Synthesis and Characterization of Bis[1]Benzothieno[3,2-B:20,30-D]Pyrroles: Quantitative Effects of Benzannulation on Dithieno[3,2-B:20,30-D]Pyrroles

molecules

Article Synthesis and Characterization of Bis[1]benzothieno[3,2-b:20,30-d]pyrroles: Quantitative Effects of Benzannulation on Dithieno[3,2-b:20,30-d]pyrroles

Rylan M. W. Wolfe , Evan W. Culver and Seth C. Rasmussen * Department of Chemistry and Biochemistry, North Dakota State University, NDSU Dept. 2735, P.O. Box 6050, Fargo, ND 58108-6050, USA; [email protected] (R.M.W.W.); [email protected] (E.W.C.) * Correspondence: [email protected]; Tel.: +1-701-231-8747

Received: 14 August 2018; Accepted: 3 September 2018; Published: 6 September 2018

Abstract: The synthesis of four N-functionalized bis[1]benzothieno[3,2-b:20,30-d]pyrroles (BBTPs) is reported in order to provide a more detailed characterization of these fused-ring units, as well as increase the scope of known BBTP units available for application to conjugated materials. The optical, electronic, and structural properties of the resulting BBTP units have been compared to the parent N-alkyl- and N-aryl-dithieno[3,2-b:20,30-d]pyrroles (DTPs), as well as their corresponding 2,6-diphenyl derivatives, in order to fully quantify the relative electronic effects resulting from benzannulation of the parent DTP building block. Such comparative analysis reveals that benzannulation results in a red-shifted absorbance, but to a lesser extent than simple phenyl-capping of the DTP. More surprising is that benzannulation results in stabilization of the BBTP HOMO, compared to the destabilization normally observed with extending the conjugation length of the backbone.

Keywords: fused-ring thiophenes; heteroacenes; dithieno[3,2-b:20,30-d]pyrroles; benzannulation; structure-function relationships

1. Introduction Conjugated organic systems can provide materials that exhibit the electronic and optical properties of inorganic semiconductors, while retaining various attractive properties typically associated with organic plastics, including mechanical flexibility and low costs for production [1–3]. While such organic semiconductors are typically viewed as very modern materials, the study of these polymers dates as far back as the early 19th century [4–6], although significant interest did not arise until the first reports of their conductive nature in the early 1960s [6–10]. Since that point, the continuing academic and technological interest in these materials has given rise to the current field of organic electronics. Such development of technological applications has focused on organic photovoltaics (OPVs), field effect transistors (FETs), organic light-emitting diodes (OLEDs), electrochromic devices, and sensors [1–3]. One of the powerful aspects of these materials has been the ability to tune their electronic and optical properties at the molecular level via synthetic modification. This is especially true for thiophene-based materials, in which the ease and versatility of form via synthetic manipulation has made them especially popular [2,3]. A particularly popular approach for the synthetic modification of thiophene-based materials has been the application of fused-ring building blocks, especially the various fused 2,20-bithiophenes shown in Figure1a, including cyclopenta[2,1-b:3,4-b’]dithiophene (CDT) [2,3,11–13], silolo[3,2-b:4,5-b’]dithiophene (SDT) [3,11,12], dithieno[3,2-b:20,30-d]pyrrole (DTP) [2,3,11–16], phospholo[3,2-b:4,5-b’]dithiophene (PDT) [3,11,12], germolo[3,2-b:4,5-b’]dithiophene (GTP) [3,17–19], and arsolo[3,2-b:4,5-b’]dithiophene (ADT) [20–22].

Molecules 2018, 23, 2279; doi:10.3390/molecules23092279 www.mdpi.com/journal/molecules Molecules 2018, 23, x 2 of 14 Molecules 2018, 23, 2279 2 of 14 [2,3,11–16], phospholo[3,2‐b:4,5‐b’]dithiophene (PDT) [3,11,12], germolo[3,2‐b:4,5‐b’]dithiophene (GTP) [3,17–19], and arsolo[3,2‐b:4,5‐b’]dithiophene (ADT) [20–22]. The fused‐ring nature of these compoundsThe fused-ring provides nature a rigid, of these planar compounds ground state, provides which can a rigid, lead planarto improved ground conjugation, state, which increased can lead delocalization,to improved conjugation,and decreased increased energy delocalization,gaps, while also and reducing decreased interannular energy gaps, torsional while also vibrations, reducing thusinterannular contributing torsional to increased vibrations, emission thus contributingquantum yields. to increased In addition, emission the bridging quantum unit yields. (ER Inor addition, ER2 in Figurethe bridging 1a) can unitcontribute (ER or ERvia2 inductivein Figure1 effectsa) can contributeto tune the via corresponding inductive effects HOMO to tune or LUMO the corresponding energies, whileHOMO the orcentral LUMO placement energies, whileof the the side central chains placement allows the of the use side of chainsfairly bulky allows groups the use without of fairly bulkythe introductiongroups without of unwanted the introduction steric interactions of unwanted that steric can interactions lead to reduced that can backbone lead to reducedplanarity backbone in the resultingplanarity materials in the resulting [12,13]. materials [12,13].

(a) (b)

FigureFigure 1. 1.(a)( aFused) Fused 2,2 2,2′‐bithiophene0-bithiophene building building blocks; blocks; (b) ( bVarious) Various extended extended fused fused-ring‐ring analogues analogues of of dithieno[3,2dithieno[3,2-‐b:2b′:2,30′‐,3d0]pyrrole-d]pyrrole (DTP). (DTP).

MoreMore recently, recently, variousvarious groups groups have have begun begun investigating investigating analogous analogous building blocksbuilding incorporating blocks incorporatingfurther annulated further rings. annulated This is rings. especially This trueis especially for the popular true for DTP the units,popular for DTP which units, a number for which of such a numberextended of analoguessuch extended have analogues been reported have (Figure been 1reportedb) [ 23–32 (Figure]. Of these 1b) extended [23–32]. analogues,Of these extended the oldest 0 0 analogues,and most the simple oldest are and the most bis[1]benzothieno[3,2- simple are the bis[1]benzothieno[3,2b:2 ,3 -d]pyrroles (BBTPs),‐b:2′,3′‐d which]pyrroles were (BBTPs), first introduced which wereby Liu first in introduced 2008 [23]. In by the Liu following in 2008 [23]. decade, In the additional following reports decade, of BBTPs additional have appearedreports of [ 24BBTPs–27], buthave the appearedscope of [24–27], these building but the blocks scope isof still these somewhat building limited, blocks is with still only somewhat a total oflimited, seven with members only reporteda total ofto seven date. members It should reported be pointed to out date. that It theshould nomenclature be pointed used out tothat refer the to nomenclature these units changes used to from refer paper to theseto paper, units changes none of whichfrom paper correspond to paper, to none the correct of which and correspond proper IUPAC to the or correct CAS nomenclature. and proper IUPAC This is orperhaps CAS nomenclature. not surprising This as is the perhaps nomenclature not surprising of such as the complicated nomenclature species of such is rarely complicated covered species in even is graduaterarely covered coursework, in even such graduate that most coursework, in the field such are that not most very in familiar the field with are the not subtleties very familiar of correctly with thenaming subtleties fused-ring of correctly systems. naming In order fused to assist‐ring the systems. field with In such order complexities, to assist anthe educational field with guide such to complexities,fused-ring nomenclature an educational has guide been to recently fused‐ring published nomenclature [33]. has been recently published [33]. AlthoughAlthough BBTPs BBTPs have have been been studied studied as as both both molecular molecular materials materials [23,24] [23,24 and] and incorporated incorporated into into polymericpolymeric materials materials [26,34], [26,34 ],only only four four BBTPs BBTPs have have currently currently been been characterized characterized in interms terms of of their their opticaloptical and and electronic electronic properties properties [23,24]. [23,24 ].Perhaps Perhaps more more critically, critically, at atno no point point have have the the resulting resulting propertiesproperties of of BBTPs BBTPs been been directly directly compared compared to to the the parent parent DTPs, DTPs, although although some some limited limited attempts attempts to to comparecompare analogous analogous polymeric polymeric materials materials have have been been reported reported [23]. [23 ].In In order order to to provide provide a more a more detailed detailed characterizationcharacterization of ofthese these fused fused-ring‐ring units, units, as as well well as as increase increase the the scope scope of of BBTPs, BBTPs, the the synthesis synthesis and and characterizationcharacterization of offour four N N-substituted‐substituted BBTPs BBTPs are are reported reported herein. herein. Of Of these these four four BBTPs, BBTPs, three three are are previouslypreviously unreported, unreported, and and one one is isonly only the the second second example example of ofan an N‐NarylBBTP.-arylBBTP. Lastly, Lastly, the the optical, optical, electronic,electronic, and and structural structural properties properties of of the the BBTP BBTP units units have have been been compared compared to to the the parent parent N‐Nalkyl-alkyl-‐ and and N‐Naryl-aryl-DTPs,‐DTPs, as as well well as as their their corresponding corresponding 2,6 2,6-diphenyl‐diphenyl derivatives, derivatives, in inorder order to to fully fully quantify quantify the the relativerelative electronic electronic effects effects resulting resulting from from benzannulation benzannulation of of the the parent parent DTP DTP building building block. block.

