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ABSTRACT Triplet arylnitrenes may provide direct access to aryl azo‐dimers, which have broad commercial applicability. Herein, the photolysis ofp‐azidostilbene (1) in argon‐saturated methanol yielded stilbene azo‐dimer (2) through the dimerization of tripletp‐nitrenostilbene (31N). The formation of31Nwas verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λmax ~ 375 nm) in cryogenic 2‐methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of1in methanol formed31N(λmax ~ 370 nm, 2.85 × 107 s−1). On shorter timescales, a transient absorption (λmax ~ 390 nm) that decayed with a similar rate constant (3.11 × 107 s−1) was assigned to a triplet excited state (T) of1. Density functional theory calculations yielded three configurations for T of1, with the unpaired electrons on the azido (TA) or stilbene moiety (TTw, twisted and TFl, flat). The transient was assigned to TTwbased on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol−1for1 N; thus, intersystem crossing of11Nto31Nis unlikely at ambient temperature, supporting the formation of31Nfrom T of1. Thus, sustainable synthetic methods for aryl azo‐dimers can be developed using the visible‐light irradiation of aryl azides to form triplet arylnitrenes.
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Photophysics of Oligothiophenes End‐Capped with Platinum(II) Auxochromes
Abstract A series of four oligothiophenes end‐capped with −Pt(PBu3)2Cl moieties on both ends of the oligomers was synthesized, and their excited state properties were investigated. The observation of low fluorescence quantum yield (<2 %) for the oligomers indicates the strong effect of platinum on the intersystem crossing (ISC) efficiency. No phosphorescence was detected for any of the oligomers; however, strong triplet‐triplet absorption was observed by nanosecond transient spectroscopy for oligomers with more than one thiophene unit. The oligomers displayed short triplet lifetimes (∼1–2 μs) compared to the unmetallated oligomers, due to large spin‐orbit coupling induced by the platinum atom. The lower limits of the ISC yields were indirectly determined by measuring the singlet oxygen quantum yields. Femtosecond–picosecond transient absorption studies revealed that the ISC rate ranges from 1012–1010 s−1, decreasing with increasing oligomer length. Electrochemical studies showed that the oligomers exhibit relatively low oxidation potentials (ca. 0.1 V vs. Fc/Fc+). Quenching of the oligomers’ triplet state absorption, simultaneously with the rise of their corresponding cationic radical absorption band in nanosecond transient spectra in the presence of methyl viologen, as an electron acceptor, established that the electron transfer occurs from their triplet state.
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- Award ID(s):
- 1904288
- PAR ID:
- 10257632
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemPhotoChem
- Volume:
- 5
- Issue:
- 2
- ISSN:
- 2367-0932
- Format(s):
- Medium: X Size: p. 160-166
- Size(s):
- p. 160-166
- Sponsoring Org:
- National Science Foundation
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