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

Abstract

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.

1. 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 molecules {-D-A-D-A}_∞ or {-D-A-D-D-A-D}_∞ and (b) segregated stacks of donor and acceptor molecules {-D-D-D-}_∞ ǁ {-A-A-A-}_∞ [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].

There have been only a few studies on CTCs with 9,10-phenanthrenequinone nitro derivatives as acceptors [10,11,12]. In [11,12], a series of CTCs based on anthracene, phenanthrene, and 9,10-phenanthrenequinone derivatives was studied. Of all the derivatives considered, 1,3,6-trinitro-9,10-phenanthrenequinone showed the strongest acceptor properties.

The purpose of the present work was to study CTCs based on 1,3,6-trinitro-9,10-phenanthrenequinone (PQ) as electron acceptor and different donors. We selected 23 donors with a varying number of π-electrons (from 6 to 26) and a different aromatic system structure; some of them had N and S heteroatoms and substituents. The following donors were used: benzene (BZ), pyridine (PD), N,N,N′,N′-tetramethyl-p-phenylenediamine (TMDA), naphthalene (NA), quinoline (QN), isoquinoline (IQN), acenaphthene (ACN), azulene (AZU), tetrathiafulvalene (TTF), anthracene (AN), phenanthrene (PA), acridine (ACR), 9-methylcarbazole (MC), 5,10-dimethylphenazine (DMPZ), tetracene (TET), tetraphene (TPH), chrysene (CRS), pyrene (PYR), triphenylene (TPL), dibenzotetrathiafulvalene (DBTTF), pentacene (PEN), porphyrin (POR), coronene (COR) (Figure 1 and Supplementary Material Figure S1).

Figure 1. Configurations of acceptor PQ, one of the donors MC, and two possible CTCs [PQ-MC] and [PQ-MC]’.

Asymmetrical molecules such as PD, QN, IQN, PA, ACR, CCN, MC, TPH, and AZU produce different stable arrangements of donor and acceptor in CTCs, therefore, the quantum-chemical calculation was performed for 32 possible models (Figure 1 and Figure S2, Table 1).

Table 1. The calculated energies (eV) of frontier molecular orbitals of donors and CTCs (^DE_HOMO, ^CTCE_HOMO, ^CTCE_LUMO), energy gaps (eV) of isolated and interacting donor and acceptor (ΔE_MO, Δ ^CTCE_MO), partial NPA charges (e⁻) of complex ground and first excited states (q_NPA, q*_NPA), and association energy (ΔE_ass, kJ/mol).

Molecule	DEHOMO	ΔEMO	CTCEHOMO	CTCELUMO	ΔCTCEMO	qNPA	q*NPA	ΔEass
[PQ-PD]	−6.97	2.40	−7.43	−3.92	3.51	0.138	0.080	−59.4
[PQ-Pd]’	−6.97	2.40	−7.61	−4.46	3.14	0.015	0.001	−46.6
[PQ-BZ]	−6.96	2.39	−7.50	−4.40	3.10	0.026	0.008	−47.0
[PQ-QN]	−6.53	1.96	−7.15	−4.35	2.79	0.019	0.071	−67.6
[PQ-Qn]’	−6.53	1.96	−7.06	−4.42	2.64	0.014	0.969	−63.3
[PQ-IQN]	−6.45	1.88	−7.01	−4.45	2.56	−0.003	0.971	−61.4
[PQ-IQn]’	−6.45	1.88	−7.05	−4.45	2.60	0.018	0.967	−64.2
[PQ-TPL]	−6.10	1.53	−6.58	−4.21	2.37	0.037	0.969	−106.5
[PQ-NA]	−6.04	1.47	−6.62	−4.43	2.19	−0.001	0.976	−63.0
[PQ-PA]	−5.98	1.41	−6.65	−4.22	2.43	0.054	0.994	−92.6
[PQ-Pa]	−5.98	1.41	−6.47	−3.89	2.57	0.028	0.992	−89.9
[PQ-ACR]	−5.92	1.35	−6.48	−4.30	2.18	0.014	0.982	−79.2
[PQ-ACr]’	−5.92	1.35	−6.50	−4.38	2.11	0.023	0.981	−77.1
[PQ-CRS]	−5.76	1.19	−6.36	−4.21	2.15	0.060	1.007	−111.3
[PQ-ACN]	−5.71	1.14	−6.31	−4.43	1.88	0.005	0.983	−74.7
[PQ-aCn]’	−5.71	1.14	−6.25	−4.53	1.72	0.047	0.985	−75.8
[PQ-COR]	−5.70	1.13	−6.13	−4.21	1.92	0.031	1.010	−115.3
[PQ-PYR]	−5.58	1.01	−6.20	−4.20	2.00	0.068	0.992	−97.5
[PQ-MC]	−5.57	1.00	−6.27	−4.31	1.96	0.070	1.012	−94.0
[PQ-Mc]’	−5.57	1.00	−6.16	−4.23	1.92	0.071	1.008	−87.0
[PQ-TPH]	−5.57	1.00	−6.17	−4.18	1.99	0.066	1.006	−104.8
[PQ-TPh]’	−5.57	1.00	−6.15	−4.21	1.94	0.065	1.005	−107.0
[PQ-AN]	−5.47	0.90	−6.12	−4.29	1.83	0.046	0.990	−82.3
[PQ-AZU]	−5.44	0.87	−6.02	−4.45	1.57	−0.004	0.969	−63.5
[PQ-AZu]’	−5.44	0.87	−6.27	−4.18	2.10	0.114	0.971	−73.3
[PQ-POR]	−5.39	0.82	−5.88	−4.07	1.82	0.077	1.033	−119.2
[PQ-TET]	−5.10	0.53	−5.70	−4.24	1.45	0.066	1.018	−106.1
[PQ-DBTTF]	−4.89	0.32	−5.53	−4.00	1.53	0.216	1.081	−124.0
[PQ-PEN]	−4.85	0.28	−5.45	−4.08	1.37	0.134	1.042	−118.0
[PQ-TTF]	−4.52	−0.05	−5.48	−4.03	1.45	0.240	1.089	−102.3
[PQ-DMPZ]	−4.39	−0.18	−5.26	−4.22	1.04	0.135	1.058	−107.5
[PQ-TMDA]	−4.37	−0.20	−5.38	−4.09	1.29	0.224	1.075	−104.3