Molecules 2018, 23, 2279 3 of 14 Molecules 2018, 23, x 3 of 14 Molecules 2018, 23, x 3 of 14 2. Results2. Results and and Discussion Discussion 2. Results and Discussion 2.1. Synthesis2.1. Synthesis 2.1. Synthesis To date,To date, several several different different approaches approaches have have been been utilized utilized forfor the synthesis of of BBTPs, BBTPs, as as outlined outlined in Schemein 1.SchemeTo Although date, several1. theAlthough earliestdifferent methodsthe approaches earliest involved havemethods been the utilized cyclizationinvolved for thethe of synthesis 3-nitro-2,2cyclization of 0BBTPs,-dibenzo[ of 3as‐nitro outlinedb]thiophene‐2,2′‐ in Scheme 1. Although the earliest methods involved the cyclization of 3‐nitro‐2,2′‐ to givedibenzo[ the unfunctionalizedb]thiophene to give BBTP the1a [unfunctionalized23], which could BBTP then be1a N[23],-functionalized, which could the then majority be N of‐ the functionalized,dibenzo[b]thiophene the majorityto give theof unfunctionalizedthe known synthetic BBTP methods1a [23], whichutilize coulda 3,3 ′‐thendihalo be‐2,2 N′‐‐ known synthetic methods utilize a 3,30-dihalo-2,20-bi(benzo[b]thiophene) (either 4 or 5) from which the bi(benzo[functionalized,b]thiophene) the majority(either 4 orof 5)the from known which syntheticthe central methodspyrrole ringutilize is formed a 3,3 ′‐viadihalo catalytic‐2,2′‐ central pyrrole ring is formed via catalytic amination [24–27]. The intermediate 4 is not only the most aminationbi(benzo[b ]thiophene)[24–27]. The (eitherintermediate 4 or 5 4) isfrom not onlywhich the the most central common pyrrole route ring to BBTPs,is formed but alsovia catalyticto other commonbenzannulatedamination route [24–27]. to BBTPs, species The but intermediate [35–40] also to such other 4 is as benzannulatednot thieno[3,2 only the most‐b:4,5 species common‐b’]bis[1]benzothiophene [35 route–40] to such BBTPs, as thieno[3,2- but[36,38,40] also tob other:4,5-andb ’]bis [1]benzothiophenephospholo[3,2benzannulated‐ b [:4,5species36,‐38b’]bis[1]benzothiophene,40 [35–40]] and phospholo[3,2- such as thieno[3,2 [37].b :4,5-‐bb’]bis[1]benzothiophene:4,5‐b’]bis[1]benzothiophene [37 ].[36,38,40] and phospholo[3,2‐b:4,5‐b’]bis[1]benzothiophene [37].

SchemeScheme 1. Previously 1. Previously reported reported synthetic synthetic routes routes toto bis[1]benzothieno[3,2-bis[1]benzothieno[3,2‐b:2b′,3:2′‐0,3d]pyrroles0-d]pyrroles (BBTPs). (BBTPs). Scheme 1. Previously reported synthetic routes to bis[1]benzothieno[3,2‐b:2′,3′‐d]pyrroles (BBTPs). In theIn the synthetic synthetic efforts efforts reportedreported here, here, BBTPs BBTPs were were produced produced via analogy via analogy with the withmore thewell‐ more well-developeddevelopedIn the DTPssynthetic DTPs (Scheme (Scheme efforts 2)2 reported)[[13].13 ].This This here, begins begins BBTPs with with were the the synthesisproduced synthesis of via 3 of‐ bromobenzo[analogy 3-bromobenzo[ withb ]thiophenethe bmore]thiophene well (3),‐ (3), developed DTPs (Scheme 2) [13]. This begins with the synthesis of 3‐bromobenzo[b]thiophene (3), whichwhich can becan reproducibly be reproducibly generated generated in in high high yield yield via via previously previously reported reported methods methods [23]. [23 Precursor]. Precursor 3 which can be reproducibly generated in high yield via previously reported methods [23]. Precursor can then3 can be then dimerized be dimerized to generate to generate the the critical critical intermediate intermediate 44 viavia methods methods our our group group had had previously previously optimized3 can then befor dimerized generation to of generate the analogous the critical 3,3′‐ bromointermediate‐2,2′‐bithiophene 4 via methods [15]. our These group methods had previously allow the optimized for generation of the analogous 3,30-bromo-2,20-bithiophene [15]. These methods allow the productionoptimized for of generation4 as a high purity,of the analogous white crystalline 3,3′‐bromo solid.‐2,2 ′‐bithiophene [15]. These methods allow the productionproduction of 4 ofas 4 a as high a high purity, purity, white white crystalline crystalline solid. solid.

Scheme 2. Optimized general synthesis of bis[1]benzothieno[3,2‐b:2′,3′‐d]pyrroles (BBTPs). SchemeScheme 2. Optimized 2. Optimized general general synthesis synthesis of of bis[1]benzothieno[3,2-bis[1]benzothieno[3,2‐b:2b′:2,30′‐,3d]pyrroles0-d]pyrroles (BBTPs). (BBTPs). It should be highlighted that previous reports have claimed the high yield (ca. 90%) production Itof should4 fromIt should either be highlighted be 3 highlighted or 2,3‐dibromobenzo[ that that previous previousb]thiophene reports reports have (2) via claimed claimed more thesimple the high high methods yield yield (ca. [24–26]. (ca. 90%) 90%) production However, production of 4 from either 3 or 2,3‐dibromobenzo[b]thiophene (2) via more simple methods [24–26]. However, of 4 from either 3 or 2,3-dibromobenzo[b]thiophene (2) via more simple methods [24–26]. However, these reports either provide no synthetic procedure or details to support the claim [25,26], or the reported data is indicative of material of low purity, with one such report stating 4 to be a “purple Molecules 2018, 23, x 4 of 14 Molecules 2018, 23, x 4 of 14

Moleculesthese reports2018, 23 ,either 2279 provide no synthetic procedure or details to support the claim [25,26], or4 of the 14 thesereported reports data eitheris indicative provide of no material synthetic of low procedure purity, withor details one such to support report statingthe claim 4 to [25,26], be a “purple or the reportedbrown solid” data [24].is indicative Most reports of material give more of low realistic purity, yields with of one 40–56% such viareport these stating basic 4methods to be a “purple[35–39]. brownbrownIn the optimized solid”solid” [[24].24]. methods Most reports reported give here, more formation realistic yields of the ofcopper 40–56% intermediate via these basic is facilitated methods by [35–39]. [35 initial–39]. InIn thethe optimized methodsmethods reportedreported here,here, formationformation ofof thethe coppercopper intermediateintermediate isis facilitatedfacilitated byby initialinitial transmetallation with ZnCl2, followed by sequential reaction with CuCl2 [15,41]. Oxidative coupling transmetallationtransmetallation withwith ZnClZnCl2,, followedfollowed byby sequentialsequential reactionreaction withwith CuCl CuCl2 [[15,41].15,41]. Oxidative couplingcoupling of the copper intermediate 2is then assisted with the addition of dry O2,2 to allow the production of 4 ofofin thethehigh coppercopper yield (80–85%). intermediateintermediate In addition, isis thenthen assistedassisted the purity withwith of thethe 4 generated additionaddition of ofvia drydry these OO22, methods to allow theisthe high production enough ofthat 44 ininX‐ highrayhigh quality yieldyield (80–85%). (80–85%).crystals were InIn addition, easily obtained, thethe purity thus of allowing 44 generatedgenerated its structural viavia thesethese methodsmethodsdetermination isis highhigh (Figure enoughenough 2). thatthat X-rayX‐ray qualityquality crystalscrystals werewere easilyeasily obtained,obtained, thusthus allowingallowing its its structural structural determination determination (Figure (Figure2 ).2).

