Photoreactivity Control Mediated by Molecular Force-Probes in Stilbene

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Spectroscopy and Photochemistry; General Theory

Photoreactivity Control Mediated by Molecular Force-Probes in Stilbene Cristina Garcia-Iriepa, Diego Sampedro, Francisco Mendicuti, Jérémie Léonard, and Luis Manuel Frutos J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03802 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry Letters

Photoreactivity Control Mediated by Molecular Force-Probes in Stilbene Cristina García-Iriepa,*a,b# Diego Sampedro,b Francisco Mendicuti,a,c Jérémie Léonardd and Luis Manuel Frutos*a,c a

Departamento de Química Analítica, Química Física e Ingeniería Química. Universidad de

Alcalá, E- 28871 Alcalá de Henares, Madrid, Spain. b

Departamento de Química. Centro de Investigación en Síntesis Química (CISQ), Universidad

de La Rioja. E-26006, Logroño, Spain. c

Instituto de Investigación Química ‘‘Andrés M. del Río’’. Universidad de Alcalá. 28805 Alcalá

de Henares, Madrid, Spain. d

Institut de Physique et Chimie des Matériaux de Strasbourg. Université de Strasbourg, CNRS,

UMR 7504 and Labex NIE, 67034 Strasbourg, France. AUTHOR INFORMATION Corresponding Author *Cristina Garcia-Iriepa: [email protected] *Luis Manuel Frutos: [email protected]

ACS Paragon Plus Environment

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We report theoretical and experimental evidences showing that photochemical reactivity of a chromophore can be modified by applying mechanical forces via molecular force-probes. This mechanical action permits to modulate main photochemical properties as fluorescence yield, excited state life-time or photoisomerization quantum yield. The effect of molecular force probes can be rationalized in terms of simple mechanochemical models, establishing a qualitative frame for understanding the mechanical control of photoreactivity in stilbenes.

TOC GRAPHICS

Force

MECHANO-PHOTOCHEMISTRY

F= 0nN

Fluor Photoisom S1 (ps)

F= 1.7nN

F= 1.9nN


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Mechanochemistry has emerged in the last decade as a novel and promising way of activating chemical reactions and modifying physical and chemical properties. 1,2 In this regard, molecular nano or subnano[Newton] external forces have been exerted on covalent bonds (i.e. covalent mechanochemistry, CMC) and on noncovalent interactions. Within the CMC framework, the mechanical external force performs a bond-specific work on the mechanophore (i.e. strained molecule). This mechanical work is added to the internal energy of the system, altering the potential energy surface (PES) of the molecule and, eventually, its reactivity. Different approaches have been reported up to now to apply a mechanical force: i) atomic-force microscopy,3 ii) molecular force probes4 and iii) sonication.5 Although mechanochemistry is a relatively incipient field, it has rapidly appeared as a valuable tool to control and modify the thermal reactivity and properties of molecules. For instance, a clear external force effect was demonstrated on the activation energies and kinetics of triazoles cycloreversion and cyclobutane ring-opening processes.4,6 Moreover, mechanochemistry can be considered as a valuable synthetic tool as it can alter the reaction products.7 Noticeably, although mechanochemistry of ground-state reactions has been widely studied, the effect on photochemical and photophysical properties remains largely unexplored. Not many investigations are available regarding the force effect on the spectroscopic properties of chromophores, particularly on the vibrational spectra,8 the electronic excitation energies9–11 and the fluorescence spectra.12 In this line, even if the environment can play a mechanical role in photoreactivity, as is the case of space-saving photoisomerization of rhodopsin,13,14 only few studies have been reported concerning the effect of controlled forces on photoreactivity, covering the modulation of azobenzene


