Electronic Circular Dichroism Spectroscopy of Jet-Cooled Phenylalanine and Its Hydrated Clusters

We obtained resonant two-photon ionization circular dichroism (R2PICD) spectra of jet-cooled phenylalanine (Phe) and its hydrated clusters (Phe­(H₂O)_<i>n</i>, <i>n</i> = 1–2) near the origin band of the S₀–S₁ transition. The R2PICD spectra of Phe exhibit well-resolved CD bands of six different conformers present in the jet, which vary in sign and magnitude depending on their conformations. We revised the previous structural assignments of the Phe conformers based on the comparison between the experimental and theoretical CD signs, infrared spectra, and rotational band contours. The R2PICD spectra of Phe­(H₂O)_<i>n</i> reveal that hydration with one or two water molecule(s) does not affect the CD signs of Phe conformers but significantly increases their CD magnitudes. Furthermore, conformational selection by solvation alters relative populations of Phe conformers, leading to a sign inversion in the CD spectra of Phe­(H₂O)_<i>n</i> compared with that of Phe monomer

Isomer-Specific Induced Circular Dichroism Spectroscopy of Jet-Cooled Phenol Complexes with (−)-Methyl l‑Lactate

Induced circular dichroism (ICD) is the CD observed in the absorption of an achiral molecule bound to a transparent chiral molecule through noncovalent interactions. ICD spectroscopy has been used to probe the binding between molecules, such as protein–ligand interactions. However, most ICD spectra have been measured in solution, which only exhibit the averaged CD values of all conformational isomers in solution. Here, we obtained the first isomer-selective ICD spectra by applying resonant two-photon ionization CD spectroscopy to jet-cooled phenol complexes with (−)-methyl l-lactate (PhOH-(−)­ML). The well-resolved CD bands in the spectra were assigned to two conformers, which contained different types of hydrogen-bonding interactions between PhOH and (−)­ML. The ICD values of the two conformers have different signs and magnitudes, which were explained by differences both in the geometrical asymmetries of PhOH bound to (−)­ML and in the electronic coupling strengths between PhOH and (−)­ML

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, (L)₂Ir-dpp-PtCl [<b>1</b>, L = 2-(4′,6′-difluorophenyl)­pyridinato-<i>N</i>,<i>C</i>² (dfppy); <b>2</b>, L = 2-phenylpyridinato-<i>N</i>,<i>C</i>² (ppy)] and [(L)₂Ir]₂(dpp) [<b>3</b>, L = dfppy; <b>4</b>, L = ppy] compared to monometallic complexes, (L)₂Ir-dpp (<b>5</b>, L = dfppy; <b>6</b>, L = ppy) and dpp-PtCl (dpp-Pt^IICl₂; <b>7</b>). The single-crystal X-ray crystallographic structures of <b>1</b>, <b>3</b>, <b>5</b>, and <b>6</b> showed that <b>1</b> and <b>3</b> have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in <b>5</b> and <b>6</b> is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes (<b>1</b>–<b>4</b>) could be explained by assuming the involvement of crossing to platinum- and iridium-based d–d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d–d states from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state, followed by cleavage of the dpp-Pt and (L)₂Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based ³MLCT to the Ir d–d or Pt d–d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d–d or Pt d–d) with a significant dependency on the relative ³MLCT levels of the (L)₂Ir-dpp component

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, (L)₂Ir-dpp-PtCl [<b>1</b>, L = 2-(4′,6′-difluorophenyl)­pyridinato-<i>N</i>,<i>C</i>² (dfppy); <b>2</b>, L = 2-phenylpyridinato-<i>N</i>,<i>C</i>² (ppy)] and [(L)₂Ir]₂(dpp) [<b>3</b>, L = dfppy; <b>4</b>, L = ppy] compared to monometallic complexes, (L)₂Ir-dpp (<b>5</b>, L = dfppy; <b>6</b>, L = ppy) and dpp-PtCl (dpp-Pt^IICl₂; <b>7</b>). The single-crystal X-ray crystallographic structures of <b>1</b>, <b>3</b>, <b>5</b>, and <b>6</b> showed that <b>1</b> and <b>3</b> have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in <b>5</b> and <b>6</b> is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes (<b>1</b>–<b>4</b>) could be explained by assuming the involvement of crossing to platinum- and iridium-based d–d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d–d states from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state, followed by cleavage of the dpp-Pt and (L)₂Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based ³MLCT to the Ir d–d or Pt d–d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d–d or Pt d–d) with a significant dependency on the relative ³MLCT levels of the (L)₂Ir-dpp component

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)–Platinum(II) and Iridium(III)–Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine

We investigated the electrochemical and excited-state properties of 2,3-bis­(2-pyridyl)­pyrazine (dpp)-bridged bimetallic complexes, (L)₂Ir-dpp-PtCl [<b>1</b>, L = 2-(4′,6′-difluorophenyl)­pyridinato-<i>N</i>,<i>C</i>² (dfppy); <b>2</b>, L = 2-phenylpyridinato-<i>N</i>,<i>C</i>² (ppy)] and [(L)₂Ir]₂(dpp) [<b>3</b>, L = dfppy; <b>4</b>, L = ppy] compared to monometallic complexes, (L)₂Ir-dpp (<b>5</b>, L = dfppy; <b>6</b>, L = ppy) and dpp-PtCl (dpp-Pt^IICl₂; <b>7</b>). The single-crystal X-ray crystallographic structures of <b>1</b>, <b>3</b>, <b>5</b>, and <b>6</b> showed that <b>1</b> and <b>3</b> have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in <b>5</b> and <b>6</b> is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp). Their electrochemical and photophysical properties revealed that efficient electron-transfer processes predominated in the bimetallic systems regardless of the second metal participation. The low efficiencies of photoluminescence of dpp-bridged Ir–Pt and Ir–Ir bimetallic complexes (<b>1</b>–<b>4</b>) could be explained by assuming the involvement of crossing to platinum- and iridium-based d–d states from the emissive state. Such stereochemical and electronic situations around dpp allowed thermally activated crossing to platinum- and iridium-based d–d states from the emissive triplet metal-to-ligand charge-transfer (³MLCT) state, followed by cleavage of the dpp-Pt and (L)₂Ir-dpp bonds. The transient absorption study further confirmed that the planarity of the dpp bridging ligand, which was defined as the magnitude of tilt between the pyridine ring and pyrazine, had a direct correlation with the degree of nonradiative decay from the emissive iridium-based ³MLCT to the Ir d–d or Pt d–d state, leading to photoinduced dissociation of bimetallic complexes. From the dissociation pattern of metal complexes analyzed after photoirradiation, we found that their dissociation pathways were directly related to the quenching direction (either Ir d–d or Pt d–d) with a significant dependency on the relative ³MLCT levels of the (L)₂Ir-dpp component