Hypsochromic solvent shift of the charge separation band in ionic donor-acceptor Li + @C 60 ⊂ [10]CPP

: Donor-acceptor dyads with C 60 as electron acceptor and a cycloparaphenylene (CPP) unit assembled via non-covalent interactions as electron donor have been reported in the literature. In this work, we study computationally using the DFT/TDDFT approach the photoinduced electron transfer (PET) in CPP ‐ based donor– acceptor supramolecule C 60 ⊂ [10]CPP and Li + @C 60 ⊂ [10]CPP. As it is well known, the energy of charge separated (CS) states generated in donor-acceptor systems by PET are significantly stabilized by polar environment. Based on the analysis of initial and final states after PET in various fullerene complexes, we find a system, Li + @C 60 ⊂ [10]CPP, that shows anomalous solvent effects, i.e. , destabilization of charge separated states by polar medium. To our knowledge, this is the first example of fullerene based systems where a hypsochromic shift of the CS band is demonstrated. The CS reaction in the Li + @C 60 ⊂ [10]CPP complex is found to occur in the inverted Marcus regime. In spite of that the CS reaction rate becomes faster with increasing solvent polarity of the environment.

Conversion of sunlight into more accessible forms of energy, such as electrical or chemical ones, is a one of primary challenge for the human race. A lot of attention and efforts have been paid to design and prepare model compounds that mimics natural photosynthetic systems. [1] Generation of a long-lived chargeseparated (CS) state with high quantum yield and separation of radical ion pairs over long distances to prevent their recombination are extremely important conditions for photosynthetic systems. [2] Most of photosynthetic reactions, both natural and artificial, take place in solution or media that significantly influences this process. [3] Multi-component systems mimicking natural photosynthetic processes have been designed and extensively studied.
Among potentially suitable chromophores, much attention has been paid to boron dipyrromethane (BODIPY) and its derivatives, [4] porphyrinoids [1a, 5] and other rigid and planar structures with a high extinction coefficient and fluorescence quantum yield. As an acceptor unit, one uses often fullerenes which demonstrate a strong electron withdrawing power, high electron delocalization ability, and small reorganization energy. [6] Electronic communication between donor (D) and acceptor (A) is a key feature in the design of photosystems. Electronic and structural nature of bridging spacer between donor and acceptor can dramatically influence the photoinduced electron transfer (PET) dynamics. [7] D-A systems assembled via non-covalent interactions are expected to give rise to efficient PET, thus representing an attractive alternative to covalently linked analogs. [8] The dispersion interaction is responsible for stabilization of such complexes. [9] Because of that, the use of concave-convex complementarity is an excellent strategy to increase the stability of the fullerene complexes. [8d,10] Cycloparaphenylenes (CPPs) are hoop-shaped π-conjugated molecules in which paraphenylene units form a cycle. CPPs have attracted the attention of scientists due to their interesting properties arising from distorted and strained aromatic system and radially oriented π-orbitals. [11] Nanosized concave cavities of CPPs, so-called carbon nano-rings, can act as hosts for πconjugated molecules with convex surfaces, such as fullerenes. [12] In 2011, Yamago and co-workers [12d] demonstrated that C60 can be selectively encapsulated by [10]CPP forming stable C60⊂ [10]CPP complex in toluene with a binding constant (Ka) of 2.79±0.03×10 6 mol/L. Later, Itami and co-workers [13] successfully synthesized and characterized Li + @C60⊂ [10]CPP complex with Ka = (4.78±0.66)×10 4 mol/L in dichloromethane. Electrochemical measurements and spectroscopic analysis revealed strong charge-transfer interaction between [10]CPP and Li + @C60 parts. Herein we report a comprehensive analysis of photoinduced charge separated states in C60⊂ [10]CPP and Li + @C60⊂ [10]CPP complexes using Time-Dependent DFT theory coupled with conductor-like polarizable continuum model (CPCM) to account for environmental effects. Lowest-lying CS states of Li + @C60⊂ [10]CPP complex demonstrate unique behavior towards solvation. They are strongly destabilized by the solvent. Moreover, destabilization effect increases with increasing