3


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photoisomerization under strain,15 the stress-induced chemiluminescence in a dioxetanebased polymer16 and the photoisomerization quantum yield modulation of a retinal-like switch.17 All these studies on mechanochemistry resulted in a wide range of applications from fluorescence force sensors,12 to chemical synthesis,18 mechanobiology19 and material science.20 To shed light on the possibility of tuning photochemical reactivity using mechanical forces, we have studied the photochemical and photophysical properties of a series of strained trans-stilbenes where forces are applied intramolecularly via force probes bridging the chromophore (i.e. stilbenophane derivatives) at the para positions (Figure 1). Using molecular force probes to strain the system permits to stay in the molecular chemistry avoiding for instance solid-state chemistry, but keeping the applied force controled, as these forces can be determined computationally and clearly depend on the specific bridge employed. Moreover, trans-stilbene offers an excellent opportunity to explore mechano-photochemistry (i.e. photochemistry affected by mechanical forces) due to its very well studied photochemistry,21 simple structure and to the application of stilbene derivatives for spectroscopic studies and as celebrated synthetic molecular motors.22,23 Additionally, several computational studies of stilbene have been reported, providing a detailed picture of the photochemical mechanism. 24–27 In this work, we have synthesized and characterized three different derivatives: the unstrained trans-4,4'-dimethoxystilbene (1) and two trans-stilbenophane derivatives with alkyl chains of twelve (2) and ten (3) CH2 groups, providing increasing strain (Figure 1) respect to 1. In order to explore the effect of increasing strain on the compounds, absorption and emission spectra as well as fluorescence and photoisomerization quantum


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yields have been obtained. Transient absorption (TA) spectroscopy was used to investigate their excited state dynamics and lifetimes. Additionally, multiconfigurational chemistry has been employed to theoretically predict the excitation, emission energies and photochemical pathways. Moreover, molecular dynamic simulations have been performed to evaluate the photoisomerization quantum yields (see Supporting Information for methodological details).

Figure 1. Structure of the unstrained (1) and strained (2 and 3) trans-stilbene derivatives. A schematic representation of the applied mechanical force (F) as well as the photochemical processes (fluorescence and photoisomerization)

is

given.

The

main

photoisomerization

coordinates,

the

dihedral



and

pyramidalization  angles, are depicted.

Compound (1) presents an absorption band (,*) in the 260-350 nm range in acetonitrile, as predicted also computationally (Table S1 and Figure S2). Similar absorption bands are observed for 2 and 3, but red-shifted by ca. 600 cm-1 (Figure 2A). The same red-shift is observed in the structured emission spectra of 2 and 3 as compared to 1 (Figure S3). The fluorescence quantum yields (F) have been measured to be 0.168, 0.011 and 0.004 for 1,


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2 and 3 respectively, showing the strong influence of the alkyl chain exerted on the molecule (Figure 2B). Additionally, the photoisomerization quantum yields φcorr along P the non-radiative decay channel (i.e. corrected from the fluorescence quantum yields, see details in the Supporting Information) have been measured to be 0.44, 0.48 and 0.53 for 1, 2 and 3 respectively (Figure 2B). To further investigate the strain effect on the photochemical properties, the excited state dynamics have been studied by TA spectroscopy using a set-up described elsewhere.28,29 Following excitation at 320 nm, all three compounds display (Figure S4) a spectrally similar excited state absorption (ESA, λmax ~ 580 to 630 nm), and stimulated emission (SE, λmax ~ 360 to 380 nm). However, the decay time scale for the three compounds is very different, as illustrated in Figure 2C by the decay of the ESA and SE band integrals. For compounds 1 and 2, both bands decay biexponentially with time constants of 370 ps and 55 ps for 1 (average lifetime of 300 ps), and 4.4 ps and 20 ps for 2 (average lifetime of 8 ps) (Figure S5, S6 and Table S2). For compound 3, the S1 lifetime is even shorter, resulting in an average bright state lifetime < 0.6 ps. What is more, a new, short-lived, positive, induced absorption band rises and decays on top of the SE band, at λ max = 370 nm, as seen in the SE band decay kinetics of 3 in Figure 2C (open blue squares). We call this band P* and attribute it to the so-called phantom state, already identified in other stilbene derivatives as a short-lived, perpendicular S1 structure, transiently populated before the system decays to S0.30–35 We

argue that P* becomes detectable only in

compound 3 because its photoreaction accelerates so much whereas in compounds 1 and 2, the P* signature is not detected because the population transfer from the initial bright state to P* is much slower than the P* decay, and so no detectable.


