Efficient Intersystem Crossing in Heavy-Atom-Free Perylenebisimide Derivatives

Efficient intersystem crossing (ISC) in heavy-atom-free organic chromophores remains rare because of the lack of strong spin–orbit coupling effects in such compounds. Finding organic chromophores with ISC ability is important for applications in several areas, e.g., photocatalysis and photodynamic therapy. Herein, we report new perylenebisimide (PBI) chromophores with tetraphenylethynyl substituents at the 2,5,8,11-positions of the PBI core (<i>ortho</i>-positions, not the usually reported <i>bay</i>-positions of PBI), which show efficient ISC without the presence of any heavy atoms. Steady-state and picosecond–nanosecond transient absorption spectroscopies as well as time-dependent density functional theory computations were used to reveal the photophysical properties. For one of the PBI derivatives, excitation wavelength-dependent ISC was observed. The efficient ISC was attributed to the S₁/S₂ → T_<i>n</i> (<i>n</i> > 1) processes. Photochemical reduction of the PBI derivatives in the presence of a sacrificial electron donor (triethanolamine) produced a stable PBI radical anion

Broad-Band N^∧N Pt(II) Bisacetylide Visible Light Harvesting Complex with Heteroleptic Bodipy Acetylide Ligands

Pt­(II) dbbpy bisacetylide (dbbpy = 4,4′-di­(<i>tert</i>-butyl)-2,2′-bipyridine) complex (<b>Pt-1</b>) with two different Bodipy ligands was prepared with the goal to attain broad-band visible light absorbing, efficient funneling of the photoexcitation energy (via resonance energy transfer, RET) to the energy acceptor and high triplet formation quantum yields. Construction of the above-mentioned molecular structural motif is challenging because two different arylacetylide ligands are incorporated in the complex; normally two homoleptic acetylide ligands were used for this kind of N^∧N Pt­(II) complexes. A reference complex with trans bis­(tributylphosphine) Pt­(II) bisacetylide protocol (<b>Pt-4</b>) was prepared for comparison of the photophysical properties. The two different Bodipy ligands in <b>Pt-1</b> and <b>Pt-4</b> constitute singlet/triplet energy donor/acceptor, as a result the harvested photoexcitation energy can be funneled to the triplet state confined on one of the two Bodipy ligands. The photophysical properties of the complexes were studied with steady state UV–vis absorption and luminescence spectroscopies, femto- and nanosecond transient absorption spectroscopies, cyclic voltammetry, as well as DFT/TDDFT calculations. Fluorescence/phosphorescence dual emission were observed for the complex. The ultrafast intramolecular singlet/triplet energy transfer in <b>Pt-1</b> was confirmed by the transient absorption spectroscopy (<i>k</i>_FRET = 2.6 × 10¹¹ s^–1, Φ_FRET = 87.1%) followed by an intersystem crossing (<i>k</i>_ISC = 1.9 × 10¹⁰ s^–1), and the triplet state lifetime (τ_T) is 54.1 μs. The reference complex <b>Pt-4</b> shows drastically different kinetics with <i>k</i>_FRET = 6.9 × 10¹⁰ s^–1, Φ_FRET = 81.0%, <i>k</i>_ISC = 5.83 × 10⁹ s^–1, and τ_T = 147.9 μs. Different singlet oxygen (¹O₂) quantum yields (Φ_Δ = 75% and 70%) and triplet state quantum yields (Φ_T = 91% and 69%, respectively) were observed for complexes <b>Pt-1</b> and <b>Pt-4</b>

DiiodoBodipy-Perylenebisimide Dyad/Triad: Preparation and Study of the Intramolecular and Intermolecular Electron/Energy Transfer

2,6-diiodoBodipy-perylenebisimide (PBI) dyad and triad were prepared, with the iodoBodipy moiety as the singlet/triplet energy donor and the PBI moiety as the singlet/triplet energy acceptor. IodoBodipy undergoes intersystem crossing (ISC), but PBI is devoid of ISC, and a competition of intramolecular resonance energy transfer (RET) with ISC of the diiodoBodipy moiety is established. The photophysical properties of the compounds were studied with steady-state and femtosecond/nanosecond transient absorption and emission spectroscopy. RET and photoinduced electron transfer (PET) were confirmed. The production of the triplet state is high for the iodinated dyad and the triad (singlet oxygen quantum yield Φ_Δ = 80%). The Gibbs free energy changes of the electron transfer (Δ<i>G</i>_CS) and the energy level of the charge transfer state (CTS) were analyzed. With nanosecond transient absorption spectroscopy, we confirmed that the triplet state is localized on the PBI moiety in the iodinated dyad and the triad. An exceptionally long lived triplet excited state was observed (τ_T = 150 μs) for PBI. With the uniodinated reference dyad and triad, we demonstrated that the triplet state localized on the PBI moiety in the iodinated dyad and triad is not produced by charge recombination. These information are useful for the design and study of the fundamental photochemistry of multichromophore organic triplet photosensitizers

