Fulgides as light driven molecular rotary motors: Computational

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Spectroscopy and Photochemistry; General Theory

Fulgides as light driven molecular rotary motors: Computational design of a prototype compound Michael Filatov, Marco Paolino, Seung Kyu Min, and Kwang S. Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02268 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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The Journal of Physical Chemistry Letters

Fulgides as Light Driven Molecular Rotary Motors: Computational Design of a Prototype Compound Michael Filatov,∗,† Marco Paolino,‡ Seung Kyu Min,† and Kwang S. Kim† †Department of Chemistry, School of Natural Sciences, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea ‡Dipartimento di Biotecnologie, Chimica e Farmacia (Dipartimento di Eccellenza 2018-1022), Università di Siena, Via A. Moro 2, 53100 Siena, Italy E-mail: [email protected]

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The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A new family of light driven molecular rotary motors utilizing the fulgide motif is proposed and its prototype molecule is studied by quantum chemical calculations and non-adiabatic molecular dynamics simulations. The new motor performs pure unidirectional axial rotation of the rotor blade with high quantum efficiency (φ ∼ 0.55– 0.68) and ultrafast dynamics (htiS1 ∼ 200–300 fs) of its successive photoisomerization steps. The photocyclization reaction typical of fulgide compounds is blocked by the design of the new motor and never occurred in the molecular dynamics simulations. The new motors can be synthesized from easily available precursors. In view of its remarkable photoisomerization ability the new motor represents a prospective class of compounds for the use in nanosized molecular devices.

Graphical TOC Entry O

unidirectional rotary motor

O

O

CH2

quantum yield: φ ~ 0.55-0.68

O

O

S

O H 2C

kT

O O H2C

O

O

O

CH2

hν kT

O

S O

O

O

S

CH2

hν S

S

Keywords non-adiabatic dynamics, conical intersection, photoisomerization, unidirectional rotation, computational quantum chemistry 2 ACS Paragon Plus Environment

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Light-driven molecular rotary motors (further on called molecular motors, for simplicity) are capable of converting the light energy into mechanical (rotary) motion of one part of the motor with respect to another. 1–6 The design of currently available synthesized molecular motors is based on overcrowded alkene framework, where the two blades of the motor are connected by a double bond, C=C 7–12 or C=N. 13,14 The rotation of the rotor blade is achieved through a sequence of steps, where photoexcitation of the π electrons of the double bond is central for motor’s operation. 15–19 Photoexcitation breaks the πcomponent of the double bond and releases the strain imposed by the overcrowding; which leads to rotation of the rotor through ca. 180◦ . 7–10,15–17 In Feringa-type motors, 7–10 the photoisomerization is followed by the thermal helix inversion step, which resets the motor to a conformation suitable for the next photochemical step; the subsequent photoisomerization and helix inversion steps complete a full 360◦ loop. The unidirectionality of rotation is achieved by breaking the inversion symmetry of the motor through introduction of chiral centers or axial chirality (pre-twist). 7–10,20,21 The described 4-stroke working cycle is the most common in synthetic molecular motors; e.g., it is implemented in recently synthesized hemithioindigo-based molecular motors 11,12 and imine-based molecular motors. 13,14 The latter motors have an ability to alternate between the 4-stroke (360◦ rotation) and 2-stroke (180◦ rotation) working cycles depending on the conformational flexibility of the stator blade. 13,14 One of the key requirements for practically useful molecular motor is high quantum efficiency of its photoisomerization steps. 12,17 Indeed, as the energy of the light quanta is ultimately dissipated into the environment, obtaining mechanical work from a motor with low quantum efficiency may require too high an intensity of the incident light; thus overheating the whole molecular device. 12 The quantum yield of E/Z photoisomeriza-



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tion of the currently synthesized motors is in the range of 10–20 %; 12,19 chemical modulation of the existing synthetic motors did not lead to noticeable increase of their quantum efficiency. 19 Consequently, there remains a substantial room for improvement of their operational efficiency. In this work, we propose a new family of molecular motors where the stator and the rotor blades are redesigned to achieve high quantum efficiency of photoisomerization. Theoretical simulations in this Letter predict that the prototype compound can achieve a quantum yield of 68%; far more than the existing synthetic motors. By contrast to other theoretical simulations, 22–25 mostly targeting hypothetical molecules, the motors presented here are synthetically viable. Hence, we believe that our work should stimulate further research, both synthetic and computational, in this area. The motor described here, (E)-3-(3,5-Dimethyl-1-thia-5,6-dihydropentalen-4-ylidene)4-methylene-2,5-furandione, abbreviated as DTMF, employs fulgide motif, see Scheme 1 for chemical formula. Fulgides 26 owe their photochromism to the formation of the cyclic form (see the chart below). 27 Although fulgides can undergo the E/Z photoisomerization, this process was considered as energy wasting, 26 as it did not lead to photochromism. O

