1 Communicion Selective Intramolecular Dehydrocyclizion Co-Porphyr on Au(111) Cen Y, Zhantao Peng, Dan Liu, Huanjun Song, Hao Zhu, Qiwei Chen * Kai Wu * BNLMS, College Chemistry Molecular Engeerg, Pekg University, Beijg , Cha; (C.Y.); (Z.P.); (D.L.); (H.S.); (H.Z.) * Correspondence: (Q.C.); (K.W.) Academic Edirs: M. Salomé Rodríguez-Morgade Soji Shimizu Received: 29 July 2020; Accepted: 17 August 2020; Published: 19 August 2020 Abstract: on-surface C H bond activion reaction is a powerful approach constructg fe-tuned surface nano. It is quite challengg control its regioselectivity due ertness C H bond volved. With scanng tunnelg microscopy/spectroscopy oretical calculions, C H activion sequential tramolecular dehydrocyclizion meso-tetra(p-phenyl)porphyrocobalt(ii) was explored on Au(111), showg th molecule could ketically medie selectivity tramolecular reaction over its counterpart. experimental results demonstre th troduced protectg group could help augment selectivity such on-surface reaction, which can be applied precise fabricion functional surface nano. Keywords: C H activion; Co-porphyr; on-surface reaction; scanng tunnelg microscopy 1. Introduction On-surface synsis emerged last decade as an efficient approach achievg novel reactions/ fabricg elabore surface molecular nano [1 6]. Among various types on-surface reactions, C H bond activion (abbrevied as C H BAC) reaction [7,8] has been one concerned, not only due its difficulty traditional wet chemistry, but also for its convenience omic efficiency fabricg surface nano such as graphene nanoribbons [9,10], conjuged porphyr wires  -dimensional covalent polymers [12,13]. Due ertness ubiquity C H bonds molecular precursors, C H BAC reaction on surface normally performs poorly both reactivity selectivity. New on-surface stregies such as surface confement , self-assembly mediion , rmal control , halogen-adom-promoted reactivity  so on so forth, have successfully been applied enhance reactivity improve regioselectivity some specific cases. Recently, metalloporphyr or tetrapyrrole derivives have been reported undergo C H BAC reactions, such as dehydrocyclizion () [18 21], reactions graphene [21,22] dehydro-s [11,16,18,20]. Owg wide applicions porphyr its derivives molecular electronics [23,24] calysis [25,26], such studies have drawn tense terest. For stance, β-h pyrrole rg or para-h meso-phenyl group se can be actived eleved temperures terlked adjacent, nano-entities on surface. leads fact th tramolecular s take place along different reaction phs, resultg regioselectivity C H BAC reaction porphyrs on surface. Several reports have described selective control tra ter-molecular porphyrs rmal  surface  effects. However, our Molecules 2020, 25, 3766; doi: /
2 Molecules 2020, 25, knowledge, few studies [10,20] have vestiged troduction protectg promote selectivity Molecules 2020, 25, tra x FOR or PEER ter-molecular REVIEW reactions on surface, possibly because high 2 10 C H activion energy requires an eleved reaction temperure th few substituent could survive on surface. porphyrs rmal  surface  effects. However, our knowledge, few studies [10,20] have vestiged troduction protectg promote selectivity In this study, low temperure scanng tunnelg microscopy (LT-) was employed tra or ter-molecular reactions on surface, possibly because high C H activion energy vestige on-surface reactions meso-tetra(p-phenyl)porphyrocobalt(ii)  requires an eleved reaction temperure th few substituent could survive on surface. ( A stg In this for study, anisyl, low an temperure alias for phenyl) scanng tunnelg on Au(111). microscopy In, (LT-) was para-positions employed its four meso-phenyl vestige on-surface reactions substituted meso-tetra(p-phenyl)porphyrocobalt(ii). As bond strength is comparable between ( A stg C O for anisyl, C Han bonds, alias for such phenyl) on Au(111). likely In, survive para-positions high temperure trement its four th meso-phenyl necessarily required substituted active C H bonds.. refore, As bond strength is comparable between para-sites C O meso-phenyl C H