Triplet pathways in diarylethene photochromism: Photophysical and computational study of dyads containing ruthenium(II) polypyridine and 1,2-bis(2-methylbenzothiophene-3-yl)maleimide units

A 1,2-bis(2-methylbenzothiophene-3-yl)maleimide model (DAE) and two dyads in which this photochromic unit is coupled, via a direct nitrogen-carbon bond (Ru-DAE) or through an intervening methylene group (Ru-CH2-DAE), to a ruthenium polypyridine chromophore have been synthesized. The photochemistry and photophysics of these systems have been thoroughly characterized in acetonitrile by a combination of stationary and time-resolved (nano- and femtosecond) spectroscopic methods. The diarylethene model DAE undergoes photocyclization by excitation at 448 nm, with 35% photoconversion at stationary state. The quantum yield increases from 0.22 to 0.33 upon deaeration. Photochemical cycloreversion (quantum yield, 0.51) can be carried out to completion upon excitation at lambda > 500 nm. Photocyclization takes place both from the excited singlet state (Si), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T-1) in the microsecond time scale. In Ru-DAE and Ru-CH2-DAE dyads, efficient photocyclization following light absorption by the ruthenium chromophore occurs with oxygen-sensitive quantum yield (0.44 and 0.22, in deaerated and aerated solution, respectively). The photoconversion efficiency is almost unitary (90%), much higher than for the photochromic DAE alone. Efficient quenching of both Ru-based MLCT phosphorescence and DAE fluorescence is observed. A complet e kinetic characterization has been obtained by ps-ns time-resolved spectroscopy. Besides prompt photocyclization (0.5 ps), fast singlet energy transfer takes place from the excited diarylethene to the Ru(II) chromophore (30 ps in Ru-DAE, 150 ps in Ru-CH2-DAE). In the Ru(II) chromophore, prompt intersystem crossing to the MLCT triplet state is followed by triplet energy transfer to the diarylethene (1.5 ns in Ru-DAE, 40 ns in Ru-CH2-DAE). The triplet state of the diarylethene moiety undergoes cyclization in a microsecond time scale. The experimental results are complemented with a combined ab initio and DFT computational study whereby the potential energy surfaces (PES) for ground state (S-0) and lowest triplet state (T-1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on So and T-1 are similar and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S-0 and T-1 PES are almost degenerate. Whereas on S-0 a large barrier (ca. 45 kcal mol(-1)) separates the open- and closed-ring minima, on T-1 the barriers to isomerization are modest, cyclization barrier (ca. 8 kcal mol(-1)) being smaller than cycloreversion barrier (ca. 14 kcal mol(-1)). These features account for the efficient sensitized photocyclization and inefficient sensitized cycloreversion observed with Ru-DAE. Triplet cyclization is viewed as a nonadiabatic process originating on T-1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S-0 at close-dring geometry. A computational study of the prototypical model 1,2-bis(3-thienyl)ethene is used to benchmark DFT results against ab initio CASSCF//CASPT2 results and to demonstrate the generality of the main topological features of the S-0 and T-1 PES obtained for DAE. Altogether, the results provide strong experimental evidence and theoretical rationale for the triplet pathway in the photocylization of photochromic diarylethenes.