O

O

O

O

O

UV

O

UV

O

UV

O

Vis S S

S

In some cases, the photocyclization pathway was completely blocked, 28,29 which could possibly provide for an E/Z switch, however no systematic study into this matter was undertaken. The maleic anhydride, a building block of fulgide moiety, has sufficiently large posi-


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tive electron affinity (1.440±0.087 eV); 30 hence the fulgide moiety can act as an electron withdrawing group and this makes it attractive for designing E/Z switches and motors. Isomerization about a double bond occurs through S1 /S0 conical intersections (CIs) at which the molecule may attain two types of geometric distortions. 17,31 One type is the ethylenic CI or torsion–pyramidalization (tor–pyr, for brevity) CI where to reach the degeneracy between the S1 and S0 electronic states the molecule undergoes near 90◦ torsion accompanied by pyramidalization of one of the atoms connected by the double bond. The other type is torsion–bond length alternation (tor–BLA) CI, where the S1 /S0 degeneracy is reached by ca. 90◦ torsion and stretching/compression of the double/single bonds in the molecule, i.e., BLA distortion. Tor–BLA CIs occur in molecules where the homolytic and heterolytic breaking of the π component of the double bond are nearly isoenergetic. 17 By contrast, the molecules where homolytic π bond breaking is preferred feature tor–pyr CIs. To stabilize the heterolytic π bond breaking the electron affinity of one of the motor’s blades needs to be increased; thus stabilizing the resulting ionic Lewis structures. 17 This is precisely the requirement fulfilled by the fulgide-type rotor blade, see bonding analysis in the Supporting Information. In synthetic molecular motors of Feringa’s type, occurrence of the tor–pyr CIs results in a movement of the rotor blade that resembles butterfly wings flapping, rather than pure axial rotation. 15–17 A substantial displacement of the rotation axle from the upright position seems to be the likely cause of the low quantum efficiency of photoisomerization in these motors. 15–17,23 Replacing the tor–pyr CIs by the tor–BLA CIs results in a nearly pure axial rotation of the rotor blade and in an increased isomerization quantum yield. 17,23 Hence, the occurrence of the tor–BLA CIs in the E/Z isomerizable fulgides can potentially provide for a high quantum yield of photoisomerization.



8' O

O O

R3

R1

definition of central dihedral angle

O 1'

R4

O

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7' O

O

5'

2'

2'

4'

6' CH2

3'

θ

θ R2

S

R1 = R2 = -CH3 R3 = R4 = -H

8

5

3a 6

4'

7

4 3

6a

3'

5

4 3a

2 1 S

Scheme 1: Chemical formula and atomic numbering of DTMF motor studied in this work. Also shown are possible synthetic precursors of the DTMF motor and definition of the central dihedral angle θ. The proposed DTMF motor is a member of a family of E/Z isomerizable fulgides where the cyclization path is blocked by chemical design; no closed-shell Lewis structure for cyclic form can be obtained without considerable rearrangement (see the Supporting Information for the analysis of the respective Lewis structures).

The DTMF motor can

be synthesized from accessible precursor compounds, e.g., the commercially available 4-methylene-3H-furan-2,5-dione and the previously reported 3,5-dimethyl-5,6-dihydro4H-cyclopenta[b]thiophen-4-one, 32 see Scheme 1. Furthermore, a series of molecules based on the DTMF scaffold could be prepared combining analogous structures already reported in literature, as described in the Supporting Information. The electronic structure of the S0 equilibrium conformations, the S1 /S0 CIs, and the minimum energy pathways (MEPs) on the S1 and S0 potential energy surfaces (PESs) were studied at the DFT REKS/SSR level, 33,34 see the Supporting Information for details. In the S0 state, DTMF may attain one of four equilibrium conformations defined by the orientation of the rotor blade with respect to the stator, E or Z, and by the helicity, P or M. The most stable conformation is EP, see Scheme 1 and Figure 2; changing helicity to M 6 ACS Paragon Plus Environment