Figure 2. Ellipsoid plot of 3,30′‐dibromo‐2,20′‐bi(benzo[b]thiophene) (4) at the 50% probability level. FigureFigure 2.2. Ellipsoid plotplot ofof 3,33,3′‐-dibromo-2,2dibromo‐2,2′‐-bi(benzo[bi(benzo[b]thiophene) (4) at the 50% probability level.level. The intermediate 4 can then be smoothly transformed into the BBTP products via a double C‐N TheThe intermediate 44 cancan then then be be smoothly smoothly transformed transformed into into the the BBTP BBTP products products via a via double a double C‐N C-Nbond bond formation formation through through Buchwald Buchwald-Hartwig‐Hartwig amination. amination. This This was was accomplished accomplished using using standardstandard bondconditions formation previously through developed Buchwald for DTPs,‐Hartwig using amination. a palladium This pre was‐catalyst accomplished coupled with using the chelatingstandard conditionsconditions previouslypreviously developeddeveloped forfor DTPs,DTPs, usingusing aa palladium pre-catalystpre‐catalyst coupledcoupled withwith thethe chelating diphosphine ligand 0 2,2′‐bis(diphenylphosphino)0 ‐1,1′‐binaphthyl (BINAP) [13]. The use of diphosphinediphosphine ligand ligand 2,2 -bis(diphenylphosphino)-1,12,2′‐bis(diphenylphosphino)-binaphthyl‐1,1′‐binaphthyl (BINAP) (BINAP) [13]. The [13]. use ofThe conventional use of longconventional chain alkylamines long chain resulted alkylamines in BBTPs resulted in yields in BBTPs of 65–80%, in yields while of 65–80%, the application while the ofapplication aniline and of conventionalaniline and allylamine long chain gave alkylamines reduced resultedyields. In in the BBTPs case in of yields aniline, of 65–80%,it is believed while that the theapplication amination of allylamineaniline and gave allylamine reduced gave yields. reduced In the yields. case In of the aniline, case of it isaniline, believed it is that believed the amination that the amination proceeds smoothly,proceeds smoothly, but the resulting but the BBTP resulting1i is quite BBTP rigid 1i is andquite tends rigid to and aggregate, tends to which aggregate, complicates which complicates purification proceedspurification smoothly, and leads but to the lower resulting isolated BBTP yields. 1i is quiteThe final rigid application and tends toof aggregate, allylamine which is complicated complicates by andpurification leads to and lower leads isolated to lower yields. isolated The final yields. application The final of application allylamine of is allylamine complicated is bycomplicated the very low by the very low boiling point of the small molar mass◦ amine (55–58 °C), thus causing loss of the volatile boilingthe very point low ofboiling the small point molar of the mass small amine molar (55–58 mass amineC), thus (55–58 causing °C), loss thus of causing the volatile loss reagent of the volatile during thereagent high temperatureduring the high amination temperature step that amination leads to quite step low that yields. leads This to couldquite potentiallylow yields. be This overcome could reagentpotentially during be overcome the high through temperature the future amination use of sealedstep that pressurized leads to conditions.quite low yields. This could throughpotentially the be future overcome use of through sealed pressurized the future use conditions. of sealed pressurized conditions. 2.2.2.2. X-rayX‐ray CrystallographyCrystallography andand StructuralStructural AnalysisAnalysis ofof BBTPsBBTPs 2.2. X‐ray Crystallography and Structural Analysis of BBTPs TheThe rigid,rigid, extendedextended fused-ringfused‐ring structurestructure ofof thethe BBTPBBTP compounds make them highly crystalline, The rigid, extended fused‐ring structure of the BBTP compounds make them highly crystalline, andand thethe crystalcrystal structuresstructures havehave beenbeen determineddetermined forfor threethree ofof thethe fourfour BBTPsBBTPs reportedreported here.here. EllipsoidEllipsoid and the crystal structures have been determined for three of the four BBTPs reported here. Ellipsoid plotsplots ofof BBTPs BBTPs1g 1g, ,1e 1e,, and and1i 1iare are shown shown in in Figures Figures3 and 3 and4. As 4. canAs can be seen be seen for BBTP for BBTP1g in1g Figure in Figure3, the 3, plots of BBTPs 1g, 1e, and 1i are shown in Figures 3 and 4. As can be seen for BBTP 1g in Figure 3, backbonesthe backbones of these of these structures structures are extremely are extremely flat withflat with deviations deviations of only of only ca. 1 ca.◦ from 1° from planarity. planarity. the backbones of these structures are extremely flat with deviations of only ca. 1° from planarity.

0 0 FigureFigure 3.3. FaceFace andand edgeedge ellipsoidellipsoid plotsplots ofof NN-octylbis[1]benzothieno[3,2-‐octylbis[1]benzothieno[3,2‐bb:2:2′,3,3′‐-d]pyrrole 1g at the 50% probabilityFigureprobability 3. Face level.level. and edge ellipsoid plots of N‐octylbis[1]benzothieno[3,2‐b:2′,3′‐d]pyrrole 1g at the 50% probability level.

Molecules 2018, 23, 2279 5 of 14 Molecules 2018, 23, x 5 of 14

(a) (b) 0 0 Figure 4. EllipsoidEllipsoid plots plots of of bis[1]benzothieno[3,2 bis[1]benzothieno[3,2-‐bb:2:2′,3,3′‐d-]pyrrolesd]pyrroles 1e1e (a(a) )and and 1i1i ((bb)) at at the the 50% probability level.

Selected bond distances for these three BBTPs are given in Table 1 1,, alongalong withwith thosethose ofof the the previously reportedreported BBTPs BBTPs1a 1a, ,1c 1c, ,1d 1d, and, andN N-octylDTP‐octylDTP for for comparison. comparison. The The fused-ring fused‐ring structures structures of allof sixall six BBTP BBTP compounds compounds show show excellent excellent agreement, agreement, with with deviations deviations of of ca. ca. 0.01 0.01 Å Å oror lessless for each bond. bond. A slightly greater difference is observed in the bond between the pyrrole and the corresponding side chain (i.e., N1-C9),N1‐C9), in which the two aryl derivatives exhibit a shortening of ca. 0.03 Å in comparison to the alkyl derivatives. This difference is less than that observed in in the the analogous DTP compounds, however, whichwhich can can potentially potentially be be attributed attributed to differences to differences in the in orientation the orientation of the phenylof the phenyl rings relative rings ◦ torelative the fused to the ring fused backbone. ring backbone. While While the N-phenyl the N‐phenyl ring of ring DTPs of DTPs exhibits exhibits a dihedral a dihedral angle angle of ca. of 46 ca. relative46° relative to the to DTP the backbone DTP backbone [14], the [14],N-phenyl the N rings‐phenyl of the rings analogous of the BBTPs analogous is nearly BBTPs perpendicular is nearly ◦ (ca.perpendicular 85 ). This difference(ca. 85°). This is most difference likely due is most to increased likely due sterics to increased between the stericsortho between-hydrogens the onortho the‐ Nhydrogens-phenyl ring on the and N the‐phenyl C-H ring bonds and at the the C 1-‐H and bonds 10-positions at the 1‐ and of the 10‐positions BBTP backbone. of the BBTP The backbone. enhanced dihedralThe enhanced angle dihedral would further angle would limit conjugation further limit between conjugation the phenyl between ring the and phenyl the fused-ring ring and the backbone, fused‐ thusring leadingbackbone, to elongationthus leading of theto elongation N-C bond. Asof the such, N one‐C bond. would As also such, expect one less would electronic also expect differences less betweenelectronic the differencesN-alkyl- andbetweenN-aryl-BBTPs the N‐alkyl as‐ aand result. N‐aryl‐BBTPs as a result.

