Unraveling a Chemically Enhanced Photoswitch: Bridged Azobenzene

Abstract

The possibility to reversibly photoisomerize azobenzene (AB) has made it one of the most ubiquitous light-sensitive molecular switches. [1][2][3][4][5][6][7][8][9][10] Opto-and nanomechanical devices [9,10] that convert light to mechanical action, for instance, exploit the considerable stretching of AB on Z!E isomerization. In biochemical studies, AB has been integrated into synthetic peptides and foldamers to photocontrol their conformational dynamics. [6,11] Photoaddressable materials such as image-storage media are often based on AB functional units. [12,7] For all of the above applications of the AB photoswitch it is of paramount importance to ensure that the photoisomerization process [13][14][15] is fast and has a high quantum yield.Recently, a greatly enhanced E!Z quantum yield F ABÀC 2 E!Z was reported for a bridged azobenzene (AB-C 2 in Figure 1) in the S 1 state compared to the parent molecule AB. [16] This finding may seem surprising at first, as the structural changes involved in isomerization are expected to be hindered by the restriction due to the presence of a bridge interconnecting the phenyl rings. It thus seems remarkable to observe a much larger quantum yield of bridged AB-C 2 compared to AB itself. Herein we demonstrate that, counterintuitively, the bridge does not hinder photoisomerization. On the contrary, it suitably preorients the phenyl rings such that AB-C 2 can more easily undergo E!Z isomerization, so that not only an enhanced quantum yield F ABÀC 2 E!Z but also ultrashort S 1 lifetimes result.Similar to earlier work [17][18][19] we investigated the photodynamics of E-AB-C 2 in the S 1 excited state in the gas phase employing nonadiabatic [20,21] ab initio molecular dynamics (AIMD), [22] whereby "on-the-fly" nonadiabaticity is introduced by using Tullys fewest-switches surface hopping [23] to couple S 1 and S 0 . In particular, the fewest-switches method has been demonstrated [13][14][15] to describe satisfactorily, both mechanistically and quantitatively, photoisomerization of the parent compound AB, including the E!Z and Z!E quantum yields and the timescales involved. Here, all calculations were carried out with the CPMD package [24] by employing a cubic 18 periodic box, the PBE functional, and plane waves with dual-space pseudopotentials. [22] A total of 30 nonadiabatic simulations of at least 500 fs were performed microcanonically in the Born-Oppenheimer mode with a time step of 2 a.u., with initial conditions sampled from a canonical ground state run at 300 K.A prerequisite for the dynamics is an extensive validation of the S 0 and S 1 potential energy surfaces, which was done in much detail [18] for the parent compound by comparison with CASPT2 data. [17][18][19] An analogous analysis for the E-AB-C 2 derivative confirms that the DFT excitation energy (2.59 eV) is again in good agreement with both CASPT2 (2.52 eV) and experiment (2.53 eV). On chemical modification of AB to form AB-C 2 (i.e., addition of a À CH 2 CH 2 À bridge in ortho position of the phenyl rings) the E isomer b...

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.…”

“…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. 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.…”

Nonadiabatic dynamics study of bridged-azobenzene by the time-dependent density functional tight-binding method

“…The S 1 state decay time result comes close to the experiment time scales of about 50 fs by Siewertsen and co-workers [25,26]. The ultrafast excited state decays are also observed in other theoretical studies with different methods [17,28,[30][31][32]66]. Fig.…”

“…For the excited state, the SCC-DFTB approach is extended to the time-dependent case with the timedependent density functional linear response theory [11]. Azobenzene and its derivatives have received considerable attention experimentally and theoretically during the past three decades, due to their potential applications in molecular switches, molecular motors, optical information storage and processing devices [12][13][14][15][16][17][18][19][20][21][22][23][24]. The pioneering studies of the photochromic properties of bridged-azobenzene (abbreviated as Br-AB and shown in Fig.…”

Ultrafast Dynamics of UV‐Excited Imidazole

“…Although the basis for these dynamical simulations was set early in the 1970s, [2,3] only in the last fifteen years, [4][5][6][7][8] with the development of computational capabilities and efficient quantum chemical methods, have they become a routine tool for research in photochemistry and photophysics. [9][10][11] Much of the research in nonadiabatic dynamics simulations has focused on the relaxation processes following photoexcitation of organic molecules. [1,12,13] These simulations have provided a mechanistic view of ultrafast processes by determining the relative importance and time constants for each available reaction pathway.…”