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destabilizes the EM conformation by 3.4 kcal/mol. The ZP conformation is slightly, by 0.4 kcal/mol, less stable than EP; the energy difference between ZM and ZP is 3.4 kcal/mol. Hence, at an ambient temperature, the chemical equilibrium is shifted toward the EP and ZP conformations. The energy splitting between the P and M helicities of DTMF is similar to that observed for the Feringa’s type motors (i.e., ca. 3 kcal/mol). 10,15,16 The S1 state at all four equilibrium conformations of DTMF corresponds to a oneelectron π → π ∗ transition localized mostly on the central C4 =C30 bond. The excitation spectrum of DTMF was calculated by TD-DFT (see the Supporting Information) and there is a good agreement for the S1 ← S0 vertical excitation energy between the SSR and TDDFT calculations. According to TD-DFT, the S2 state has charge transfer character and is located ca. 0.6–0.9 eV above the S1 state. As the S2 state has a very low oscillator strength (ca. 8 times less than S1 ), it is unlikely that this state will receive substantial population during the photoexcitation. The calculated excitation wavelength for the static EP and ZP geometries is 308 nm and 311 nm, respectively. To simulate the effect of nuclear vibrational motion at these geometries, the absorption spectra were also calculated for an ensemble of geometries generated by sampling the Wigner function at T=300K (see the Supporting Information for detail). Nuclear vibrations result in broadening of the absorption band and in a slight red shift (ca. 10 nm) of the band maximum to 318 nm for both conformations. At typical UV-A wavelengths, i.e., around ca. 350 nm, the absorbance of the EP and ZP conformations decreases by approximately a factor of two; which suggests that the DTMF motor should be operational in UV-B as well as UV-A regions. Excitation to the S1 state results in breaking of the π component of the C4 =C30 bond; thus releasing the steric strain. Torsion of the rotor blade through ca. ±90◦ leads to a



geometry at which S1 /S0 CIs occur. The geometries of the CIs obtained by 90◦ and 270◦ CCW rotation of the rotor blade, further on denoted as CI90 and CI270 , were optimized by the SSR method and the branching plane (BP) vectors of both CIs were calculated, see Figure SI-5 of the Supporting Information. The BP vectors describe all nuclear displacements lifting the S1 /S0 degeneracy at the CI; hence they define all the directions of exiting the strong non-adiabatic coupling region where the population transfer occurs during the excited state relaxation. The optimized CI90 and CI270 geometries feature the C4 =C30 bond in an upright orientation, i.e., no pyramidalization is involved. The BP vectors of both CIs correspond to torsion of the rotor (the h-vector) and to BLA distortion (the g-vector); i.e., the tor-BLA type CIs. The BLA distortion describes predominantly stretching/compression of the bonds along the C6a =C3a –C4 =C30 –C40 =C60 chain.

O

92.7

ΔE, kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O

O O

O

CH2

S

S

50.2

50.4

CI90

17.0 3.8

0.0

O

O

5.7

O

ZM

ZP

O

S

O

H2C

CH2

O

CH2

Δ

hν S

O

O

O

O

S

EP

O

Δ

hν

0.0

EM

O

O

H2C

CH2

3.4

0.4

O

O

CI270

45.9

46.9

EP

92.7

O

92.5

CH2

S

S

Figure 1: Schematic representation of the S0 (blue) and S1 PESs of the DTMF motor and working cycle of the motor. The relative energies (with respect to the EP conformation) are given in kcal/mol. Using the optimized S0 equilibrium geometries and the CI geometries, the MEPs con8 ACS Paragon Plus Environment

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necting the geometries arranged in the counterclockwise direction of rotation of the rotor blade (see the Supporting Information for detail) were optimized on the S0 and S1 PESs. Based on the MEPs, shown schematically in Figure 1, the working cycle of the DTMF motor comprises the following steps. Upon the photoexcitation of the EP conformation, the molecule slides down the S1 PES towards CI90 , where it relaxes to the S0 state and continues on the S0 PES to the ZM conformation. From the ZM conformation, the S0 path back to EP is guarded by a high (43.1 kcal/mol) potential barrier and the molecule moves forward to the ZP conformation hopping over a 1.9 kcal/mol hummock. The photoexcitation in the ZP conformation brings the molecule on the slope of the S1 PES inclined to CI270 . From the EM conformation, reached upon S1 → S0 relaxation through CI270 , the way back to ZP is obstructed by a barrier 42.5 kcal/mol tall and the way to the EP conformation leads across a much lower hill of 13.6 kcal/mol. Hence, the working cycle of the DTMF motor is a classical 4-stroke cycle, 7–10 where the photoisomerization steps are followed by thermally activated helix inversion steps. In DTMF, the ZM → ZP helix inversion is expected to be extremely fast and the rotation rate limiting step is the EM → EP inversion. The difference between the ZM → ZP and EM → EP helix inversion barriers (1.9 vs. 13.6 kcal/mol) is caused by the steric repulsion between the exocyclic methyl group at the ˚ C3 atom, see Scheme 1, and the extremities of the rotor blade; the C40 =C60 bond (1.345 A) ˚ A similar difference (2.5 vs. 9 kcal/mol) between is longer than C20 =O70 bond (1.194 A). the helix inversion barriers was also found for the hemithioindigo-based motors. 11,12 A recent modification of Feringa-type motor has helix inversion barrier of ca. 13.1 kcal/mol and the inversion half-life t1/2 = 1 ms. 35 A more detailed representation of the optimized S1 and S0 MEPS is shown in Figure