bonds, such could be suppressed. likely Our survive measurements high clearly temperure identifiedtrement tramolecular th necessarily required active on Au(111) C H bonds. eleved refore, temperures. Combed scanng tunnelg spectroscopy para-sites (STS) meso-phenyl oretical could calculions be suppressed. furr supported Our measurements clearly identified tramolecular on Au(111) eleved reaction formed. Experimentally resolved reaction diced th temperures. Combed scanng tunnelg spectroscopy (STS) oretical calculions furr substituents could stay tact a temperure where reaction started. peculiar supported reaction formed. Experimentally resolved reaction observion diced th substituents could demonstred stay tact a temperure pround selectivity where reaction based reaction stistical started. analyses. peculiar observion demonstred pround selectivity reaction based on stistical analyses. 2. Results Discussion 2. Results Discussion 2.1. Dehydrocyclizion on Au(111) 2.1. Dehydrocyclizion on Au(111) structure is sly depicted left side 1a. Upon its adsorption on Au(111) structure surface, is sly molecule depicted adopts a saddle-like left side configurion 1a. Upon . its adsorption on an dividual Au(111) surface, molecule molecule (set adopts a saddle-like 1b) present configurion several key. feures th well mch an dividual structure : molecule (1) (set A bright rod-like 1b) present protrusion several key feures molecule th well mch structure : (1) A bright rod-like protrusion molecule center represents up-ward tilted pyrrole rgs central Co om. (2) Four dim spots center represents up-ward tilted pyrrole rgs central Co om. (2) Four dim spots both sides bright rod due meso-phenyl. (3) Claw-shaped protrusions around both sides bright rod due meso-phenyl. (3) Claw-shaped protrusions around four dim four spots dim spots stem stem from from . . At At room room temperure, temperure, low-coverage low-coverage adsorption adsorption led led formion formion sparsely sparsely distributed distributed self-assembled on on surface, surface, accordance accordance our previous our previous study study . . 1. (a) 1. Schemic (a) Schemic illustrion reaction reaction reaction product product 750 K 750 K on Au(111); on Au(111); (b) (b) adsorbed adsorbed on Au(111) on Au(111) room room temperure temperure low coverage; low coverage; set: enlarged an dividual molecule; (c) set: enlarged an dividual molecule; (c) product product genered from 750 K annealg sample (b); set: enlarged an genered from 750 K annealg sample (b); set: enlarged an dividual dividual product. Imagg conditions for s 0.3 V pa. Image size product. Imagg conditions for s 0.3 V pa. Image size for (b,c): for (b) (c): nm 2 ; size for set s: 4 4 nm nm 2 ; size for set s: 4 4 nm 2.
3 Molecules 2020, 25, After beg about 750 K for 2 m, surface molecular coverage did not decrease distctly. s 1b,c A3 show general appearance sample surface before after reaction. However, exhibited an d feure different from th molecule deposited room temperure. A typical such species is set 1c. In contrast untreed, different regions rmally treed molecule possess a similar contrast, suggestg th rmally treed molecule adopted a planar adsorption configurion. Besides, although rmally treed molecule shows a four-lobed feure similar th for untreed, it is obvious th angle formed left lobes is larger than th right counterparts. Moreover, claw-shaped protrusions not observable, implyg th four dissocied. Accordg above-described experimental observions, structure treed molecule was (right side 1a). It shows th rmally treed molecule was actually a product between β-h pyrrole group ortho-h meso-phenyl group. Similar previously reported for 5,15-diphenylporphyr (2H-DPP) on Cu(111) , tetrakis-(4-fluorophenyl)porphyr (2H-4FTPP) , Fe-tetra(4-bromophenyl)porphyr chloride [Br 4 -FeTPP(Cl)]  on Au(111) Zn-β-tetrabromo-tetraphenylporphyr (ZnTPPBr 4 ) wet synsis . Due fact th meso-phenyl may react pyrrole group eir on its left or its right side, re exist four structural for product (Appendix A). Additionally, we note th hydroxy group is normally unidentifiable imagg. Consequently, possibility group s partial dissociion a hydroxy group cannot be completely ruled out. Neverless, structural analysis on traces diced th free hydroxyl species because no er-type product was found (Appendix B). STS oretical calculions furr supported formion. 