C Dearlove

T Clausen

7188 BAE Systems ATC Updates: 6130, 7181

2 (OLSRv2) and the MANET Neighborhood Discovery Protocol (NHDP) to increase their abilities to accommodate protocol extensions. This document updates RFC 7181 (OLSRv2) and RFC 6130 (NHDP).

Introduction

The MANET Neighborhood Discovery Protocol (NHDP) [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and the Optimized Link State Routing Protocol version 2 (OLSRv2) [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] are protocols for use in Mobile Ad Hoc Networks (MANETs) [RFC2501], based on the Generalized MANET Packet/Message Format [START_REF] Clausen | Generalized MANET Packet/Message Format[END_REF]. This document updates [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF], specifically their use of TLV (Type-Length-Value) elements, to increase the extensibility of these protocols and to enable some improvements in their implementation.

This specification reduces the latitude of implementations of [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] to consider some messages, which will not be created by implementations simply following those specifications, as a reason to consider the message as "badly formed", and thus as a reason to reject the message. This gives greater latitude to the creation of extensions of these protocols, in particular extensions that will interoperate with unextended implementations of those protocols. As part of that, it indicates how TLVs with unexpected value fields must be handled, and adds some additional options to those TLVs.

Note that TLVs with unknown type or type extension are already specified as to be ignored by [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] and also are not a reason to reject a message.

Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

Additionally, this document uses the terminology of [START_REF] Clausen | Generalized MANET Packet/Message Format[END_REF], [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF], and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF].

Applicability Statement

This document updates the specification of the protocols described in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF].

Specifically, this specification updates [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] in the following ways: o Removes the latitude of rejecting a message with a TLV with a known type, but with an unexpected TLV Value field, for the TLV Types defined in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF].

TLV Values

NHDP [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and OLSRv2 [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] define a number of TLVs within the framework of [START_REF] Clausen | Generalized MANET Packet/Message Format[END_REF]. These TLVs define the meaning of only some of the contents that can be found in a TLV Value field. This limitation may be either defining only certain TLV Values or considering only some lengths of the TLV Value fields (or a singlevalue field in a multivalue Address-Block TLV). This specification describes how NHDP [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and OLSRv2 [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] are to handle TLVs with other TLV Value fields.

Unrecognized TLV Values

NHDP and OLSRv2 specify that, in addition to well-defined reasons (in the respective protocol specifications), an implementation of these protocols MAY recognize a message as "badly formed" and therefore "invalid for processing" for other reasons (Section 12.1 of [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and Section 16.3.1 of [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF]). These sections could be interpreted as allowing rejection of a message because a TLV Value field is unrecognized. This specification removes that latitude: It should be stressed that this is not a change to [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] or [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF], except with regard to not allowing this to be a reason for rejection of a message. [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] or [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] are specified in terms such as "if an address is associated with a value of LOST by a LINK_STATUS TLV". Association with an unrecognized value has no effect on any implementation strictly following such a specification.

o

TLV Value Lengths

The TLVs specified in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] may be either singlevalue or multivalue TLVs. In either case, the length of each item of information encoded in the TLV Value field is the "single-length", defined and calculated as in Section 5.4.1 of [START_REF] Clausen | Generalized MANET Packet/Message Format[END_REF]. All TLVs specified in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] have a one-or two-octet singlelength. These are considered the expected single-lengths of such a received TLV.

Other single-length TLV Value fields may be introduced by extensions to [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF]. This document specifies how implementations of [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF], or extensions thereof, MUST behave on receiving TLVs of the TLV types defined in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF], but with TLV Value fields with other single-length values.

The following principles apply:

o If the received single-length is greater than the expected singlelength, then the excess octets MUST be ignored.

o If the received single-length is less than the expected singlelength, then the absent octets MUST be considered to have all bits cleared (0).

Exception:

o A received CONT_SEQ_NUM with a single-length < 2 SHOULD be considered an error.

Undefined TLV Values

[RFC6130] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] define a number of TLVs, but for some of these TLVs they specify meanings for only some TLV Values. This document establishes IANA registries for these TLV Values, with initial registrations reflecting those used by [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF], and as specified in Section 4.3.3. and3): 1 and 2 represent the presence of two different attributes associated to an address, and 3 represents "both 1 and 2".

These TLV Value fields are, by this specification, converted to bit fields and MUST be interpreted as such. As the existing definitions of values 1, 2, and 3 behave in that manner, it is likely that this will involve no change to an implementation, but any test of (for example) Value = 1 or Value = 3 MUST be converted to a test of (for example) Value bitand 1 = 1, where "bitand" denotes a bitwise AND operation.

