bpy)(mnt) (mnt = maleonitriledithiolate, 4) moiety, have been synthesized and studied by UV−vis steady-state absorption, transient absorption, and emission spectroscopies and cyclic voltammetry. Comparison of the absorption spectra and cyclic voltammograms of dyads 3, 4, and 6 and those of their model compounds 1a, 2, 5, and 7 shows that the spectroscopic and electrochemical properties of the dyads are essentially the sum of their constituent chromophores, indicating negligible interaction of the constituent chromophores in the ground state. However, emission studies on 3 and 6 show a complete absence of both Bodipy-based fluorescence and the characteristic luminescence of the Pt(bpy)(bdt) unit. Dyad 4 shows a weak Pt(mnt)-based emission. Transient absorption studies show that excitation of the dyads into the Bodipy-based 1ππ* excited state is followed by singlet energy transfer (SEnT) to the Pt(dithiolate)-based 1MMLL′CT (mixed metal-ligand to ligand charge transfer) excited state (τSEnT3 = 0.6 ps, τSEnT4 = 0.5 ps, and τSEnT6 = 1.6 ps), which undergoes rapid intersystem crossing to the 3MMLL′CT state due to the heavy Pt(II) ion. The 3MMLL′CT state is then depopulated by triplet energy transfer (TEnT) to the low-lying Bodipy-based 3ππ* excited state (τTEnT3 = 8.2 ps, τTEnT4 = 5 ps, and τTEnT6 = 160 ps). The transition assignments are supported by TD-DFT calculations. Both energy-transfer processes are shown to proceed via a Dexter electron exchange mechanism. The much longer time constants for dyad 6 relative to 3 are attributed to the significantly poorer coupling and resonance of charge-separated species that are intermediates in the electron exchange process

Meso-substituted Porphyrin Derivatives via Palladium-Catalyzed Amination Showing Wide Range Visible Absorption: Synthesis and Photophysical Studies

In recent years, there has been a growing interest in the design and synthesis of chromophores, which absorb in a wide region of the visible spectrum, as these constitute promising candidates for use as sensitizers in various solar energy conversion schemes. In this work, a palladium-catalyzed coupling reaction was employed in the synthesis of molecular triads in which two porphyrin or boron dipyrrin (BDP) chromophores are linked to the meso positions of a central Zn porphyrin (<b>PZn</b>) ring via an amino group. In the resulting conjugates, which strongly absorb over most of the visible region, the electronic properties of the constituent chromophores are largely retained while detailed emission experiments reveal the energy transfer pathways that occur in each triad

Electron vs Energy Transfer in Arrays Featuring Two Bodipy Chromophores Axially Bound to a Sn(IV) Porphyrin via a Phenolate or Benzoate Bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn­(IV) porphyrin center either via a phenolate (3) or via a carboxylate (6) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of 3 are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in 3 results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on 6 show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in 3 is suppressed. Instead, Bodipy excitation in 3 and 6 results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state

Electron vs Energy Transfer in Arrays Featuring Two Bodipy Chromophores Axially Bound to a Sn(IV) Porphyrin via a Phenolate or Benzoate Bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn­(IV) porphyrin center either via a phenolate (<b>3</b>) or via a carboxylate (<b>6</b>) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of <b>3</b> are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in <b>3</b> results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on <b>6</b> show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in <b>3</b> is suppressed. Instead, Bodipy excitation in <b>3</b> and <b>6</b> results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state

Cyclometalated 6-Phenyl-2,2′-bipyridyl (CNN) Platinum(II) Acetylide Complexes: Structure, Electrochemistry, Photophysics, and Oxidative- and Reductive-Quenching Studies

Three cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (CNN-Ph-R) Pt(II) acetylide complexes, Pt(CNN-Ph-R)(CCPh), where R = Me (1), COOMe (2), and P(O)(OEt)2 (3), have been synthesized and studied. Compounds 1 and 3 have been structurally characterized by single crystal X-ray crystallography and are found to exhibit distorted square planar geometries about the Pt(II) ions. The electrochemical properties of the compounds, as determined by cyclic voltammetry, have also been examined. Complexes 1−3 are brightly emissive in fluid CH2Cl2 solution and in the solid state with λemmax of ca. 600 nm and lifetimes on the order of ca. 500 ns in fluid solution. The emissions are assigned to a 3MLCT transition. The complexes undergo oxidative quenching by MV2+ with quenching rates near the diffusion-controlled limit (kq ∼ 1.4 × 1010 M−1 s−1) in CH2Cl2 solution. Reductive-quenching experiments of complexes 1−3 by the amine donors N,N,N′,N′-tetramethylphenylenediamine (TMPD), phenothiazine (PTZ), and N,N,N′,N′-tetramethylbenzidine (TMB) follow Stern−Volmer behavior, with very fast quenching rates on the order of 109−1010 M−1 s−1 in CH2Cl2 solution. When the complexes are employed as the sensitizer in multiple component systems containing MV2+, TEOA, and colloidal Pt in aqueous media, approximately one turnover of H2 (TN vs mol of chromophore) is produced per hour upon irradiation with λ > 410 nm but only after at least a 2 h induction period

Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar Hydrogen Production

A series of halogenated boron-dipyrromethene (Bodipy) chromophores with potential applications in solar energy conversion were synthesized and characterized by steady state and ultrafast laser spectroscopy. The ultrafast dynamics of the chromophores were compared between a series containing H, Br, or I at the 2,6 positions of the Bodipy dye. The parent Bodipy has a fluorescent lifetime (τ_fl) of 3−5 ns, a fluorescence quantum yield (Φ_fl) of 0.56, and negligible triplet state yield. Bromination enhances the intersystem crossing (ISC) such that τ_fl and Φ_fl decrease to ∼1.2 ns and 0.11, respectively, while iodination further accelerates ISC so that τ_fl is only ∼130 ps and Φ_fl is 0.011. Transient absorption experiments lead to the observation of excited state absorption bands from the singlet (S₁) and triplet (T₁) states at ∼345 and 447 nm, respectively, and characterization of ISC via the dynamics of these bands and the decay of S₁ stimulated emission

Electron vs Energy Transfer in Arrays Featuring Two Bodipy Chromophores Axially Bound to a Sn(IV) Porphyrin via a Phenolate or Benzoate Bridge

In this report we describe the synthesis of multichromophore arrays consisting of two Bodipy units axially bound to a Sn­(IV) porphyrin center either via a phenolate (<b>3</b>) or via a carboxylate (<b>6</b>) functionality. Absorption spectra and electrochemical studies show that the Bodipy and porphyrin chromophores interact weakly in the ground state. However, steady-state emission and excitation spectra at room temperature reveal that fluorescence from both the Bodipy and the porphyrin of <b>3</b> are strongly quenched suggesting that, in the excited state, energy and/or electron transfer might occur. Indeed, as transient absorption experiments show, selective excitation of Bodipy in <b>3</b> results in a rapid decay (τ ≈ 2 ps) of the Bodipy-based singlet excited state and a concomitant rise of a charge-separated state evolving from the porphyrin-based singlet excited state. In contrast, room-temperature emission studies on <b>6</b> show strong quenching of the Bodipy-based fluorescence leading to sensitized emission from the porphyrin moiety due to a transduction of the singlet excited state energy from Bodipy to the porphyrin. Emission experiments at 77 K in frozen toluene reveal that the room-temperature electron transfer pathway observed in <b>3</b> is suppressed. Instead, Bodipy excitation in <b>3</b> and <b>6</b> results in population of the first singlet excited state of the porphyrin chromophore. Subsequently, intersystem crossing leads to the porphyrin-based triplet excited state