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P-containing Polycyclic Aromatic Rózsa Szűcs, Pierre-Antoine Bouit, Laszlo Nyulászi, Muriel Hissler

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Rózsa Szűcs, Pierre-Antoine Bouit, Laszlo Nyulászi, Muriel Hissler. P-containing Polycyclic Aromatic Hydrocarbons. ChemPhysChem, Wiley-VCH Verlag, 2017, 18 (19), pp.2618-2630. ￿10.1002/cphc.201700438￿. ￿hal-01538723￿

HAL Id: hal-01538723 https://hal.archives-ouvertes.fr/hal-01538723 Submitted on 14 Jun 2017

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P-containing Polycyclic Aromatic Hydrocarbons Rózsa Szűcs,[a,b] Pierre-Antoine Bouit,*,[a] László Nyulászi,*,[b] Muriel Hissler*,[a]

aromatic synthons using simple organic reactions (C-C cross- coupling, oxidations…). This method cannot be used to prepare micrometric fragments, but it allows the preparation of well-defined up to 10 nm (NGs or Graphene Abstract: Polycyclic Aromatic Hydrocarbons (PAHs) are highly NanoRibbons (GNRs)). In this field, pioneering work was appealing functional materials in the field of molecular performed by the group of K. Müllen during the 1990’s. They electronics. In particular, of these showed that with suitable polyaromatic precursors, the Scholl derivatives using organic is a powerful method to tune reaction can be used to “graphenize” large polyaromatic their properties from the point of view of the bandgap and frameworks.[ 5 ] They thus synthesized defect-free and soluble supramolecular assemblies. Another way to achieve such molecular ranging from the prototype hexa-peri- control is to take advantage of the specific reactivity of benzocoronene to GNRs up to 100 - 200 kg.mol-1 (1 and 2, 2 heteroatoms placed within the C-sp framework. This strategy Figure 1).[6] A precise structure-property study on the effect of has been successfully applied to N, S or B . In this review, cycle number and size, shape (disc shape ribbons), edge we will detail the examples of P-containing PAHs and the effect structure (zig-zag, bay, cove, fjord…), can be properly of the P-environment on their electronic properties from both performed as the compounds are monodisperse.[7] Furthermore, experimental and theoretical points of view. the efficient hole transport properties of these compounds make them particularly appealing for these applications such as field- effect and organic solar cells.[5] 1. Introduction

The isolation of graphene by the Nobel Prize winners Geim and Novoselov opened new perspectives in the field of molecular materials. Its two-dimensional of sp2 atoms confers to graphene exceptional electronic, thermal and mechanical properties that make it one of the most promising classes of carbon-based materials for various applications ranging from to .[1,2] Given its large range of applications, the synthesis of graphene- based materials is a rapidly expanding research field. Since its first isolation numerous research groups tried to find more [1,2] reproducible and efficient synthetic methods. In this context, Figure 1. Examples of reported PAHs (upper part) and heteroatom-containing [3] two main synthetic strategies emerged. The first one, based on PAHs (lower part). exfoliation or oxidation, is referred to as “top-down” approach. One of its main advantages is that scaling up the Over the last ten years, another strategy emerged for the gap quantities is possible. However, structural defects, which may tuning of PAHs consisting in the introduction of heteroatoms into appear during growth or processing, are difficult to avoid and the polycyclic backbone. [ 8 , 9 , 10 ] Even though some polycyclic can deteriorate the performance of graphene-based devices.[4] heteroaromatic molecules are known since the beginning of the Even though these results are highly important, a precise control 20th century,[8] pioneering work in this area has been achieved of the structure and therefore the properties is not yet possible. by Draper et al.. [11] Hence, their N-doped benzocoronene (3, The second approach takes advantage of the possibility to Figure 1), also referred to as N-heterosuperbenzene) clearly perform a molecular engineering of the polycyclic aromatic reflects the strategy of inserting N atoms inside a hexa-peri- hydrocarbons (PAHs) by using the power of benzocoronene scaffold. The authors used the basicity and the organic/organometallic synthesis to overcome these problems. coordination ability of the moiety to tune the optical This stepwise “total synthesis” of molecular graphene fragments properties of the molecular graphene. Other N-containing PAHs is referred as to as the “bottom-up approach”. Indeed PAHs including extended have also been prepared.[ 12 ] (such as hexa-peri-benzocoronene (HBC) 1, Figure 1) or Following this work, S-doped molecular graphenes were nanographenes (NGs) are prepared by connecting small synthesized.[13] Such compounds were recently used as active layer in organic or hybrid solar cells, taking advantage of the charge transport properties of both the PAH backbone and the [14] [a] R. Szűcs, P.-A. Bouit, M. Hissler thienyl rings (4, Figure 1). O-doped PAH (namely O-doped Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS - benzorylenes, 5) were also prepared by Bonifazi et al.[ 15 ] Université de Rennes 1 Recently, a new step was reached with the insertion of highly Campus de Beaulieu, 35042 Rennes Cedex France reactive B into these PAH structures. B-doped PAHs (6, [16] [b] R. Szűcs, L. Nyulászi Figure 1) were reported by the groups of S. Yamaguchi and M. Department of Inorganic and Analytical Chemistry, Wagner[17]. These -deficient molecular graphenes can Budapest University of Technology and Economics, react with anions and display tuneable properties, demonstrating Szt. Gellert ter 4 H-1111 Budapest, Hungary