Radical-Enhanced Intersystem Crossing in New Bodipy Derivatives and Application for Efficient Triplet–Triplet Annihilation Upconversion

A long-lived triplet excited state of the well-known fluorophore boron dipyrromethene (Bodipy) was observed for the first time via efficient radical-enhanced intersystem crossing (EISC). The triplet state has been obtained in two dyads in which the Bodipy unit is linked to a nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), with two different length spacers. The photophysical properties were studied with steady-state and time-resolved transient optical spectroscopies and electron spin resonance (cw-ESR and TR-ESR). The fluorescence of Bodipy units is significantly quenched in the dyads, and the spin-polarized TEMPO signals were observed with TR-ESR, generated by a radical triplet pair mechanism. Efficient EISC (Φ_T = 80%) was observed for the dyad with a shorter linker, and the triplet state lifetime of the Bodipy chromophore is exceptionally long (62 μs). The EISC takes 250 ps. Poor ISC was observed for the dyad with a longer linker. The efficient ISC and long-lived triplet excited state in this flexible system are in stark contrast to the previously studied rigid EISC systems. The EISC effect was employed for the first time to perform triplet–triplet annihilation (TTA) upconversion (quantum yield Φ_UC = 6.7%)

Near<b>-</b>IR Broadband-Absorbing <i>trans</i>-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

Broadband near-IR absorbing <i>trans</i>-bis­(trialkylphosphine) Pt­(II) bisacetylide binuclear complex (<b>Pt–1</b>) was prepared with boron-dipyrromethene (Bodipy) and styrylBodipy acetylide ligands. <b>Pt–1</b> shows strong absorption bands at 731 and 503 nm. Singlet energy transfer (EnT) and efficient intersystem crossing of the central coordinated Bodipy ligand were proposed to be responsible for the efficient funneling of the excitation energy to the triplet-state manifold. Reference complexes containing only a single Bodipy ligand were prepared for comparison (with styrylBodipy ligand <b>Pt–0</b> or Bodipy ligand <b>Pt–2</b>). The molecular structures were confirmed by single-crystal X-ray diffraction. The photophysical properties were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for <b>Pt–1</b>. Singlet EnT in <b>Pt–1</b> was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient absorption spectra (energy transfer rate constant <i>k</i>_EnT = 2.2 × 10¹⁰ s^–1). With nanosecond transient absorption spectra, intramolecular <i>triplet</i>-state energy transfer in <b>Pt–1</b> was proved. Gibbs free energy changes of charge separation indicate that the photoinduced intramolecular electron transfer in <b>Pt–1</b> is thermodynamically prohibited. Intermolecular triplet transfer between <b>Pt–2</b> and <b>L–1</b> was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were determined as 3.6 × 10⁴ s^–1 and 94.5%, respectively. The singlet oxygen (¹O₂) photosensitizing of <b>Pt–1</b> was improved as compared to the complexes containing only a single visible-light-absorbing chromophore

Near<b>-</b>IR Broadband-Absorbing <i>trans</i>-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

Broadband near-IR absorbing <i>trans</i>-bis­(trialkylphosphine) Pt­(II) bisacetylide binuclear complex (<b>Pt–1</b>) was prepared with boron-dipyrromethene (Bodipy) and styrylBodipy acetylide ligands. <b>Pt–1</b> shows strong absorption bands at 731 and 503 nm. Singlet energy transfer (EnT) and efficient intersystem crossing of the central coordinated Bodipy ligand were proposed to be responsible for the efficient funneling of the excitation energy to the triplet-state manifold. Reference complexes containing only a single Bodipy ligand were prepared for comparison (with styrylBodipy ligand <b>Pt–0</b> or Bodipy ligand <b>Pt–2</b>). The molecular structures were confirmed by single-crystal X-ray diffraction. The photophysical properties were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for <b>Pt–1</b>. Singlet EnT in <b>Pt–1</b> was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient absorption spectra (energy transfer rate constant <i>k</i>_EnT = 2.2 × 10¹⁰ s^–1). With nanosecond transient absorption spectra, intramolecular <i>triplet</i>-state energy transfer in <b>Pt–1</b> was proved. Gibbs free energy changes of charge separation indicate that the photoinduced intramolecular electron transfer in <b>Pt–1</b> is thermodynamically prohibited. Intermolecular triplet transfer between <b>Pt–2</b> and <b>L–1</b> was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were determined as 3.6 × 10⁴ s^–1 and 94.5%, respectively. The singlet oxygen (¹O₂) photosensitizing of <b>Pt–1</b> was improved as compared to the complexes containing only a single visible-light-absorbing chromophore