Charge Transfer Complexes of 1,3,6-Trinitro-9,10-phenanthrenequinone with Polycyclic Aromatic Compounds

Understanding the interactions of organic donor and acceptor molecules in binary associates is crucial for design and control of their functions. Herein, we carried out a theoretical study on the properties of charge transfer complexes of 1,3,6-trinitro-9,10-phenanthrenequinone (PQ) with 23 aromatic π-electron donors. Density functional theory (DFT) was employed to obtain geometries, frontier orbital energy levels and amounts of charge transfer in the ground and first excited states. For the most effective donors, namely, dibenzotetrathiafulvalene, pentacene, tetrathiafulvalene, 5,10-dimethylphenazine, and tetramethyl-p-phenylenediamine, the amount of charge transfer in the ground state was shown to be 0.134−0.240 e−. Further, a novel charge transfer complex of PQ with anthracene was isolated in crystalline form and its molecular and crystal structure elucidated by single-crystal synchrotron X-ray diffraction.


Introduction
Organic π-π charge transfer complexes (CTCs) form a special class of binary compounds stabilized by partial electron transfer between noncovalently interacting donor (D) and acceptor (A) molecules. The degree of electron transfer in CTCs is governed by the difference between the donor ionization potential and the acceptor electron affinity which can be approximated as the difference between the donor highest occupied molecular orbital (HOMO) and the acceptor lowest unoccupied molecular orbital (LUMO) [1]. The HOMO-LUMO energy gap can be obtained from DFT calculations.
Individual CTCs may undergo self-assembly and form crystalline or supramolecular structures [2]. The properties of such assemblies depend on the stoichiometric composition of the complexes [3,4] and their polymorphism [5,6]. CTCs in crystals tend to form one of two types of molecular stacks: (a) mixed-type stacks with alternating donor and acceptor  [3,7].
CTCs exhibit a wide range of physical properties therefore the search for new effective electron donors, acceptors, and synthesis of new CTCs on this basis is of high relevance [8]. At the same time, quantum-chemical modeling is one of the main approaches to study structure and properties of CTCs. Computer modeling allows a large number of complexes to be examined in a short period of time to select only a few of the most promising for further experimental research [9].