2a,b show optimized free space density functional ory (DFT) calculions. A side view pots out th product adopts a planar configurion while molecule adopts a saddle-like one. explas uniformity contrast for an dividual product. STS spectra correspondgly acquired on Au(111) centers ( 2c). surface-ste-reled spectral feure Au(111) substre appears about 0.3 V. For, characteristic peaks positioned about 0.8 V 1.6 V (marked red les), correspondg highest occupied molecular orbital (HOMO) lowest unoccupied molecular orbital (LUMO) molecule adsorbed on Au(111). surface ste Au(111) is possibly buried tail 0.8 V peak hence becomes distguishable spectrum. For product, peak 0.3 V is remarkably distct from th for Au(111) surface ste both peak shape tensity, feurg spectroscopic properties product. Anor small hump was about 1.2 V. se spectroscopic peaks identified as HOMO LUMO product on Au(111), as marked blue le. Compd, HOMO LUMO gap product narrows down from ev. is understable fact th product is more conjugive than. DFT calculions for molecule product free space also support such a narrowg-down HOMO LUMO gap, as DFT calculed HOMO LUMO gaps 2.9 ev 2.1 ev for molecule product, respectively. absolute values calculed HOMO LUMO gaps different from those measured STS. reason is quite obvious: DFT calculions conducted free space where surface effect on molecule was not cluded.
4 Molecules 2020, 25, x FOR PEER REVIEW Molecules 2020, 25, 3766 Molecules 2020, 25, x FOR PEER REVIEW Bird side views optimized (a) (b) free space; (c) STS acquired on Au(111), product. 2. Bird spectra sideside views optimized free 2. Bird views optimized (a)(a) (b) (b) (c) STS acquired spectra acquired on Au(111), product. space;free (c) space; STS spectra on Au(111), product Intermedies Dehydrocyclizion Reaction 2.2. Intermedies Dehydrocyclizion 2.2. Intermedies DehydrocyclizionReaction Reaction Once surface was a medium temperure, i.e., 523 K, for 2 m, partially yielded Once surface was amedium medium temperure, temperure, i.e., 523 K,K, for 2 m, partially yielded surface a surface m, partially yielded Once was on As i.e., 3a,for on surface. surface. As As 3a,3a, on remaed tact formed small domas self-assembly. remaed tact formed small domas self-assembly. remaed tact formed small domas self-assembly.. presented presented as claw-shaped feures In addition, several presented as claw-shaped feurescan canalso also be be identified identified. In addition, several as claw-shaped feures can also be identified. In addition, several novel morphologies identified, as marked white circles. Enlarged novel morphologies identified, as marked white circles. Enlarged novel morphologies identified, as marked white circles. Enlarged s s ( 3b,c) dice th partially genered s ( 3b,c) dice thsuch suchnovel novel partially genered ( 3b,c) dice th such novel partially genered coupled pyrrole.. meso-phenyl meso-phenyl coupled ir ir adjacent adjacent pyrrole meso-phenyl coupled ir adjacent pyrrole. se sly well mch sese sly 3d,e, 3d,e,which which well mch sly 3d,e, which well mch morphologies. morphologies. morphologies. 3. (a) self-assembly on surface 523 K; (b c) types as dehydrocyclizion where only four meso-methyl 3. self-assembly on types types coupled pyrrole ; (d e) for (b) 3. (a) (a) self-assembly on surface surface K; K; (b c) (b,c) 2 2 as dehydrocyclizion dehydrocyclizion where only four meso-methyl ; (b, c) 4 4 nm. (c), respectively. Imagg conditions: 0.3 V, 20 pa. Image size: (a) 20 four 20 nm as where only meso-methyl coupled pyrrole pyrrole ; ; (d,e) (d e) for for (b) coupled (b,c), 2; (b, c) 42 4 nm2. 2 (c), respectively. Imagg conditions: 0.3 V, 20 pa. Image size: (a) nm respectively. Imagg conditions: 0.3 V, 20 pa. Image size: (a) nm ; (b,c) 4 4 nm.