2.1. Theoretical

The formation of charge transfer complexes is controlled by the energy difference (ΔE_MO) between the LUMO of the isolated acceptor (^AE_LUMO) and the HOMO of the isolated donor (^DE_HOMO) [3,13]. Since ^AE_LUMO is constant for all the considered CTCs and is equal to −4.57 eV, ΔE_MO depends only on ^DE_HOMO which varies from −6.97 to −4.37 eV for selected donors. Based on the ^DE_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 that donors become stronger as the number of aromatic rings grows. The ΔE_MO value decreases in the series of donors with linear arrangement of rings: BZ (2.39 eV), NA (1.47 eV), AN (0.90 eV), TET (0.53 eV), PEN (0.28 eV). The same goes for Δ^CTCE_MO: the HOMO–LUMO energy difference in CTCs decreases from 3.10 eV to 1.37 eV for complexes of PQ with benzene and pentacene, respectively.

Δ^CTCE_MO values for anthracene [PQ-AN] (1.83 eV) and phenanthrene [PQ-PA] (2.43 eV)/[PQ-PA]’ (2.57 eV) complexes demonstrate that donors with a linear arrangement of aromatic rings are stronger than those with a non-linear arrangement. Similarly, Δ^CTCE_MO values increase when replacing tetracene [PQ-TET] (1.45 eV) to, tetraphene [PQ-TPH] (1.99 eV), pyrene [PQ-PYR] (2.00 eV), chrysene [PQ-CRS] (2.15 eV), or triphenylene [PQ-TPL] (2.37 eV). Δ^CTCE_MO for the pentacene complex [PQ-PEN] (1.37 eV) is lower than that for coronene [PO-COR] (1.92 eV) (Table 1).

Introduction of a nitrogen heteroatom into donor molecules leads to an increase of Δ^CTCE_MO values for the corresponding complexes, which points to a decrease of donor properties. The same trend for Δ^CTCE_MO holds true when changing BZ for PD, NA for QA/IQA, and AN for ACR (Table 1).

Δ^CTCE_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 Δ^CTCE_MO values, but also the formation energies ΔE_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 Δ^CTCE_MO from 3.10 to 1.29 eV. When TTF is replaced with DBTTF Δ^CTCE_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. Δ^CTCE_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: ^CTCE_HOMO lies below ^DE_HOMO, while ^CTCE_LUMO is higher than ^AE_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, Δ^CTCE_MO values are significantly larger than the corresponding ∆E_MO but less than the HOMO-LUMO gaps of isolated PQ or the donors. Figure 2 illustrates this difference for CTCs of PQ with AN and TMDA: Δ^CTCE_MO values (1.83 and 1.29 eV) are smaller than the HOMO-LUMO gaps of PQ (3.37 eV), AN (3.56 eV) and TMDA (4.19 eV). The energy difference values ΔE_MO of isolated acceptor and donors in the series decrease from 2.40 to −0.20 eV (Table 1). Notably, ^DE_HOMO of the most pronounced donors, e.g., TTF, TMPZ, and TMDA, lies higher than ^AE_LUMO as graphically exemplified in Figure 2.

