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In Search of the Perfect Photocage: Structure–Reactivity Relationships in <i>meso</i>-Methyl BODIPY Photoremovable Protecting Groups
A detailed investigation of the photophysical
parameters and photochemical
reactivity of <i>meso</i>-methyl BODIPY photoremovable protecting
groups was accomplished through systematic variation of the leaving
group (LG) and core substituents as well as substitutions at boron.
Efficiencies of the LG release were evaluated using both steady-state
and transient absorption spectroscopies as well as computational analyses
to identify the optimal structural features. We find that the quantum
yields for photorelease with this photocage are highly sensitive to
substituent effects. In particular, we find that the quantum yields
of photorelease are improved with derivatives with higher intersystem
crossing quantum yields, which can be promoted by core heavy atoms.
Moreover, release quantum yields are dramatically improved by boron
alkylation, whereas alkylation in the <i>meso</i>-methyl
position has no effect. Better LGs are released considerably more
efficiently than poorer LGs. We find that these substituent effects
are additive, for example, a 2,6-diiodo-<i>B</i>-dimethyl
BODIPY photocage features quantum yields of 28% for the mediocre LG
acetate and a 95% quantum yield of release for chloride. The high
chemical and quantum yields combined with the outstanding absorption
properties of BODIPY dyes lead to photocages with uncaging cross sections
over 10 000 M<sup>–1</sup> cm<sup>–1</sup>, values
that surpass cross sections of related photocages absorbing visible
light. These new photocages, which absorb strongly near the second
harmonic of an Nd:YAG laser (532 nm), hold promise for manipulating
and interrogating biological and material systems with the high spatiotemporal
control provided by pulsed laser irradiation, while avoiding the phototoxicity
problems encountered with many UV-absorbing photocages. More generally,
the insights gained from this structure–reactivity relationship
may aid in the development of new highly efficient photoreactions