Excited-State Dynamics and Two-Photon Absorption of Regioisomeric Benzothiadiazole and Isobenzothiadiazole Derived Dyes with Dipolar and Quadrupolar Architectures

Abstract We investigate the excited-state dynamics and two-photon absorption (TPA) properties of two regioisomeric triphenylamine end-capped quadrupolar dyes, Qbtd and Qibtd, with a D-π-A-π-D setup and compare them with their dipolar counterparts, Dbtd and Dibtd. These S,N-heteroaromatic derivatives comprise either the widely used benzo[2,1,3]thiadiazole core (benzothiadiazole, BTD) or its regioisomer─benzo[1,2,3]thiadiazole (isobenzothiadiazole, iBTD). Femtosecond fluorescence upconversion and transient absorption spectroscopy revealed faster and more efficient spectral relaxation for the BTD-derived dyes compared to their iBTD congeners. This feature, together with red-shifted absorption and emission maxima and more pronounced positive solvatochromism, indicates a larger excited-state dipole moment and greater efficiency of the excited-state symmetry-breaking in the quadrupolar BTD-derived dye than in the iBTD analogue. Interestingly, two-photon excitation spectroscopy revealed that, despite the stronger electron-acceptor strength of the BTD unit relative to iBTD, larger TPA cross sections in the near-infrared region are observed for Qibtd than for Qbtd (the more so in polar tetrahydrofuran than in apolar toluene), reaching values up to ca. 1500 GM for Qibtd in THF. Both quadrupolar molecules outperform their dipolar analogues by an order of magnitude. The experimental results are rationalized by quantum chemical calculations, which account for the weaker electron-accepting efficiency of the iBTD core, while highlighting its superior TPA properties when arylamine donors are linked to the C-4 and C-7 positions.