0 0 Table 1.1. SelectedSelected bond bond lengths lengths (Å) (Å) for for various various bis[1]benzothieno[3,2 bis[1]benzothieno[3,2-‐b:2′b,3:2′‐d,3]pyrroles-d]pyrroles and and N‐ 0 0 Noctyldithieno[3,2-octyldithieno[3,2-‐b:2b′,3:2′‐,3d]pyrrole.-d]pyrrole. 1 1 2 3 Bond 1a1a 1 1c1c 1 1g1g 1e1e 1i1i 1d1d 2 N-octylDTP‐octylDTP 3 S1-C1S1‐C1 1.732(2) 1.7362(16) 1.7338(14) 1.731(6)1.731(6) 1.7319(11)1.7319(11) 1.7325(13)1.7325(13) 1.719(3) S1-C4S1‐C4 1.759(3) 1.7582(17) 1.7560(14) 1.763(6)1.763(6) 1.7546(12)1.7546(12) 1.7536(14)1.7536(14) 1.716(3) C1-C2 1.389(3) 1.3974(19) 1.3960(18) 1.404(8) 1.3924(15) 1.3894(17) 1.384(4) C1‐C2 1.389(3) 1.3974(19) 1.3960(18) 1.404(8) 1.3924(15) 1.3894(17) 1.384(4) C2-N1 1.373(3) 1.384(2) 1.3842(17) 1.378(8) 1.3844(13) 1.3802(16) 1.379(5) C2N1-C9‐N1 1.373(3) 1.4706(16)1.384(2) 1.4650(16)1.3842(17) 1.464(7)1.378(8) 1.4347(14)1.3844(13) 1.4367(15)1.3802(16) 1.451(4)1.379(5) N1C2-C3‐C9 1.431(3)1.4706(16) 1.431(2) 1.4382(19)1.4650(16) 1.443(8)1.464(7) 1.4334(14)1.4347(14) 1.4307(18)1.4367(15) 1.416(5)1.451(4) C3-C4C2‐C3 1.431(3) 1.407(3) 1.431(2) 1.422(2) 1.4201(19)1.4382(19) 1.412(8)1.443(8) 1.4171(15)1.4334(14) 1.4159(18)1.4307(18) 1.349(6)1.416(5) C1-C5C3‐C4 1.407(3) 1.412(3) 1.422(2) 1.411(2) 1.4080(19)1.4201(19) 1.410(8)1.412(8) 1.4123(14)1.4171(15) 1.4163(18)1.4159(18) 1.420(4)1.349(6) C3-C10 1.402(3) 1.404(2) 1.4079(18) 1.406(8) 1.4050(16) 1.3984(19) C4-C13C1‐C5 1.412(3) 1.392(3) 1.411(2) 1.394(2) 1.4080(19) 1.393(2) 1.387(9)1.410(8) 1.3991(15)1.4123(14) 1.3951(19)1.4163(18) 1.420(4) C10-C11C3‐C10 1.402(3) 1.372(4) 1.404(2) 1.382(3) 1.4079(18) 1.384(2) 1.387(9)1.406(8) 1.3877(16)1.4050(16) 1.3984(19) 1.386(2) C11-C12C4‐C13 1.392(3) 1.396(4) 1.394(2) 1.397(2) 1.399(2)1.393(2) 1.386(9)1.387(9) 1.4011(18)1.3991(15) 1.3951(19) 1.389(2) C10C12-C13‐C11 1.372(4) 1.384(4) 1.382(3) 1.378(2) 1.386(2)1.384(2) 1.381(9)1.387(9) 1.3893(17)1.3877(16) 1.384(2)1.386(2) C11‐C12 1.396(4)1 Data from1.397(2) reference [23]. 21.399(2)Data from reference1.386(9) [24]. 3 Adapted1.4011(18) from reference1.389(2) [14 ]. C12‐C13 1.384(4) 1.378(2) 1.386(2) 1.381(9) 1.3893(17) 1.384(2) 1 Data from reference [23]. 2 Data from reference [24]. 3 Adapted from reference [14].

Molecules 2018, 23, 2279 6 of 14

In comparison to N-octylDTP as a representative example, the BBTP units exhibit elongation of nearly all of the bonds that make up their central DTP core. As previously reported [14], the parent DTPs incorporate a central pyrrole that agrees well with the isolated heterocycle and this central Molecules 2018, 23, x 6 of 14 ring then seems to force the fused thiophenes to match its geometry. In contrast, the bond lengths of the BBTP pyrroleIn comparison deviate fromto N‐octylDTP that of as the a representative isolated heterocycle, example, the and BBTP the units structure exhibit elongation is now muchof more consistentnearly with all that of the of bonds dibenzothiophene that make up their [42 central]. For DTP example, core. As previously the fused reported bonds [14], C1-C2 the parent (1.396 Å) and DTPs incorporate a central pyrrole that agrees well with the isolated heterocycle and this central ring C3-C4 (1.420 Å), of 1g more closely match the fused bond of dibenzothiophene (1.409 Å) [42] than then seems to force the fused thiophenes to match its geometry. In contrast, the bond lengths of the either thatBBTP of isolated pyrrole deviate thiophene from (1.370that of the Å) isolated or pyrrole heterocycle, (1.382 and Å) [the43 ].structure As such, is now the much greater more aromaticity of the terminalconsistent benzene with that rings of dibenzothiophene now seems to [42]. dictate For example, the overall the fused geometry bonds C1 of‐C2 the (1.396 BBTP Å) and backbone. C3‐ The primary exceptionC4 (1.420 Å), to of this 1g more general closely bond match lengththe fused elongation bond of dibenzothiophene is seen in the (1.409 central Å) [42] than bond either between the that of isolated thiophene (1.370 Å) or pyrrole (1.382 Å) [43]. As such, the greater aromaticity of the thiophenes, C1-C5, which is slightly shorter than that observed in DTP (ca. 1.41 vs. 1.42 Å). This terminal benzene rings now seems to dictate the overall geometry of the BBTP backbone. The primary 0 bond is alsoexception shorter to thanthis general the interannular bond length elongation bonds of is eitherseen in the 2,2 central-bithiophene bond between (1.45 the Å) thiophenes, [44] or 4 (1.458 Å), implying strongC1‐C5, which coupling is slightly through shorter the than central that observed pyrrole in ring. DTP (ca. 1.41 vs. 1.42 Å). This bond is also shorter than the interannular bonds of either 2,2′‐bithiophene (1.45 Å) [44] or 4 (1.458 Å), implying 2.3. Absorptionstrong Spectroscopy coupling through the central pyrrole ring. One of2.3. the Absorption goals ofSpectroscopy the current study was to fully quantify the relative electronic effects resulting from benzannulationOne of the of goals the of parent the current DTP study building was to block.fully quantify As such, the relative the photophysical electronic effects data resulting of the BBTPs were directlyfrom compared benzannulation to of both the parent the parent DTP buildingN-octyl- block. and As such,N-phenyl-DTPs, the photophysical and data to of theirthe BBTPs 2,6-diphenyl derivativeswere (Figure directly5). compared Photophysical to both the data parent for N all‐octyl species‐ and N are‐phenyl collected‐DTPs, and in Table to their2 and2,6‐diphenyl representative derivatives (Figure 5). Photophysical data for all species are collected in Table 2 and representative UV-vis spectraUV‐vis of spectra1g, 6a of, 1g and, 6aN, and-octylDTP N‐octylDTP are are shown shown in Figure Figure 6. 6.

Figure 5. FigureAnalogous 5. Analogous DTP-based DTP‐based species species utilized utilized to compare compare the the relative relative effects effects of benzannulation of benzannulation verses simpleverses phenyl simple phenyl substitution. substitution.

