Controlling the Excited-State Reaction Dynamics of a Photochromic Molecular Switch with Sequential Two-Photon Excitation

Abstract

Sequential two-photon excitation increases the cycloreversion yield of a diarylethene-type photochromic molecular switch compared with one-photon excitation. This letter shows for the first time that an optimal delay of ∼5 ps between primary and secondary excitation events gives the largest enhancement of the ring-closing reaction. Pump-probe (PP) and pump-repump-probe (PReP) measurements also provide detailed new information about the excited-state dynamics. The initially excited molecule must first cross a barrier on the excited-state potential energy surface before secondary excitation enhances the reaction. The PReP experiments demonstrate that the reaction path of a photochromic molecular switch can be selectively controlled through judicious use of time-delayed femtosecond laser pulses.

Cycloreversion Dynamics of a Photochromic Molecular Switch via One-Photon and Sequential Two-Photon Excitation

“…On a slightly longer time scale, however, the PP measurements reveal an ∼3 ps evolution of the excited-state absorption spectrum that we attribute to an activated barrier crossing along the C−C bond-breaking coordinate based on comparison with theory. 10,29 The excited-state absorption spectrum changes on an ∼3 ps time scale due to changing Franck−Condon overlap with the higher excited states as the molecule crosses the S 1 barrier. 10 Our previous one-color PReP measurements (500 nm pump−500 nm repump) clearly resolved the C−C stretching motion, because the cycloreversion yield only increased for molecules that were re-excited after passing over the barrier.…”

“…We recently confirmed this hypothesis and showed that two time-delayed femtosecond laser pulses provide a sensitive means of controlling the cycloreversion reaction. 10 Our pump−repump−probe (PReP) measurements also revealed additional insight on the fundamental excited-state dynamics of DMPT-PFCP by mapping the motion along the S 1 reaction coordinate onto the higher excited states of the molecule.…”

Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators

“…The excited wavepacket moves away from the Franck−Condon region along the 1B surface and falls from the 1B to 2A surface via a conical intersection (1B/2A CI (c)) in ∼100 fs. 32 The molecules excited with light of shorter wavelengths having vibrational excess energy on the 2A surface go over the energy barrier 2ATS more easily and reach 2A/1A conical intersection (2A/1A CI(o)), where fast deactivation to the ground state takes place and thus the openring isomers are formed. The above route suggests that thermal activation or excess kinetic energy on the 2A surface is required for the cycloreversion reaction to take place.…”

“…Table 1 summarizes photocyclization and cycloreversion quantum yields in n-hexane together with absorption maximum wavelengths and coefficients of typical diarylethenes. 32 solution. The conrotatory cyclization can proceed only from the antiparallel conformation.…”

Inverse versus Normal Dithienylethenes: Computational Investigation of the Photocyclization Reaction

“…Recent efforts have been devoted to the conception of highly efficient and sensitive photoswitches, and different strategies have been proposed such as the construction of multiphotochromic molecules 7 or the control of the photochromic properties with sequential multiphoton excitation. 8 Regarding the photochromic efficiency, two DTE categories can be distinguished, bis(3-thienyl)ethenes 9,10 or normal (N)type DTE, and bis(2-thienyl)ethenes 11 or inverse (I)-type DTE (see Scheme 1). Concerning the cycloreversion reaction, the quantum yield (QY) of N-DTE is smaller than its I-type counterpart, that is, closed I-DTE are easier to ring-open.…”