Theoretical
The formation of charge transfer complexes is controlled by the energy difference (∆E MO ) between the LUMO of the isolated acceptor ( A E LUMO ) and the HOMO of the isolated donor ( D E HOMO ) [3,13]. Since A E LUMO is constant for all the considered CTCs and is equal to −4.57 eV, ∆E MO depends only on D E HOMO which varies from −6.97 to −4.37 eV for selected donors. Based on the D E HOMO values we can expect an increase in donor properties in the following series: PD < BZ < QN < IQN < TPL < NA < PA < ACR < CRS < ACN < COR < PYR < MC < TPH < AN < AZU < POR < TET < DBTTF < PEN < TTF < DMPZ < TMDA ( Table 1).
The most important structural features that determine electron donor properties of a molecule are the number and position of the condensed aromatic rings and also the presence of heteroatoms and functional groups. It is evident from Table 1 Table 1).
Introduction of a nitrogen heteroatom into donor molecules leads to an increase of ∆ CTC E MO values for the corresponding complexes, which points to a decrease of donor properties. The same trend for ∆ CTC E MO holds true when changing BZ for PD, NA for QA/IQA, and AN for ACR (Table 1).
∆ CTC E MO values for complexes with acridine (1.88 and 1.72 eV) and azulene (1.57 and 2.10 eV) are found to be less than those of the complex with naphthalene (2.19 eV). Therefore, ACN and AZU are more active donors than NA. It is worth noting that the relative spatial arrangement of AZU and PQ molecules in CTCs ( Figure S3) determines not only ∆ CTC E MO values, but also the formation energies ∆E ass (−63.5 and −73.3 kJ/mol) as well as the mean distance between donor and acceptor planes R (3.16 and 3.11 Å).
Substitution of benzene [PQ-BZ] for tetramethyl-p-phenylenediamine [PQ-TMDA] in the complex changes ∆ CTC E MO from 3.10 to 1.29 eV. When TTF is replaced with DBTTF ∆ CTC E MO of the corresponding complexes increases from 1.45 to 1.53 eV. The strongest electron donor in the series considered in this work is DMPZ. ∆ CTC E MO of [PQ-DMPZ] complex has the lowest value of 1.04 eV.
Donor and acceptor orbitals constitute the HOMO and LUMO in complexes ( Figure S4). However, upon complexation their energy levels change: CTC E HOMO lies below D E HOMO , while CTC E LUMO is higher than A E LUMO (Figure 2). For the CTCs having the highest degree of charge transfer in the series, namely, [PQ-TTF] and [PQ-TMDA], the magnitude of these changes reaches 1.50 eV and 1.49 eV, respectively. As a result, ∆ CTC E MO values are significantly larger than the corresponding ∆E MO but less than the HOMO-LUMO gaps of isolated PQ or the donors.  (Table 1). Notably, D E HOMO of the most pronounced donors, e.g., TTF, TMPZ, and TMDA, lies higher than A E LUMO as graphically exemplified in Figure 2.  [12], while [PQ-AN] spectra show peaks at 658 and 641 nm for dichloromethane and toluene, respectively [11]. The presence of the charge-transfer bands in the absorption spectra of the complexes is a reliable confirmation of the CTC formation. The amounts of ground state charge transfer in CTCs q NPA , calculated as the sum of NPA charges on donor atoms in CTC, is in the range from −0.004 to 0.249 e − . Assuming alternating molecular stacking in crystals, these CTCs can be classified as neutral and mixed-valence CT solids [14]. Even a small charge transfer amount of 0.2 is enough for materials to exhibit conducting properties and neutral ionic phase transition in the mixed valent state [14]. For the first excited states the amounts of charge transfer q * NPA lie between 0.967 and 1.089 e − (Table 1). This indicates that the electron transitions upon excitation in the CTCs are mainly associated with the transfer of electron density from the donor to the acceptor atoms.
Absolute values of the calculated association energies ∆E ass increase as the π-conjugated system grows. In the series BZ, NA, AN, TET, and PEN ∆E ass values are −47.0, −63.5, −82.3, −106.1, and −118.0 kJ/mol, respectively. The stability of CTCs with TTF, TMDA, DMPZ, POR, and DBTTF is determined not by the size of molecule, but by the presence of heteroatoms ( Table 1).
The calculated intermolecular separation distances (R) in CTCs lie in the range from 2.87 to 3.25 Å. The complexes with the lowest distance values exhibit substantial deviation from a planar structure ( Figure S5). The calculated R for [PQ-AN] complex (3.24 Å) agrees with the interplanar distance of the X-ray structure (3.49 ± 0.26 Å). The bond lengths of [PQ-AN] predicted by DFT are close to those of both calculated structures of isolated PQ and AN and the X-ray structure ( Table 2). Table S1 Table 2. Bond lengths d (Å) and valence angles ω (deg.) of complex I (X-ray diffraction data), complex [PQ-AN] and isolated PQ and AN molecules (DFT calculations). Atom numbering scheme is given in Figure 3.

Experimental
Dark green single prism-shaped crystals of [PQ-AN] complex (I) were grown by slow evaporation from equimolar solution of PQ and AN in CH 2 Cl 2 . The X-ray diffraction study confirmed the 1:1 ratio of PQ and AN in complex I and revealed the monoclinic structure (space group P2 1/c ).
The molecular structure of PQ was determined for the first time, although in a complex with AN. It is interesting to discuss and compare the main geometric features of PQ and 2,4,7-trinitro-9,10-phenathrenequinone (TNPQ), especially in complexes with AN (II [11]) and PA (III [12]).
The main structural features of NO 2 groups in I (Table 2), II [11], and III [12] differ only slightly and are close to the average values [16]: the N-O bond lengths in I are in the range from 1.2198 (13) [12]. Unlike structures II and III, where the greatest rotation angle was observed in the NO 2 group at the C 1 atom, experiencing significant steric repulsion from atoms C 10 and H 10 , in structure I steric difficulties arise between the nitro group at the atom C 4 and the carbonyl group O 5 -C 5 , which causes a~63 • rotation of the NO 2 group and a significant non-planarity of the carbonyl. It should be noted that the C-N bonds near the heavily rotated nitro groups are somewhat elongated relative to other similar bonds: N 2 -C 4 1.4791(13) Å in I (Table 2), N 1 -C 1 1.480(2) Å in II [11], and N 1 -C 1 1.483(2) Å in III [12].
In crystal I, the molecules of the acceptor PQ and the donor AN are arranged parallel to each other and form stacks of mixed type {···[A-D]···[A-D]'···} ∞ along the crystallographic axis a (Figure 4). Every second PQ molecule in the stack is rotated in a plane by 180 • relative to the previous one (A and A'), which was observed for TNPQ molecules in II [11]. The AN molecules in I are displaced relative to each other (D and D') only slightly and their central ring practically overlaps, which distinguishes them significantly from II, where the AN molecules are rotated by 60 • relative to each other [11].  In I, the PQ and AN molecules form two types of shortened contacts which are less than the sum of the van der Waals radii (Table S2). [A-D] and, [D-A]' contacts are found in the same stack while A···D and A···A' contacts are between the adjacent stacks. In one stack, each acceptor molecule establishes six C···C contacts with molecules D and D' in the range from 3.263(2) to 3.363(2) Å, which may indicate strong π-π interactions between the molecules. The molecules D and D' form a different number of shortened C···C contacts in the stack: each D molecule has four C···C contacts with acceptor molecules A and A', while each D' has only two such contacts.
Each PQ molecule in I interacts with AN and PQ molecules from adjacent stacks via O···H-C shortened contacts in the range of 2.45−2.64 Å. There are also O···C contacts between the PQ molecules from adjacent stacks-from 2.921(2) to 3.097(2) Å (Table S2). The presence of such a significant number of various intermolecular interactions in I, the number of which for each PQ molecule reaches twenty-five, and the observed geometric characteristics of PQ can be due to its high acceptor capacity.