the power of the heteroatomic approach. Finally, significant interactions in conjugated π-systems, resulting in polyheteroatom-containing PAH such as NS,[18] BN,[19] and BO[20] significant stabilization energies.[33] have also been prepared. The case of BN - PAH is particularly We and others showed that chemical modifications and interesting as the B-N bond is isoelectronic with the C-C bond coordination to transition metals of a 3,3-P center[ 34 ] is a and nitride itself is a promising “graphene like” 2D- powerful method to develop new functional -conjugated material.[ 21 ] These different examples show that the chemists molecules.[ 35 ] Their HOMO and LUMO are thus precisely have been able to develop synthetic methods for the insertion of tuneable and they are chemically and thermally stable enough to heteroatoms into PAH structures having different Lewis and be inserted in OLEDs or solar cells.[36] The high reactivity of the nucleophilic characters leading to specific physical properties. P-atom toward organic reagents and transition metals offers an Effectively, a crucial point in this field is the need to adapt the almost unlimited way of tuning the properties of synthetic approaches developed for the “classical” PAHs to the organophosphorus derivatives.[37] Furthermore, the diversity of specific reactivity of the heteroatoms. Generally speaking, the conjugated P-heterocycles (3-membered phosphirene[ 38 ] 4- preparation of such compounds always represents a synthetic membered phosphetes,[ 39 ] 5-membered ,[ 40 ] 6- challenge. membered phosphinines[ 41 ], 7-membered phosphepine[ 42 ]) as In the present minireview, we will focus on PAH structures well as the variety of bonding modes (3,3-P, 4,5-P, 2,3-P) incorporating a P atom and the impact of this heteroatom on the make this heteroatom highly versatile and adept to tune the physical properties of the PAH. In the past years, physical properties of -systems and PAHs in particular. has turned out to be an excellent building block for the [22] construction of extended conjugated π-systems. For example, 2 3 the  , -P=C double bond has matching ionization energies in all hitherto investigated systems with their C=C bonded [23] counterparts making phosphorus in conjugated π-systems a [24] “carbon copy”. It is worthy to mention that the unoccupied π*- levels are significantly stabilized with respect to the all carbon [ 25 ] analogues, thus for these compounds a narrower HOMO- LUMO gap is expected. The situation of the compounds with 3 3  , -P is rather complex. As isovalent hybridization is not [ 26 ] favoured for heavier elements, the phosphorus’ 3 3 retains a significant “s” character (consequently  , - phosphorus has a large barrier to pyramidalization), and conjugation with the π-system is reduced. With enforced 3 3 [ 27 ] planarity of the  , -phosphorus, an excellent conjugative [ 28 ] building block can be formed. The pyramidal phosphorus, however, has sigma orbitals with proper symmetry to overlap Figure 2. Reported “ortho-fused” phospholes and scope of this review (“peri- with the π-system, via hyperconjugative interaction. In case of fused” P-heterocycles) the five-membered ring, the interaction with the σP-C orbital contributes to (altogether 6 π ), More precisely, this minireview describes the electronic interaction with the σPC* orbital (negative hyperconjugation) properties of P-containing PAHs. As the definition of PAHs contributes to antiaromaticity (altogether 4 π electrons). As a varies depending on the reference, this review will be limited to combination of all these effects phosphole is weakly aromatic.[22] PAHs featuring P-heterocycles that are “peri-fused” (also called [43] Since the σPC* orbital is rather low-lying, its interaction with the “ortho- and peri-fused” according to the IUPAC nomenclature) π* levels reduces the LUMO of the interacting conjugated and featuring at least 18 C-sp2 atoms in the -, making phosphole an interesting building block in framework. This thus excludes the P-linear and the P- conjugated π-systems.[ 29 ] Variation of the substituents at helicenes which are ortho-fused PAHs and have already been phosphorus, or utilization of the lone pair either by changing the recently reviewed (Figure 2).[ 44 ] Hitherto, only 5- and 6- oxidation state, or by complexation results in changes in the membered P-heterocycles have been incorporated in PAHs. The hyperconjugative interaction. Accordingly, the optoelectronic following paragraphs will detail these contributions including properties can be fine-tuned, together with the variation of the theoretical investigations. aromaticity. For example, phospholes with 4,5-phosphorus atom turned out to be slightly antiaromatic.[30 ] This is also in accordance with the charge distribution of the 4,5-P=X (X can 2. 5-membered P-heterocyclic PAHs 4 5 + be: O, S, NH, CH2) bonds, represented often as  , -P - - ,[ 31 ]. X having a formal positive charge at phosphorus leaving 2.1. Synthesis and electronic properties of PAHs containing again 4 π-electrons in the cyclic system. In this case, however, one phosphole unit the 4,5-P=X bond itself is not a part of the conjugated 32 system. Interestingly, such building blocks also show As previously discussed, benzenoid PAHs’ are usually synthesized via the Scholl reaction. However this strategy does

