The
recently developed technique of femtosecond stimulated Raman
spectroscopy, and its variant, femtosecond Raman-induced Kerr effect
spectroscopy (FRIKES), offer access to ultrafast excited-state dynamics
via structurally specific vibrational spectra. We have used FRIKES
to study the photoexcitation dynamics of nickel(II) phthalocyanine
with eight butoxy substituents, NiPc(OBu)8. NiPc(OBu)8 is reported to have a relatively long-lived ligand-to-metal
charge-transfer (LMCT) state, an essential characteristic for efficient
electron transfer in photocatalysis. Following photoexcitation, vibrational
transitions in the FRIKES spectra, assignable to phthalocyanine ring
modes, evolve on the femtosecond to picosecond time scales. Correlation
of ring core size with the frequency of the ν10 (asymmetric
C–N stretching) mode confirms the identity of the LMCT state,
which has a ∼500 ps lifetime, as well as that of a precursor
d-d excited state. An even earlier (∼0.2 ps) transient is observed
and tentatively assigned to a higher-lying Jahn–Teller-active
LMCT state. This study illustrates the power of FRIKES spectroscopy
in elucidating ultrafast molecular dynamics.