Allenes as Stimuli-Responsive Chromophores for Visible to Near-Infrared Optical Modulation

Despite their rigid, orthogonal π-systems and synthetic modularity, allenes remain underexplored in functional material design. Here, we report that tetraaryl-substituted allenes undergo clean and reversible protonation at the central sp-hybridized carbon, triggering substantial π-electron reorganization. To investigate how substitution patterns modulate the halochromic response, we synthesized a series of five tetraarylallenes, enabling a systematic structure-property relationship study of their optoelectronic behavior. Protonation induces large bathochromic shifts in absorption of up to 500 nm, with extinction coefficients exceeding 30,000 M^-¹ cm^-¹ in the near-infrared region. Frontier molecular orbital (FMO) analysis and time-dependent density functional theory (TD-DFT) calculations reveal pronounced delocalization upon protonation, resulting in polymethine-like electronic structures with tunable donor-acceptor character. Protonation occurs even with weak acids such as acetic acid, and the process remains fully reversible through base-mediated deprotonation. UV-vis-NIR spectroscopy and ultrafast transient absorption spectroscopy confirm the formation of charge-transfer states. These findings establish allenes as a synthetically accessible and tunable platform for reversible, stimuli-responsive optical modulation across the visible to NIR spectrum.