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 λ > 500 nm. Photocyclization takes place both from the excited singlet state (S1), as an ultrafast (ca. 0.5 ps) process, and from the triplet state (T1) 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 complete 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 (S0) and lowest triplet state (T1) of the diarylethene are investigated along the reaction coordinate for photocyclization/cycloreversion. At the DFT level of theory, the transition-state structures on S0 and T1 are similar and lean, along the reaction coordinate, toward the closedring form. At the transition-state geometry, the S0 and T1 PES are almost degenerate. Whereas on S0 a large barrier (ca. 45 kcal mol-1) separates the open- and closed-ring minima, on T1 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 T1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S0 at closedring 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 S0 and T1 PES obtained for DAE. Altogether, the results provide strong experimental evidence and theoretical rationale for the triplet pathway in the photocyclization of photochromic diarylethenes.