Symmetry-Selective Franck-Condon and Herzberg-Teller Coupling Enables Ultra-Narrow Blue Phosphorescence in Pt(II) Complexes.

High color purity remains a critical limitation in deep-blue phosphorescent materials. The origin of ultranarrow emission (10-14 nm) in trans-(NHC)2Pt(C≡C-C≡C-R)2 complexes (Pt-TMS, R = TMS; Pt-Ph, R = Ph) is elucidated through a symmetry-guided vibronic coupling analysis. Direction-dependent Pt-ligand donation interaction, reinforced by the trans effect, localizes the excitation on the butadiynyl ligand (Bdiy) and confines the T1→S0 transition dipole moment to the x-axis. Under C2V symmetry constraints, vibronic coupling becomes strongly mode-selective, yielding a dominant 0-0 emission character. In Pt-TMS, σ-π isolation induces vibrational mode decoupling, preserving the low-frequency TMS rotational modes with large displacements, which generate vibrational focusing and enable Franck-Condon (FC) activity, while symmetry suppresses Herzberg-Teller (HT) contributions, minimizing vibronic broadening. In Pt-Ph, π-conjugation reduces out-of-plane phenyl vibrational displacements that are symmetry-allowed for HT coupling, leading to vibrational confinement that suppresses HT-induced spectral broadening and maintains narrowband emission. Moreover, vibrational focusing in Pt-TMS enhances both radiative and nonradiative decay rates, whereas vibrational confinement in Pt-Ph suppresses structural relaxation and internal conversion. These results identify vibrational focusing and vibrational confinement as complementary mechanisms underlying vibronic regulation in ultranarrowband phosphorescent emitters.