not work for pentaphenylphosphole oxide derivatives which are too sensitive for this method.[ 45 ] The -fused thiooxidized phosphole 7 was thus prepared by Fagan-Nugent method and used as key intermediate. Photocyclization in optimized conditions performed on compound 7 led to a mixture of two products 8 and 9 which could be easily separated.[46] In the case of the thiophene containing phospholes 10-11, the irradiation led to the formation of the dissymmetric derivatives 12-13 (Figure 3).[ 47 ] X-ray diffraction revealed that the sp2-C backbone of compound 9 is planar, thus validating the strategy of placing the P-atom on the edge of the PAH. The different C-C bond lengths are classical for HBC-like molecules. Derivative 9 possesses all the structural features of both phospholes and PAHs. In the packing, head-to-tail -dimers are observed without any long-range arrangement.

Figure 4. Reactivity of PAH 15.

In the crystallographic structures of coordination complexes M15, the PAH conserves its structural characteristics (planarity, angles, bond lengths…)[ 48 ] and the coordination spheres are classical for a coordinated to transition metals. The most interesting aspect comes from the study of the solid-state packing (Figure 4). In most of the cases (Au(15),

Au(15)2, Re(15), Cu(15)2), long range -columns are observed in the solid state due to the -stacking ability of the ligand (Figure 5). In the case where two are in the coordination sphere,

the -stacking can be intermolecular (Au(15)2) or both inter- and intramolecular (Cu(15) ). Hence, using the same PAH based Figure 3. Structures of 7-13 (a); crystallographic structure (b) and packing of 2 compound 9 (c) (CCDC 861180).[46] ligand, a large diversity of supramolecular structures is obtained only by varying the metallic cation. This property is promising considering the potential optoelectronic applications of these To the best of our knowledge, compound 9 was the first example derivatives and validates the strategy of introducing a of planar PAH framework incorporating a P atom. Importantly, coordinating P-atom inside the PAH backbone. this compound is thermally and air-stable. It was then converted into its reactive analogue 15 (Figure 3) through a two-step alkylation-reduction method. Interestingly, despite the presence of the 2D -extended framework, this compound really displays a « phosphine-like » reactivity as it can be easily oxidized (17), alkylated (16) or coordinated to transition metals (M15, M = Au(I),

Re(I)) and M(15)2 ; M = Au(I), Pd(II), Cu(I)) (Figure 4).

