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Two-photon activation of the p-hydroxyphenacyl (pHP) photoactivated protecting group is demonstrated for the first time using visible light at 550 nm from a pulsed laser. Broadband two-photon absorption measurements reveal a strong two-photon transition (>10 GM) near 4.5 eV that closely resembles the lowest-energy band at the same total excitation energy in the one-photon absorption spectrum of the pHP chromophore. The polarization dependence of the two-photon absorption band is consistent with excitation to the same S3 ((1)ππ*) excited state for both one- and two-photon activation. Monitoring the progress of the uncaging reaction under nonresonant excitation at 550 nm confirms a quadratic intensity dependence and that two-photon activation of the uncaging reaction is possible using visible light in the range 500-620 nm. Deprotonation of the pHP chromophore under mildly basic conditions shifts the absorption band to lower energy (3.8 eV) in both the one- and two-photon absorption spectra, suggesting that two-photon activation of the pHP chromophore may be possible using light in the range 550-720 nm. The results of these measurements open the possibility of spatially and temporally selective release of biologically active compounds from the pHP protecting group using visible light from a pulsed laser.

The photoisomerization dynamics of trans-stilbene have been well studied in the lowest excited state, but much less is known about the behavior following excitation to higher-lying electronically excited states. This contribution reports a combined study of the spectroscopy and dynamics of two-photon accessible states above S1. Two-photon absorption (2PA) measurements using a broadband pump-probe technique reveal distinct bands near 5.1 and 6.4 eV. The 2PA bands have absolute cross sections of 40 ± 16 and 270 ± 110 GM, respectively, and a pump-probe polarization dependence that suggests both of the transitions access Ag-symmetry excited states. Separate transient absorption measurements probe the excited-state dynamics following two-photon excitation into each of the bands using intense pulses of 475 and 380 nm light, respectively. The initially excited states rapidly relax via internal conversion, leading to the formation of an S1 excited-state absorption band that is centered near 585 nm and evolves on a time scale of 1-2 ps due to intramolecular vibrational relaxation. The subsequent evolution of the S1 excited-state absorption is identical to the behavior following direct one-photon excitation of the lowest excited state at 4.0 eV. The complementary spectroscopy and dynamics measurements provide new benchmarks for computational studies of the electronic structure and dynamics of this model system on excited states above S1. Probing the dynamics of molecules in their higher-lying excited states is an important frontier in chemical reaction dynamics.