This specification creates registries for recording reservations of the individual bits in these bit fields, with initial registrations for values defined and used by [RFC7181] --see Section 5.

Other TLVs defined by [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF] are not affected by this specification.

Unspecified TLV Values

The registries defined in Section 5 for the LOCAL_IF, LINK_STATUS, and OTHER_NEIGHB TLVs each include an additional TLV Value UNSPECIFIED. This TLV Value represents a defined value that, like currently undefined TLV Values, indicates that no information is associated with this address; the defined value will always have this meaning. Such a TLV Value may be used to enable the creation of more efficient multivalue Address Block TLVs or to simplify an implementation.

Dearlove & Clausen Standards Track [Page 6]

RFC 7188

NHDP and OLSRv2 Extension TLVs April 2014

The similar requirement for the MPR and NBR_ADDR_TYPES TLVs is already satisfied by the TLV Value zero, provided that each bit in the TLV Value is defined as set ('1') when indicating the presence of an attribute, or clear ('0') when indicating the absence of an attribute. Therefore, this is required for registrations from the relevant registries; see Section 5.

For the LINK_METRIC TLV, this is already possible by clearing the most significant bits (0 to 3) of the first octet of the TLV Value.

It is RECOMMENDED that in this case the remaining bits of the TLV Value are either all clear ('0') or all set ('1').

IANA Considerations

IANA has completed the ten actions set out in the following sections. IANA has populated this registry as specified in Table 1. --------+-------------+-------------------------------+----------- New assignments are to be made by Expert Review [START_REF] Narten | Guidelines for Writing an IANA Considerations Section in RFCs[END_REF].

+---------+-------------+-------------------------------+-----------+ | Value | Name | Description | Reference | +-
The Designated Experts are required to use the guidelines specified in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF].

Modification to Existing Registry

IANA maintains a sub-registry called "LOCAL_IF Address Block TLV Type Extensions" within the "Mobile Ad hoc NETwork (MANET) Parameters" registry. This sub-registry already had an entry for value 0. IANA has replaced the entry in the Description column for this value with the text "This value is to be interpreted according to the registry LOCAL_IF TLV Values". The resulting table is as specified in Table 2. ----------+------------------------------------------+------------+ ----------+------------------------------------------+------------ IANA has populated this registry as specified in Table 5. IANA maintains a sub-registry called "OTHER_NEIGHB Address Block TLV Type Extensions" within the "Mobile Ad hoc NETwork (MANET) Parameters" registry. This sub-registry already had an entry for value 0. IANA has replaced the entry in the Description column for this value with the text "This value is to be interpreted according to the registry OTHER_NEIGHB TLV Values". The resulting table is as specified in Table 6. IANA has populated this registry as specified in Table 7. ----+-------+----------+------------------------------+-----------+ | 7 | 0x01 | Flooding | The neighbor with that New assignments are to be made by Expert Review [START_REF] Narten | Guidelines for Writing an IANA Considerations Section in RFCs[END_REF].

+-----------+-----------------------------------------+-------------+ | Type +-----------+-----------------------------------------+-------------+
+-----------+------------------------------------------+------------+ | Type | Description | Reference | | Extension | | | +-
+---------+-------------+-------------------------------+-----------+ | Value | Name | Description | Reference | +---------+-------------+-------------------------------+-----------+ | 0 | LOST | The neighbor relationship
+-----+-------+----------+------------------------------+-----------+ | Bit | Value | Name | Description | Reference | +-
The Designated Experts are required to use the guidelines specified in [START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and [START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF]. Additionally, the Designated Experts are required to ensure that the following sense is preserved:

o For each bit in the field, a set bit (1) means that the address has the designated property, while an unset bit (0) means that no information about the designated property is provided. In particular, an unset bit must not be used to convey any specific information about the designated property.

Modification to Existing Registry

IANA maintains a sub-registry called "MPR Address Block TLV Type Extensions" within the "Mobile Ad hoc NETwork (MANET) Parameters" registry. This sub-registry already had an entry for value 0. IANA has replaced the entry in the Description column for this value with the text "This value is to be interpreted according to the registry MPR TLV Bit Values". The resulting table is as specified in Table 8. o For each bit in the field, a set bit (1) means that the address has the designated property, while an unset bit (0) means that no information about the designated property is provided. In particular, an unset bit must not be used to convey any specific information about the designated property.