FCEP

CI90

EP

FCEP

FCZP

ΔE, kcal/mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CI270

ZM

ZP

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EM

EP BLA, Å

θ, deg.

Figure 2: Minimum energy pathways on the S0 (blue) and S1 (red) PESs of the DTMF motor. The adiabatic reaction path is highlighted in green. Vertical excitations are shown by magenta arrows. 2, where the energy is depicted as a function of the torsion angle θ and the BLA distortion defined as BLA = ( R(C3a − C4 ) + R(C30 − C40 ))/2 − ( R(C6a − C3a ) + R(C4 − C30 ) + R(C40 − C60 ))/3, i.e., a difference between the average lengths of formally single and formally double bonds in the C6a =C3a –C4 =C30 –C40 =C60 chain. When the S1 MEP departs from the FC points, the initial motion corresponds to considerable decrease of BLA which is caused by breaking of the central π-bond and rearrangement of the bonding pattern along the chain. The S1 state has diradicaloid character (see the S1 natural orbitals in Figure SI-2 of the Supporting Information) and remains diradicaloid until reaching the proximity of the CIs. The S0 state is a closed shell covalent state at the equilibrium geometries, however it becomes increasingly ionic as the S0 MEP approaches ca. ±90◦ of torsion. Near ±90◦ of torsion, heterolytic breaking of the central π-bond occurs in the S0 state; which is confirmed by the total charges on the rotor and the stator blades, see


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Figure SI-6 of the Supporting Information. The EP and ZP FC points occur in the regions of the S1 PES that are inclined towards CI90 and CI270 , respectively. The shape of the S1 PES in the vicinity of the FC points suggests that, in the adiabatic regime, rotation in the clockwise sense (ZP → EP) is hindered and the molecule proceeds in the counterclockwise direction (EP → ZP). The slope on S1 PES near the EP FC point is sufficiently steep, however a shoulder occurs near the ZP FC point. The latter is caused by steric repulsion between the exacyclic CH2 group and the methyl group at C5 in the ZP conformation. No barrier results from the repulsion and an unhindered rotation in the CCW direction is possible. Torsion in the CCW direction brings the molecule to CI points, CI90 and CI270 , both of which have pronounced peaked topography. Hence, both CIs represent very efficient funnels for the S1 → S0 population transfer. Upon relaxation on the S0 PES the way is open to the metastable ZM and EM conformations. In the vicinity of the CIs, folding of the S0 PES, see Figure 2, hinders propagation in the opposite direction, back to EP and ZP. The folding is caused by the occurrence of the CI90 and CI270 behind the respective transition states for the central π-bond breaking on the S0 PES. Hence, upon passage through the CIs the molecule continues rotation in the CCW direction and ends up with the respective metastable conformations, ZM or EM. This completes the photoisomerization steps, which are followed by the thermally activated helix inversions. The estimated adiabatic reaction path is highlighted in Figure 2 by green color. The non-adiabatic molecular dynamics (NAMD) simulations of the photodynamics of the DTMF motor (see the Supporting Information for detail) were carried out using the SSR-BH&HLYP/6-31G* method in connection with the DISH-XF surface hopping formalism. 36 The parameters of the EP → ZM and ZP → EM photoreactions, the average S1



Table 1: Average lifetimes (htiS1 ) of the S1 state, parameters of the monoexponential fit of the S1 populations, and quantum yields (φ) of photoisomerization reaction of the EP and ZP conformations of the DTMF motor.

