Self-Adaptive Switch Enabling Complete Charge Separation in

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Self-Adaptive Switch Enabling Complete Charge Separation in Molecular-Based Opto-Electronic Conversion Ziye Wu, Peng Cui, Guozhen Zhang, Yi Luo, and Jun Jiang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00119 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018

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The Journal of Physical Chemistry Letters

Self-Adaptive Switch Enabling Complete Charge Separation in Molecular-Based Opto-Electronic Conversion Ziye Wu,† Peng Cui,† Guozhen Zhang,† Yi Luo, Jun Jiang* Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China Supporting Information Placeholder

ABSTRACT: Achieving high charge recombination probability has been the major challenge for the practical utilization of molecule-based solar harvesting. Molecular switches were introduced to stabilize the charge separation state in donoracceptor systems, but it is difficult to seamlessly incorporate the ON/OFF switching actions into the opto-electronic conversion cycle. Here we present a self-adaptive system where the donor and acceptor are bridged by a switchable moiety that enables a complete charge separation repeatedly. Calculations are presented for a platinum(II) terpyridyl complex with an azobenzene bridge. The charge transfer induced by light extracts electrons from the azobenzene group, automatically triggering a trans→cis isomerization. The resulting conformation suppresses charge recombination. Energized charges are trapped in the acceptor, ready for charge collection by electrodes. The bridge then goes through inverse isomerization to restore the conjugation and conductance. This self-adaptive design provides a novel way to improve the performance of opto-electronic conversion and realize practical solar harvesting applications in organic molecular systems.

TOC Graphic

Charge separation (CS) is crucial for practical opto-electronic applications based on the photo-induced charge transfer (CT). For example, organic solar cell (OSC), which converts solar energy into electric energy through organic molecular photo-excitation, holds great potential for sustainable technology thanks to its advantages of low cost, lightweight, structural flexibility, easy fabrication, and green utilization.1-3 However, utilization of OSCs has long been hampered by the low efficiency of opto-electronic conversion, due to the fast charge recombination (CR) before the collection/depletion of energetic charges by electrodes.4 Inorganic solar cells are successful because of high carrier mobility, in which photo-excited charges can be separated into free electron and hole carriers.5 In contrast, although many OSCs with the donor-acceptor (D-A) framework realize charge transfer state by moving an electron from donor to acceptor,6-8 their charge

separation is reversible because of the delocalized nature of molecular orbitals in well-conjugated D-A system.9 Instead of creating complete CS state, molecular excitation merely generates an exciton, where a positive charge is bound to a negative electron through D-A conjugation. Because of the low mobility of molecular carriers to adjacent molecules or electrodes, the backward CT process can easily happen when excited electrons move from the acceptor part back to the donor through radiative or non-radiative CR.4,10 One strategy to achieve the complete CS state in D-A system is to design a sequential CT pathway with negative energy gradient that prohibits backward CT.11 This however causes substantial energy loss. It was therefore proposed to shut down the reverse CT channel by incorporating a molecular bridge to control electric conductance. Monti et al. designed a donor-antenna-acceptor molecular rectifier to realize a stable CS state,12 which was subjected to extra energy loss because of the need for energy level matches.13 Castellanos et al. have shown that a D-A system sandwiched by a non-conductive bridge exhibits much slower CR than that of another D-A with a conductive bridge.14 Gust et al. reported an acceptor-donor-switch design in which excited charges transfer through two pathways with different conductivities depending on the isomerization of a molecular switch.15 Unfortunately, those designs based on inserting bridge molecule between D-A, cannot switch the ON and OFF conductivity in a cyclic manner, implying that both CT and CR are equally suppressed by the “OFF” state. Recently, Bao et al. have made a D-A dyad that rectifies the forward and backward CT through dipole-mediated conformation changes,16 suggesting possible switch control with electric field. Nevertheless, a major remaining challenge for switchable D-A lies in the precise control of the ON/OFF action time sequence. That is, to seamlessly couple into the loop sequence of light harvesting, charge transfer, charge separation, charge collection/depletion, and system recovering for the next cycle of opto-electronic conversion/storage. For instance, action of switching “OFF” needs to take place right after the photo-induced CT, whose lifetime in molecule is normally within nano-micro second (ns~µs) scale. This requires precise timings of response and action for external electric field control in ns scale, which is impractical now. Undoubtedly, the CS state achieved by rectifying molecule is unrepeatable. Therefore, switchable D-A systems are not feasible without the self-adaptive switching “OFF” after the instant detection of each photo-excitation and switching “ON” before each light harvest. In this work, we propose a self-adaptive donor-switch-acceptor (D-S-A) system. Using a first-principles theoretical study we demonstrate that the light-induced CT (in ~2 ps timescale)