Modification to Existing Registry

IANA maintains a sub-registry called "NBR_ADDR_TYPE Address Block TLV Type Extensions" within the "Mobile Ad hoc NETwork (MANET) Parameters" registry. This sub-registry already had an entry for value 0. IANA has replaced the entry in the Description column for this value with the text "This value is to be interpreted according to the registry NBR_ADDR_TYPE TLV Bit Values". The resulting table is as specified in Table 10. o Created, in some registries, a registration for "UNSPECIFIED" values for more efficient use of multivalue Address Block TLVs. The interpretation of an address being associated with a TLV of a given type and with the value "UNSPECIFIED" is identical to that address not being associated with a TLV of that type. Thus, this update does not give rise to any additional security considerations.

o Reduced the latitude of implementations of the two protocols to reject a message as "badly formed" due to the value field of a TLV being unexpected. These protocols are specified in terms such as "if an address is associated with a value of LOST by a LINK_STATUS TLV". Association with an unknown value (or a value newly defined to mean no link status information) has no effect on such a specification. Thus, this update does not give rise to any additional security considerations.

  5.1. LOCAL_IF Address Block TLVs 5.1.1. New Registry IANA has created a new sub-registry called "LOCAL_IF TLV Values" within the "Mobile Ad hoc NETwork (MANET) Parameters" registry.

  There are different cases of TLV Values with different characteristics. These cases are considered in this section.

	RFC 7188	NHDP and OLSRv2 Extension TLVs	April 2014
	4.3.1. NHDP TLVs: LOCAL_IF, LINK_STATUS, and OTHER_NEIGHB
	For the Address-Block TLVs LOCAL_IF, LINK_STATUS, and OTHER_NEIGHB
	TLVs, defined in [RFC6130], only a limited number of values are
	specified for each. These are converted, by this specification, into
	extensible registries with initial registrations for values defined
	and used by [RFC6130] --see Section 5.
	An implementation of [RFC6130] that receives a LOCAL_IF, LINK_STATUS,
	or OTHER_NEIGHB TLV with any TLV Value other than the values that are
	defined in [RFC6130] MUST ignore that TLV Value, as well as any
	corresponding attribute association to the address.
	4.3.2. OLSRv2 TLVs: MPR and NBR_ADDR_TYPE
	The Address-Block TLVs MPR and NBR_ADDR_TYPE, defined in [RFC7181],
	are similar to those defined in [RFC6130] in having only limited
	values specified (1, 2,
	Dearlove & Clausen	Standards Track	[Page 5]

Table 2

 2

	: LOCAL_IF Address Block TLV Type Extensions Modifications
	5.2. LINK_STATUS Address Block TLVs
	5.2.1. New Registry
	IANA has created a new sub-registry called "LINK_STATUS TLV Values"
	within the "Mobile Ad hoc NETwork (MANET) Parameters" registry.
	IANA has populated this registry as specified in Table 3.
	Dearlove & Clausen	Standards Track	[Page 8]

Table 3 :

 3 LINK_STATUS TLV ValuesNew assignments are to be made by Expert Review[START_REF] Narten | Guidelines for Writing an IANA Considerations Section in RFCs[END_REF].

	The Designated Experts are required to use the guidelines specified
	in [RFC6130] and [RFC7181].
	5.2.2. Modification to Existing Registry
	IANA maintains a sub-registry called "LINK_STATUS Address Block TLV
	Type Extensions" within the "Mobile Ad hoc NETwork (MANET)
	Parameters" registry. This sub-registry already had an entry for
	value 0. IANA has replaced the entry in the Description column for
	this value with the text "This value is to be interpreted according
	to the registry LINK_STATUS TLV Values". The resulting table is as
	specified in Table 4.
	Dearlove & Clausen	Standards Track	[Page 9]

Table 4 :

 4 + LINK_STATUS Address Block TLV Type Extensions Modifications 5.3. OTHER_NEIGHB Address Block TLVs 5.3.1. Create New Registry IANA has created a new sub-registry called "OTHER_NEIGHB TLV Values" within the "Mobile Ad hoc NETwork (MANET) Parameters" registry.

Table 5 :

 5 OTHER_NEIGHB Address Block TLV ValuesNew assignments are to be made by Expert Review[START_REF] Narten | Guidelines for Writing an IANA Considerations Section in RFCs[END_REF].

	The Designated Experts are required to use the guidelines specified
	in [RFC6130] and [RFC7181].
	Dearlove & Clausen	Standards Track	[Page 10]

Table 6 :

 6 OTHER_NEIGHB Address Block TLV Type Extensions Modifications

	5.4. MPR Address Block TLVs
	5.4.1. New Registry
	IANA has created a new sub-registry called "MPR TLV Bit Values"
	within the "Mobile Ad hoc NETwork (MANET) Parameters" registry.