reaction EP → ZM ZP → EM

monoexp. fita t0 , fs τ, fs φ c 157(2) 57(4) 0.55 198(4) 104(8) 0.68

htiS1 b , fs 203(35) 277(53) t − t0

Monoexponential regression f (t) = e− τ of the S1 populations. b) Average S → S hop time. The standard deviation σ is given in parentheses. t 0 1 c) Standard error of the estimate is given parenthetically.

a)

lifetime, parameters of a monoexponential fit f (t) = e−

t − t0 τ

of the S1 populations, and the

quantum yields of the photoreactions obtained in the DISH-XF/SSR-BH&HLYP/6-31G* NAMD simulations are shown in Table 1. The DTMF motor displays an ultrafast dynamics (htiS1 ∼ 200–300 fs) of the S1 decay. Photoexcitation at the EP or ZP conformations results in rotation of the rotor blade which occurs exclusively in the CCW direction, i.e., EP → ZM and ZP → EM; this is precisely what was anticipated from inspection of the MEPs. No other photoreactions, e.g., cyclization, were observed in the NAMD simulations. The NAMD trajectories were propagated for up to 480 fs, which provided sufficient time window for relaxation of the reaction products on the S0 PES. The geometries along the trajectories were inspected visually (see the multimedia files in the Supporting Information) and the central dihedral angle θ (see Scheme 1 for definition) along the trajectories was calculated and plotted in Figure 3 (see Figure SI-4 for the θ values of different conformations). The quantum yields of the EP → ZM and ZP → EM photoreactions were calculated as a fraction of the successfully isomerized (i.e., ended up in the ZM


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or the EM conformations) trajectories in the total number of trajectories started at the respective conformation. Both photoisomerization steps display sufficiently high quantum yield, φEP→ ZM = 0.55 and φZP→ EM = 0.68, see Table 1. The total quantum yield of the working cycle of DTMF, which includes both isomerization steps, can be estimated as Φtot = 21 φEP→ ZM φZP→ EM /(φEP→ ZM + φZP→ EM − φEP→ ZM φZP→ EM ); this yields Φtot ≈ 0.22, see the Supporting Information. Had a motor have both photoisomerizations with φ < 0.3, the total quantum yield would be < 0.09. At the beginning of the EP → ZM and ZP → EM photoreactions, the π component of the C4 =C30 bond is broken and this leads to rapid oscillations of the adjacent single and double bonds as reflected by the BLA distortion. Evolution of the BLA distortion and the central torsion angle θ for all the NAMD trajectories is shown in Figure 3. The BLA distortion undergoes rapid (with the period of ca. 25 fs) oscillations; the oscillations are synchronized at the very beginning of the isomerization, however loose synchronicity after ca. 100 fs. Simultaneously with the BLA oscillations, rotation about the C4 –C30 axis sets off. For the EP → ZM reaction, the rotation progresses rapidly and after ca. 100 fs it reaches nearly 30◦ . For the ZP → EM reaction, the rotation is stalled initially (due to steric hindrance from the methyl group at C5 ) and the torsion angle θ begins to deviate noticeably from its initial value after ca. 150 fs. When the torsion angle θ reaches ca. ±90◦ , the NAMD trajectories approach the respective CIs at which the S1 → S0 population transfer occurs. Interestingly, the population transfer takes place within a narrow range (± 4◦ ) of torsion angle θ values. Both forward propagating (EP → ZM and ZP → EM) trajectories and trajectories turning back (EP → EP and ZP → ZP) undergo nonadiabatic transition within the same range of torsion angles, see Figure 3. Hence, the fate of the trajectories seems to be decided upon the S1 →



a)

140

ZM EP

time, fs

120

BLA, Å

c)

1.0 0.8 0.6

100

0.4

80

0.2

60

0.0

40 20

θ,

EP

de

g.

e, tim

fs

200

300

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600

λ, nm

b)

140

EM ZP

time, fs

120

BLA, Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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d)

1.0 0.8 0.6

100

0.4

80

0.2

60

0.0

40 20

θ,

de

ZP g.

, me

200

fs

300

400

500

600

λ, nm

ti

˚ with Figure 3: Variations of the central dihedral angle θ (deg.) and the BLA distortion (A) respect to propagation time (fs) along the NAMD trajectories initiated at the EP conformation (a) and the ZP conformation (b). Purple dots in panels a) and b) show the S1 → S0 surface hops for the trajectories propagating in the direction of isomerization (to the ZM and EM conformations, respectively) and cyan dots show the surface hops for the trajectories turning back to the initial conformation. Simulated time-resolved emission spectra of the EP (c) and ZP (d) conformations of the DTMF molecular motor. The emission spectra show fast (period ca. 25 fs) oscillations of the intensity at the beginning of photoisomerization.