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triggers “OFF” switching through isomerization of the switch moiety (in ps~ns timescale). A long-lived CS state (lifetime in ~µs scale) is established. The depletion of energized charges automatically recovers the “ON” state, making it ready for next light harvesting. A smooth loop sequence of light harvesting is achieved without external control (Figure 1a). The D-S-A system can realize complete CS and repeated energy conversion. The complete CS requires the breaking of exciton bonding, preserving of energetic carriers, and repeating in opto-electric conversion cycle. The D-S-A model system is displayed in Figure 1a, illustrating the working mechanism as follows: (1) Light absorption induces CT excitation. (2) The excited electron transfers from donor to acceptor through the conductive “ON” switch channel (with a timescale of ~2 ps). (3) The CT process turns “OFF” the switch (in ps~ns timescale) before the annihilation of excited state (lifetime in ~µs scale). The key is to ensure the switch action being self-adapted to the CT process, automatically realize ON/OFF motion without external control. (4) The CS state is an energized electron in acceptor is prohibited from moving back to donor. By suppressing both radiative and non-radiative CRs, we achieve long-lived CS states with free energetic electron and hole carriers. (5) Energized charges are collected and depleted by the battery electrode for future utilization, resulting in the uncharged D-S-A with the switch “OFF”. (6) The molecular switch is restored to the “ON” state after charge depletion, to make the D-S-A ready for the next cycle of light harvesting and converting. We expect that no external control is needed in the whole process thanks to the selfadaptive nature of the azobenzene (Azo) moiety. By achieving recyclable and durable CS states in such a self-switchable D-S-A system, we anticipate substantial improvement on the efficiency of opto-electronic conversion.

Figure 1. (a) Schematic diagram for the working mechanism of a D-S-A system. (b) A typical D-A system made of (–C≡C–Ph)n and [Pt(tBu3tpy)]+, together with the photoswitch Azo whose electric conductive state (trans-Azo) can be switched to the OFF state (cis-Azo) by ultraviolet (UV) excitation, and vice versa by adding visible (Vis) light. (c) The D-A system of [Pt(tBu3tpy)(– C≡C–Ph)1]+ harvests visible light via CT excitation when the donor and the acceptor are conjugated in the same plane, while the CT is prohibited when they are not conjugated in the same plane. Red and purple bubbles illustrate molecular orbitals in the CT state.