Table 7 :

 7 MPR Address Block TLV Bit Values

	Dearlove & Clausen	Standards Track	[Page 11]

Table 8

 8

	: MPR Address Block TLV Type Extensions Modifications
	5.5. NBR_ADDR_TYPE Address Block TLVs
	5.5.1. New Registry
	IANA has created a new sub-registry called "NBR_ADDR_TYPE Address
	Block TLV Bit Values" within the "Mobile Ad hoc NETwork (MANET)
	Parameters" registry.
	IANA has populated this registry as specified in Table 9.
	Dearlove & Clausen	Standards Track	[Page 12]

Table 9 :

 9 NBR_ADDR_TYPE Address Block TLV Bit ValuesNew assignments are to be made by Expert Review[START_REF] Narten | Guidelines for Writing an IANA Considerations Section in RFCs[END_REF].The Designated Experts are required to use the guidelines specified in[START_REF] Clausen | Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP)[END_REF] and[START_REF] Clausen | The Optimized Link State Routing Protocol Version 2[END_REF]. Additionally, the Designated Experts are required to ensure that the following sense is preserved:

Table 10

 10 Created IANA registries for retaining TLV values for TLVs, already defined in the already published specifications of the two protocols, and with initial registrations for the TLV values defined by these specifications. This does not give rise to any additional security considerations. o Enabled protocol extensions for registering TLV values in the created IANA registries. Such extensions MUST specify appropriate security considerations.

	: NBR_ADDR_TYPE Address Block TLV Type Extensions
	Modifications
	6. Security Considerations
	The updates made to [RFC6130] and [RFC7181] have the following
	implications on the security considerations:
	o

  Did not introduce any opportunities for attacks on the protocols through signal modification that are not already present in the two protocols.

	RFC 7188	NHDP and OLSRv2 Extension TLVs	April 2014
	o
	Dearlove & Clausen	Standards Track	[Page 14]

Dearlove & ClausenStandards Track [Page 2]

Acknowledgments

The authors would like to gratefully acknowledge the following people for intense technical discussions, early reviews, and comments on the specification (listed alphabetically): Ulrich Herberg (Fujitsu Laboratories of America) and Henning Rogge (Frauenhofer FKIE).

The authors would also like to express their gratitude to Adrian Farrel for his assistance and contributions to the successful and timely completion of this specification.

Authors' Addresses


Dr Jan Holec

Beatrice Cogliati

Dr James Lawrence

Alejandro Berdonces-Layunta

Dr Pablo Herrero

Dr Yuuya Nagata

Dr Marzena Banasiewicz

Dr Boleslaw Kozankiewicz

Dr Martina Corso

Dimas G De Oteyza

Andrej Jancarik
email: andrej.jancarik@cemes.fr

Andre Gourdon
email: andre.gourdon@cemes.fr

Dr A Jancarik

Dr A Gourdon

A large starphene comprising pentacene branches

starphene in preparative scale. Our approach is based on a synthesis of a carbonyl-protected starphene precursor that is thermally converted in a solid-state form to the neat [16]starphene, which is then characterised with a variety of analytical methods, such as 13C CP-MAS NMR, TGA, MS MALDI, UV-Vis and FTIR spectroscopy. Furthermore, high-resolution STM experiments unambiguously confirm its expected structure and reveal a moderate electronic delocalisation between the pentacene arms. Nucleus-independent chemical shifts NICS(1) are also calculated to survey its aromatic character.

Introduction

Starphenes are 2D polyaromatic hydrocarbons (PAHs) comprising three acene [1] arms connected through a single benzene ring. [2] These structures can be used in single-molecule electronics as NAND [3] or NOR [4] gates. Furthermore, as a sort of 2D PAHs, starphenes can represent attractive materials for applications in organic electronics, as components in organic field-effect transistors (OFETs) or organic light-emitting diodes (OLEDs). [5] In general, the synthesis of starphenes is a challenging task due to their limited solubility, caused by strong intermolecular π-π stacking. [6] Therefore, only a few examples of unsubstituted starphenes have been reported until now and the largest one, decastarphene- (3,3,3), was described as far back as 1968 by Clar and Mullen. [7] To overcome this solubility problem, pendant groups are often introduced at the molecular peripheries, enhancing both solubility and stability of the starphenes. [6] For example, [7]starphene derivatives with n-butyl [8] or hexyloxy groups [9] were synthesized in this manner. Recently, Bunz et al. have prepared [13]-and [16]starphene derivatives with bulky triisopropylsilyl-ethynyl groups, [10] which are the longest substituted starphenes prepared to date. However, these bulky substituents can modify the electronic properties of the molecules, their packing and moreover affect interactions with metal surfaces, which can be limiting factors in the applications of the substituted starphenes in organic and single molecule electronics. The most applied synthetic strategy towards these soluble starphenes is a palladium-catalysed [2+2+2] cycloaddition reaction of in situ generated arynes from appropriate arene derivatives. [11][12][13] Another often used approach is a nickel-mediated Yamamoto coupling reaction of ortho-dibrominated arenes. [2,6,10,14] As an alternative, on-surface synthesis (OSS) in ultra-high vacuum (UHV) allows the preparation, although in minute amounts, of organic structures that are hardly attainable by traditional in-solution chemistry. [15][16][17] Recently, Fasel et al. [18] prepared a [13]starphene from ortho-dibromotetracene by OSS on a Ag(111) surface. Then, Grill et al. [19] reported OSS of [10]starphene from orthodibromoantracene controlled by the surface structure. However, in order to apply these unsubstituted starphene molecules as semi-conducting materials in organic electronic devices, or as single molecular gates, new practical methods of their preparation are still of great interest.