polarity of the solvent. Stability and ground state properties. Geometries of C60⊂ [10]CPP and Li + @C60⊂[10]CPP ( Figure 1) were optimized using BLYP-D3(BJ)/def2-SVP functional. [14] Subsequent excited state calculations were performed using time-dependent DFT formalism with range-separated CAM-B3LYP [15] functional (see full computational details in the SI). For the C60⊂ [10]CPP system, the interaction energy is found to be -53.8 kcal/mol, in a good agreement with previously estimated values by Rodríguez-Otero [16] and Zhao [17] using other DFT functionals. It is worth mentioning that doping of this system by Li + leads to noticeable stabilization of complex associated with superadditive effect similar to that found for carbon nano-onions (CNOs). [18] The stabilization energy of Li + @C60⊂ [10]CPP complex is -94.8 kcal/mol, which is almost 11 kcal/mol higher than sum of energies of the individual species, i.e. Li + @C60 and C60⊂ [10]CPP (Table  S1). As can be seen from Figure 1, LUMO in the Li + -doped C60 is by 3.26 eV lower than in the original C60 mainly due to electrostatic potential of Li + . In final complexes, electron-withdrawing abilities of pristine and Li + -doped fullerenes change only slightly compared to their free state. At the same time, the effect of association is more pronounced for the energy of the HOMO localized on [10]CPP fragment. The energy of the HOMO does not change when complex with C60 is formed, whereas Li + @C60 counterpart lowers the HOMO energy of [10]CPP by nearly 2 eV. Overall, HOMO-LUMO gap in Li + @C60⊂ [10]CPP complex is 1 eV smaller than in C60⊂ [10]CPP (3.15 and 4.15 eV, respectively). Electronic structure of the C60⊂ [10]CPP complex in the ground state (GS) can be described as two neutral fragments without charge separation. In case of Li + -doped complex, most of charge is localized on Li (0.77 e), while [10]CPP and C60 units are almost neutral. Singlet excited states. In order to characterize properties of excited states, C60⊂ [10]CPP and Li + @C60⊂ [10]CPP systems were divided into 2 and 3 fragments, respectively (Figure S1, SI): 1) [10]CPP; 2) C60; and 3) Li + (only in case of Li + @C60⊂[10]CPP). Exciton delocalization and charge transfer contributions were analyzed for 60 lowest excited states of each complex. Three types of excited states were identified: locally excited states (LE) exciton is mostly localized on single fragment (charge separation value < 0.1e); CS states where electron density is transferred between two fragments (charge separation > 0.9e); and mixed states with significant contributions of LE and CS (charge separation is between 0.1e and 0.9e). In the gas phase, the vertical singlet excitation energies of C60⊂ [10]CPP range from 2.50 to 4.30 eV. The LE state localized on C60 has the lowest energy and is associated with a HOMO-3 to LUMO+1 transition. LE states on [10]CPP unit are located at 3.46 eV and correspond to HOMO to LUMO+7 transitions. The first CS state found at 2.76 eV corresponds to electron transfer from [10]CPP to C60 (Table 1). This state can be described as purely HOMO-LUMO transition with 0.98e transferred. Therefore, the CS structure corresponds to C60 -⊂[10]CPP + . This CS transition has a relatively weak oscillator strength of about 0.001. Table 1. Singlet excitation energies (E, eV), the main singly excited configuration (HOMO(H)-LUMO(L)) and its weight (W), the oscillator strength (f), and the extent of charge separation (CS, e) or exciton localization ().  [11] Structure of the CS state can be described as Li + @C60 -⊂[10]CPP + . The frontier molecular orbitals representing the LE and CS states are shown in Figure S2. Environment effect on CS states. To evaluate the effect of solvation on the excited states, a COSMO-like model [19] was applied with dichloromethane (DCM) as the solvent. The GS solvation energies are -0.86 and -1.91 eV for C60⊂ [10]CPP and Li + @C60⊂ [10]CPP, correspondingly. The solvation energy of Li +doped complex is noticeably larger because system is charged. Usually for LE states, changes in the solvation energies with respect to the GS are relatively weak due to similarity in dipole moments, while for CT states the effect can be significant. [20] However, high symmetry of the studied complexes and ability of both fragments to effectively delocalize the charge allowed us to assume that even for the CS states the difference in solvation energies compared to the ground state should not be large.

