Extended Data Fig. 9: Electronic structure and wavelength-resolved impulsive vibrational spectra of M₂TTM-3PCz/2PCz.

a, The transition dipole moment of the two lowest energy electronic transition of the M₂TTM-3PCz and M₂TTM-2PCz obtained from TDDFT with experimental absorption spectra. b, Frontier molecular orbitals of the M₂TTM-3PCz and M₂TTM-2PCz and the arrows indicate lowest bright charge-transfer transition. c, The wavelength resolved impulsive vibrational maps of M₂TTM-3PCz and M₂TTM-2PCz in CHCl₃. d, radiative and non-radiative rate of the isomers obtained from PLQE and photoluminescence decay rates (see Extended Data Fig. 10 for details). It is important to note that depending on attachment on 2 or 3 positions of the phenyl-carbazole (PCz), the HOMO (N-nonbonding) and HOMO-1 (carbazole-π) molecular orbitals have different extent of leakage into the phenyl ring of the M₂TTM. HOMO-1 (carbazole-π) orbitals is delocalised to the adjacent phenyl ring of the M₂TTM in M₂TTM-2PCz whereas for M₂TTM-3PCz, HOMO (N-nonbonding) shows the similar phenomenon. N-nonbonding type HOMO for M₂TTM-2PCz and π-type HOMO-1 for M₂TTM-3PCz are extremely localised on the carbazole as the corresponding molecular orbitals have weak orbital coefficient on the linking carbon atom. As the electron accepting level (SOMO) is located on the M₂TTM core, only delocalised occupied orbitals makes charge-transfer electronic transitions with non-negligible transition dipole moment which explains the placement of the arrows in the Extended Data Fig. 9b. Also due to the leakage on the adjacent phenyl ring of M₂TTM, N-nonbonding and the carbazole-π molecular orbitals has higher orbital energy in M₂TTM-3PCz and M₂TTM-2PCz respectively in comparison to alternative isomer (Extended Data Fig. 9b). That explains the smallest energy gap between two lowest energy transitions for M₂TTM-2PCz (Extended Data Fig. 9b).