5 Molecules 2020, 25, x 3766 FOR PEER REVIEW 5 10 Anor fdg was th some partially produced survived, Anor as fdg claw wasfeure th can be some clearly partially produced ( 3c). means survived, th as dissociion claw feure group can be requires clearly an activion ( energy 3c). higher means than th or least dissociion nearly equivalent th group requires reaction. an activion refore, energyre higher is a than chance or th least nearly equivalent serve thas a protectg reaction. group refore, prevent re is a chance th dehydro- reactions serve as afrom protectg happeng group eleved prevent temperures. dehydro- reactions from happeng eleved temperures Intra Intra Inter-molecular Inter-MolecularReaction ReactionSelectivity In In addition widespread dividual ( 1c), 1c), traces on on Au(111) surface. surface. As As marked marked white white oval oval 4a, 4a, product morphologically morphologically connected connected each or each via or one via ir one four lobes ir four lobes be be product. product. structure such structure species is such illustrively species is superimposed illustrively superimposed on an enlarged on an enlarged, which displays, which well displays morphological well consistency. morphological consistency (a) (a) An An self-assembly self-assembly on on surface surface 750 K. 750 K. product product is marked is marked white white oval oval superimposed its structure on an enlarged. (b) Stistic hisgrams, tramolecular different reaction temperures. Imagg conditions: 0.3V, V, 50 pa. Image size: nm Stistics on portions unreacted, tramolecular furr analyzed, aimg aimg a quantitive a quantitive understg understg reaction reaction molecule molecule on Au(111). on Au(111). Before Before stistics, stistics, rule for countg rule for each countg type each type is defed as is follows. defed as Forfollows. For reaction, a reaction, molecule a eir molecule partiallyeir or fully partially turnedor fully turned product was counted product as one was molecule counted as one stistical molecule process. stistical For process. For reaction, wher or reaction, not wher or an not ly connected an ly cha alreadyconnected underwentcha reaction, already underwent y counted reaction, as y counted as only. ensured thonly. sum molecular ensured th proportions sum molecular for, proportions tramolecular for, tramolecular exactly equaled 100%. exactly For eachequaled sample100%. For each a different sample temperure, a different temperure, s collected different s regions collected different substre regions surface substre used for surface stistical analyses. used for In tal, stistical analyses. In tal, counted for each sample. counted As displayed for each sample. 4b, As displayed tramolecular 4b, reaction tramolecular 523 K volved reaction about 9% 523 K volved about. 9% As annealg. temperure As creased annealg temperure 750 K, all creased 750 K, eventually turned ventually, turned only a small, portion (about only 6%) a small furr portion underwent (about 6%) furr underwent reactions. reactions. Accordg above-described results, a a scenario scenario for for reaction reaction process process molecule molecule on Au(111) on isau(111) as follows: is as At follows: an eleved At an temperure eleved temperure about 523 K, about C H 523 bonds K, C H bonds meso-phenyl pyrrole can be actived. Additionally,