Figure 5. packing of coordination complexes 15M (adapted with permission from ref 48. Copyright 2017 American Chemical Society).

Figure 6. UV-vis absorption and emission and TD-DFT simulated spectra (vertical lines) of 7-9, 14-17 and 15M (adapted with permission from ref 46. Copyright 2012 American Chemical Society). excitation, thus the properties of the frontier MOs were In conclusion, all the general reactions (oxidation and discussed in detail. These data show that the molecular coordination) known for simple could be reproduced engineering of both polycyclic backbone and P-environment with the P-containing PAH. This observation shows that despite extend the absorption over the entire visible spectrum. its 2D -extended framework, the P-atom retains its Regarding the properties, P-modifications in the series of characteristic reactivity. compounds 15, 9, 17, 16, 14 lead to a gradual increase of both The optical/electrochemical properties of the compounds were oxidation and reduction potentials (except for compound 15, all then investigated taking into account the size of the -system reduction waves are quasi-reversible). The modulation is and the nature of P-substituent. Together with the C-C stronger for the reduction potentials, which leads to a decrease connectivity and edge structure, the size of the 2D -framework of the gap in this series, in agreement with the optical of PAHs has strong impact on the optical/electrochemical measurements and the DFT calculations. properties.[5] The absorption spectrum of phosphole 7 has the Furthermore, the utilization of derivative 15 as a two-electron typical shape of -conjugated phospholes with a broad * donor towards different metallic (Pd(II), Cu(I), Re(I), Au(I), transition in the visible range.[22] A gradual red-shift accompanied Figure 4) which possess different coordination numbers permits by a hyperchromic shift appears from compound 8 to compound [48] a fine-tuning of the absorption properties. The complexes M15 9 (Figure 3). This latter shows the typical of (Re15 and Au15), having one ligand 15 in their coordination PAHs.[ 49 ] Unlike precursors 7-8, the derivative 9 is highly sphere, present a red-shifted (Figure 6) absorption band with fluorescent in solution (> 20%). Interestingly, the picture is similar vibrational fine structures as for the ligand 15 due to the rather different in the solid state as compound 9 is not Lewis acidity of the metal. The complexes Au(15)2, Pd(15)2, and fluorescent in powder / contrary to compound 7. This Cu(15)2, having two PAH ligands 15 in the coordination sphere phenomenon is rationalized on the basis of the aggregation- present a red-shifted absorption in comparison with M15-type induced emission (AIE)/aggregation caused quenching (ACQ) complexes. The polyaromatic vibronic fine structure can still be phenomena.[ 50 ] The redox properties were studied by Cyclic somewhat observed despite the broadening of the bands (Figure Voltammetry (CV). 7 displays an irreversible oxidation and a 6). Effectively, the TD-DFT calculations (B3LYP/def2-TZVP) quasi-reversible reduction processes. The effect of two indicate the presence of four transitions involving the excitations consecutives intramolecular bond formations (resulting in from the HOMO−1 and HOMO to the LUMO and LUMO+1., compounds 8 and 9) is a gradual increase of both oxidation and These resulting four excited states show the largest splitting – reduction potentials. As the effect is more intense on oxidation and consequently spectral broadening - in case of Pd(15)2, potentials, the electrochemical gap decreased. After looking at which has a cisoid arrangement of the ligands, resulting in large the effect of the polycyclic backbone, the effect of P-substitution intramolecular interaction. This nicely illustrates the strong on the optical properties was studied. Derivative 15 displays a impact of a localized chemical transformation on the P-atom for large and structured absorption band in the visible ( = 472 abs the tuning of the optical/redox properties. nm) and a structured emission (em = 489 nm). A marked As an example of device application, P-containing PAHs 9 and bathochromic shift of the absorption and emission maxima are 17 were inserted as orange of a blue emitting matrix (- observed with a (thio)oxidation reaction (9, 17), the formation of NPB, N,N’-diphenyl - N,N’-bis (1-naphthylphenyl) -1,1‘- biphenyl- the cationic phospholiums 14 and 16 (Figure 6) exhibits an even 4,4’-diamine) in white-emitting OLEDs (WOLEDs)..[49] more pronounced effect.[46] TD-DFT calculations of the vertical Electroluminescence spectrum observed with compound 9 in - excitation energies carried out at the B3LYP/6-31+G* level are in NPB displays both blue emission characteristic of -NPB and excellent agreement with the observed band maxima (Figure 6), orange emission of the P-containing PAH. The tuning of the while the reproduction of the band maxima from the emission emission colour from the orange (CIE coordinates: x = 0.45; y = spectra by the vertical emission energies is less accurate. 0.44, rate of 5.5%) to the white area (x = 0.32; y = 0.37, Apparently, the TD-DFT optimized excited state geometries are doping rate of 1.1%) is achieved through careful tuning of the less satisfactorily predicted than the ground state geometry. In doping rate. Even if the device performances remain moderate, these data demonstrate that P-containing PAHs can be used in any case the first excited state is basically a HOMO-LUMO opto-electronic devices such as OLEDs..[49] However, given their