Herein, we report a synthesis of planar and fully conjugated [16]starphene 1 without any attached substituents (Figure 1). Furthermore, we describe its comprehensive characterisation by FTIR, UV-Vis spectroscopy, 13C CP-MAS NMR, TGA and MS MALDI. In addition, high-resolution scanning tunnelling microscopy (HR-STM) experiments precisely confirm structure 1, and are complemented by scanning tunnelling spectroscopy (STS) measurements that provide an estimate of the energy gap between the molecule's highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively). To gain further insight into its electronic properties, we also carried out theoretical calculations. To the best of our knowledge, compound 1 is the largest unsubstituted starphene that has been synthesised to date.

Results and Discussion

Synthesis of [16]Starphene 1. We have synthesised [16] First, a Diels-Alder (DA) reaction of benzyne, generated in situ from commercially available 2-(trimethylsilyl)phenyl-trifluoromethanesulfonate, with one diene side of 2, gave 3 in 64% yield. After oxidation by 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), diene 4 was produced in 95% yield. Under a controlled DA reaction of the monoaryne species, selectively generated from 1,4bis(trimethylsilyl)-phenyl-2,5-bis(triflate) [22] and diene 4, compound 5 was prepared in 54% yield. Oxidation of 5 by DDQ gave 6 in 89% yield. The bridging ketal moiety in 6 ensures its solubility by preventing strong intermolecular -stacking, which is crucial for the construction of extended polyaromatic structures. The preserved trimethylsilyl and triflate groups in 6 are in a place for in situ formation of the aryne employed in the next synthetic step. It is worth mentioning that compound 6 is a very valuable building block for the preparation of a large variety of PAHs. [23,24] Under the optimised palladium-catalysed [2+2+2] cycloaddition reaction, [6,11] the aryne precursor 6 was converted to a mixture of two expected isomers 7a and 7b (in a ratio 2:1, see Supporting Information for a chromatogram, Figure S30) with a total yield of 35%. The subtle difference in polarity between 7a and 7b allowed us to separate these isomeric products by standard column chromatography and their distinct symmetry enabled the determination of their structures by comparing their 1H NMR spectra. The spectrum of 7a was consistent with its lower C2v symmetry (chair-like structure). Four signals (3.27, 2×3.28, and 3.29 ppm) of methyl groups of ketal moieties appeared in the spectrumtwo signals for the inner methyl groups pointing to the central part of the starphene structure and then another two signals of outer methyls pointing to the ends of the arms. In the case of isomer 7b, with C3v point symmetry (table-like structure), only two of those signals (3.27 and 3.29 ppm) were observed (See Supporting Information, Figures S11 andS13). The last reaction step in solution was the cleavage of three ketal groups by trimethylsilyl iodide, to afford the corresponding (tris)carbonylated compounds 8a and 8b, both as pale-yellow solids in yields of 81% and 95%, respectively. The final step of the whole synthesis was a thermal cheletropic decarbonylation of 8a/8b in solid-state form. During this process, only the planar [16]starphene 1 was formed in quantitative yield accompanied by formation of carbon monoxide as a volatile by-product.

Characterisation of [16]starphene 1.

Following the final solid-state decarbonylation with thermal gravimetric analysis (TGA) (Figure 2 for 8a and Figure S31 for 8b) revealed that the decarbonylation, determined at 150°C, is a one-step process, through which all three carbonyls are simultaneously removed. A weight loss of 9.1% for 8a and 8.6% for 8b agrees well with the theoretical (9.2 %) weight loss of 3 carbonyls per molecule. Despite the different geometries of the isomers 8a/8b, their FTIR analyses in KBr pellets provide identical spectra and confirm the presence of carbonyl groups with a strong peak being recorded at 1774 cm 1 (Figure 3a for 8a and Figure S25 for 8b). The isomers 8a/8b in KBr pellets were transformed to starphene 1 by heating the sample at 180 C for 30 s. The complete decarbonylation is evident by the disappearance of a strong CO adsorption peak at 1774 cm 1. To confirm the starphene formation, we compared the IR spectrum of 1 with that of pentacene [20] (see Supporting Information, Figure S26), which assured the presence of the pentacene moieties in the structure of 1.

