Chiral Pathways and Periodic Decay in cis-Azobenzene Photodynamics

Abstract

Azobenzenes are candidates for efficient, photochemically triggered switching in devices of molecular size. The cis-azobenzene isomer is inherently chiral because of its helicity. Applying OM2/MRCI surface-hopping molecular dynamics simulations, we analyze chiral photoisomerization pathways in cis-azobenzene and correlate oscillatory features in the population decay to modes that trigger motion toward and from the S1/S0 crossing region.

A multireference perturbation study of the NN stretching frequency of trans-azobenzene in nπ* excitation and an implication for the photoisomerization mechanism

“…In our surface hopping AIMD simulation 47 at the state-averaged CASSCF (SA-CASSCF) level, it was shown that cis to trans isomerization in nπ * excitation occurs via two-step rotation mechanism, accompanying rotations of the central NN part and two phenyl rings, and this process can be classified into two types with respect to the orientation of the rotation, namely, clockwise and counterclockwise rotation pathways; the calculated quantum yields and lifetime in the excited states are in very good agreement with the corresponding experimental results. 47 The similar reaction mechanism was also reported in the dynamics simulations by Doltsinis et al 45,49,50 and Thiel et al 55 As described above, the transient Raman spectra indicate that the NN stretching frequency of trans-azobenzene is almost unchanged (decreases by only 12 cm −1 ) in the S 1 (nπ * ) state. 12 There are several theoretical reports on vibrational frequencies for trans-azobenzene in the ground state at the MP2, density functional theory (DFT), and CASSCF levels, 20,21,23,25,30 while, to our knowledge, there is only one report on frequencies of trans-azobenzene in the S 1 (nπ * ) state, which employed the CASSCF method.…”

“…This finding can explain, why in dynamics simulations the isomerization occurs through a rotation of the -N=N-fragment rather than a true torsion along the N-N bond. 47,55 Finally, we summarize significant results from the present calculations. First, we should mention that we employ the CASPT2 method in determinations of geometrical structures, vibrational frequencies, and reaction pathways.…”

“…After Ishikawa's report, 24 all theoretical studies at the CASSCF, multireference complete-active-space second-order perturbation theory (CASPT2), second-order approximated coupledcluster model with the resolution-of-the-identity approximation (RI-CC2), and time-dependent density functional theory (TDDFT) levels indicate that the rotation is the preferred pathway for nπ * excitation in the gas phase. 27,28,30,31,42,58 On-the-fly dynamics simulations were also performed for the photoisomerization of azobenzene on the basis of semiempirical molecular orbital calculations with the surface hopping method 29,33,55,59,61 and with the multiple spawning method. 32 Recently, ab initio molecular dynamics (AIMD) simulations at the CASSCF level 26,47,54,57 and Car-Parrinello molecular dynamics simulations 34,45,49,50 were also performed for the photoisomerization of azobenzene in nπ * excitation.…”

Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited‐State Relaxation

“…As shown in the Supporting Information Figure S4, the S 1 path from M-Z11(S 0 )t oM -CI(S 1 S 0 )i sb arrierless,i mplying that the S 1 decay should be efficient and ultrafast. Thep otential energy surfaces thus have the same topology as in the case of cis azobenzene; [11,17,22] however, the dynamical roles of the two enantiomeric S 1 /S 0 conical intersections in the 1 np*e xcitedstate decay are distinct in azobenzenes and arylazopyrazoles (see below).…”

Generalized trajectory surface-hopping method for internal conversion and intersystem crossing

“…[4][5][6][7][8][9][10][11][12][13][14][15][16] It has been applied to study many ultrafast photophysical and photochemical processes in the gas phase and the condensed phase. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Despite the growing popularity and success of the trajectory-based FSSH method, it remains approximate and has some well-known drawbacks. [34][35][36][37] One major limitation is that the original FSSH method has been formulated with a focus on nonadiabatic internal conversion processes, disregarding intersystem crossing events that are often encountered in photochemistry and may even occur on an ultrafast timescale in systems with non-negligible spin-orbit couplings.…”