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Pure 2D lead‐iodide perovskites typically demonstrate poor charge transport and compromised visible light absorption, relative to their 3D congeners. This hinders their potential use as solar absorbers. Herein, the systematic tuning of pyridinium‐based templating cations is reported to introduce intermolecular interactions that provide access to a series of new 2D lead‐iodide perovskites with reduced inter‐octahedral distortions (largest Pb‐(μ‐I)‐Pb bond angles of 170–179°) and very short inorganic interlayer separations (shortest I⋅⋅⋅I contacts ≤4.278–4.447 Å). These features manifest in reduced band gaps (2.35–2.46 eV) and relaxed dielectric confinement (excitonic binding energies of 130–200 meV). As a consequence, they demonstrate (more than ten‐fold) improved photo‐ and electrical conductivities relative to conventional 2D lead‐iodide perovskites, such as that templated by 2‐(1‐naphthyl)ethylammonium. Through computational studies, the origin of this behavior was shown to derive from a combination of short iodoplumbate layer separations and the aromaticity of the organic dications. [ABSTRACT FROM AUTHOR]

Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self‐assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton–photon coupling, etc. Detailed understanding into the strong light–matter interactions in these hybrid organic–inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C6H5C2H4NH3)2PbI4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton–phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin–quasiparticle interactions in these layered 2D halide perovskites with large spin–orbit coupling. The intricate coherent interplay of excitons, spins, and phonons in 2D perovskites (C6H5C2H4NH3)2PbI4 are studied using transient optical spectroscopy. New insights into the hotly debated origins of transient spectral features, exciton dynamics, and relaxation pathways; ultrafast spin relaxation; and coherent exciton–phonon coupling are revealed from the detailed phenomenological modeling of the transient dynamics. [ABSTRACT FROM AUTHOR]