We have simulated a model molecular system at the timedependent density function theory (TDDFT) level using the Gaussian 09 program.17 The intramolecular CT process is based on elastic electron tunneling mechanism, which can be modeled by using TDDFT and ab. initio molecular dynamics. We compared results predicted by different functionals, including representative hybrid B3LYP,18 PBE0,19 and CAM-B3LYP20 that contains long-range exchange correction.21 Although long-rang exchange correction often helps to improve CT state simulation, we found that CAM-B3LYP result has a big deviation from experiment in absorption spectra of Dn-A system (n=1-3), while B3LYP and PBE0 are better in capturing the features of experimental spectra (Figure S1). Besides, B3LYP and PBE0 have been proved reliable for the strongly bonded Pt complexes and commonly used in theoretical studies.22-24 The square planar platinum(II) terpyridyl complex of [Pt(tBu3tpy)(–C≡C–Ph)n]+ (Figure 1b), a typical D-A photosensitizer system which we have studied before,22 is chosen as the starting point of exploring the D-S-A model. Here (–C≡C–Ph)n is the donor and [Pt(tBu3tpy)]+ is the acceptor. This D-A is known for excellent optoelectronic properties for visible (Vis) light harvesting,25 and characterized by low-lying CT states with a long lifetime of ~3 µs.26 Another interesting property was recently revealed by our study that its Vis-induced CT ability relies on the D-A conjugation feature.22 Specifically, the CT probability reaches the maximum when donor and acceptor are well conjugated in the same plane, while the perpendicular orientation between donor and acceptor results in poor conjugation that prevents CT (Figure 1c and details in Figure S2). This angular-dependent behavior thus enables a CT control through the D-A conformation tuning with a sandwiched azobenzene (Azo) photoswitch. The Azo molecule (Figure 1b) has the trans and cis forms, which can interconvert by photo or thermal excitation.27 It has a remarkable photo-stability and reversibility.28 Its photo-induced structural variations result in global conformational changes, leading to useful photo-control applications.29-31 At room temperature the Azo group is 100% in the thermally more stable trans form. After UV light radiation, about 90% molecules turn to the cis form.32 The trans→cis isomerization converts the planar molecule to the non-planar form with two phenyl rings in ~65º dihedral angle, which breaks the conjugation and dramatically reduces the conductivity. As we notice the small structural difference between the Azo (Ph–N=N–Ph) and donor unit (Ph– C≡C–Ph), we expect easy of synthesis to substitute one C≡C with a N=N group. Using a N=N to replace the second C≡C near the acceptor in the D-A, we built the D-S-A complex n of [Pt(tBu3tpy)–C≡C–Azo(–C≡C–Ph)n]+. Geometry optimizations identified the most stable conformation as trans-n, which is characterized by the trans-Azo bridge, planar geometry and strong D-A conjugation. Unlike many cases that neighboring metal centers inhibit molecular photoisomerizations,33 Azo has been successfully integrated into Pt(II) complexes,34 especially Pt(II) terpyridine complexes35 in which the isomerization of Azo moiety can still be induced by light. Consequently, the trans-n and cis-n can interconvert to each other through the Azo bridge switching, enabling us to tune the backward CT ability and D-A conductance. As a photovoltaic device, complex n will be docked to conducting leads. Azo moiety has been successfully integrated into conducting polymers as molecular electronic devices, exhibiting good flexibility and retaining the photoisomerization properties.36,37 We expect that complex n that inducing an Azo moiety into an organic molecule can be also applied to the electrical circuit in a similar fashion. Considering the isomerization of Azo moiety will result in a change of distance between the conducting leads, flexible electrodes will be used to mitigate this issue.


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The Journal of Physical Chemistry Letters Following the mechanism in Figure 1a, we had first validated the first two steps of effective light harvesting and CT in trans-1, the smallest D-S-A [Pt(tBu3tpy)–C≡C–Azo(–C≡C–Ph)1]+ (Figure 2a). The simulated ultraviolet-visible (UV-Vis) spectrum (Figure 2b) of trans-1 displays strong visible light absorptions at ~457 and ~541 nm, respectively. The latter is attributed to the first excited state S1 as computed by TDDFT, corresponding to electronic transitions from the highest occupied molecular orbital (HOMO: located on donor) and the one below HOMO (HOMO1: located on donor) to the lowest unoccupied molecular orbital (LUMO: located on acceptor) (Figure 2c). Both transitions exhibit typical CT features with electron motion to the [Pt(tBu3tpy)]+ moiety. It is noted that the Azo bridge partially oxidized during the CT process (Figure 2c), serving the role of co-donor. The CT extracts ~0.19 effective negative charge from the two central N atoms of Azo, creating a cationic Azo moiety. This has a strong excitation probability as reflected by the oscillation strength of f =1.06, which is stronger than the pure D-A of [Pt(tBu3tpy)(– C≡C–Ph)3]+ with similar size. Electronic transitions of another two higher excitations also involve CT from the trans-Azo to acceptor (Figure S3). Results of B3LYP calculations are consistent with PBE0 and CAM-B3LYP (Figure S4) calculations. To quantitatively describe the electron dynamics associated with charge transfer, we carried out time dependent non-adiabatic molecular dynamics with the surface hopping scheme in the PYXAID (PYthon eXtension for Ab Initio Dynamics) program.38,39 The time-dependent electron distribution indicates that significant charge transfer from the part of donor and switch to the part of acceptor of trans-1 takes places within 2 ps (Figure S5). This is quicker than the isomerization time of ~5 ps in comparative Azo derivatives,40 making sure that photo-excited electrons can migrate to acceptor before the bridge isomerization. More importantly, negative charge in the acceptor barely migrate back to the donor through the cis-bridge within 2 ps (Figure S5), indicating a “complete” charge separation and a consequent trap of photo-excited electron in the acceptor.