Figure 2. HOMO-LUMO energy levels for complexes of PQ with AN and TMDA according to DFT calculations.

The calculated wavelengths of electronic transitions based on the ∆^CTCE_MO values vary from 468 nm for the [PQ-QN]’ complex, to 509 and 481 nm for the [PQ-PA] and [PQ-PA]’ complexes, to 676 nm for the [PQ-AN] complex and further to 959 and 1186 nm for the [PQ-TMDA] and [PQ-DMPZ] complexes, respectively. Two sets of experimentally determined CT absorption bands for complexes [PQ-PA] and [PQ-AN] measured in CH₂Cl₂ and toluene are available in the literature. Solutions of complex with phenanthrene absorb at 490 and 495 nm [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. For a series of CTCs based on the same acceptor, the shift of charge-transfer bands to long wavelengths indicates an increase in donor power. According to this criterion, donor properties increase in the following sequence of molecules: QN, IQN, PA, TPL, NA, ACR, CRS, PYR, TPH, MC, COR, ACN, AZU, AN, POR, DBTTF, TET, TTF, PEN, TMDA, DMPZ.

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 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.

Bond	d			Angle	ω
	I	[PQ-AN]	PQ, AN		I	[PQ-AN]	PQ, AN
O5–C5	1.2103(13)	1.208	1.207	O1–N1–O2	124.64(10)	125.5	125.9
O6–C6	1.2135(13)	1.211	1.209	O2–N1–C2	117.20(9)	117.1	117.0
O1–N1	1.2258(13)	1.219	1.217	O1–N1–C2	118.16(9)	117.4	117.1
O2–N1	1.2257(13)	1.218	1.217	O3–N2–O4	125.46(10)	126.7	127.2
O3–N2	1.2198(13)	1.214	1.215	O4–N2–C4	117.13(9)	116.4	116.4
O4–N2	1.2211(13)	1.217	1.214	O3–N2–C4	117.32(9)	116.7	116.3
O7–N3	1.2289(13)	1.220	1.218	O7–N3–O8	123.79(10)	125.2	125.6
O8–N3	1.2264(13)	1.219	1.217	O7–N3–C9	118.31(9)	117.4	117.3
N1–C2	1.4712(14)	1.479	1.485	O8–N3–C9	117.89(9)	117.4	117.1
N2–C4	1.4791(13)	1.480	1.482	O5–C5–C4A	122.52(10)	123.0	122.8
N3–C9	1.4717(13)	1.477	1.484	O5–C5–C6	119.50(10)	120.0	119.8
C5–C6	1.5379(15)	1.537	1.540	C4A–C5–C6	117.91(9)	117.0	117.3
C10A–C10B	1.4819(15)	1.480	1.484	O6–C6–C6A	123.53(10)	123.3	123.2
C11–C12	1.3606(18)	1.373	1.373	O6–C6–C5	118.92(10)	119.6	119.5
C12–C13	1.4213(17)	1.427	1.428	C6A–C6–C5	117.54(9)	117.1	117.2
C11–C14B	1.4303(17)	1.431	1.431	C3–C4–N2	115.79(9)	115.7	115.9
C14A–C15	1.3968(17)	1.403	1.402	C4A–C4–N2	120.83(9)	121.4	121.4
C14A–C14B	1.4400(15)	1.446	1.446	C12–C11–C14B	120.73(11)	120.8	121.0

Table S1 demonstrates the NPA charges on atoms of complex [PA-AN] and isolated molecules PQ and AN in both ground and first excited states. In the ground state of [PQ-AN] complex the amount of charge transferred from AN to PQ equals 0.046 e⁻. Interestingly, this value hides the fact that AN carbon atoms gain −0.062 e⁻ while hydrogen atoms lose 0.111 e⁻ when the complex forms. At the same time oxygen and nitrogen atoms are the acceptor centers of PQ.

Upon excitation the charge on the donor increases to 0.990 e⁻ indicating the formation of [D⁺-A⁻] complex. This is accompanied by the electron density transfer from AN carbon atoms to PQ oxygen and nitrogen atoms (0.369 e⁻) and C-atoms (0.624 e⁻).

2.2. 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 CH₂Cl₂. 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 C=O bond lengths in I (1.2103(13) and 1.2135(13) Å, Table 2, Figure 3) do not differ from those in II (1.211(2) и 1.217(2) Å) [11] and III (1.211(2) и 1.216(2) Å) [12]. The bond C⁵–C⁶ in I (1.5379(15) Å) is significantly longer than the standard single bond of C(sp²)–C(sp²) type (1.479 Å) [15]. This deviation is determined by anti-bonding interactions of oxygen atoms in o-quinones as was shown earlier in II (1.528(2) Å) [11] and III (1.526(2) Å) [12]. Valence angles of C⁵ and C⁶ in I have values close to those of II and III. In complex I atoms O⁵ и O⁶ are found to deviate notably from the plane of the central ring of PQ (torsion angle O⁵–C⁵–C⁶–O⁶ is 15.54(16)°), which is different from TNPQ where atoms O⁵ and O⁶ lie on the plane of the central ring (0.4(2)° for II [11] and 0.8(2)° for III [12]).