To further investigate the thermal transformation of 8a/8b into 1, we recorded the carbon solid-state cross-polarisation magic angle spinning (13C CP-MAS) NMR spectra before and after decarbonylation. In the spectrum of 8a (Figure 3b, i), three groups of signals are present. The signal around 56 ppm corresponds to the bridgehead sp3 carbons, the peaks between 116 and 139 ppm belong to the aromatic carbons and a signal of carbonyl carbons is located at 194 ppm. After heating 8a at 180 C for 15 min under an argon atmosphere, the relatively intensive signals of the bridgehead and carbonyl carbons disappeared and a narrowed signal between 114 and 140 ppm of aromatic carbons remained (Figure 3b, ii). The same behaviour was observed also for isomer 8b (Figure S21,S22). Notably, in Figure 3b, ii, a small broadened signal detectable around 55 ppm presumably belongs to the bridgehead sp3 carbons of a negligible amount of starphene dimer or higher oligomers that could be formed during the thermal decarbonylation. [25] Storing 1 in a glove box for one month at room temperature did not lead to any change in the 13C CP-MAS NMR spectrum (See Supporting Information, Figure S20) confirming its good stability in the solid-state form.

To elucidate the electronic properties of 1, we measured its absorption spectrum in a frozen matrice and compared it with that of pentacene [20,26] (Figure 3c) prepared under similar conditions from a carbonylated pentacene precursor (see Figure S29 for its structure). Solutions of 8a, 8b, respectively, were prepared in toluene with concentrations 10-4 mol.l-1 and the spectra recorded at room temperature (Figure S27a, S28a). The samples were then cooled to 5K, irradiated with UV light (310 nm) for 15 min to generate starphene 1 and measured again (see Supporting Information for further experimental details). A strong and broad absorption band with maximum at around 479 nm and then three weaker absorption bands with maxima at 558, 595 and 641 nm appear in the spectrum of 1. The obvious red shift (55 nm) of the longest wavelength absorption band of 1 compared to that of pentacene (at 586 nm) indicates electronic conjugation among the pentacene arms in 1. Based on the comparison with the literature data [26], it seems that the electronic properties of 1, diluted in frozen matrices, are similar to those of hexacene (rather than of longer acenes)Finally, MS MALDI analysis (Figure S23) of 1 revealed a mass of 828.5 Da, which agrees with the predicted value of 828.3 Da.