Synthesis
1,3,6-trinitro-9,10-phenathrenequinone (PQ, melting point 261−263 • C) was obtained by nitration and subsequent decomposition of 9,10-sulfonyldioxyphenanthrene in concentrated nitric acid (d = 1.51) [19]. Pure-grade anthracene was used without additional purification. The solvent, namely, pure-grade CH 2 Cl 2 was purified by standard methods. To obtain CTC in the crystalline state, the solutions of acceptor (PQ, 0.2 mmol in 12 mL of CH 2 Cl 2 ) and donor (AN, 0.2 mmol in 5 mL of CH 2 Cl 2 ) were mixed in equimolar amounts. Single crystals of the [PQ-AN] complex suitable for the X-ray diffraction studies were grown by slow evaporation of the solvent.

X-ray Crystallography and Structure Refinement
The X-ray diffraction study of [PQ-AN] complex was carried out at the "BELOK" beamline of the Kurchatov Institute Synchrotron Radiation Source. The parameters of the unit cell and the reflection intensities were measured using a Rayonix SX165 CCD two coordinate detector (λ = 0.79272 Å, ϕ-scanning in 1.0 • steps) (Rayonix LLC, 1880 Oak Ave UNIT 120, Evanston, IL 60201, USA). The structure was solved by direct methods and refined by the full-matrix least squares technique on F 2 with anisotropic displacement parameters for all non-hydrogen atoms using the iMOSFLM (CCP4) [20], SCALA [21], and SHELXL [22] programs. All hydrogen atoms were placed in calculated positions and included in the refinement within the riding model with fixed isotropic displacement parameters U iso (H) = 1.5U eq (O), 1.2U eq (N), and 1.2U eq (C). The crystallographic data as well as the experimental and refinement parameters are summarized in Table 3. Crystallographic data is available online at the Cambridge Crystallographic Data Centre (CCDC 2099997).

Quantum Chemical Calculations
Quantum chemical simulation of the electronic structure of donor, acceptor, and CTC molecules was performed in the framework of the density functional theory using the B3LYP hybrid functional and the def2-SV(P) basis set. TDDFT methodology was used to explore the low-lying excited states. The Boys-Bernardi method was used for BSSE correction [23]. All D4 dispersion correction was used in all calculations [24]. The amount of charge transfer from a donor to an acceptor was calculated using the natural populations analysis (NPA) [25] as the difference between the sum of charges on the acceptor atoms in free state and in complex for both the ground (∆q NPA , e − ) and first excited (∆q* NPA , e − ) states. The CTC association energies are defined as follows: where CTC E tot , A E tot and D E tot are total energies (in kJ/mol) of the CTC, acceptor, and donor, respectively. All the calculations were performed using the Firefly 8.20 software package [26].

Conclusions
In this study we explored a series of 23 charge transfer complexes based on 1,3,6trinitro-9,10-phenanthrenequinone and different electron donors by means of density functional theory. Complexes with dibenzotetrathiafulvalene, pentacene, tetrathiafulvalene, 5,10-dimethylphenazine, and tetramethyl-p-phenylenediamine were shown to be in a mixed-valence state with a ground state charge transfer degree of 0.134-0.240 e − . A charge transfer complex with anthracene was synthesized, isolated as a single crystal, and the structure determined by X-ray diffraction experiment. Geometric and electronic structure features and their influence on the charge transfer properties of the complexes are discussed.
Supplementary Materials: The following are available online. Figure S1: Chemical structure depiction of PQ and 23 donors used in this work, Figure S2: Configurations of charge transfer complexes considered in this work, Figure

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.