Table 2. UV-visibleTable 2. UV data‐visible for data various for various bis[1]benzothieno[3,2- bis[1]benzothieno[3,2‐b:2:2′,30,3′‐d0-]pyrrolesd]pyrroles and anddithieno[3,2 dithieno[3,2-‐b:2′,3′‐ b:20,30-d] d]pyrrole. 1 pyrrole. 1 Compound λmax (nm) ε (M−1 cm−1) Eopt (eV) 2 −1 −1 2 CompoundN‐octylDTP λmax (nm)309 ε (M26,100cm )E3.88 opt (eV) N-octylDTP 309296 30,400 26,100 3.88 N‐phenylDTP 296310 32,000 30,400 3.87 299 35,200 N-phenylDTP1g 310341 44,300 32,000 3.52 3.87 299325 39,200 35,200 1g 341267 21,000 44,300 3.52 1e 325341 44,100 39,200 3.52 267325 39,100 21,000 267 20,900 1e 341 44,100 3.52 1h 340 48,000 3.52 325 39,100 324 45,100 267 20,900 266 24,900 1h 1i 340341 40,400 48,000 3.52 3.52 324325 36,700 45,100 266265 22,500 24,900 1i 6a 341381 61,000 40,400 2.95 3.52 6b 325378 58,400 36,700 2.96 1 In CH3CN.265 2 Calculated from the onset 22,500 of absorption. 6a 381 61,000 2.95 6b 378 58,400 2.96 1 2 In CH3CN. Calculated from the onset of absorption. Molecules 2018, 23, x 7 of 14 Molecules 2018, 23, 2279 7 of 14 Molecules 2018, 23, x 7 of 14

Figure 6. UV‐visible spectra of N‐octylbis[1]benzothieno[3,2‐b:2′,3′‐d]pyrrole (1g), N‐octyl ‐2,6‐ diphenyldithieno[3,2Figure 6.6.UV-visible UV‐visible‐ spectrab:2 ′,3spectra′‐d]pyrrole of N -octylbis[1]benzothieno[3,2-of (6aN‐),octylbis[1]benzothieno[3,2 and N‐octyldithieno[3,2b:20,3‐0b‐-b:2d:2]pyrrole′,3′,3′‐′‐dd]pyrrole]pyrrole (1g), inN -octyl-2,6-dipheny(CH1g),3CN. N‐ octyl‐2,6‐ 0 0 0 0 ldithieno[3,2-diphenyldithieno[3,2b:2 ,3 -d‐]pyrroleb:2′,3′‐d]pyrrole (6a), and (6aN),-octyldithieno[3,2- and N‐octyldithieno[3,2b:2 ,3 -‐db:2]pyrrole′,3′‐d]pyrrole in CH in3CN. CH3CN. The BBTP series all exhibit two closely‐spaced transitions at ca. 341 and 325 nm, as well as a higherThe energy BBTP shoulder series all at exhibitexhibit ca. 311 two nm. closely-spacedclosely Due to‐spaced the close transitions energetic at spacing ca. 341 and of these 325 nm, transitions, as well it as is a logicalhigher energytoenergy assign shoulder shoulder these atas at ca.vibrational ca. 311 311 nm. nm. Due components Due to theto the close ofclose energetic a single energetic spacingelectronic spacing of these transition. of transitions,these transitions,This itis is further logical it is supportedtological assign to these assignby the as these vibrationalfact that as vibrationalthe components observed components spacings of a single of of ca. electronic a single1400 cm electronic transition.−1 are in closetransition. This isagreement further This supported iswith further the − −1 −1 breathingbysupported the fact modes by that the the of fact observedfunctionalized that the spacings observed thiophenes of spacings ca. 1400 (1354–1422 of cm ca.1 1400are cm in cm) closeand−1 are pyrroles agreement in close (1415–1491 agreement with the cm breathing with) [45]. the − − Amodesbreathing separate of functionalized modes electronic of functionalized transition thiophenes is thiophenes (1354–1422also observed cm(1354–1422 1at) andhigher pyrroles cm −energy1) and (1415–1491 pyrroles(ca. 266 cm (1415–1491nm).1)[ The45]. Aextinctioncm separate−1) [45]. coefficientselectronicA separate transition forelectronic all isof also transitionthese observed transitions is atalso higher fallobserved energyroughly at (ca. higherbetween 266 nm). energy 20,000–50,000 The extinction(ca. 266 nm).M coefficients−1 cmThe−1, extinctionwith for all the of − − coefficientsthesecoefficients transitions offor the fallall low roughlyof energythese between transitionstransition 20,000–50,000 nearlyfall roughly twice M that1betweencm of1 ,the with 20,000–50,000parent the coefficients DTPs, Mconsistent− of1 cm the−1 low, with energy thethe increasetransitioncoefficients in nearlythe of crossthe twice low‐sectional energy that of area transition the of parent the BBTPnearly DTPs, relative twice consistent thatto DTP. of with the All the parent of increasethese DTPs, transitions in consistent the cross-sectional are assignedwith the asareaincrease simple of the in π the BBTPπ cross* transitions, relative‐sectional to DTP.which area All of agree the of these BBTPwith transitions relativethe calculated to areDTP. assignedelectron All of these density as simple transitions distributionsπ→π *are transitions, assigned of the HOMOwhichas simple agree and π  withLUMOπ* transitions, the shown calculated inwhich Figure electron agree 7, density andwith the the distributions measured calculated extinction electron of the HOMO densitycoefficients and distributions LUMO correspond shown of theto in stronglyFigureHOMO7 , andallowed and theLUMO measuredtransitions. shown extinction inThe Figure energies coefficients 7, and of thesethe correspondmeasured transitions extinction to are strongly essentially coefficients allowed independent transitions. correspond of The the to natureenergiesstrongly of ofallowedthe these N‐substitution, transitionstransitions. aresimilar The essentially energies to that of independentof DTPs these [14]. transitions ofThis the is nature consistentare essentially of the with N-substitution, theindependent frontier orbitals similar of the showntonature that in of Figure DTPsthe N‐ [ substitution,7,14 which]. This show is consistent similar that the to with thatN‐substituents theof DTPs frontier [14]. reside orbitals This at is showna consistent node in the Figure with BBTP 7the, which HOMO, frontier show andorbitals that the sidetheshown N-substituents chain in Figure contributes 7, residewhich to at showneither a node that the in the the HOMO N BBTP‐substituents HOMO,or LUMO, reside and both the at sideaof node which chain in the contributesare BBTP similar HOMO, toto neither previous and the calculationsHOMOside chain or LUMO, contributesof DTP both [13–15]. ofto whichneither are the similar HOMO to previous or LUMO, calculations both of ofwhich DTP are [13 –similar15]. to previous calculations of DTP [13–15].

Figure 7. Electronic density contours, calculated at B3LYP/6-31G(d,p) level, for the HOMO and LUMO Figure 7. Electronic density contours, calculated at B3LYP/6‐31G(d,p) level, for the HOMO and frontier molecular orbitals of bis[1]benzothieno[3,2-b:20,30-d]pyrroles 1g and 1i. LUMOFigure frontier7. Electronic molecular density orbitals contours, of bis[1]benzothieno[3,2 calculated at B3LYP/6‐b:2′,3‐′‐31G(d,p)d]pyrroles level, 1g and for 1ithe. HOMO and InLUMO addition frontier to molecular the enhanced orbitals absorbance of bis[1]benzothieno[3,2 intensities, the‐b:2 BBTP′,3′‐d]pyrroles spectra 1g are and also 1i. red-shifted by ca. In addition to the enhanced absorbance intensities, the BBTP spectra are also red‐shifted by ca. 30 nm in comparison to the parent DTPs, consistent with the greater conjugation length resulting 30 nm in comparison to the parent DTPs, consistent with the greater conjugation length resulting fromIn benzannulation addition to the ofenhanced the DTP absorbance backbone. intensities, As shown the in BBTP Figure spectra6, however, are also the red effect‐shifted of by this ca. from benzannulation of the DTP backbone. As shown in Figure 6, however, the effect of this benzannulation30 nm in comparison is limited to the in parent comparison DTPs, to consistent that resulting with fromthe greater end-capping conjugation the DTP length with resulting phenyl benzannulation is limited in comparison to that resulting from end‐capping the DTP with phenyl rings.from benzannulation The corresponding of red-shiftthe DTP resultingbackbone. from As theshown phenyl in end-cappingFigure 6, however, is more thanthe effect twice of that this of rings.benzannulation The corresponding is limited red in‐ shiftcomparison resulting to from that theresulting phenyl from end ‐endcapping‐capping is more the thanDTP twicewith phenylthat of rings. The corresponding red‐shift resulting from the phenyl end‐capping is more than twice that of

Molecules 2018, 23, 2279 8 of 14 Molecules 2018, 23, x 8 of 14 benzannulation, with a corresponding increase in absorbance intensity. Although the phenyl phenyl-capped‐capped DTPs 6a and 6b consists of an additional four π‐π-electronselectrons and a larger cross-sectionalcross‐sectional area, this is not enough to account for the drastic difference in absorptionabsorption energies.energies. Nevertheless, this suggests that benzannulation of such fused-ringfused‐ring heterocycles is not as effective as simple aryl-functionalizationaryl‐functionalization in extending the delocalization of the conjugated backbone.