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To get insight into the photoreaction pathway of 1, 2 and 3 the energy profiles have been computed at the CASSCF/6-31G(d) level of theory with CASPT2//CASSCF/6-31G(d) single point corrections for stationary points (Figure 2D). The photoreactive pathway of the unstrained system (1) closely resembles that of trans-stilbene.24–26 After excitation, the molecule relaxes along the stretching coordinate (i.e. single-double bond length alternation) reaching an almost planar minimum (Min1 in Figure 2D). This minimum is the responsible of the observed fluorescence, as the computed emission energy (389 nm), agrees with the experimental emission spectra in the 340-460 nm window. Further evolution along the torsion coordinate drives the molecule over an energy barrier (TS) of 2.8 kcal·mol-1, towards a second intermediate (Min2) characterized by an almost perpendicular geometry (Figure 2D). From Min2, an accessible S1/S0 conical intersection (CI) appears along the pyramidalization of one of the ethylene carbons,  (Figure 1), allowing the system to decay to S0 by completing the isomerization or reverting to the initial isomer.26 For the strained compounds, the reaction path is qualitatively similar, except that the TS energy barrier is decreased to ca. 0.5 kcal·mol-1 for 2, and vanishes for 3, for which the reaction path to Min2 becomes barrierless. In order to investigate whether mechanochemistry can provide a rational explanation for the observed changes in the photochemical properties from 1 to 3, it is necessary to determine the exerted forces by the alkyl bridges in 2 and 3, and check if a mechanochemical model is able to account for the observed variations. We have determined computationally the bridge induced force by removing the alkyl chains from the ground state structures of 2 and 3, while keeping fixed the C-C distance of the methoxy groups (see Supporting Information). Similar structures are predicted for 2 and 3 with and


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without the alkyl chain. Then, following a procedure described elsewhere, 4 the magnitude of the force on each side has been evaluated to be 1.7 nN in 2 where the alkyl chain contains 12 CH2 groups, and up to 1.9 nN in 3 with an alkyl chain reduced to 10 CH2 units (see Supporting Information). Hence, we conclude that the effect of the alkyl chain is essentially equivalent to an external force pair applied to the two terminal -CH3 groups of 1, which remains approximately constant along the reaction path, with a standard deviation of 15% as determined for different structures along this path.

Figure 2. A) Absorption spectra of 1-3 in acetonitrile. B) Graphical representation of the F and Pcorr quantum yields variations. C) S1 population decay kinetics monitored by the ESA (solid symbols) and SE & P* (open symbols) band integrals. D) Photochemical path computed for compounds 1-3 along the torsion α and pyramidalization  coordinates at the CASSCF level of theory. E) Cross section of S1/S0 conical intersection at zero gradient difference for 1 and 3.

Once determined the type and magnitude of the force-probes exerted forces, appropriate mechanochemical models have to be applied to investigate the observed differences


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between 1, 2 and 3 on a purely mechanochemical basis. The first property to analyze is the absorption and emission spectra. Previous studies show that the force-probe effect on the absorption spectra of different chromophores 11-13 can be mainly understood as a structural effect. For both 2 and 3, the absorption and emission spectra are red-shifted compared to 1. The predicted absorption/emission energies for 2 and 3 are basically equivalent to that of 1 when conveniently restrained to mimic 2 and 3 (Table S3), showing that observed shifts are due to the structural changes induced by the force probes, discarding a significant electronic effect of the alkyl chain on these optical properties. Regarding the fluorescence quantum yields and excited state lifetimes, both decreases by, at least, two orders of magnitude when reducing the alkyl chain length (i.e. when increasing the applied force) from compound 1 to 3. These observations can be understood by analyzing the computed photochemical paths. For 1, a significant energy barrier is computed along the torsion coordinate, which is responsible for the relatively slow (300 ps) SE and ESA decay and significant fluorescence quantum yield F=0.17. This energy barrier is reduced in 2 and disappears in 3, in line with the fact that 2 and 3 SE and ESA signatures decay much faster (8 ps and 0.27 ps, respectively) and F drops. In all three compounds, the reaction path beyond this barrier is predicted to reach the twisted Min2 structure which is lying close to a conical intersection with S0. We therefore assign Min2 to the 370-nm-absorbing, short-lived phantom state, which is only observed in 3 as discussed above.