planar -conjugated structure, applications in OFETs are also envisaged. However, the preparation of OFETs using organophosphorus derivatives as the semi-conductor remains a [51] scientific challenge, as only one example is reported. In conclusion, the reported synthetic method allows fine-tuning of the optical/redox properties of the compounds as well as controlling their coordination driven assembly was achieved. As proof of concept for their utilization in opto-electronic devices, WOLEDs were prepared.

2.2. Theoretical investigation on phosphole-containing PAHS

To gain a deeper understanding of the effect of P-modifications 15 9 14 on the optical and redox properties, the Kohn-Sham FMOs of compounds 15, 9, 17, 16 and 14 were investigated Figure 7 B3LYP/6-31+G* Kohn-Sham FMOs and their energies of 1H- computationally at the B3LYP/6-31+G level. All P-modified PAHs phosphole, 15, 9 and 14. show the characteristic phosphole-type HOMO and LUMO orbitals since the orbitals of the parent displayed in

Figure 7 are clearly recognizable throughout the entire series, with a contribution of the polycyclic backbone. In a very simple picture, this explains why the modifications on the P atom have such an impact on the optical properties. From compounds 15, 9,

17, 16 and 14, the decrease in the HOMO energy level is accompanied by a stronger stabilization of the LUMO (Figure 7) as a consequence of the increased negative hyperconjugation discussed above resulting altogether in the reduction of the Figure 8 NICS(1) local aromaticities (in ppm) of compounds 15, 9 and 14. HOMO-LUMO gap. This point is in full agreement with the gradual red shift observed in the absorption spectra and the 2.3. Theoretical investigation on different heterole- results of the TD-DFT calculations. It is worthy to note that the containing PAHs effect of the energy variation of the LUMO on the HOMO-LUMO gap is relatively larger for the extended PAH than for the non- The effect of the variation of the aromatic/antiaromatic character conjugated heterocycle; the latter having a much larger energy within the five-membered heterocycle embedded in a PAH, was separation between the occupied and unoccupied levels. further investigated in a theoretical study on heterocycles Studying the aromaticity inside each ring leads to a deeper containing different heteroatoms and different substituents on insight into the conjugation within the whole structure. To this them (Figure 9). Among five-membered heterocycles it is well end, Nucleus Independent Chemical Shift (NICS (1)[52] values known that pyrrole (6 π electrons) is highly aromatic, borole (4 π were calculated for compounds 15, 9, 17, 16 and 14 (Figure 7, electrons) is highly antiaromatic, and by variation of the negative values indicate aromaticity, while positive numbers heteroelements[ 55 ] and their substituents, the stand for antiaromaticity). The advantage of NICS is its ability to aromaticity/antiaromaticity can gradually be modified. The provide local aromaticity values in polycondensed systems investigation of the local NICS aromaticities in the heterole- describing both aromatic and antiaromatic rings. The tendencies modified PAHs revealed that the aromatic character can indeed within the investigated series of the compounds containing the be varied, in a broad range.[56] Quite importantly, there is a good P-heterocycles are in good accordance with the aromaticities of correlation between the NICS(1) values of the built in the parent phospholes themselves.[ 53 ].In all of the cases, the heterocycles and those of the isolated rings (Figure 9), indicating cationic P-heterocycles exhibit the biggest antiaromaticity also the appropriateness of using NICS as a local aromaticity (Figure 8), in agreement with Clar’s sextet rule.[54] Interestingly, measure. It is noteworthy that not only the NICS(1) values of the the increasing antiaromaticity of the P-ring is accompanied by an five membered heterocycles correlate with each other, but there increase of the antiaromaticity of its endocyclic 6-membered ring is also a connection between the aromaticity of the A ring and as well. This is another proof that the modifications performed at the NICS(1) values of the B and C rings (Figure 9). This the P-atom impact the entire π-system. This gradual switch behaviour, however, does not hold for all six-membered rings, between aromaticity/antiaromaticity upon chemical modification the NICS(1) values of the D and E rings show little dependence (which is in good accordance with the decrease of the HOMO- on the modification of the five membered heterocycle. Further LUMO gap) performed on a single atom of a PAH does not have efforts are needed to understand these effects, including the role any equivalent in the literature in non-porphyrinoid series.[8] of the Clar aromatic positions[54] within the PAHs.