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S0 transition, when the molecule is back in the S0 state. It is likely that the shape of the S0 PES in the vicinity of ± 90◦ of torsion (and in the vicinity of the CIs), see Figure 2, is the decisive factor for the branching ratio of the photoreaction. Interestingly, the S1 and S0 MEPs in Figure 2 allow to anticipate a higher quantum yield for the ZP → EM photoreaction; which is indeed observed in the NAMD simulations (φEP→ZM = 0.55, φZP→EM = 0.68). The rapid synchronous oscillations of the BLA distortion observed at the beginning of the NAMD trajectories can potentially manifest themselves in the emission spectra of the excited species. The simulated fluorescence spectra (see the Supporting Information for detail) of the EP and the ZP conformations show rapid oscillations of intensity occurring at the same frequency as the BLA oscillations. The intensity oscillations are caused by the oscillations of the S1 –S0 energy gap. As the torsional motion progresses at the initial stage of the trajectories, the gap narrows down and this causes the emission intensity to quickly fade away. 37 Although the intensity oscillations occur on a rapid time scale of ca. 25 fs, experimental ultrafast fluorescence upconversion measurements with the appropriate time resolution have been already reported in the literature. 38 In summary, the study of the S1 and S0 PESs and of the non-adiabatic dynamics of the S1 decay of the proposed DTMF molecule provides convincing evidence that 1) DTMF is a unidirectional molecular rotary motor; 2) DTMF has a reasonably high isomerization quantum yield (φ ∼ 0.55–0.68) of its photoisomerization steps, which enables its utilization in molecular devices; 3) DTMF (and related compounds) can be synthesized from readily available precursor compounds. The key to high quantum efficiency of the DTMF motor is in the topography of its S1 /S0 conical intersections. The latter have pronounced peaked topography and occur behind the S0 barrier to E/Z isomerization; thus hindering



falling back to the initial conformation upon the S1 → S0 population transfer. The theoretical methodology employed in this work has previously been used to predict the mechanism of photoisomerization in the Feringa-type molecular motors 15,16 (these predictions were fully confirmed by the subsequent experiments 18 ) and to design a new family of ultrafast synthetic photoswitches. 39 Hence, we believe that the predicted properties of the DTMF motor are sufficiently reliable and that this work should stimulate further research into the design and synthesis of new families of molecular rotary motors.

Acknowledgement This work was supported by Brain Pool Program (2018H1D3A2000493) and National Honor Scientist Program (2010-0020414) through the National Research Foundation of Korea(NRF) funded by the Ministry of Science and ICT. SKM acknowledges financial support by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2016R1C1B2015103).

Supporting Information Available Synthetic procedure, computational details, Cartesian coordinates and pictorial representations of all relevant species, simulated UV absorption and emission spectra, results and analysis of the NAMD simulations.


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References (1) Browne, W. R.; Feringa, B. L. Making Molecular Machines Work. Nature Nanotech. 2006, 1, 25–35. (2) Balzani, V.; Credi, A.; Venturi, M. Light Powered Molecular Machines. Chem. Soc. Rev. 2009, 38, 1542–1550. (3) Coskun, A.; Banaszak, M.; Astumian, R. D.; Stoddart, J. F.; Grzybowski, B. A. Great Expectations: Can Artificial Molecular Machines Deliver on Their Promise? Chem. Soc. Rev. 2012, 41, 19–30. (4) Kay, E. R.; Leigh, D. A. Rise of the Molecular Machines. Angew. Chem. Int. Ed. 2015, 54, 10080–10088. (5) Kassem, S.; van Leeuwen, T.; Lubbe, A. S.; Wilson, M. R.; Feringa, B. L.; Leigh, D. A. Artificial molecular motors. Chem. Soc. Rev. 2017, 46, 2592–2621. (6) Baroncini, M.; Casimiro, L.; deVet, C.; Groppi, J.; Silvi, S.; Credi, A. Making and Operating Molecular Machines: A Multidisciplinary Challenge. ChemistryOpen 2018, 7, 169–179. (7) Koumura, N.; Zijlstra, R. W. J.; van Delden, R. A.; Harada, N.; Feringa, B. L. LightDriven Monodirectional Molecular Rotor. Nature 1999, 401, 152–155. (8) Koumura, N.; Geertsema, E. M.; van Gelder, M. B.; Meetsma, A.; Feringa, B. L. Second Generation Light-Driven Molecular Motors. Unidirectional Rotation Controlled by a Single Stereogenic Center with Near-Perfect Photoequilibria and Acceleration



