Impact of position of electron withdrawing cyano groups on nonlinear optical properties of centrosymmetric donor‐π‐acceptor system

Abstract

Two symmetrically substituted phenylenevinylene D‐A‐D′‐A‐D type siblings, (2Z,2′Z)‐2,2′‐(2,5‐dimethoxy‐1,4‐phenylene)bis(3‐(4‐(dimethylamino)phenyl)acrylonitrile) (↑‐dscn) and (2Z,2′Z)‐3,3′‐(2,5‐dimethoxy‐1,4‐phenylene)bis(2‐(4‐(dimethylamino)phenyl)acrylonitrile) (↓‐dscn), are prepared. We investigate the effect of different but symmetrical location of these cyano groups in vinylene bridges on the 1‐photon and 2‐photon absorption behaviors. We report that the closeness of CN group on the vinyl group to the central phenyl ring in ↑‐dscn induces an intramolecular geometric distortion between the central phenyl ring and vinylene group, preventing the effective π‐conjugation length in ground and excited states. Thus, the transition energy that is observed in 1‐photon absorption and fluorescence is larger in ↑‐dscn than in ↓‐dscn. This leads to a different intramolecular charge distribution, as a result of which the linear and nonlinear optical properties strongly depend on the position of acceptors. These results are theoretically unraveled in terms of charge transfer pathways, charge distribution, and charge distribution differences on transition.

Solid State Luminescence Enhancement in π-Conjugated Materials: Unraveling the Mechanism beyond the Framework of AIE/AIEE

“…The stabilization of the LUMO is stronger when the cyano-group is introduced in the β-position of DSB compared to the α-position; this results in a stronger bathochromic shift of the electronic transition for the βversus the α-series in agreement with experiment. In a comparison of the S 1 state energies for planar and nonplanar ground-state geometries by TD-DFT (see Table S3), a small steric contribution is found; however (and in contrast to what was previously suggested 91 ), the main effect is electronic in nature. For instance, the calculated energy difference ΔE vert between αand β-DBDCS for the relaxed ground-state geometries is 0.29 eV, whereas for planar geometries it is still 0.23 eV.…”

“…Qualitatively, the situation does not change by BuOsubstitution in the terminal rings (α-β-DBDCS), however, with somewhat varying twists due to slight changes in the electronic structure (i.e., frontier MO topologies, see Figure S1). The effect of MeO-substitution in the central ring on the resulting geometry is sensitively dependent on whether the αor the β-series, 91 and whether P-or S-rotamers (Scheme 1), are considered; see Table 2. In the α-series, the torsional angles are hardly changed upon MeO-substitution for the P-rotamers, i.e., α-(DB)MODCS; on the other hand, for S-rotamers, C CN ••• O MeO interactions between the CN-and MeO-groups lead to a significantly stronger twist of the inner vinyl−phenyl bonds, i.e., ϕ i ≈ 49°(it should be stressed at this point that alkoxysubstitution in the central ring does not provide substantially different geometries than alkyl-substitution as previously suggested 24 ).…”