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The authors model the strong-field, intensity-dependent electron dynamics of single thiophene (1T) and quaterthiophene (4T) using real-time time-dependent density functional theory paired with determinant-overlap population analysis. This ab initio approach resolves long-standing questions about the non-equilibrium population transfers that govern optical limiting and high-harmonic generation in conjugated polymers. The simulations reveal that extended $\pi$-conjugation in 4T slashes nonlinear excitation thresholds by orders of magnitude compared to 1T, driving complete ground-state depletion and sequential excited-state absorption across the $\pi^*$-manifold.
Extending $\pi$-conjugation from a single thiophene ring to quaterthiophene slashes nonlinear optical excitation thresholds by orders of magnitude, triggering total ground-state depletion via sequential excited-state absorption.
The interaction between conjugated molecules and intense electric field pulses drives a plethora of intriguing nonlinear phenomena, including optical limiting. While excited-state absorption was recently identified as the primary mechanism for this effect in thiophene oligomers, open questions remain regarding field-dependent population dynamics under intense laser driving. Using real-time time-dependent density functional theory combined with a determinant-overlap population framework, we track the intensity-dependent nonlinear response of a single thiophene ring (1T) and quaterthiophene (4T) as prototypical members of the oligothiophene family with different length and symmetry. We demonstrate that extended $\pi$-conjugation in 4T lowers nonlinear excitation thresholds by orders of magnitude compared to 1T. State-resolved population dynamics reveal complete ground-state depletion accompanied by sequential excited-state absorption channels under intense driving. High-harmonic generation spectra explicitly reflect molecular symmetry constraints, while energy-resolved occupation densities confirm continuous orbital redistribution across the $\pi^*$-manifold. This work provides dynamic physical insights for engineering organic materials with tailored nonlinear optical properties.