Photoinduced Forward and Backward Pedalo-Type Motion of a Molecular Switch

Abstract

Photoresponsive molecular switches enable spatial and temporal control of molecular processes and are therefore crucial for the development of smart functional materials. Because the light-induced dynamics of these switching units are at the core of the resulting functionality, a detailed insight into their structural time evolution is fundamental for molecular embedding. Here, we performed a hybrid quantum mechanics (CASPT2 and TDDFT)/ molecular mechanics (QM/MM) study to elucidate the photodynamics of an azodicarboxamide-based molecular switch, which is a promising candidate for implementation in highly dense environments such as polymers. In particular, we report a detailed picture of the molecular motion at the atomic level based on a relevant number of excited-state trajectories. We show that the azodicarboxamide-based molecular switch undergoes both a forward and backward pedalo-type motion upon excitation. Trans−cis photoisomerization on the other hand, which is well-known to occur for other azo-based chromophores, is shown to be a negligible pathway. By validating the volume-conserving pedalo-type motion, we provide a rational basis for the design of novel types of photoresponsive functional materials in which the active component must operate in a confined space.

Modus Operandi of a Pedalo-Type Molecular Switch: Insight from Dynamics and Theoretical Spectroscopy

“…GS optimization in gas-phase at the DFT and MP2 level of theory show, in line with calculations on the solvated model [ 15 ], that the GS equilibrium of the photochromic unit is characterized by 75° distortion from planarity around the N 1 -C 2 -N 3 -N 4 ( ) and N 3 -N 4 -C 5 -N 6 ( ) dihedrals ( Table 1 , see Figure 1 for atom notation). It is worth noting that the two carbonyl groups are decoupled from the central C 2 -N 3 -N 4 -C 5 ( ) fragment, i.e., the molecule does not exhibit a conjugated π-system in the GS.…”

“…The S 1 ( ) PES of the isolated azodicarboxamide resembles the one reported for the solvated model [ 15 ], exhibiting CIs with comparable geometrical features. Contrarily to what was reported in chloroform (CI cis-trans and CI plan lying ca.…”

“…Due to the decoupling of the carbonyl and N=N fragments, the oscillator strengths associated with these transitions are rather weak ( Table 2 ). Overall, the electronic structure of isolated azodicarboxamide resembles closely that in chloroform [ 15 ].…”

“…To support atomistic interpretation, we simulated the transient IR spectra both in gas-phase and in chloroform (using the dynamics reported in our previous study [ 15 ]), following the procedure illustrated in the Methods section. In the following, we compare the simulated spectrum in chloroform with the experiment (the gas-phase spectrum is reported in Figures S4 and S5 in the SI ) [ 14 ].…”

“…Recently, a novel molecular photo-switch based on the azodicarboxamide chromophore unit ( Figure 1 ) stepped into the spotlight thanks to a unique feature: an ultrafast photoinduced volume-conserving motion. Time-resolved UV-pump/IR-probe spectroscopy [ 14 ] supplemented with QM(TDDFT)/MM non-adiabatic molecular dynamics simulation on the solvated model [ 15 ] demonstrated that the chromophore, in contrast to other azo-compounds, does not undergo the usual trans-cis photoisomerization but rather a volume-conserving pedalo -type motion, rendering possible the application of this new class of chromophores under constrained conditions such as in solid state [ 16 ], on surfaces [ 17 , 18 ] and inside polymers [ 19 ].…”