Low-Temperature Luminescence in Organic Helicenes: Singlet versus Triplet State Circularly Polarized Emission

Abstract

The experimental measurement of the photophysical and chiroptical properties of helicene-based π-conjugated emitters with electron-accepting (−CN, −py, −NO2) or donating (TMS, NMe2, NH2) moieties is reported at low temperature (77 K). The samples exhibit strong circularly polarized phosphorescence in frozen solution of 2-MeTHF, with a luminescence dissymmetry factor reaching 1.6 × 10–2 and a lifetime of over 0.46 s for the most active molecule, the nitro compound. The theoretical investigation shows that although the singlet (S 1) and triplet (T 1) excited-state emissions mainly arise from the helicene core, the rotatory strengths of the spin-allowed versus spin-forbidden emission have opposite signs. Further analysis of the spin–orbit coupling matrix elements shows that there is no strong mixing between S 1 and T 1, justifying the different signs of the rotatory strengths. In the case of the nitro compound, the enhanced phosphorescence emission is due to an efficient intersystem crossing.

Enantiopure cycloplatinated pentahelicenic N-heterocyclic carbenic complexes that display long-lived circularly polarized phosphorescence

“…The CPL responses were then also recorded in a glass of 2-MeTHF at 77 K (Figure 4c). 21 To our delight, the (P) and (M) enantiomers of both systems revealed mirror-image signals in the same region as the non-polarized fluorescence. In this case, the measured g lum values were 1.3 × 10 -2 for Pt1 and 6.2 × 10 -3 for Pt2, the former being one of the highest values reported up to now for Pt-helicene derivatives.…”

“…Of importance, might be also the aforementioned mixing of different (higher-energy) electronic states into the emitting excited-state. 21,22 Further computational studies, including spin–orbit coupling effects and possibly higher-order correlated methods, are needed to provide a detailed rationalization of all the experimental results for Pt1 and Pt2 , 20 c ,21,23 which we plan to report for these and other related systems elsewhere in the future.…”

“…This very commonly leads to rapid variations in the g lum value across the emission band. 21,22 The electronic states that get mixed into the lowest excited state each bring their own electric and magnetic transition dipole moments.…”

“…Of importance, might be also the aforementioned mixing of different (higher-energy) electronic states into the emitting excited-state. 21,22 Further computational studies, including spin-orbit coupling effects and possibly higher-order correlated methods, are needed to provide a detailed rationalization of all the experimental results for Pt1 and Pt2, 20c,21,23 which we plan to report for these and other related systems elsewhere in the future. Due to the long-lived phosphorescence of these helicene-NHC-based platinum(II) complexes, and hence the potential for triplet-triplet quenching, the solid-state luminescence properties of compounds Pt1 and Pt2 were better studied in amorphous poly(methyl methacrylate) (PMMA) films at room temperature with 2 wt% emitter concentration under inert conditions.…”

Sparkling Organic Phosphorescence from Fluorinated Tetrathia[7]helicenes: Synthesis and Photophysical, Electrochemical and Computational Studies

“…Unfortunately, there is not such an extended literature data concerning these studies on helical systems. According to some literature references, [45] the shape of the T 1 emission is the result of vibration and breathing of the helical motif in the triplet state (i. e., changes in the helical pitch). In fact, the millisecond phosphorescence decay regime would allow helix breathing modes which, by geometry rearrangements (see Huang-Rhys parameter below), broaden the vibronic satellites.…”