Synthesis and Optical Characterization of Hydrazone-Substituted Push-Pull-Type NLOphores

Abstract

Two distinct families of NLOphores featuring hydrazone donors were synthesized using click-type [2 + 2] cycloaddition retroelectrocyclizations (CA-RE). Despite the limitations in the substrate scope, it was shown for the first time that hydrazone-activated alkynes could undergo reactions with TCNE/TCNQ. The electrochemical, photophysical, and second-order nonlinear optical (NLO) characteristics of the chromophores were analyzed utilizing experimental and computational approaches. Chromophores 17-21 and 23-27 exhibited two reduction waves, along with one oxidation wave that can be attributed to the hydrazone moiety. All chromophores exhibit charge-transfer bands extending from the visible to the near-infrared region. The λ_max of hydrazone-based chromophores falls within the range of 473 to 725 nm. Additionally, all chromophores exhibited positive solvatochromism. Computational studies have been performed to elucidate the origin of the low-energy absorption bands. Parameters such as dipole moment, band gaps, electronegativity, global chemical hardness/softness, average polarizability, and first hyperpolarizability were calculated to obtain information about NLO properties of the target structures. The thermal stabilities of the NLOphores were assessed through TGA. Experimental NLO measurements were conducted using the electric field-induced second harmonic generation (EFISHG) technique. The studied structures demonstrated NLO responses, with μβ values between 520 × 10^-48 esu and 5300 × 10^-48 esu.

Scheme 1. Synthesis of Hydrazone-Substituted Terminal Alkyne 7

Scheme 2. Synthesis of Disubstituted Alkynes 12–15 via Sonogashira Cross-Coupling Reactions

Scheme 3. CA-RE Reactions between Hydrazone-Substituted Alkynes 7, 12–15 and TCNE

Scheme 4. CA-RE Reactions between Hydrazone-Substituted Alkynes 7, 12–15 and TCNQ

Figure 1

ORTEP representation of 24 with vibrational ellipsoids shown at the 50% probability level, arbitrary numbering. H atoms and solvent molecules are omitted for clarity.

Figure 2

UV/vis spectra of NLOphores 17–21 in CH₂Cl₂ (2 × 10^–5 M) at 25 °C.

Figure 3

UV/vis spectra of NLOphores 23–27 in CH₂Cl₂ (2 × 10^–5 M) at 25 °C.

Figure 4

UV/vis spectra of the representative NLOphores (a) 18 and (b) 24 in CH₂Cl₂/n-hexane mixtures at 25 °C.

Figure 5

TGA curves for (a) 18 and (b) 24.

Figure 6

Cyclic voltammograms of the representative NLOphores 21 and 27 at a scan rate of 100 mVs^–1 in DCM + 0.1 M Bu₄NPF₆. All potentials are indicated versus ferrocene/ferrocenium redox couple used as an internal reference (given as IUPAC convention).

Figure 7

Energy level diagram of the frontier orbitals (HOMOs and LUMOs) of NLOphores 17–21 and 23–27.

Figure 8

Calculated (red line) and experimental (blue line) UV/vis spectra of (a) 18 and (b) 24 [TD-DFT:CAM-B3LYP/6-31G++(d,p) in DCM].