Figure 2. (a) The optimized structure of the ground state of molecule trans-1. (b) The computed UV-Vis absorption spectrum of trans-1. (c) Photo-excitation to the first excited state of trans-1, characterized by molecular orbitals (bubbles) from H (HOMO) and H-1 (HOMO-1) to L (LUMO), excitation energy (Ex), absorption wavelength (λx), and oscillator strength (f). (d) Structure optimization of the first excited state of trans-1 results in cis-1 conformation. (e) Schematic illustration of the Azo trans→cis isomerization induced by UV light (black straight arrow for excitation and curved arrows for relaxation), and the one induced by ionization of removing an electron (red straight arrow). The upper-right inset graph displays the simulated potential energy surfaces along both ways. Here and

trans Strans , S1 , 0

Strans represents the ground, first excited and second excited 2

state of the trans-Azo, and

Scis is the ground state of the cis-Azo. 0

[Strans ]+ and [Scis ]+ stand for positive-charged ground states in 0 0 trans and cis forms, respectively. All results are based on B3LYP simulations, which are consistent with those from PBE0 and CAM-B3LYP (Figure S4). We then examined the third step (switching “OFF”) of selfadapted D-S-A switching through Azo bridge modulation. Since the CT of trans-1 also involves Azo excitation, which can induce isomerization of the isolated Azo, we conducted geometry optimization to relax the first excited state of trans-1. Noteworthy, the relaxation results in a cis-1 structure, automatically reaching the “OFF” state with non-planar conformation and nonconjugated D-A frame (Figure 2d, Figure S4c and S5c for three DFT levels). Such CT induced D-S-A switching can be rationalized by the isomerization pathways of the isolated Azo in Figure 2e. For the direct photo-switching of trans-Azo, the trans

ground state ( S0

) is excited by UV light to the second lowest

trans 42 . 2

Strans relaxes quickly to the lowest 2

excited state of

S

excited state of

S1trans within 200 fs.28,42 Then the isomerization

from

The

S1trans to Scis takes place (Figure 2e) with a time constant 0

of 0.4~3 ps,28,32 while that of Azo derivatives is ~5 ps.40,41 Here the creation of

S1trans holds the key to overcome the isomerization

energy barrier and the generated

S1trans could also relax to the

Strans which lowers the quantum yield of trans→cis 0 isomerization. While in the D-S-A system, the CT excitations extracts electrons from the trans-Azo bridge, producing a partial ionized state at the Azo moiety. The

[Strans ]+ of the isolated 0

trans-Azo was found to resemble well the neutral excited state

S1trans , in terms of removing of an electron from the HOMO of Strans and the associated increasing of C–N=N angle from 115º in 0 Strans to ~130º (Figure 2e). There are two possible isomerization 0 pathways (Figure S6). Along the inversion pathway (Figure S7), the energy barrier for trans→cis transformation of Azo is ~2.5 eV, which can be overcome via the photo-excitation from