of the Speed of Rotation by Structural Modification. J. Am. Chem. Soc. 2002, 124, 5037– 5051. (9) Pijper, D.; van Delden, R. A.; Meetsma, A.; Feringa, B. L. Acceleration of a Nanomotor: Electronic Control of the Rotary Speed of a Light-Driven Molecular Rotor. J. Am. Chem. Soc. 2005, 127, 17612–17613. (10) Pollard, M. M.; Meetsma, A.; Feringa, B. L. A Redesign of Light-Driven Rotary Molecular Motor. Org. Biomol. Chem. 2008, 6, 507–512. (11) Guentner, M.; Schildhauer, M.; Thumser, S.; Mayer, P.; Stephenson, D.; Mayer, P. J.; Dube, H. Sunlight-powered kHz rotation of a hemithioindigo-based molecular motor. Nat. Comm. 2015, 6, 8406(8). (12) Wilcken, R.; Schildhauer, M.; Rott, F.; Huber, L. A.; Guentner, M.; Thumser, S.; Hoffmann, K.; Oesterling, S.; de Vivie-Riedle, R.; Riedle, E. et al. Complete Mechanism of Hemithioindigo Motor Rotation. J. Am. Chem. Soc. 2018, 140, 5311–5318. (13) Greb, L.; Lehn, J.-M. Light-Driven Molecular Motors: Imines as Four-Step or TwoStep Unidirectional Rotors. J. Am. Chem. Soc. 2014, 136, 13114–13117. ¨ (14) Greb, L.; Eichhofer, A.; Lehn, J.-M. Synthetic Molecular Motors: Thermal N Inversion and Directional Photoinduced C=N Bond Rotation of Camphorquinone Imines. Angew. Chem. Int. Ed. 2015, 54, 14345–14348. (15) Kazaryan, A.; Kistemaker, J. C. M.; Schäfer, L. V.; Browne, W. R.; Feringa, B. L.; Filatov, M. Understanding the Dynamics Behind the Photoisomerization of a LightDriven Fluorene Molecular Rotary Motor. J. Phys. Chem. A 2010, 114, 5058–5067.


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Kazaryan, A.; Lan, Z.; Schäfer, L. V.; Thiel, W.; Filatov, M. Surface Hopping ExcitedState Dynamics Study of the Photoisomerization of a Light-Driven Fluorene Molecular Rotary Motor. J. Chem. Theory Comput. 2011, 7, 2189–2199. (17) Filatov, M.; Olivucci, M. Designing conical intersections for light-driven single molecule rotary motors: From precessional to axial motion. J. Org. Chem. 2014, 79, 3587–3600. (18) Conyard, J.; Addison, K.; Heisler, I. A.; Cnossen, A.; Browne, W. R.; Feringa, B. L.; Meech, S. R. Ultrafast Dynamics in the Power Stroke of a Molecular Rotary Motor. Nature Chem. 2012, 4, 547–551. (19) Conyard, J.; Cnossen, A.; Browne, W. R.; Feringa, B. L.; Meech, S. R. Chemically Optimizing Operational Efficiency of Molecular Rotary Motors. J. Am. Chem. Soc. 2014, 136, 9692–9700. ˇ (20) Kistemaker, J. C. M.; Stacko, P.; Visser, J.; Feringa, B. L. Unidirectional rotary motion in achiral molecular motors. Nat. Chem. 2015, 7, 890–896. (21) Marchand, G.; Eng, J.; Schapiro, I.; Valentini, A.; Frutos, L. M.; Pieri, E.; Olivucci, M.; Léonard, J.; Gindensperger, E. Directionality of Double-Bond Photoisomerization Dynamics Induced by a Single Stereogenic Center. J. Phys. Chem. Lett. 2015, 6, 599– 604. (22) Garc´ıa-Iriepa, C.; Marazzi, M.; Zapata, F.; Valentini, A.; Sampedro, D.; Frutos, L. M. Chiral hydrogen bond environment providing unidirectional rotation in photoactive molecular motors. J. Phys. Chem. Lett. 2013, 4, 1389–1396.