The authors also studied the effect of phosphine moieties on the out-of-plane anisotropy by DFT calculations. The calculated electrostatic potential (ESP) map showed that 18-syn isomer presents a negative ESP on the -surface containing the three atoms and a positive ESP on the -surface containing the phenyls groups indicating a Janus-type -surface and leading to a high permanent out-of-plane moment (12.0 D). These properties of compound 18-syn induce a specific organization of these molecules on surface. On Au(111), the X-ray photoelectron shows that the three sulfur atoms of 18-syn interact equally with the Au atoms forcing the - framework to be aligned in parallel with the Au surface (Figure 11). The 18-anti compound presents weaker interactions because only two sulfur atoms interact with the Au surface. The scanning tunneling microscope measurements confirm that the strength of these interactions will induce different morphologies. While 18-anti presents aggregated states, 18-syn presents clear molecular features with 1 to 2 nm in size. The molecular absorption on the Au surface for 18-syn (Figure 11) prompts the authors to perform single-molecule transport experiments -4 leading to conductance (5 x10 G0) higher than those of 1,4- -3 Figure 9 Molecular structure of the series of heteroatom-containing PAHs and benzenedithiol (4 x10 G0). the correlation between the aromaticity of the five-membered ring and the connecting units (B3LYP/cc-pVTZ).

2.4. Synthesis and electronic properties of poly-phosphole- containing PAHs

In 2017, Furukawa, Tada, Fujii, Saito and co-workers synthesized triphosphasumanene trisulfide based on a synthetic approach using the hexalithiation of hexaalkoxytriphenylenes (Figure 10).[57] This synthesis afforded syn and anti isomers of triphosphasumanene trisulfide 18 and triphenylodiphosphole disulfide 19 which could be separated by repeated chromatography. The X-ray diffraction analyses Figure 11 Representation of the junction Au-18-syn-Au. (Adapted with support all the proposed molecular structures and show that the permission from ref 57. Copyright 2013 American Chemical Society). -framework of 18-syn has a bowl shape and 18-anti has almost a planar structure. This study shows the importance of using P-chemistry for the development of new -systems which are key building blocks for the molecular engineering of interface for organic electronics.

3. 6-membered P-heterocyclic PAHs

3.1. Internally P-doped PAHs

Kivala and Clark studied computationally the effect of internal heteroatom-doping on PAHs.[58] In particular, they studied the P- doped PAH 18 (Figure 12) in which the 3,3 P atom occupies the central position. According to DFT calculations (B97XD/6- 31G(d)), this compound possesses a distorted geometry (Figure 12) due to the presence of the pyramidal P-atom. This behaviour is opposite to the case of the planar B and N-doped PAHs. As a consequence, compound 18 displays a rather high optical bandgap (3.01 eV calculated by TD-DFT (B3LYP/6- Figure10 Synthesis of syn/anti 18-19 (a) and crystallographic structure of 18- 311++G(d,p)) and semi-empirically (MNDO UNO-CIS)) but still anti (b) and 18-syn (c). behaves as a semi-conductor. This was attributed to the low