Strans to 0

S1trans . Noticeably, this barrier is sharply reduced to ~0.6 eV for a cationic Azo (Figure 2e), as validated at the PBE0/6-31G(d) and CAM-B3LYP/6-31G(d) levels, respectively (Figure S7b). Similar



trends were found with the rotation pathway (Figure S8). The conclusion that the energy barrier can be reduced by the ionization of Azo is consistent with previous experiments of lowering the barrier in oxidized Azo.43-46 To make sure it is the same with whole D-S-A system 1, we performed the energy scanning for 1 and found that the energy barrier for trans→cis isomerization is reduced from ~2.4 eV to ~0.9 eV with the ionization of 1 (Figure S9). The decrease of energy barrier leads to the increase of trans→cis isomerization rate. Consequently, the phenomenon that the light harvesting and CT of trans-1 cause ionization for the trans-Azo bridge substantially facilitates the self-adapted switching “OFF” action to cis-1. It also indicated less solar energy consumption for the isomerization. It is well known that the reduction of energy barrier for cationic Azo moiety is equally effective in both forward and backward directions of trans→cis isomerization. Similar to the trans→cis isomerization occurring with photo-induced CT, the cis→trans isomerization also needs an extra external stimulus to overcome the barrier. It should be noted that the excited state of cis-1 is difficult to be excited again (Figure S10). Only after the excited electron is consumed, the cis-1 can harvest photo energy and use that to become trans-1. The ns~µs lifetime of most CT excited states is much longer than the Azo derivatives isomerization timescale of ~5 ps,40,41 allowing the switch action to trap energized electrons before CR takes place. Thus, by making use of such self-adapted switching action, we circumvented the technical difficulty to add (withdraw) external control on Azo switching before (after) CT occurring. The switching “OFF” step leads to the next two steps of achieving a complete charge CS and consequently ensuring charge collection. All of the photo-excited states of trans-1 relax to the lowest excited state in the cis-1 form, in which ~0.79 echarge migrates from the donor and Azo bridge to the acceptor part. To ensure separated ~0.79 e- and ~0.79 h+ carriers in cis-1, both the non-radiative and radiative CRs have to be restrained. The intrinsic non-radiative CR usually involves energetic electrons moving back from acceptor to donor through the bridge channel. Such backward CT ability is investigated by simulating electric conductivity and ultrafast electron evolution process. It was found that the conductivity is dramatically reduced from trans-1 to cis-1, as reflected by a 98 : 1 ON/OFF ratio in electron transmission ability of the first conducting peak for electron with ~1.3 eV, and two more peaks with higher trans/cis ratios (Figure 3a). The similar ON/OFF ratio of 34:1 was found with conductivity computed with the excited states (Figure S11). As the time-dependent electron distribution indicates, electrons can hardly migrate from acceptor to donor through cis-1 while that through trans-1 is very efficient (Figure S5), thus the isomerization of switch enables control on charge migration between donor and acceptor. Meanwhile, the radiative CR in cis-1 owing to S1→S0 transition is trivial, as indicated by a small oscillator strength of 0.01 for this transition (Figure 3b). The poorly conjugated cis-1 thus turns exciton to free carriers, while preserving the excitation energy. This ensures the collection of energetic charges, by allowing slowly-conducting carriers to be eventually collected by electrodes, which in turn converts the cis1 with free carriers to the ground state. The last step for D-S-A mechanism is to restore cis-1 isomer to trans-1, making it ready for the next cycle of light-harvesting and energy conversion. The step is self-adapted upon the depletion of energized carriers, which produces the cis-1 ground state available for restoring switching “ON” status. Although lightinduced CT excitation in cis-1 is negligible, its cis-Azo bridge can response to visible light. The UV-Vis spectrum of the isolated cisAzo molecule shows effective Vis-light absorptions (Figure 3c), in which the 473 nm absorption is due to electron transition from

HOMO to LUMO (Figure 3d). Such transition is known to drive cis→trans isomerization, switching “ON” the conductivity of the isolated cis-Azo (Figure 1b).28-30 For the cis-1, Vis-light absorption is inherited from its cis-Azo bridge (Figure 3e). Here the lowest excitation exhibits almost the same electronic transition behavior as the isolated cis-Azo (Figure 3f), suggesting high probability for transformation from cis-1 to trans-1. Therefore, we can conveniently restore the switch back to “ON” status via visible light after energetic carriers are collected.