2.4. Electrochemistry In order to determine the effects of DTP benzannulation on the frontier orbitals of the resulting BBTPs, the electrochemistry of the BBTP series was investigated by cyclic voltammetry (CV) and compared toto thethe respective respective DTP DTP parent parent species, species, as as well well as theas the analogous analogous diphenyl-capped diphenyl‐capped DTPs DTPs6a and 6a 6band. Representative 6b. Representative voltammograms voltammograms of 1g, of6a ,1g and, 6aN, -octylDTPand N‐octylDTP are shown are inshown Figure in8, Figure and the 8, collected and the CVcollected data is CV given data in is Tablegiven3 in. All Table BBTPs 3. All exhibit BBTPs an exhibit initial an well-deﬁned, initial well‐defined, quasi-reversible quasi‐reversible redox couple, redox followedcouple, followed by a second by a irreversible second irreversible oxidation. oxidation. Similar to Similar previously to previously reported reported DTPs [13 –DTPs15], the [13–15], potential the ofpotential the initial of the oxidation initial oxidation is dependent is dependent on the nature on the of thenatureN-functionalization, of the N‐functionalization, with N-arylBBTPs with N‐ exhibitingarylBBTPs oxidation exhibiting at oxidation slightly higher at slightly potentials. higher However, potentials. this effectHowever, is signiﬁcantly this effect diminished is significantly in the BBTPs,diminished which in exhibitthe BBTPs, shifts which of only exhibit 50 mV shifts in comparison of only 50 mV to shifts in comparison of ca. 90–100 to shifts mV for of ca. the 90–100 analogous mV DTPs.for the This analogous difference DTPs. is attributed This difference to the nearly is attributed orthogonal to orientationthe nearly oforthogonal the phenyl orientation rings observed of the in thephenyl X-ray rings structures observed of N -arylBBTPs,in the X‐ray which structures would severely of N‐arylBBTPs, limit conjugation which between would theseverely phenyl limit ring andconjugation the fused-ring between backbone. the phenyl As such,ring and the electronicthe fused‐ effectring backbone. of the N-aryl As functionalizationsuch, the electronic is assumed effect of tothe be N predominately‐aryl functionalization inductive. is assumed to be predominately inductive.

Figure 8. 8. CyclicCyclic voltammograms voltammograms of N‐octylbis[1]benzothieno[3,2 of N-octylbis[1]benzothieno[3,2-‐b:2′,3′‐d]pyrroleb:20,3 0(-1gd]pyrrole), N‐octyl‐ (2,61g),‐ Ndiphenyldithieno[3,2-octyl-2,6-diphenyldithieno[3,2-‐b:2′,3′‐d]pyrroleb:20,3 (6a0-d),]pyrrole and N‐octyldithieno[3,2 (6a), and N-octyldithieno[3,2-‐b:2′,3′‐d]pyrrole.b:20,3 0-d]pyrrole.

Although the benzannulation of DTP extends the conjugated backbone, the ﬁrstfirst oxidation of the BBTPs actuallyactually occursoccurs at at higher higher potentials potentials than than the the analogous analogous DTPs. DTPs. In In fact, fact, the the initial initial oxidation oxidation of the of Nthe-alkylBBTPs N‐alkylBBTPs is essentially is essentially the same the assame the as more the stabilized more stabilizedN-arylDTPs. N‐arylDTPs. In comparison, In comparison, the addition the ofaddition phenyl of end-caps phenyl end to DTP‐caps results to DTP in results the expected in the loweringexpected oflowering the potential of the forpotential the ﬁrst for oxidation. the first Thus,oxidation. the modulationThus, the modulation of the frontier of the orbitals frontier by orbitals benzannulation by benzannulation is directly is directly opposite opposite that of simplethat of additionsimple addition of phenyl of groups. phenyl Thisgroups. seemingly This seemingly contradictory contradictory electronic stabilization electronic stabilization with extension with of theextensionπ-backbone of the π‐ mustbackbone be at least must partially be at least related partially to the related structural to the changes structural observed changes in the observed DTP core in uponthe DTP benzannulation. core upon benzannulation.

Molecules 2018, 23, 2279 9 of 14

Table 3. Electrochemical data for various bis[1]benzothieno[3,2-b:20,30-d]pyrroles and dithieno[3,2-b:20, 30-d]pyrroles. 1

0/+1 +1/+2 2 Compound E1/2 (V) E (mV) Epa (V) EHOMO (eV) N-octylDTP 0.56 3 −5.60 N-phenylDTP 0.65 3 −5.69 1g 0.64 75 1.37 −5.68 1e 0.65 80 1.37 −5.69 1h 0.65 75 1.37 −5.69 1i 0.70 90 1.37 −5.70 6a 0.38 80 1.05 −5.42 6b 0.47 80 1.09 −5.51 1 + 2 All measurements in dry CH3CN containing 0.10 M TBAPF6. All potentials vs. Ag/Ag . EHOMO = 3 −(E[ox vs. Fc+/Fc] + 5.1)(eV) [46]. Irreversible, Epa reported.

3. Materials and Methods N-Octyldithieno[3,2-b:20,30-d]pyrrole [47], N-phenyldithieno[3,2-b:20,30-d]pyrrole [16], N-octyl-2,6- diphenyldithieno[3,2-b:20,30-d]pyrrole [16], N-phenyl-2,6-diphenyldithieno[3,2-b:20,30-d]pyrrole [16], and 3-bromobenzo[b]thiophene (3)[23] were prepared as previously reported. Dry THF and xylenes were obtained via distillation over sodium/benzophenone. Dry CH3CN was obtained via distillation over CaH2. All other chemical species were reagent grade and used without further puriﬁcation. All reactions were carried out under an inert atmosphere. All glassware for chemical synthesis was oven-dried, assembled while still hot, and cooled under dry nitrogen. 1H and 13C NMR spectra 1 13 were collected on a 400 MHz spectrometer (400 MHz H, 100 MHz C) in CDCl3 and referenced to the CHCl3 signal. Peak multiplicity is reported as follows: s = singlet, d = doublet, t = triplet, quint = quintet, sept = septet, dd = doublet of doublets, dt = doublet of triplets, ddt = doublet of doublet of triplets, m = multiplet. See ESI for copies of all NMR spectra.

