Finally, we investigated the origin of the narrowband emission of 1 via a comparison of the 1b emission spectra with its aldehyde Figure . Chemical structures, p, estimated kp, and steady-state photoluminescence spectra of 1b, 2b, and 3b in cyclohexane (4.4 × 10 -6 M, 5.0 × 10 -4 M, and 5.0 × 10 -6 M, respectively) excited at 368 nm under Ar, and corresponding full-width-at-half-maxima (FWHM). kp of 2b and 3b were derived as kp = p/p, assuming ISC = 1. R = triisopropylsilyl. Table . Calculated Factors of kp for 1a-3a a a Calculated at the TDA/uCAM-B3LYP-D3/6-311G(d) level of theory. Geometries are optimized at the T1 state. counterpart, 2b (Figure ). 1b exhibited narrowband emission, while 2b had broadband emission with a FWHM of 106 nm. The bandwidths could be strongly correlated with molecular geometry changes in the ground and excited states, which are quantified using the Huang-Rhys (HR) factor obtained from a Franck-Condon analysis of the spectra (Figure ,b and Table ). The HR factor of 1b was 0.55, while that of 2b was 2.54. The smaller HR factor in 1b corresponded to smaller geometry changes in the T1-to-S0 transition. The striking difference in the HR factors was reflected in bondlength differences in DFT calculations of the geometry changes in the T1-to-S0 transition (Figure ). Thus, the elongation ratios between the S0 and T1-relaxed geometries were large in 2a (e.g., the bond length between 3C and 4C changed by 8.4%), while the geometry of the thiophene ring in 1a had almost no changes. The large geometry change in 2a was attributed to the 3 (,*) character of the T1 state; i.e., bonding to antibonding electronic transitions, as evident in the NTOs (Figure ). In contrast, the NTOs of 1a were mainly localized on the 1,2-dicarbonyl moiety, and the electronic transition was nonbonding to antibonding (Figure ). Moreover, the transition had no net charge-transfer character because of the centrosymmetric geometry. This was in contrast to most aromatic (mono)carbonyl compounds, where 3 (n,*) states have a chargetransfer character from the carbonyl to the aromatic ring. These features minimized molecular geometry changes during phosphorescence emission. Therefore, the 1,2-diketone-based phosphor would be a promising platform for efficient narrowband RTP.