Theoretical calculations. The molecular orbitals of [16]starphene 1, including its Kohn-Sham HOMO and LUMO, respectively, were calculated using density functional theory (DFT) at the B3LYP/6-311++g(2d,p) level. The HOMO and LUMO are delocalised over the entire conjugated backbone of 1, and the HOMO-LUMO gap was determined as 1.64 eV, which is smaller than that of comparable calculations on pentacene (2.20 eV) [27] or hexacene (1.76 eV). [28] Together with the calculated HOMO-LUMO gap (1.95 eV) of angularly fused pentacenes (corresponding to two thirds of starphene 1), the values manifest evolution of the HOMO-LUMO gap when going from pentacene to starphene (Figure S32), thus supporting the hypothesis that the three pentacene arms are partially conjugated. To evaluate the local aromaticity of 1, nucleus-independent chemical shift (NICS) values were calculated. [29] The NICS values 1.0 Å above the molecular plane NICS(1) of 1 (Figure 1) on the benzene moieties along a pentacene arm range from -7.2 to -12.8, indicating their high degree of aromaticity. The NICS(1) values increase towards the middle rings of the arms suggesting an increase of aromaticity. On the other hand, the NICS(1) value of the central ring of 1 is +1.9, displaying almost no aromaticity. [30] This positive value suggests that single bonds interconnect the three pentacene arms and form the central ring. However, despite the positive value of the central ring, the coplanar arrangement of the three pentacene units likely allows their partial electronic coupling. STM experiments. In order to verify the chemical and electronic structure of the [16]starphene 1 via low-temperature STM experiments, the molecules needed to be deposited onto a clean surface under ultra-high-vacuum (UHV). Attempts to sublime precursors 8a and 8b under UHV proved unsuccessful. Instead, saturated solutions of the two precursors in acetonitrile were deposited onto an Au(111) surface in high vacuum with an atomic layer injection source ("ALI", see Supporting Information for details). The samples were then post-annealed at 150°C for 5 minutes to desorb a significant proportion of the contaminants present in the solution. Large-scale STM images recorded at 4.3 K before and after the annealing process are presented in the Supporting Information (Figure S33). Very low coverage of starphene-shaped molecules was observed after this process, identified by their distinctive shape in the STM images. To our surprise, the formation of starphene-shaped structures was observed only when the precursor with a table-like structure (8b) was deposited. The isomer 8a likely desorbs before being decarbonylated. An STM image showing two of these starphene-shaped molecules is presented in Figure 4a. Functionalisation of the STM probe with a CO molecule and scanning in the repulsive CO-adsorbate interaction regime provides HR-STM images that resolve the internal structure of molecules at surfaces. [31] Such images clearly show the distinguishable differences in the middle rings of all three arms of the starphene objects found on the surface when compared to the adjacent aromatic rings (Figure 4b). While these features coincide with the carbonylfunctionalised rings of 8b, we believe a decarbonylation has already taken place and those rings are instead hydrogenated (Figure 4c). The following findings support our hypothesis: (I) occasionally, the different ring does not coincide with the originally functionalised position, but with a neighbouring ring (Supporting Information, Figure S34); (II) the post-deposition annealing treatment has been performed at a temperature at which the decarbonylation is readily known to set in (see Figure 2); (III) finally, STM images of precursors closely related to 8b, with bridging ketones, which have been used to form long acenes at surfaces in other studies, usually yield a greatly enhanced signal at the location of the ketone bridge. [32,33] In contrast to this, we typically observe a reduced signal in the centre of the arms of these objects when imaging at a voltage that is within the energy gap of the molecule (Figure S35), in line with other studies of hydrogenated acenes on surfaces. [34] Such hydrogenation breaks the conjugation along the starphene arms, resulting in what can be seen as "independent" conjugated moieties. Indeed, it has been previously shown that each of the conjugated sections hosts molecular orbitals that are extremely similar in energy and wavefunction symmetry to those of the correspondingly sized acene. [34,35] In this case, the molecule comprises naphthalene moieties at the end of each arm and a [7]starphene in the central part. In a similar fashion to hydrogenated acenes, only when imaging at energies approaching the orbitals of these remaining conjugated moieties does the darker contrast on the central ring disappear and eventually become a maximum (Figure S36) [35].

A locally different contrast on the central rings of nonacene molecules on Au(111) has also been associated with underlying Au adatoms coordinating to the corresponding C atoms, the reason being the high reactivity displayed at those positions due to the partial open-shell character of higher acenes. [32] However, such an obviously reduced signal at the different ring, as observed in our experiments (Figure S35), was not found in the mentioned nonacene case. Based on this noticeable difference in the bias-dependent contrast, we thus discard the coordination to Au adatoms as the reason for the modified appearance. We propose that, as occurs with nonacene, starphene 1 is also a very reactive molecule once it has been formed by decarbonylation of 8a and 8b, respectively. However, under our experimental conditions, its most reactive C sites become hydrogenated during the sample processing rather than coordinating to Au. Although the source of the hydrogen atoms required for this reaction is unclear, it is presumably related to the significant presence of contaminants that were co-deposited with the molecule (Figure S33), along with the required annealing step. Note that such a pronounced reactivity as observed here for 1 has been associated with a substantial open-shell character and, in the case of polyacenes, it has only been observed with heptacene or longer molecules. [32] This can thus be readily considered a hint for a partial open-shell character of 1 and its substantially different electronic properties when compared to pentacene.

To ultimately form and characterise 1, the extra hydrogens were removed via STM tip-induced dehydrogenation. Applying bias voltage pulses of +2 V, acquiring dI/dV spectra, or even scanning at higher biases was often enough to remove the extra hydrogen atoms. An example of the sequential removal of hydrogens from each of a hydrogenated starphene is shown in Figure S37, with clear differences observed in STM imaging. An HR-STM image of 1 is presented in Figure 5, alongside dI/dV point spectra that display multiple resonances marked with green dashed lines. The resemblance of the conductance maps of the various resonances to the calculated wavefunctions of molecular orbitals of free-standing starphene 1 allows for their unambiguous assignment, as shown in Figure 5c for the LUMO, Figure 5d for the HOMO and Figure S38 for all the resonances together. The experimental energy gap between the HOMO (at -0.61 eV) and LUMO (at 0.9 eV) as obtained from STS is thus 1.51 eV, which is in a good agreement with the calculated value of 1.64 eV (vide supra) and is comparable to the gap measured with STS for heptacene on gold. [36], [37]. The HOMO-LUMO gap reduction by about 30% with respect to that of pentacene on Au(111) (2.20 eV as measured by STS) [38] provides further evidence that the molecule does not entirely behave as three isolated pentacene arms, but instead shows a notable electronic coupling between them.