3.1. Synthesis of 3,30-Dibromo-2,20-bi(benzo[b]thiophene) (4) The following is a modification of previously reported methods for the synthesis of 3,30-dibromo-2,20-bithiophene [15]. To a 250 mL 3-necked flask was added THF (120 mL), which was then cooled to 0 ◦C. Diisopropylamine (3.9 mL, 27.5 mmol) was added, followed by BuLi (11.0 mL, 2.5 M in hexanes, 27.5 mmol), and the mixture allowed to stir for 30 min. 3-Bromobenzo[b]thiophene (5.32 g, 25.0 mmol) was added and the solution was stirred for another 2 h. ZnCl2 (3.75 g, 27.5 mmol) ◦ was added in one portion and stirred for 15 min. The solution was cooled to −78 C and CuCl2 (3.70 g, 25.0 mmol) was added in one portion, and stirred for 30 min. Dry O2 was bubbled through the solution for 2 min, and the reaction was stirred until completion (as monitored by TLC, ~1 h). The reaction was then warmed to room temperature and quenched with saturated aqueous NH4Cl. The organic layer was separated, and the aqueous layer was extracted with diethyl ether. The combined organic layers were dried with MgSO4, concentrated via rotary evaporation, and purified by silica gel chromatography (hexanes) to give the isolated product as a white solid (80–85%). mp 169.0–171.0 ◦C ◦ 1 (lit. mp 178 C[35]); H NMR (CDCl3) δ 7.93 (dd, J = 1.2, 7.2 Hz, 2H), 7.87 (dd, J = 1.2, 7.2 Hz, 2H), 7.53 13 (dt, J = 1.2, 7.2 Hz, 2H), 7.49 (dt, J = 1.2, 7.2 Hz, 2H); C NMR (CDCl3) δ 139.2, 138.1, 129.4, 126.4, 125.5, 124.1, 122.3, 110.9. NMR data agree well with previously reported values [35–40].

3.2. General Synthesis of N-Functionalized Bis[1]benzothieno[3,2-b:20,30-d]pyrroles To a 50 mL 3-necked flask was added compound 4 (0.424 g, 1.0 mmol), sodium tert-butoxide (0.233 g, 2.4 mmol), Pd2(dba)3 (0.046 g, 0.050 mmol), and BINAP (0.062 g, 0.10 mmol), followed by evacuation of the flask and backfilling with nitrogen. Xylenes (15 mL) was then added, followed by the chosen amine (1.1 mmol), and the reaction mixture was heated to slightly under reflux (ca. 138 ◦C) with stirring for overnight. The reaction was then cooled to room temperature, water was added, and Molecules 2018, 23, 2279 10 of 14

the mixtureextracted with chloroform. The combined organic layers were dried with MgSO4, ﬁltered, concentrated via rotary evaporation, and puriﬁed by silica gel chromatography, to give the isolated product as a crystalline solid.

3.2.1. N-Octylbis[1]benzothieno[3,2-b:20,30-d]pyrrole (1g) The crude material was eluted with hexanes to give 1g as a white solid (75–80% yield). mp ◦ 1 93.5–94.0 C; H NMR (CDCl3) δ 7.91 (d, J = 8.0 Hz, 2H), 7.89 (d, J = 8.0 Hz, 2H), 7.42 (t, J = 8.0 Hz, 2H), 7.29 (t, J = 8.0 Hz, 2H), 4.80 (t, J = 7.6 Hz, 2H), 2.06 (quint, J = 7.6 Hz, 2H), 1.53 (quint, J = 7.6 Hz, 2H), 13 1.37 (quint, J = 7.6 Hz, 2H), 1.28–1.21 (m, 6H), 0.86 (t, J = 7.6 Hz, 3H); C NMR (CDCl3) δ 141.9, 137.5, 127.5, 124.5, 124.4, 123.1, 118.8, 114.5, 47.7, 32.0, 31.3, 29.6, 29.3, 27.0, 22.8, 14.3.

3.2.2. N-(2-Ethylhexyl)bis[1]benzothieno[3,2-b:20,30-d]pyrrole (1e) The crude material was eluted with hexanes to give 1e as a white solid (65–70% yield). mp ◦ ◦ 1 148.2–149.0 C (lit. mp 147–148 C[25]); H NMR (CDCl3) δ 7.92 (d, J = 8.0 Hz, 2H), 7.86 (d, J = 8.0 Hz, 2H), 7.42 (t, J = 8.0 Hz, 2H), 7.30 (t, J = 8.0 Hz, 2H), 4.70 (dd, J = 8.0, 14.0 Hz, 1H), 4.66 (dd, J = 8.0, 14.0 Hz, 1H), 2.24 (sept, J = 7.6 Hz, 2H), 1.60–1.20 (m, 8H), 0.90 (t, J = 7.6 Hz, 3H), 0.84 (t, J = 7.6 Hz, 3H); 13 C NMR (CDCl3) δ 141.9, 137.7, 127.6, 124.5, 124.2, 123.1, 119.0, 114.6, 52.1, 39.2, 31.8, 31.7, 25.0, 22.8, 14.3, 11.1. NMR data agree well with previously reported values [25].

3.2.3. N-Allylbis[1]benzothieno[3,2-b:20,30-d]pyrrole (1h) The crude material was eluted with 1% ethyl acetate in hexanes to give 1h as a white solid (25–30% 1 yield). H NMR (CDCl3) δ 7.88 (dd, J = 0.92, 7.4 Hz, 2H), 7.86 (dd, J = 0.92, 7.4 Hz, 2H), 7.42 (dt, J = 0.92, 7.4 Hz, 2H), 7.32 (dt, J = 0.92, 7.4 Hz, 2H), 6.28 (ddt, J = 4.4, 10.7, 16.9 Hz, 1H), 5.44 (dt, J = 1.8, 4.4 Hz, 13 2H), 5.29 (ddt, J = 0.9, 1.8, 10.7 Hz, 1H), 5.18 (ddt, J = 0.9, 1.8, 16.9 Hz, 1H); C NMR (CDCl3) δ 142.0, 137.9, 132.8, 127.5, 124.6(1), 124.5(8), 123.5, 119.1, 117.6, 115.1, 49.4.

3.2.4. N-Phenylbis[1]benzothieno[3,2-b:20,30-d]pyrrole (1i) The crude material was eluted with hexanes to give 1i as a white solid (30–35% yield). mp ◦ 1 201.6–202.8 C; H NMR (CDCl3) δ 7.83 (d, J = 8.0 Hz, 2H), 7.72–7.65 (m, 5H), 7.23 (d, J = 8.0 Hz, 2H), 7.16 (t, J = 8.0 Hz, 2H), 7.12 (t, J = 8.0 Hz, 2H); 13C NMR not obtained due to aggregation at suitable concentrations.

3.3. X-ray Crystallography X-ray quality crystals of 4, 1g, 1e, and 1i were grown by the slow evaporation of a hexane solution. The X-ray intensity data of the crystals were measured at 100 K on a Bruker Kappa Apex II Duo CCD-based X-ray diffractometer system equipped with a Mo-target X-ray tube (λ = 0.71073 Å) operated at 2000 W of power. The detector was placed at a distance of 5.000 cm from the crystal and data collected via the Bruker APEX2 software package. The frames were integrated with the Bruker SAINT software package. The unit cell was determined and refined by least-squares upon the refinement of XYZ-centeroids of reflections above 20σf(I). The structure was refined using the Bruker SHELXTL (Version 5.1) Software Package. Crystal data for 4 (C16H8Br2S2, M = 424.16 g/mol): triclinic, space group P-1, a = 7.4904(2) Å, b = 7.6660(2) Å, c = 14.4986(3) Å, α = 102.0470(10)◦, β = 91.9550(10)◦, γ = 118.3950(10)◦, V = 707.58(3) 3 −1 3 Å , Z = 2, T = 100(2) K, µ = 6.007 mm ,Dcalc = 1.991 g/cm , 25256 reflections measured, 3377 unique (Rint = 0.0157) which were used in all calculations. The final R1 was 0.0138 (I > 2σ(I)) and wR2 was 0.0357 (all data). Crystal data for 1g (C24H25NS2, M = 391.57 g/mol): monoclinic, space group P2(1)/n, a = 5.5843(4) Å, b = 16.9653(12) Å, c = 20.5440(15) Å, β = 92.396(1)◦, V = 1944.6(2) Å3, Z = 4, T =100(2) K, Molecules 2018, 23, 2279 11 of 14

−1 3 µ = 0.283 mm ,Dcalc = 1.337 g/cm , 19766 reflections measured, 4644 unique (Rint = 0.0307) which were used in all calculations. The final R1 was 0.0325 (I > 2σ(I)) and wR2 was 0.0832 (all data). Crystal data for 1e (C24H25NS2, M = 391.57 g/mol): triclinic, space group P-1, a = 8.6365(6) Å, b = 10.7755(8) Å, c = 11.1074(8) Å, α = 94.297(3)◦, β = 105.748(3)◦, γ = 92.076(3)◦, V = 990.36(12) Å3, −1 3 Z = 2, T = 100(2) K, µ = 0.278 mm ,Dcalc = 1.313 g/cm , 28835 reflections measured, 3509 unique (Rint = 0.0533) which were used in all calculations. The final R1 was 0.0784 (I > 2σ(I)) and wR2 was 0.2361 (all data). Crystal data for 1i (C22H13NS2, M = 355.45 g/mol): monoclinic, space group P2(1)/n, a = 16.2108(5) Å, b = 10.6136(3) Å, c = 20.8222(6) Å, β = 110.374(2)◦, V = 3358.44(17) Å3, Z = 8, T =100(2) K, −1 3 µ = 0.320 mm ,Dcalc = 1.406 g/cm , 68878 reflections measured, 10272 unique (Rint = 0.0224) which were used in all calculations. The final R1 was 0.0339 (I > 2σ(I)) and wR2 was 0.0928 (all data). CCDC 1861451–1861454 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: [email protected]. ac.uk).

