Unconventional Synthesis of a Cu^I Rotaxane with a Superacceptor Stopper: Ultrafast Excited‐State Dynamics and Near‐Infrared Luminescence

Abstract

A Cu bis-phenanthroline rotaxane was prepared by using the [2+2] cycloaddition-retroelectrocyclization (CA-RE) reaction to graft a bulky dicyanoquinodimethane (DCNQ) stopper. The electronic properties were investigated with electrochemical and photophysical techniques, in parallel with three reference compounds, namely, the DCNQ derivative alone, the DCNQ-based phenanthroline ligand, and an analogue Cu complex lacking the DCNQ moiety. In all the systems containing the DCNQ unit, the lowest electronic excited states are centered thereon, with the singlet level (S ) located at about 1.0 eV, as suggested by TDDFT calculations. Accordingly, in the DCNQ-equipped rotaxane, the typical metal-to-ligand charge-transfer luminescence of the Cu center is totally quenched. Ultrafast transient absorption and emission studies show that, in the rotaxane, the final sink of photoinduced processes is the lowest singlet state of the DCNQ moiety (S ), which exhibits strong charge-transfer character and a lifetime of 0.4 ps. Its deactivation leads to population of another excited state with a lifetime of 1.3 ps, which can be the related triplet state (T ) or a vibrationally hot level (hot-S ). Notably, S also shows stimulated fluorescence in the near-infrared (NIR) region between 1100 and 1500 nm, corroborating the TDDFT prediction. This unusual finding opens up the study of ultrashort-lived NIR luminescence in organic donor-acceptor systems.

Isoxazole Strategy for the Synthesis of 2,2′-Bipyridine Ligands: Symmetrical and Unsymmetrical 6,6′-Binicotinates, 2,2′-Bipyridine-5-carboxylates, and Their Metal Complexes

“…To the degassed solution of Cu­[MeCN] 4 BF 4 (15.7 mg, 50 μmol) in acetonitrile (5 mL) was added the degassed solution of dimethyl 6-(4-chlorophenyl)-6′-(4-fluorophenyl)-[2,2′-bipyridine]-5,5′-dicarboxylate 7b (47.7 mg, 100 μmol) in DCM under an Ar atmosphere. The solution was stirred overnight; the solvents were evaporated, and the residue was treated with benzene by decantation and crystallized from ether . The precipitate was filtered off and dried in vacuo to give a dark violet solid: 49.7 mg (90% yield), mp 125–126 (dec, ether); 1 H NMR (CDCl 3 , 400 MHz, 23 °C) δ 3.69 (s, 12H), 6.49–6.53 (m, 4H), 6.70–6.87 (m, 12H), 8.48–8.65 (m, 8H); 13 C NMR (CDCl 3 , 100 MHz, 23 °C) δ 53.00, 53.02, 114.6 (d, J = 21.7 Hz), 122.3, 122.7, 127.6, 128.7, 129.6 (d, J = 8.3 Hz), 129.7, 130.2, 134.4 (d, J = 3.3 Hz), 135.2, 136.9, 140.3, 140.7, 151.8, 152.2, 156.4, 156.6, 162.7 (d, J = 252.1 Hz), 165.3, 165.7; 19 F NMR (CDCl 3 , 376 MHz, 23 °C) δ −153.0 (BF 4 ), −109.8 (ArF); ESI/HRMS ( m / z ) 1017.1157 calcd for C 52 H 36 Cl 2 CuF 2 N 4 O 8 + [M – BF 4 ] + , found 1017.1138.…”

Electrochemically switchable rotaxanes: recent strides in new directions

“…The groups of Armaroli and Diederich synthesized a Cu I bisphenanthroline rotaxane by using tetracyano-p-quinodimethane as the stopper unit in a [2 + 2]cycloaddition-retroelectrocyclization. 72 Electrochemical experiments revealed for the resulting rotaxane a reversible Cu I/II oxidation and two reversible one-electron reductions for the dicyanoquinodimethane moiety, which was generated during the stoppering reaction.…”

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

“…We mention, however, that stepwise Suzuki couplings on 396 are documented 162,430 and that similar molecules to 437 can be obtained from 396. 438 The Sonogashira coupling reaction is another popular crosscoupling reaction allowing the coupling of alkynes to halogenated substrates, e.g., halogenated phen (2,9)sub (Scheme 76). Historically, the first example of Sonogashira coupling was performed on 404 with ethynyltrimethylsilane in the presence of Pd(PPh 3 ) 2 Cl 2 and CuI as catalysts and triisopropylamine as the base in THF at room temperature (Scheme 76A).…”

“…409 was utilized to prepare non-symmetrical phen 437 , taking advantage of the difference of reactivities of C–I vs C–Cl bonds (Scheme C). We mention, however, that stepwise Suzuki couplings on 396 are documented , and that similar molecules to 437 can be obtained from 396 …”