6 Molecules 2020, 25, meso-phenyl pyrrole can be actived. Additionally, about dissocie as well. However, because requires dissociion advance, tramolecular reaction becomes more feasible. Given th annealg time is relively short (2 m), it is th steric hdrance effect group ketically promotes selectivity tramolecular reaction agast counterpart. Once all dissocie, furr rmal annealg as-prepd sample 750 K results crease (Appendix B). 3. Merials Methods All experiments performed on a LT- (Unisoku USM1300) system a base pressure lower than Torr. An omically fl Au(111) sgle crystal (MaTeck GmbH) surface was prepd cycled argon ion beam bombardment 2 kv subsequent high temperure annealg K. cleanness surface was confirmed imagg; only traces (<3% ) contames (adsorbed H or C oms) on Au(111) surface. If present, contamg H or C oms normally apped as dark depressions s. (Sigma-Aldrich, Darmstadt, Germany, 97%) was deposited on surface via rmal evaporion from a home-made tantalum bo. deposition re molecule was monired a quartz crystal microbalance (Inficon SQM-160) kept 1 2 ML/m durg deposition. rmal annealg sample was conducted radiion heg back sgle crystal. sample temperure was measured an frd rmometer which was calibred a rmocouple advance. used tips mechanically shed Pt/Ir alloy wires. All STS measurements carried out liquid helium temperure (~4.4 K). All s raw s directly exported from Rev9 stw RHK Technology. STS signals recorded constant tip height usg a lock- amplifier (SRS SR830) a bias modulion V rms = 10 mv 1720 Hz. STS da processed Orig stw. DFT calculions performed Gaussian 09 stw (Revision A.02, Wallgford, CT, USA) , usg B3LYP functional. For molecule product, geometrical optimizion was performed usg a combed 6 31 g (d, p) LANDL2DL basis set. HOMO LUMO analyses based on correspondgly optimized models. 4. Conclusions In summary, we employed LT- DFT calculions explore reactions on Au(111). Both STS measurements DFT calculions strongly supported. At 523 K, reaction carried on dissociion. Rapid annealg higher temperures led selective formions tramolecular over, implyg th substitution a protective group such as could ketically medie selectivity tra ter-molecular reactions. Author Contributions: Conceptualizion, K.W.; methodology, C.Y. H.S.; validion, C.Y.; formal analysis, C.Y. Q.C.; vestigion, C.Y., D.L., Z.P. H.Z.; resources, Q.C.; da curion, C.Y.; writg origal draft preparion, C.Y. Q.C.; writg review editg, K.W.; visualizion, C.Y. Q.C.; supervision, K.W.; project admistrion, K.W.; fundg acquisition, K.W. All authors have read agreed published version manuscript. Fundg: research was funded Mistry Science Technology Cha (grant number 2017M620495) Nional Nural Science Foundion Cha (grant number , ). Acknowledgments: Q.C. thanks Jg Liu from Beijg Academy Quantum Informion Sciences for helpful discussions. Conflicts Interest: authors decl no conflict terest.
7 Molecules FOR PEER REVIEW Molecules 2020, 2020, 25, 25, x3766 Molecules 2020, 25, x FOR PEER REVIEW Appendix A Appendix A As meso-phenyl group may react pyrrole group on eir its left or right side, re meso-phenyl meso-phenyl may react pyrrole group onaseir its or side, right side, may react pyrrole on eir its left orleft right exist As tal four types group group structural for group, A1. re re exist th tal four types structural, as A1. exist tal four types structural for for, as A1. rionalizes not uniformly alike. rionalizes notuniformly uniformly alike. rionalizes th1cth not alike. set represents type 1 isomer. Or also identifiable accordg ir 1c represents type set isomer. Or also identifiable ir d morphological shapes 1 isomer. symmetric feures. For example, 1 accordg 2 mirror d morphological morphological shapes symmetric feures. For example, example, mirror For symmetric, isomer 3 is centrosymmetric, isomer 4 is asymmetric. structure each isomer 4 is asymmetric. symmetric, 3 is centrosymmetric, structure each molecule isomer product (enlarged 4a) is accordg accordg accordg molecule as product such shapes symmetric feures well. (enlarged 4a) is such shapes symmetric feures as well. A1. Four structural on Au(111). A1. Four Four structural structural on Au(111). Appendix B Appendix B B Appendix Furr annealg as-achieved leads crease Furr annealg isas-achieved as-achieved leads dissocied crease Furr annealg leads crease along ( A2a). because th ( A2a). is because th dissocied along formions ( A2a). is because th dissocied, dissocied along