Nonlinear Optical Studies of Conjugated Organic Dyes for Optical Limiting Applications

Abstract

D-π-A conjugated organic dyes made of different donor/acceptor moieties namely Hexylcarbazole (HC), 4,7- Dithiophene-2-yl) benzo[c][1,2,5]thiadiazole (DTBT) and Poly-4-(5-(9-Hexyl-9H-carbazol-2-yl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (PCTB) were prepared by coupling reactions. FTIR, NMR, and mass spectrometry were used to determine the molecular structure of synthesized compounds. Both HC and DTBT show a wide optical transmittance window (400-600 nm) with absorption maxima in the UV region. However, PCTB shows an altered band structure with a broad absorption in the visible domain (448 nm). Density functional theory (DFT) calculations expose that the bandgap between HOMO and LUMO decreases as the conjugation extends, and redshift occurs in UV-Vis absorption spectra. The emission spectrum of PCBT shows strong emission maxima at 541 nm with the corresponding large Stokes shift 3837 cm−1 due to its enhanced charge transfer property in the excited state. Nonlinear optical absorption was studied by open-aperture Z-scan technique using Nd:YAG laser (532 nm, 10 Hz, 9 ns). Here HC, DTBT exhibit a genuine two-photon absorption (TPA) process while PCTB possesses sequential TPA involving excited-state absorption. All dyes exhibit energy-absorbing optical limiting behaviour ascribed due to the two-photon absorption process. Among the investigated dyes, PCTB polymer shows a higher TPA coefficient (1.22 × 10−10 m/W) and a lower onset optical limiting threshold (1.06 × 1012 W/m2). Thus, the tunability of band structure and nonlinearity with a change in the molecular arrangement was successfully demonstrated in thiophene-benzothiadiazole-based organic dyes.