Re-absorption and excitation energy transfer of N,N ′-bis(2,5-di-tert-butylphenyl)-3,4:9,10-perylenebis(dicarboximide) (DBPI) laser dye

Spin-Correlated Luminescence of a Carbazole-Containing Diradical Emitter: Single-Molecule Magnetoluminescence and Thermally Activated Emission

Luminescent radicals have been intensively studied as a new class of materials exhibiting novel photofunctions unique to open-shell systems. When luminescent radicals are assembled, intriguing spin-correlated luminescence phenomena emerge, including excimer-like emission and magnetic-field effects on luminescence (i.e., magnetoluminescence, MagLum). However, the underlying mechanisms of these phenomena arising from spin multiplicity and spin-dependent excited-state dynamics are poorly understood due to the limited number of luminescent polyradical systems available for study. In particular, the correlation between stronger intramolecular exchange interactions (|2J/kB| > ∼10 K, where J and kB are the intramolecular exchange coupling constant and the Boltzmann constant, respectively) and luminescence properties has not been fully explained. In this study, a novel carbazole-containing diradical emitter (1) and the corresponding monoradical (2) were prepared for the in-depth study of spin-correlated luminescence properties, with luminescence measurements under magnetic fields of up to 18 T. Diradical 1 has a negative 2J/kB value of several tens of kelvin and exhibits a single-molecule MagLum and thermally activated luminescence, whereas 2 does not. Detailed quantitative analyses revealed that both the spin-correlated luminescence properties of 1 are strongly dominated by ground-state spin statistics based on the Boltzmann distribution (i.e., 2J/kB values). Furthermore, diradical 1 exhibits external heavy-atom effects in heavy-atom-containing solvents such as iodobenzene, whereas monoradical 2 does not. This is the first experimental verification of external heavy-atom effects in polyradical emitters. This work demonstrates that polyradical emitters can be designed based on spin degrees of freedom in both ground and excited states.

Comment on "Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution"

A recent Letter in this Journal (Langhals and Schlücker, J. Phys. Chem. Lett. 202213, 7568-7573) reported a dependence of the fluorescence lifetime of a dye on concentration and attributed it to "electromagnetic interactions with distant resonating structures" on a length scale of more than 100 nm. We show that their results can be explained as a straightforward result of absorption and re-emission of the fluorescence ("radiative energy transport"), which lengthens the apparent lifetime at higher concentrations. This effect has been well-documented in the literature many times. We show that simulations of the fluorescence decays accounting for radiative transport can reproduce the authors' results without postulating any new electromagnetic mechanism.

Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution

Long-range interactions between electronically excited molecules and molecules at the ground state were found for distances of much more than 100 nm, as indicated by the dependence of the fluorescence lifetime on the concentration of dyes in diluted solutions. In contrast to this experimental result, the fluorescence lifetimes of distant isolated molecules should be independent from the concentration according to basic theory for light emission, such as that reported by Förster and Strickler-Berg. As a consequence, the theory of such emission should be modified for real systems to include electromagnetic interactions with distant resonating structures. Consequences of these findings concern many subjects, such as imaging methods (FLIM) in biochemistry.

π-π-Induced aggregation and single-crystal fluorescence anisotropy of 5,6,10b-tri-aza-acephenanthrylene

The structural origin of absorption and fluorescence anisotropy of the single crystal of the π-conjugated heterocyclic system 5,6,10b-tri-aza-acephenan-thrylene, TAAP, is presented in this study. X-ray analysis shows that the crystal framework in the space group P [Formula: see text] is formed by centrosymmetric dimers of face-to-face mutually oriented TAAP molecules joined by π-π non-covalent interactions. The conformation of the TAAP molecule is stabilized by intramolecular C-H⋯N(sp2), N(sp2)H⋯π(CN), and C-H⋯O(sp2) hydrogen bonds. The presence of weak π-π interactions is confirmed by quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analysis. The analysis of the optical spectra of TAAP in solution and in the solid state does not allow the specification of the aggregation type. DFT calculations for the dimer in the gas phase indicate that the lowest singlet excitation is forbidden by symmetry, suggesting H-type aggregation, even though the overall absorption spectrum is bathochromically shifted as for the J-type. The experimental determination of the permanent dipole moment of a TAAP molecule in 1,4-dioxane solution indicates the presence of the monomer form. The calculated absorption and emission spectra of the crystal in a simple approximation are consistent with the experimentally determined orientation of the absorption and emission transition dipole moments in TAAP single crystals. The electrostatic interaction between monomers with a permanent dipole moment (ca 4 D each) could result in the unusual spectroscopic JH-aggregate behaviour of the TAAP dimer.