The Impact of C2 Insertion into a Carbazole Donor on the Physicochemical Properties of Dibenzo[a,j]phenazine-Cored Donor–Acceptor–Donor Triads

Novel electron donor–acceptor–donor (D-A-D) compounds comprising dibenzo[a,j]phenazine as the central acceptor core and two 7-membered diarylamines (iminodibenzyl and iminostilbene) as the donors have been designed and synthesized. Investigation of their physicochemical properties revealed the impact of C₂ insertion into well-known carbazole electron donors on the properties of previously reported twisted dibenzo[a,j]phenazine-core D-A-D triads. Slight structural modification caused a drastic change in conformational preference, allowing unique photophysical behavior of dual emission derived from room-temperature phosphorescence and triplet–triplet annihilation. Furthermore, electrochemical analysis suggested sigma-dimer formation and electrochemical polymerization on the electrode. Quantum chemical calculations also rationalized the experimental results.     

1-Azathioxanthone Appended Lanthanide(III) DO3A Complexes That Luminesce Following Excitation at 405 nm**

Since the pioneering report by Selvin, we have been fascinated by the potential of using lanthanide luminescence in bioimaging. The uniquely narrow emission lines and long luminescence lifetimes both provide the potential for background free images together with full certainty of probe localization. General use of lanthanide based bioimaging was first challenged by low brightness, and later by the need of UV (<405 nm) excitation sources not present in commercial microscopes. Here, we designed three lanthanide-based imaging probes based on a known motif to investigate the limitations of 405 nm excitation. These were synthesized, characterized, investigated on dedicated as well as commercial microscopes, and the photophysics was explored in detail. It was proven without doubt that the lanthanide complexes enter the cells and luminesce internally. Even so, no lanthanide luminescence were recovered on the commercial microscopes. Thus, we returned to the photophysical properties that afforded the conclusion that – despite the advances in light sources and photodetectors – we need new designs that can give us brighter lanthanide complexes before bioimaging with lanthanide luminescence becomes something that is readily done.     

Exciplex Emission from a Boron Dipyrromethene (Bodipy) Dye Equipped with a Dicyanovinyl Appendage

The photophysical properties of a prototypic donor–acceptor dyad, featuring a conventional boron dipyrromethene (Bodipy) dye linked to a dicyanovinyl unit through a meso‐phenylene ring, have been recorded in weakly polar solvents. The absorption spectrum remains unperturbed relative to that of the parent Bodipy dye but the fluorescence is extensively quenched. At room temperature, the emission spectrum comprises roughly equal contributions from the regular π, π* excited‐singlet state and from an exciplex formed by partial charge transfer from Bodipy to the dicyanovinyl residue. This mixture moves progressively in favor of the locally excited π, π* state on cooling and the exciplex is no longer seen in frozen media; the overall emission quantum yield changes dramatically near the freezing point of the solvent. The exciplex, which has a lifetime of approximately 1 ns at room temperature, can also be seen by transient absorption spectroscopy, in which it decays to form the locally excited triplet state. Under applied pressure (P<170 MPa), formation of the exciplex is somewhat hindered by restricted rotation around the semirigid linkage and again the emission profile shifts in favor of the π, π* excited state. At higher pressure (170<P<550 MPa), the molecule undergoes reversible distortion that has a small effect on the yield of π, π* emission but severely quenches exciplex fluorescence. In the limiting case, this high‐pressure effect decreases the molar volume of the solute by approximately 25 cm³ and opens a new channel for nonradiative deactivation of the excited‐state manifold.     

Solvent‐Driven Conformational Exchange for Amide‐Linked Bichromophoric BODIPY Derivatives

The fluorescence lifetime and quantum yield are seen to depend in an unexpected manner on the nature of the solvent for a pair of tripartite molecules composed of two identical boron dipyrromethene (BODIPY) residues attached to a 1,10‐phenanthroline core. A key feature of these molecular architectures concerns the presence of an amide linkage that connects the BODIPY dye to the heterocyclic platform. The secondary amide derivative is more sensitive to environmental change than is the corresponding tertiary amide. In general, increasing solvent polarity, as measured by the static dielectric constant, above a critical threshold tends to reduce fluorescence but certain hydrogen bond accepting solvents exhibit anomolous behaviour. Fluorescence quenching is believed to arise from light‐induced charge transfer between the two BODIPY dyes, but thermodynamic arguments alone do not explain the experimental findings. Molecular modelling is used to argue that the conformation changes in strongly polar media in such a way as to facilitate improved rates of light‐induced charge transfer. These solvent‐induced changes, however, differ remarkably for the two types of amide.     

Charge‐Recombination Fluorescence from Push–Pull Electronic Systems Constructed around Amino‐Substituted Styryl–BODIPY Dyes

A small series of donor–acceptor molecular dyads has been synthesized and fully characterized. In each case, the acceptor is a dicyanovinyl unit and the donor is a boron dipyrromethene (BODIPY) dye equipped with a single styryl arm bearing a terminal amino group. In the absence of the acceptor, the BODIPY‐based dyes are strongly fluorescent in the far‐red region and the relaxed excited‐singlet states possess significant charge‐transfer character. As such, the emission maxima depend on both the solvent polarity and temperature. With the corresponding push–pull molecules, there is a low‐energy charge‐transfer state that can be observed by both absorption and emission spectroscopy. Here, charge‐recombination fluorescence is weak and decays over a few hundred picoseconds or so to recover the ground state. Overall, these results permit evaluation of the factors affecting the probability of charge‐recombination fluorescence in push–pull dyes. The photophysical studies are supported by cyclic voltammetry and DFT calculations.