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Since, as has been discussed above, the alkyl-chain effect on 2 and 3 is basically mechanical, the PES of 2 and 3 should be equivalent to that of 1 after adding the energy term due to the mechanical work. This work can be determined in the usual way36 as the dot product between force and displacement vector projection along the mechanical coordinate, which in our case corresponds to the distance between the carbon atoms of the methoxy groups (Figure 1). By computing the developed work due to the forces along the path (i.e. 1.7 nN for 2 and 1.9 nN for 3), it is possible to construct their PES just by adding the corresponding work term to the PES of 1. In Figure 3 we show this construction of the approximate S1 PES for 2 and 3. The match between these PES and those found for 2 and 3 along the reaction path (see Figure 2.D) is significant. This becomes apparent when comparing activation energies for 2 and 3 (i.e. 0.5 kcal/mol for 2 and almost vanishing energy for 3) with those predicted for the approximate S1 PES obtained by adding the mechanical work to 1 (i.e. 0.6 kcal/mol for 2 and also vanishing energy for 3). Therefore, the topography of the PES for 2 and 3 is basically that of 1 when adding the corresponding work term (mechanical considerations), which implies that all the photochemical information of 2 and 3 can be predicted from the properties of the unstrained compound 1. Finally, a dynamical study of 1 and 3 has been performed to investigate whether the increase of Pcorr can also be rationalized in terms of an applied force. Unfortunately, running the trajectories from the FC structure is not computationally feasible as the molecule gets trapped in the excited state minimum. For this reason, a set of 40 non-adiabatic molecular dynamics trajectories starting directly from the minimum energy S1/S0 CI structure has been run for 1 and 3 (see Supporting Information for details).


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Energy (kcal·mol-1)

Page 11 of 16

Ea(1)

1

TS TS

2 3

MIN 1

Ea(2)

Figure 3. Approximate S 1 PESs of 1, 2 and 3 for FC and TS region. PESs of 2 and 3 are built by adding to the unstrained compound 1 PES the work term due to the force probe. The force has two components: one along the torsion coordinate (α) and one along the relative terminal C-C distance. The derived PESs qualitatively match the computed PESs of 2 and 3 (Figure 2D).

The predicted photoisomerization quantum yield increases from 0.32 in 1 to 0.80 in 3, in qualitative agreement with experimental findings. As has been previously observed for benzene,36 the CI tilt may change on strained systems, affecting the quantum yield of the photo-process. Here, the CI tilt increases with the strain from 1 to 3 (Figure 2E). Tilted CIs are usually related to high photoreactivity yields, due to acceleration in the direction of photoproducts.37 In the present case the S1/S0 CI topography of all compounds is peaked, and its tilt changes with the external force, making the path towards the photoproduct steeper for 3 (Figure 2E), due to the force pointing to the


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photoisomerization, explanation that may provide a qualitative picture for the observed trend. Concluding, we provide here experimental and theoretical investigations showing that mechanical forces, exerted by molecular force probes, can affect the photochemistry of a chromophore. Absorption/emssion spectra, and fluorescence and photoreaction quantum yields are strongly affected by mechanical forces. These changes can be understood in terms of PES variations between the unrestrained compound 1 and those subjected to mechanical forces, 2 and 3. The variations in the PES topography can be determined by applying mechanochemical models either to conical intersections, transitions states or complete reaction paths. In summary, we show that mechanical effects on photoreacting systems may strongly affect their photoreactivity, paving the way to considering mechano-photochemistry as a reliable strategy for photochemical control.

ASSOCIATED CONTENT Supporting Information. Experimental procedures. Computational details. Absorption spectra. Emission spectra. Transient absorption spectroscopy. Evaluation of the force-probe effect on the absorption and emission spectra. Procedure to evaluate the force induced by probes on transstilbene. Dynamic study. CI topography change with external forces. NMR spectra. Cartesian coordinates.


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AUTHOR INFORMATION #Present address: Université Paris-Est, Laboratoire Modélisation et Simulation Multi Échelle, MSME, UMR 8208 CNRS, UPEM, 5 bd Descartes, 77454 Marne-la-Vallée, France. The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by the Spanish MINECO via grants CTQ2016-80600-P and CTQ2017-87372-P, by University of Alcalá (CCG2016/EXP-076, CCGP2017-EXP/027) and by the French ANR via grants Labex NIE ANR-11-LABX-0058_NIE and Labex CSC ANR-10LABX-0026_CSC.

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Photoreactivity Control Mediated by Molecular Force-Probes in Stilbene

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