Chalcogenopyrylium
monomethine (CGPM) dyes represent a class of
environmentally activated singlet oxygen generators with applications
in photodynamic therapy (PDT) and photoassisted chemotherapy (PACT).
Upon binding to genomic material, the dyes are presumed to rigidify,
allowing for intersystem crossing to outcompete excited state deactivation
by internal conversion. This results in large triplet yields and hence
large singlet oxygen yields. To understand the nature of the internal
conversion process that controls the activity of the dyes, femtosecond
transient absorption experiments were performed on a series of S-,
Se-, and Te-substituted CGPM dyes. For S- and Se-substituted species
in methanol, rapid internal conversion from the singlet excited state,
S1, occurs in ∼5 ps, deactivating the optically
active excited state. The internal conversion produces a distorted
ground-state species that returns to its equilibrium structure in
∼20 ps. For Te-substituted species, the internal conversion
competes with rapid intersystem crossing to the lowest triplet state,
T1, which occurs with a ∼ 100 ps time constant in
methanol. In more viscous methanol/glycerol mixtures, the internal
conversion to the ground state slows by 2 orders of magnitude, occurring
in 500–600 ps. For Se- and Te-substituted species in viscous
environments, the slower internal conversion rate allows a larger
triplet yield. Using femtosecond stimulated Raman spectroscopy (FSRS)
and time-dependent density functional theory (TD-DFT), the internal
conversion is determined to occur by twisting of the pyrylium rings
about the monomethine bridge. Evolution from the distorted ground
state occurs by twisting back to the S0 equilibrium structure.
The environmentally dependent photoactivity of CGPM dyes is discussed
in the context of PDT and PACT applications.