Supramolecule
Calculations showed that differences in dipole moments between GS and CS states in C60⊂ [10] To get more insight into the environmental effect, we have additionally performed excited state simulations for 6 solvents of different polarity, from =2 (n-hexane) to =25 (benzonitrile). The gas-phase geometry was used in all cases to avoid any effect associated with geometry changes. Calculations of the C60⊂ [10]CPP complex, as a reference, were also performed. In all solvents, LE states for both C60⊂ [10]CPP and Li + @C60⊂ [10]CPP show the solvation energies similar to the GS, providing only negligible changes in the excitation energies (detailed data provided in Table S2). In contrast, the CS transition energies depend on the solvent polarity. As can be seen in Figure  2, the lowest CS state of Li + -doped complex demonstrates a big hypsochromic shift of about 0.4 eV. The Gibbs solvation energy estimated with the PCM model is determined by two terms: electrostatic Eel and non-electrostatic Enon energies. In turn, the last term may be divided into three parts including the dispersion, repulsion and cavitation components. [21] Because by a vertical transition, the geometry of the system remains unchanged, the solvation energy change by a difference in Eel of the initial and final states. To explain the hypsochromic shift of the CS transition, the molecular electrostatic potential (MEP) of Li + @C60⊂ [10]CPP complex in its GS and CS excited state was calculated (Figure 3). A large difference between the GS and CS MEPs is observed around the C60 unit. By PET, Li + @C60⊂ [10]CPP  Li + @C60 -⊂[10]CPP + , the total charge on Li + @C60 becomes almost neutral decreasing the MEP in its vicinity. MEP changes around [10]CPP are less pronounced due to efficient delocalization of the positive charge over the entire unit. A more elegant qualitative explanation for the observed phenomenon can also be provided. Let us divide the Li + @C60⊂ [10]CPP complex into 2 (rather than 3) parts, Li + @C60 and [10]CPP. Then the charge separation between C60 and [10]CPP can formally be described as charge shift from Li + @C60 to [10]CPP. For a charge-shift reaction, D + ···A  D···A + , a change in solvation energy is determined by the size of the D and A sites, providing the electrostatic potential is uniformly distributed around D + and A + ). The band shift can be roughly estimated using the Born equation: where  is the solvent dielectric constant. Because [10]CPP has a larger radius than Li + @C60, the final state of the ET reaction, [10]CPP + has a smaller solvation energy as compared to the initial state Li + @C60, which results in a hypsochromic shift of the CT band (positive h in eq. (1)) Electron transfer rates. Encapsulation of Li + by the fullerene is also associated by modulating the PET rate. The non-adiabatic electron transfer rate, kET, was estimated using the classical Marcus equation [22] (for details see the Supporting Information).
The computed values of ET parameters (the Gibbs energy G0, electronic coupling V, and the reorganization energy and kET for CS in C60⊂ [10]CPP and Li + @C60⊂ [10]CPP in the polar (benzonitrile and dichloromethane), and nonpolar (diisopropyl ether) medium are listed in Table 2.
The PET in C60⊂ [10]CPP occurs in normal Marcus regime (|G0| < ) on the nanosecond time scale, kET  1.1 and 1.7 ns -1 in nonpolar and polar solvents, respectively. As expected, G0 decreases as the solvent polarity increases. [a] Gibbs energy difference between LE1 and CS states in solvent.
By contrast, CS in Li + @C60⊂ [10]CPP complex takes place in inverted Marcus region (|G0 |> ), and the ET reaction becomes higher with decreasing exergonicity. The rate undergoes a rise from 1.4 ns -1 to 137 ns -1 . Note that the rate of charge recombination cannot be accurately estimated since this ET process is found to be in the deep inverted Marcus region (|G0|>> ).
In conclusion, photoinduced charge separation in the Li + @C60⊂ [10]CPP and C60⊂ [10]CPP complexes has been studied in detail using the TD-DFT approach. The anomalous solvent effect-destabilization of charge separated states by polar medium has been found for Li + @C60⊂ [10]CPP. This is the first example of fullerene based systems where a hypsochromic shift of the CS band is observed. The CS reaction in the Li + @C60⊂ [10]CPP complex is found to occur in the inverted Marcus regime. In spite of that the CS reaction rate becomes faster with increasing solvent polarity of the environment.

Experimental Section
Full computational Details are given in the supporting information.