Long-living optical gain induced by solvent viscosity in a push–pull molecule

Abstract

The combination of continuum and ultrafast pump-probe spectroscopy with DFT and TDDFT calculations, in viscous and non-viscous environments, is effective in unraveling important features of the twisted intramolecular charge transfer mechanism in a new push-pull molecule that possesses aggregation induced emission properties. Long-living optical gain is found when this mechanism is inhibited, highlighting the importance of the environment rigidity in the design of materials for photonic applications.

Structure–activity relationship for the solid state emission of a new family of “push–pull” π-extended chromophores

“…A distinctive feature of these molecular structures is their twisted conformation due to the steric hindrance both between the CN and the dimethylamino-phenyl substituents and between the phenyl rings. As previously evidenced, 12 three geometrical factors can act in a concerted way to reduce such hindrance, that is, the (N)C-C]C-C(Ph) torsion angle, the reciprocal tilting of the phenyl rings (quantied through the dihedral angle between the least-squares planes through the phenyl carbon atoms) and the central double bond, which in the present structures is signicantly elongated with respect to the value of 1.331(9)Å reported for (C 2 )-C]C-(C 2 ) unconjugated bonds, 24 denoting a high degree of conjugation. It is to be pointed out that, owing to their cross-conjugated architecture, 25 the phenyl rings, connected with each other via two single bonds, are separately conjugated to each CN group, as well as the CN groups being separately conjugated to each phenyl ring.…”

“…Crystal structures of the series 1, 10a and compounds 2b 12 and 5a 13 were previously reported by us, together with a thorough discussion, supported with DFT and TDDFT calculations, [11][12][13] of the structure-optical property relationships governing their emissive behaviour. It was found that their AIE properties were strictly related to their crystal structure, which is able to activate a RIR process.…”

“…Among the other compounds, 3b and 5a display quite interesting features with solvatochromism in solution, good solid state QYs and the presence of different components in the solid state emission, already studied in detail in the case of compound 5a. 12 The optical absorption, PL excitation proles (PLE) and emission spectra of 3b in acetonitrile solution and as powders are reported in Fig. 4.…”

“…11 Pump-probe ultrafast dynamics have also been employed to study the time evolution of the excited states of compound 2b, whose long-living optical gain was detected only when the twisted intramolecular charge transfer mechanism is inhibited on increasing the solvent viscosity. 12 Here we report ultrafast pump-probe measurements on two solutions of compound 5a displaying different viscosities, namely polyethylene glycol (PEG) and acetonitrile (ACN). In this way it is possible to temporally resolve the spectral evolution of the excited states created aer the pump excitation of the molecule dispersed in viscous and non-viscous solvents.…”

“…1) was effective in unravelling important features of the twisted intramolecular charge transfer mechanism, a peculiar characteristic of such "push-pull" structures. 12 The potential of "push-pull" molecular scaffolds for the design of innovative AIE luminogens is huge, since the molecular optical properties (absorption and emission l max ), and consequently the solid state emission, can in principle be tuned using "push" and "pull" substituents with variable electron-donating and electron-withdrawing characters. In fact, we have recently reported that one of these compounds (5a in Fig.…”