conjugation between the P fragment and the -system, an significant aromaticity even in this non fully planar form.[ 62 ] apparent consequence of the structural deformation caused by Clearly, these examples show that the incorporation of central the inherently pyramidal 3,3-phosphorus. The reported barrier phosphorus atoms into small PAHs does not necessarily destroy of planarization was 37.0 kcal mol−1 at the ωB97XD/6-31G(d) the π-conjugation, and there are many aspects to be understood level, which is indeed matching with the 35 kcal mol-1 barrier of in the future.[63] [59] planarization of PH3. The aromaticity of 18 inside the rings was also studied by NICS(0) calculations (SCF-GIAO B3LYP/6- 311+G(d,p)). The A rings (Figure 10) display considerable aromatic character ((NICS(0) = -8.4), the B rings are antiaromatic (NICS(0) = 10,3) while the C rings are almost nonaromatic (NICS(0) = -3.5). The local aromaticity distribution in this compound can thus be easily represented using the Clar sextet formalism with -sextet in the A rings and localized double bonds in the C rings. This trend is general with all doped PAHs studied in the article. The small deviation in term of aromaticity Figure 13. NICS(1) local aromaticities (in ppm) of 21-23 (B3LYP/6-311+G**). comes from structural and electronic (inductive effect) modifications. The authors also studied the electron transfer Different systems featuring a triarylphosphine bridged by 3 [ 64 ] ability of 18 in photo-induced processes. C60 and a heteroatoms or C-sp were recently described. These (porphine) were chosen as electron acceptor and concave molecules are not formally considered as PAHs, but electron donor respectively. In both cases, complexation occurs their synthesis surely represent promising advances toward the as evidenced by the calculation of the binding energy preparation of novel internally P-doped PAHs. (B97XD/6-31G(d)). Interestingly, the binding between 18 and C60 is stronger than with porphine, mainly because of the 3.2. Phosphaphenalenes favoured concave-convex -interactions.[ 60 ] This theoretical study thus describes the electronic properties of internally P- Romero-Nieto and co-workers developed a novel synthetic doped PAH and their potential application in photo-induced approach based on a noncatalyzed protocol to prepare a novel charge transfer processes. However, synthetic access to such 6-membered P-heterocycles family, including compounds is still lacking in the literature. phosphaphenalenes 24 and 25 (Figure 14).[65] In these systems, the P-atom retains its reactivity toward oxidants or transition metals. 4,5-P=O compounds such as 24-25 are air stable. The polycyclic system is fully planar as evidenced by the crystallographic structure of 25 (Figure 14). Again, this is allowed by the position of the P-atom at the edge of the structure. In the P-ring, the C-C bond lengths are slightly larger than in the aromatic phosphinines (1.43 Å < d < 1.46 Å, aromatic phosphinine: 1.40 Å) while the C-P distances are 5 4 Figure 12. Molecular structure of 20, DFT-optimized geometry and NICS(0) classical for  , -P derivatives (d = 1.79 Å). aromaticity (Adapted with permission from ref 58. Copyright 2013 American Chemical Society).

The authors conclude that the large planarization barrier prevents the use of central 3,3-phosphorus in a planar conjugated π-system. It is noteworthy, however, that for other conjugated systems with 3,3-phosphorus in bridgehead position much smaller inversion barriers were predicted. For example, in the tricyclic system 21 (Figure 13) a planar phosphorus was predicted by HF/3-21G(*) preliminary calculations[27] (at B3LYP/6-311+G** the minimum has a non- planar structure, with a barrier of 0.15 kcal mol-1, and with a somewhat antiaromatic +4.1 ppm NICS(1) value). The phosphaindolizine derivative 22 has turned out to be non-planar as well, but the barrier was only 2.8 kcal mol-1 at the B3LYP/6- Figure 14: synthetic access to compounds 24-25 (a) and crystallographic 311+G** level, both rings exhibiting significant aromaticity, even structure of 25 ( top (b) and side (c) view) (CCDC 1418421).[65] in this non-planar system.[ 61 ] This ring system is particularly noteworthy, since the 2-tBu derivative of 23 was DFT calculations (B3LYP/6-31G*) show that the electronic synthesised.[61,62] 23 was described to have a somewhat larger is fully delocalized over the entire polycyclic framework, (6.9 kcal mol-1) planarization barrier than 22, but all rings exhibit

and the P-atom contributes to both the HOMO and the LUMO -framework and the role of the P-orbitals depends on its (Figure 15). For this new family of compounds, the optical and environment. The contribution of the P-orbitals is larger in the electronical properties are still to be evaluated. Nevertheless it HOMO for the compound having a 3,3-P atom 28 and in the opens new perspectives for the design of 6-membered P- LUMO for 4,5-phosphine oxide 29 (Figure 17). heterocycle containing PAHs.

