Laser-Induced Fluorescence of 1,3,5-Trimethoxybenzene Radical Cation in Solution at Room Temperature

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.

Multiresponsive Luminescent Cationic Cyclometalated Gold(III) Amphiphiles and Their Supramolecular Assembly

A new class of amphiphilic tridentate cyclometalated gold(III) complexes has been designed and synthesized as luminescent supramolecular building blocks. Positively charged trimethylammonium (-CH₂NMe₃⁺) containing alkynyl ligands have been incorporated to introduce the electrostatic interactions. The X-ray crystal structures of two of the complexes have been determined, and the existence of π-π interactions between molecules has been observed. Steady-state and time-resolved absorption and emission studies have been carried out to investigate the nature of the excited states. The complexes are found to exhibit self-assembly properties with the assistance of π-π stacking and hydrophobic interactions and possibly weak Au···Au interaction, resulting in notable emergence of low-energy absorption bands and luminescence changes. The presence of a large hydrophobic moiety is found to be crucial for the formation of aggregates, especially in polar media where hydrophobic interactions play an important role. The nature of the counterion has been shown to have a significant effect on the extent of self-assembly in different media. Upon aggregation, nanofibers are formed in polar media, while nanorods are observed in nonpolar media in one of the representative complexes. Interestingly, a small modification on the alkynyl ligand resulted in the formation of nanoribbons instead. Intriguing luminescence mechanochromic properties have also been observed. This orthogonal and rational molecular design strategy has been shown to be effective in the construction of gold(III)-based smart and multiresponsive materials.

Site-selective bimodal absorption and emission of distonic radical cation

An acyclic 1,4-distonic dimer radical cation (DAE(2)(*+)) was generated from the dimerization of 1,1-bis(4-methoxyphenyl)ethylene radical cation (DAE(*+)) with the neutral molecule (DAE) in solution. The absorption spectrum of DAE(2)(*+) shows bimodal absorption bands with peaks at 350 and 500 nm corresponding to the 1,1-bis(4-methoxyphenyl)ethyl radical (An(2)C(*)CH(3)) and 1,1-bis(4-methoxyphenyl)ethyl cation (An(2)C(+)CH(3)), respectively. Therefore, DAE(2)(*+) in the ground state has the spin and positive charge localized on the 1- and 4-positions, respectively. The bimodal characteristic emissions by the site-selective excitation of radical and cation sites of DAE(2)(*+) were observed at 77 K, showing that the excitation energy is localized on the radical or cation site of DAE(2)(*+) in the excited state. The interaction between radical and cation sites of DAE(2)(*+) in the ground and excited states are discussed on the basis of the steady-state spectroscopic and transient absorption measurements, as well as theoretical calculations.

Bimolecular hole transfer from the trimethoxybenzene radical cation in the excited state

Bimolecular hole transfer quenching of the 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) by hole quenchers (Q) such as biphenyl (Bp), naphthalene (Np), anisole (An), and benzene (Bz) with higher oxidation potentials than that of TMB was directly observed during the two-color two-laser flash photolysis at room temperature. From the linear relationships between the inverse of the transient absorption changes of TMB*+ during the second 532-nm laser excitation versus the inverse of the concentration of Q, the rate constant of the hole transfer from TMB*+* to Q was estimated to be (8.5 +/- 0.4) x 10(10), (1.4 +/- 0.7) x 10(11), (1.3 +/- 0.6) x 10(11), and (6.4 +/- 0.3) x 10(10) M(-1)s(-1) for Bp, An, Np, and Bz, respectively, in acetonitrile based on the lifetime of TMB*+*. The estimated rate constants are larger than the diffusion-controlled rate constant in acetonitrile. Short lifetime, high energy, and high oxidation potential of TMB*+* cause the lifetime-dependent quenching process or static quenching process as the major process during the quenching of TMB*+* by Q as indicated by the Ware's theoretical model. The subsequent hole transfer from Q*+ to TMB, giving TMB*+, was found to occur at the diffusion-controlled rate for Bp and An as Q. For Q such as Np and Bz, the dimerization of Q*+ with Q to give dimer radical cation (Q2*+) occurred competitively with the hole transfer from Q*+ to TMB.

One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis

One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*.

Ultrafast Excited State Dynamics of the Perylene Radical Cation Generated upon Bimolecular Photoinduced Electron Transfer Reaction

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.

Synthesis, Structure, Characterization, and Photophysical Studies of a New Platinum Terpyridyl-Based Triad with Covalently Linked Donor and Acceptor Groups

A new terpyridyl-containing Pt triad [Pt(pytpy)(p-CC-C6H4-NH-CO-C6H2(OMe)3)](PF6)2 (4), where pytpy = 4'-(4-pyridin-1-ylmethylphenyl)-[2,2';6',2' ']terpyridine and p-CC-C6H4-NH-CO-C6H2(OMe)3 = N-(4-ethynylphenyl)-3,4,5-trimethoxybenzamide, has been synthesized and structurally characterized. The related donor-chromophore dyad [Pt(ttpy)(p-CC-C6H4-NH-CO-C6H2(OMe)3)]PF6 2, where ttpy = 4'-p-tolyl-[2,2';6',2' ']terpyridine, and the chromophore-acceptor dyad [Pt(pytpy)(CCC6H5)](PF6)2 (3), where CCC6H5 = ethynylbenzene, have also been studied. The multistep syntheses culminate with a CuI-catalyzed coupling reaction of the respective acetylene with either [Pt(ttpy)Cl]PF6 or [Pt(pytpy)Cl](PF6)2. X-ray and spectroscopic studies support assignment of a distorted square planar environment around the Pt(II) ion with three of its coordination sites occupied by the terpyridyl N-donors and the fourth coordination site occupied by the acetylenic carbon. Although the parent compound [Pt(ttpy)(CCC6H5)]PF6 (1) is brightly luminescent in fluid solution at 298 K, dyad 2 as well as triad 4 exhibit complete quenching of the emission. The chromophore-acceptor (C-A) dyad 3 displays weak solution luminescence at room temperature with a phi(rel)(em) of 0.011 (using Ru(bpy)3(2+) as a standard with phi(rel)(em) = 0.062). Electrochemically, the donor-chromophore (D-C) dyad and the donor-chromophore-acceptor (D-C-A) triad exhibit both metal-based and donor ligand-based oxidations, whereas the triad and the C-A dyad show the expected pyridinium- and terpyridine-based reductions. Transient absorption studies of the dyad and triad systems indicate that although the trimethoxybenzene group acts as a reductive donor, in the present system, the pyridinium group fails to act as an acceptor.