Page 20 of 22

(23) Nikiforov, A.; Gamez, J. A.; Thiel, W.; Filatov, M. Computational Design of a Family of Light-Driven Rotary Molecular Motors with Improved Quantum Efficiency. J. Phys. Chem. Lett. 2016, 7, 105–110. (24) Wang, J.; Oruganti, B.; Durbeej, B. Light-driven rotary molecular motors without point chirality: a minimal design. Phys. Chem. Chem. Phys. 2017, 19, 6952–6956. (25) Oruganti, B.; Wang, J.; Durbeej, B. Excited-State Aromaticity Improves Molecular Motors: A Computational Analysis. Org. Lett. 2017, 19, 4818–4821. (26) Yokoyama, Y. Fulgides for Memories and Switches. Chem. Rev. 2000, 100, 1717–1740. (27) Kaftory, of

M.

(E)

and

Photochromic (Z)

and

isomers

thienyl)ethylidene]succinic

of

thermochromic

compounds.

I.

Structures

2-isopropylidene-3-[1-(2-methyl-5-phenyl-3-

anhydride,

C20H18O3S,

and

the

photoproduct

7,7a-dihydro-4,7,7,7a-tetramethyl-2-phenylbenzo[b]thiophene-5,6-dicarboxylic anhydride (P), C20H18O3S. Acta Crystallogr. Sec. C 1984, 40, 1015–1019. (28) Glaze, A. P.; Harris, S. A.; Heller, H. G.; Johncock, W.; Oliver, S. N.; Strydom, P. J.; Whittall, J. Photochromic heterocyclic fulgides. Part 4. The thermal and photochemical reactions of (E)-isopropylidene-[[small alpha]-(2- and -(3thienyl)ethylidene]succinic anhydrides and related compounds. J. Chem. Soc., Perkin Trans. 1 1985, 957–961. (29) McCabe, R. W.; Parry, D. E.; Saberia, S. P. Organometallic fulgides. Part 1. Synthesis and photochemistry of ferrocenyl fulgides [ferrocenylethylidene(isopropylidene)succinic anhydrides]. J. Chem. Soc., Perkin Trans. 1 1993, 1023–1029. 20 ACS Paragon Plus Environment

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Paul, G.; Kebarle, P. Electron affinities of cyclic unsaturated dicarbonyls: maleic anhydrides, maleimides, and cyclopentenedione. J. Am. Chem. Soc. 1989, 111, 464–470. (31) Levine, B. G.; Mart´ınez, T. J. Isomerization Through Conical Intersections. Annu. Rev. Phys. Chem. 2007, 58, 613–634. (32) Ewen, J. A.; Elder, M. J.; Jones, R. L. Patent No.: US 6,635,779 Date of Patent: Oct. 21, 2003. (33) Filatov, M. Spin-restricted ensemble-referenced Kohn-Sham method: basic principles and application to strongly correlated ground and excited states of molecules. WIREs Comput. Mol. Sci. 2015, 5, 146–167. (34) Filatov, M. In Density-functional methods for excited states; Ferré, N., Filatov, M., HuixRotllant, M., Eds.; Top. Curr. Chem.; Springer: Heidelberg, 2016; Vol. 368; pp 97–124. (35) Bauer, J.; Hou, L.; Kistemaker, J. C. M.; Feringa, B. L. Tuning the Rotation Rate of Light-Driven Molecular Motors. J. Org. Chem. 2014, 79, 4446–4455. (36) Ha, J.-K.; Lee, I. S.; Min, S. K. Surface Hopping Dynamics beyond Nonadiabatic Couplings for Quantum Coherence. J. Phys. Chem. Lett. 2018, 9, 1097–1104. (37) The oscillator strength f i f of a i → f transition is proportional to the energy gap, f i f = 23 Ei − E f |µi f |2 , where µi f is the transition dipole moment. (38) Lee, G.; Kim, J.; Kim, S. Y.; Kim, D. E.; Joo, T. Vibrational Spectrum of an Excited State and HuangRhys Factors by Coherent Wave Packets in Time-Resolved Fluorescence Spectroscopy. ChemPhysChem 2017, 18, 670–676.



(39) Paolino, M.; Gueye, M.; Pieri, E.; Manathunga, M.; Fusi, S.; Cappelli, A.; Latterini, L.; Pannacci, D.; Filatov, M.; Léonard, J. et al. Design, Synthesis, and Dynamics of a Green Fluorescent Protein Fluorophore Mimic with an Ultrafast Switching Function. J. Am. Chem. Soc. 2016, 138, 9807–9825.


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Fulgides as light driven molecular rotary motors: Computational

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