Figure 17. Frontiers orbitals of compounds 28 (left) and 29 (right). Adapted with permission from ref 66. Copyright 2011 American Chemical Society).

All compounds display strong absorption bands around 380 – 415 nm that were assigned to HOMO-LUMO transition by TD- DFT. They all display moderate to strong luminescence in the blue range. Furthermore, the emission of 3,3-phosphine 28 can Figure 15: HOMO and LUMO orbitals of 25. (Adapted with permission from ref be tuned with concentration since this compound shows an 65. Copyright 2015 John Wiley and Sons). excimeric emission at high concentration. The authors further extend their strategy with the preparation of 3.3. Fused highly distorted PAH fragment featuring 2 heteroatoms 31-33 (Figure 18).[67] Again, the presence of the tetrahedral P-atom Hatakeyama, Nakamura et al. developed a tandem phospha- and the steric repulsion between the Me-groups afford a PAH fragment composed of 13 cycles that can also be viewed as a Friedel-Crafts reaction (in the presence of S8) that allowed them to prepare polycylic derivatives featuring 4,5-P=S at a ring double [5]-helicene as shown on the crystallographic structure junction (Figure 16).[66 ] They first developed a family of curved (Figure 18). In particular, the central phenyl ring is distorted to a triarylphosphines 27-30. Phosphine sulfide 27, which is boat-like conformation. thermally and air-stable, could be deprotected into its 3,3 analogue 28, which could further be oxidized or coordinated to Au(I) showing that the P-atom retains its reactivity. X-ray structures showed that in all the cases, the P atom adopts a pyramidal geometry leading to curved -framework. The deviation from planarity was further increased by H-H repulsion at the-position of the P (see Figure 16).

Figure 18. Synthesis and crystallographic structure (CCDC 1020103) of contorted P-containing PAH 31-33.[67]

DFT (B3LYP/6-31G(d)) calculations show efficient - delocalization over the full C-framework despite the contorted Figure 16. Synthesis and crystallographic structure of contorted P-containing structure, with a gap of 2.96 eV (Figure 19). According to [66] PAH 27-30. NICS(1) calculations (GIAO/B3LYP/6-311+G(d,p)) the aromaticity of the central ring is decreased due to the Theoretical calculations (B3LYP/6-31G(d)) showed that the deformation (-6.7) but it still keeps an aromatic character (Figure electronic density involves the entire C-backbone despite the 19). Furthermore, the two P-rings display a slightly antiaromatic curvature. The HOMO and LUMO are mainly dominated by the character (0.4 < NICS(0) < 1.5).

The optical/electrochemical properties of these -systems were P-containing PAH that would take advantage of the rich investigated. The UV-vis absorption consists of a large band in valence/coordination of organophosphorus derivatives. the visible (max= 450 nm) that is red-shifted compared to the smaller analogues 27-30. The compounds do not exhibit fluorescence in solution. Phosphine oxide 33 showed an Acknowledgements reversible reduction at moderated potential (-1.8 V vs Fc+/Fc) indicating that these compounds can act as electron acceptors. This work is supported by the Ministère de la Recherche et de These unprecedented scaffolds featuring a helical PAH l’Enseignement Supérieur, the CNRS, the Région Bretagne, backbone and a reactive P atom present promising redox and Campus France, China-French associated international optical properties than can be further used in opto-electronic laboratory in “Functional Organophosphorus Materials”, Balaton applications including chiroptical applications provided that PHC (830386K and 38522ZH), the French National Research enantiomers of 31-33 can be separated.[43b] Agency (ANR Heterographene ANR-16-CE05-0003-01, ANR

Helphos ANR-15-CE29-0012-03), OTKA K 105417 and COST

CM10302 (SIPS). Authors thanks Prof. R. Réau for his active

participation to the early stages of this project and Dr M. Duffy

for fruitful discussions.

Keywords: Polycyclic Aromatic •

Organophosphorus • Electronic properties • Aromaticity •

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