A Tethered Tolane: Twisting the Excited State

Abstract

The synthesis of a doubly bridged tolane is reported. The target is obtained in a five-step synthesis, starting from commercially available 2-amino-meta-xylene by a combination of a Sandmeyer reaction, radical bromination, and Stille-type coupling, followed by double ring closing. The doubly tethered tolane is crystalline; the two phenyl rings are highly twisted with respect to each other both in solution and in the solid state. Optical spectroscopy and quantum chemical calculations show that the doubly bridged tolane is twisted not only in the ground state, but also in the excited state, leading to emission from the twisted state in solution and in the solid state. Strong phosphorescence is observed at cryogenic temperatures.

Quantification of the Helicality of Helical Molecular Orbitals

“…There is almost free rotation for the end-groups of polyynes with the ground-state orientation determined by the type of end-groups. 40 − 44 The ground state of odd [ n ]cumulenes is the coplanar geometry, and there are considerable barriers to torsion. 27 , 45 − 49 Because the ground state for both molecules converges to coplanar end-groups, all MOs for structures with a dihedral angle >0° are single-point calculations where the dihedral angle has been set manually with the remaining internal degrees of freedom frozen.…”

A Doubly Bridged Bis(phenylethynyl)benzene: Different from a Twisted Tolan

“…Figure 3 shows the unit cell of 1 containing two independent half‐molecules. The twist angles (i.e., the angles spanned by the averaged phenyl ring plane normal vectors) amount to 40° and 44°, respectively, and are thus lower than the angles observed in A and B1 , where they are 79° and 84°, respectively [5c,d] . The interaction between the two benzene rings is quantified as overlap of the adjacent atomic p orbitals; it is proportional to (cos α) 2 with values of 0.59 at α=40°, and 0.52 at 44° twist, while at 79° twist the overlap is only 0.04.…”

“…Planar tolans absorb at around 300 nm, while twisted derivatives absorb at shorter wavelengths (around 280 nm). In contrast to planar ones, twisted tolans (Figure 1, A, B1 ) [5c,d] display strong phosphorescence with long lifetimes at low temperatures (e.g. τ =4 s for A ).…”

Fluorinated tolane-based fluorophores bearing a branched flexible unit with aggregation-induced emission enhancement characteristics

“…8 However, it has been reported that tolanes are not emissive at room temperature because the excitons generated after excitation immediately undergo internal conversion from fluorescent pp* to dark ps* excited states with a trans-bend molecular shape. [9][10][11] With a great effort to evolve tolanes into effective luminescent materials within the last decade, several reports on tolane-based fluorophores have been published thus far involving (a) fluorescence molecules in crystalline states, 12 (b) dual emission through intramolecular charge transfer (ICT) processes of donor-p-acceptor-type tolanes having an electron-withdrawing carborane unit, 13 (c) phosphorescent molecules in organic glasses, 14,15 and (d) dualstate fluorescence molecules in both solution and powder states (Fig. 1).…”