formions tramolecular. After all para-h on tramolecular. all dissocied, para-h on para-h meso-phenyl formions group tramolecular. After all dissocied, on meso-phenyl reengaged After reaction form. Similar group reengaged reaction form. Similar meso-phenyl group reengaged form. Similar also previouslyreaction reported ors [20,21,32]. also previously ors [20,21,32]. A2a) reported also previously reported ors [20,21,32]. typical (marked white ovals typical (marked white ovals A2a) typicala2b,c, (marked white ovals A2a) respectively. In addition, essentially no er-type A2b,c, respectively. In addition, essentially no er-type A2b,c, respectively. In addition, essentially no er-type accordg -d morphologies. accordg -d morphologies. excludes accordg dissociion -d. excludes possible partial morphologies group th would lead remag possible partial dissociion group th would lead remag hydroxy. excludes. possible partial dissociion group th would lead remag hydroxy hydroxy. A2. sample surface 1c furr about 750 K;750 (b,c) A2. (a) (a) sample surface 1c furr about K; for markedmarked about white A2. (a) sample surface 1c furr 750 (b,c) (b,c) for K; white ovals ovals (a). Imagg conditions: 20 pa.v,image size: 30 size: 30 nm. marked for 0.3 V, (a). Imagg conditions: pa. Image nm2. white ovals (a). Imagg conditions: 0.3 V, 20 pa. Image size: nm2.
8 Molecules 2020, 25, Molecules 2020, 25, x FOR PEER REVIEW 8 10 Appendix C Appendix C A3. A3. Large-scale Large-scale s s adsorbed adsorbed Au(111) Au(111) surface surface (a) (a) room room temperure, (b) 750 K. blue scale bars (a) (b) represent 10 nm. Imagg conditions: temperure, (b) 750 K. blue scale bars (a,b) represent 10 nm. Imagg conditions: (a) 60 pa. pa. (a) V, V, pa; pa; (b) (b) V, V, 60 Appendix D D Appendix coverage coverage dependence dependence reaction reaction eleved eleved temperures temperures was was furr furr vestiged. A4a shows -covered Au(111) surface high molecular vestiged. A4a shows -covered Au(111) surface high coverage after rmal annealg 523 K. In well-ordered molecular coverage after rmalannealg 523K. In surface a, surface a, self-assembly well-ordered  rema, while some small domas become disordered. An enlarged self-assembly  rema, while some small domas become disordered. An a disordered doma is doma Although A4b. shows a quite messyshows feure, enlarged a disordered is A4b. Although a some, as marked whiteas circle, termedie quite messy feure, some, marked similar white circle, similar 3c. Half brightrod-like protrusions becomeprotrusions dim s termedie 3c. Half bright rod-like both s 3c In addition, some In addition, well-ordered domas become dim A4b. s both s 3c A4b. someself-assembly wellunderwent reaction. As termedie similar those ordered self-assembly domas underwent A4c, reaction. As A4c, 3b marked circles. Accordg observions, termedie similar those 3b marked circles. reaction onobservions, Au(111) is not seriously affected molecular coverage. re is Accordg reaction on Au(111) is notmeanwhile, seriously affected no experimental evidence th a high molecular coverage would facilite molecular coverage. Meanwhile, re is no experimental evidence th a high molecular between. coverage would facilite between. A4. (a) (a)large-scale Large-scale adsorbed on Au(111) high A4. adsorbed on Au(111) high coverage coverage after beg 523 K. (b) Enlarged s a disordered doma (a). after beg 523 K. (b) Enlarged s a disordered doma (a). (c) Enlarged (c) Enlarged s self-assembly a well-ordered self-assembly (a). termedie s a well-ordered doma (a). doma termedie marked markedcircles white (b) correspondgly (c). blue scale bars white (b,c). blue circles scale bars (a c) represent 5, 2(a c) correspondgly represent 5, 2 1 nm. conditions: (a, b) 1.0 V, 30 pa; (c) 0.3 V, 10 pa. 1 nm. Imagg conditions: (a,b) 1.0 V, 30Imagg pa; (c) 0.3 V, 10 pa.
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