Figure 3. X-ray crystallographic structure of PQ and AN in complex I.

The main structural features of NO₂ 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) to 1.2289(13) Å and the O–N–O valence angles are from 123.79(10)° to 125.46(10)°. The nitro groups in structure I at atoms C2, C4, and C9 are rotated out of the aromatic plane by 16.84(13), 62.75(13) and 13.56(12)°, respectively, which significantly distinguishes them from similar values at atoms C1, C3, and C8 in II (69.02(19), 0.25(18), and 19.93(18)°) [11] and III (73.8(2), 1.35(15), and 0.95(15)°) [12]. Unlike structures II and III, where the greatest rotation angle was observed in the NO₂ group at the C¹ atom, experiencing significant steric repulsion from atoms C¹⁰ and H¹⁰, in structure I steric difficulties arise between the nitro group at the atom C⁴ and the carbonyl group O⁵–C⁵, which causes a ~63° rotation of the NO₂ 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: N²-C⁴ 1.4791(13) Å in I (Table 2), N¹-C¹ 1.480(2) Å in II [11], and N¹–C¹ 1.483(2) Å in III [12].

The AN molecule in complex I has C-C bond lengths in the range from 1.359(2) to 1.4400(15) Å and valence angles between 118.32(12)–122.62(11)°, which match with the corresponding values in II (bonds from 1.360(3) to 1.443(2) Å and angles within 118.38(14)−122.50(15)° [11]), in the free AN molecule [15] and in its CTC with 2,3,5,6-tetrachloro-1,4-dicyanobenzene [17].

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]. As a result of this mutual arrangement of the molecules PQ and AN in I, peculiar triads [D-A]⋅⋅⋅[D-]’ and [D-A]’⋅⋅⋅[D-] are formed, in which the π systems of the donor and the acceptor overlap almost to the same degree (Figure 4).

Figure 4. Crystal structure of complex I. (a) View along crystallographic a axis; (b) view along crystallographic b axis; (c) view along crystallographic c axis.

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.

The average interplanar distances D⋅⋅⋅A in I are about 3.434(5) Å, which is close to those in CTCs of AN with 2,3,5,6-tetrachloro-1,4-dicyanobenzene (3.427(3) Å) [17] or with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (3.379(2) Å) [18]. The calculated charge transfer values for the CTCs [PQ-AN] (0.046 e⁻) (Table 1) and [TNPQ-AN] (0.091 e⁻) [11] are consistent with the interplanar distances.

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.

3. Materials and Methods

3.1. 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 CH₂Cl₂ was purified by standard methods. To obtain CTC in the crystalline state, the solutions of acceptor (PQ, 0.2 mmol in 12 mL of CH₂Cl₂) and donor (AN, 0.2 mmol in 5 mL of CH₂Cl₂) 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.

3.2. 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 F² 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).

Table 3. Summary of crystallographic experiment data and structure refinement parameters for I.

Compound	I
CCDC	2099997
Formula	C14H5N3O8·C14H10
Crystal system	Monoclinic
Space group	P21/c
Z	4
a,b,c, Å	14.2721(14), 19.479(2), 8.1900(9)
α, β, γ, deg	90, 99.041(8), 90
V, Å3	2248.6(4)
Dx, g/cm3	1.540
Radiation, λ, Å	Synchrotron, 0.79272
μ, mm−1	0.147
T, K	100(2)
Specimen size, mm	0.18 × 0.15 × 0.03
Absorption correction	Semi-empirical
Tmin/Tmax	0.966/0.987
θmax, deg	30.95
Limits of h,k,l	−15<=h<=18; −25<=k<=18; −10<=l<=10;
Number of reflections: measured/independent (N1);	13943/5033
observed with I>2σ(I) (N2)	4596
Rint	0.0276
Number of parameters	353
Extinction coefficient	0.029(3)
R1/wR2 by N1	0.0404/0.0975
R1/wR2 by N2	0.0373/0.0950
S	1.023
Δpmin/Δpmax, eÅ−3	−0.220/0.261

3.3. 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 (Δq_NPA, e⁻) and first excited (Δq*_NPA, e⁻) states. The CTC association energies are defined as follows:

ΔE_ass = ^CTCE_tot − ^AE_tot − ^DE_tot

where ^CTCE_tot, ^AE_tot and ^DE_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].

4. Conclusions

In this study we explored a series of 23 charge transfer complexes based on 1,3,6-trinitro-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.