Figure 3. (a) Electric conductivity reflected by the electron transmission ability as the function of electron energy in trans-1 and cis-1, exhibiting a 98.3 : 1 ON/OFF ratio. (b) Radiative electron transitions from the first excited state to ground state in cis-1. (c) The computed UV-Vis spectrum of cis-Azo. (d) Electron transitions of the excitation to the first excited state of cis-Azo. (e) The computed UV-Vis spectrum of cis-1. (f) Electron transitions of the excitation to the first excited state of cis-1. Here H (L) denotes HOMO (LUMO). All transitions are characterized by molecular orbitals (bubbles), photo energy (Ex), photo wavelength (λx), and oscillator strength (f). To validate the mechanism in D-S-A in larger systems, we have investigated the complexes 3 and 5 ([Pt(tBu3tpy)–C≡C–Azo(– C≡C–Ph)n]+ with n=3 and 5, respectively) (Figure 4a), which also offer good Vis-light harvesting ability with the trans forms (Figure 4b). The electronic transitions responsible for the lowest excitations of the trans-3 and trans-5 exhibit similar CT behaviors to those of trans-1 (Figure 4c and d). Intriguingly, the CT effect is so strong that electrons can migrate for a distance of seven –C≡C– Azo and –C≡C–Ph groups to the acceptor part in trans-5 (Figure 4d). As the dihedral angle between the donor and acceptor of trans-1, 3, 5 increases from 0° to 90°, photo-absorption intensity decreases, together with the reduction of the contribution of the donor to HOMO (Figure S13-S15). Meanwhile, the absorption intensity increases with the increase of n if holding the same dihedral angle (Figure S13d). More importantly, the light


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The Journal of Physical Chemistry Letters harvesting also automatically switches “OFF” the trans-3 and trans-5, as demonstrated by the optimized structure of their lowest excited states (Figure 4e and f). These suggest generalization of the D-S-A design in larger systems towards engineering on realistic devices.

Supporting Information Available：：Computational methods, absorption properties of Dn-A system, electron transition of trans1 in higher excited states, computed results at PBE0 and CAMB3LYP level, ultrafast electron evolution process, potential energy curves of trans-cis isomerization, electric conductivity of 1 with excited state.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ORCID Jun Jiang: 0000-0002-6116-5605

Author Contributions †Z.W., P.C. and G.Z. contributed equally to the work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the 973 Program (No. 2014CB848900, 2016YFA0400904), the National Natural Science Foundation of China (No. 21633006, 21473166), and the CAS Strategic Priority Research Program B (No. XDB01020000). Figure 4. (a) Structure for the ([Pt(tBu3tpy)–C≡C–Azo(–C≡C– Ph)n]+ D-S-A system. (b) The computed UV-Vis spectra of trans1, 3, 5. Electron transitions of the excitation to the first excited state of trans-3 (c), and trans-5 (d), which resulted in the optimized structures of cis-3 (e) and cis-5 (f). Here H (L) denotes HOMO (LUMO). All transitions are characterized by molecular orbitals (bubbles) of, photo energy (Ex), photo wavelength (λx), and oscillator strength (f). We have designed a self-adaptive donor-switch-acceptor system, which demonstrated a self-switchable mechanism to ensure the repeatable and complete CS in opto-electric conversion. This would overcome the bottleneck problem of backward CT and non-negligible CR probability in traditional DA molecular system. Upon visible light exposure, our D-S-A undergoes CT from the donor and switch-bridge to the acceptor, which triggers the trans→cis isomerization of the bridge. It becomes a new molecule in which the donor and acceptor are no longer conjugated, which keeps photogenerated electron (on acceptor) and hole (on donor) separated. The D-A conductance is switched “OFF”, and thereby prevents the transferred charges from moving back. The complete CS state with long lifetime is achieved, ensuring the collection/depletion of charges. The charge depletion in turn triggers cis→trans isomerization to switch “ON” the D-S-A for the next cycle of light harvesting and conversion. There is no need to apply external control on switching actions, so that the “ON” and “OFF” switching are integrated naturally into the loop sequence of light harvesting CT switching “OFF” CS charge depletion switching “ON”. We are exploring other popular photo-switch molecules and different D-A systems in the same framework to concrete this D-S-A concept and searching for better candidates for applications. This would suggest cost-effective molecular systems for realistic optoelectronic conversion, photochemical catalysis and other solardriven applications.

ASSOCIATED CONTENT

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