Conclusion

We have demonstrated an efficient new strategy towards the preparation of unsubstituted [16]starphene 1. We believe that our general strategy is also adaptable for the construction of substituted analogues and, potentially, for even larger starphenes than 1 presented here. The multi milligram scale synthesis of 1 enabled its comprehensive characterisation by a variety of analytical methods, such as 13C CP-MAS NMR, TGA, MS MALDI and FTIR spectroscopy. UV-Vis absorption spectrum of 1 in frozen toluene was red-shifted compared to that of pentacene, indicating a partial electron conjugation of the starphene arms. The HR-STM experiments provided a characterisation of the fundamental structural and electronic properties and confirmed the moderate decrease of the HOMO-LUMO gap with respect to pentacene.

Our synthetic strategy enables to produce starphene compounds in large quantity, which is essential for the investigation of electron/hole mobilities in bulk and offers the possibility to implement 1 into electronic devices such as OFETs and OLEDs.

Figure 1 .

 1 Figure 1. Structure of the[16]starphene 1; the red numbers represent NICS(1) values calculated at the B3LYP/6-311++g(2d,p)//B3LYP/6-31G(d,p) level of theory (vide infra).

Figure 2 .

 2 Figure 2. TGA of 8a showing the weight loss of 3 CO groups (ca 9.1%; calcd 9.2 %).

Figure 3 .

 3 Figure 3. (a) FTIR spectra (KBr pellets) of 8a; (b) 13 C CPMAS NMR spectra of (i) 8a and (ii) 1; (c) Absorption spectra of 1 and pentacene in toluene at 5 K, concentrations 10 -4 mol.l -1 .

Figure 4 .

 4 Figure 4. (a) STM image (-600 mV, 50 pA) of the starphene/Au(111) sample after deposition and post-annealing to 150°C. Some of the objects on the surface were often moved around by the tip, leading to bright streaks in the image. (b) HR-STM image (CO tip, constant height, 2 mV) of the hydrogenated starphene prior to hydrogen removal via the STM tip. (c) The expected corresponding chemical structure of the hydrogenated starphene. All STM images recorded at 4.3 K.

Figure 5 .

 5 Figure 5. (a) HR-STM image (CO tip, 5 mV, constant height) of a starphene after hydrogen removal. (b) dI/dV point spectra on starphene showing various resonances marked with dashed lines. The inset marks the positions at which each of the spectra are taken, two on the molecule and one on the substrate as reference. (c) and (d) Constant height dI/dV images of a starphene recorded with a Cl tip at the energy of the lowest unoccupied and highest occupied states, respectively. Oscillation parameters: f = 828 Hz, A = 20 mV. All scale bars are 500 pm, all STM images recorded at 4.3 K. (e) and (f) Calculated wavefunctions of the HOMO and LUMO of free-standing 1 at the B3LYP/6-311++g(2d,p)//B3LYP/6-31G(d,p) level of theory in vacuum. See SI for complementary data.

Acknowledgements

This research has been funded in parts by foundation EXPERIENTIA, ERDF/ESF "UOCHB MSCA Mobility" (No. CZ.02.2.69/0.0/0.0/17 050/0008490), and has received funding from the EraNET Cofund Initiatives Quan-tERA under the European Union's Horizon 2020 research and innovation programme grant agreement ORQUID. This project has received financial support from the CNRS through the MITI interdisciplinary programs and JST-ERATO (No. JPMJER1903) and JSPS-WPI. Financial support is also acknowledged from the Spanish Ministry of Science and Innovation (MAT2016-78293-C6, PID2019-107338RB-C63), from the Basque Government (IT-1255-19), from the Spanish National Research Council (CSIC, COOPB20432), from the Euro-pean Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (contract no. EFA 194/16 TNSI) and from the European Research Council (grant agreement no. 635919). The authors thank Yannick Coppel (LCC-CNRS, Toulouse) for recording CP-MAS spectra, Jean-Francois Meunier (LCC-CNRS, Toulouse) for thermogravimetric analysis and Vladimir Vrkoslav (IOCB-Prague) for recording MS MALDI spectra.

Keywords: [16]Starphene, solid-state synthesis, decarbonylation, scanning tunnelling microscopy,conjugation, HOMO-LUMO gap, acenes.