3.4. Absorption Spectroscopy Spectra were measured on a dual beam scanning UV-visible-NIR spectrophotometer in 1 cm quartz cuvettes. Samples were prepared as dilute CH3CN solutions. Extinction coefﬁcients were determined via standard Beer-Lambert relationships using at least ﬁve standard solutions of different concentrations.

3.5. Computational Modeling Calculations were performed with the Gaussian 09 program [48]. The molecular geometries of the neutral states were calculated at the DFT level using the B3LYP functional [49,50] and the 6-31G(d,p) basis set. A single point energy calculation was then performed using methods matching the geometry optimization, from which molecular orbital diagrams were generated.

3.6. Electrochemical Measurements All electrochemical methods were performed utilizing a three-electrode cell consisting of a platinum disc working electrode, a platinum wire auxiliary electrode, and a Ag/Ag+ reference electrode (0.251 V vs. SCE) [51]. Supporting electrolyte consisted of 0.10 M TBAPF6 in dry CH3CN. Solutions were deoxygenated by sparging with argon prior to each scan and blanketed with argon during the measurements. All measurements were collected at a scan rate of 100 mV/s. EHOMO values were estimated from the oxidation in relation to ferrocene (60 mV vs. Ag/Ag+), using the value of 5.1 eV vs. vacuum for ferrocene [46].

4. Conclusions An optimized synthesis of N-functionalized BBTPs has been developed and applied to the synthesis of four benzannulated building blocks. The optical, electronic, and structural properties of the resulting BBTP units have been compared to the parent N-alkyl- and N-aryl-DTPs, as well as their corresponding 2,6-diphenyl derivatives, in order to fully quantify the relative electronic effects resulting from benzannulation of the DTP core. Although benzannulation does increase the conjugation length of the fused ring species, resulting in a corresponding red shift in absorption, this is not as effective as simple end-capping with aryl groups. In terms of modulation of the frontier orbitals, however, benzannulation results in stabilization of the BBTP HOMO, compared to the destabilization normally observed with extending the conjugation length of the backbone. Thus, while benzannulation is less effective in extending delocalization and shifting absorption to lower energy, it is an effective approach to stabilizing the HOMO of strongly electron-rich units. Molecules 2018, 23, 2279 12 of 14

Supplementary Materials: The electronic supplementary information (ESI) consisting of 1H and 13C NMR spectra for all compounds are available online. Author Contributions: R.M.W.W. performed all of the synthetic work and the bulk of the data acquisition. E.W.C. and S.C.R. collected additional characterization data. S.C.R. carried out the comparative analysis and wrote the paper. All of the authors read and approved the final version of the manuscript before submission. Funding: This research was funded by the ND EPSCoR Flexible Electronics program (EPS-0814442) and North Dakota State University. In addition, the departmental XRD instrument used in the research was funded by NSF-CRIF (CHE-0946990). Acknowledgments: We would also like to thank Angel Ugrinov (NDSU) for the collection of the X-ray crystal data. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References

1. Skotheim, T.A.; Reynolds, J.R. (Eds.) Handbook of Conducting Polymers, 3rd ed.; CRC Press: Boca Raton, FL, USA, 2007; ISBN 978-1574446654. 2. Rasmussen, S.C.; Ogawa, K.; Rothstein, S.D. Synthetic Approaches to Band Gap Control in Conjugated Polymeric Materials. In Handbook of Organic Electronics and Photonics; Nalwa, H.S., Ed.; American Scientiﬁc Publishers: Stevenson Ranch, CA, USA, 2008; Volume 1, pp. 1–50, ISBN 978-1-58883-095-5. 3. Perepichka, I.F.; Perepichka, D.F. (Eds.) Handbook of Thiophene-Based Materials: Applications in Organic Electronics and Photonics; John Wiley & Sons: West Sussex, UK, 2009; ISBN 978-0470057322. 4. Rasmussen, S.C. The Early History of Polyaniline: Discovery and Origins. Substantia 2017, 1, 99–109. 5. Rasmussen, S.C. Revisiting the Early History of Synthetic Polymers: Critiques and New Insights. Ambix 2018, 65, in press. [CrossRef][PubMed] 6. Rasmussen, S.C. Early history of Conjugated Polymers: From their Origins to the Handbook of Conducting Polymers. In Handbook of Conducting Polymers, 4th ed.; Reynolds, J.R., Skotheim, T., Thompson, B., Eds.; CRC Press: Boca Raton, FL, USA, 2018, in press. 7. Rasmussen, S.C. Electrically Conducting Plastics: Revising the History of Conjugated Organic Polymers. In 100+ Years of Plastics. Leo Baekeland and Beyond; Strom, E.T., Rasmussen, S.C., Eds.; ACS Symposium Series 1080; American Chemical Society: Washington, DC, USA, 2011; pp. 147–163, ISBN 9780841226777. 8. Rasmussen, S.C. The Path to Conductive Polyacetylene. Bull. Hist. Chem. 2014, 39, 64–72. 9. Rasmussen, S.C. Early History of Polypyrrole: The First Conducting Organic Polymer. Bull. Hist. Chem. 2015, 40, 45–55. 10. Rasmussen, S.C. Early History of Conductive Organic Polymers. In Conductive Polymers: Electrical Interactions in Cell Biology and Medicine; Zhang, Z., Rouabhia, M., Moulton, S.E., Eds.; CRC Press: Boca Raton, FL, USA, 2017; pp. 1–21, ISBN 9781482259285. 11. Baumgartner, T. π-Conjugated Heterocyclic fused Bithiophene Materials. J. Inorg. Organomet. Polym. Mater. 2005, 15, 389–409. [CrossRef] 12. Rasmussen, S.C.; Evenson, S.J.; McCausland, C.B. Fluorescent Thiophene-based Materials and Their Outlook for Emissive Applications. Chem. Commun. 2015, 51, 4528–4543. [CrossRef][PubMed] 13. Rasmussen, S.C.; Evenson, S.J. Dithieno[3,2-b:20,30-d]pyrrole-based Materials: Synthesis and Applications to Organic Electronics. Prog. Polym. Sci. 2013, 38, 1773–1804. [CrossRef] 14. Ogawa, K.; Rasmussen, S.C. A Simple and Efﬁcient Route to N-Functionalized Dithieno[3,2-b:20,30-d]- pyrroles: Fused-Ring Building Blocks for New Conjugated Polymeric Systems. J. Org. Chem. 2003, 68, 2921–2928. [CrossRef][PubMed] 15. Evenson, S.J.; Seth, C.; Rasmussen, S.C. N-Acyldithieno[3,2-b:20,30-d]pyrroles: Second Generation Dithieno[3,2-b:20,30-d]pyrrole Building Blocks with Stabilized Energy Levels. Org. Lett. 2010, 12, 4054–4057. [CrossRef][PubMed] 16. Evenson, S.J.; Pappenfus, T.M.; Delgado, M.C.R.; Radke-Wohlers, K.; Navarrete, J.T.L.; Rasmussen, S.C. Molecular Tuning in Highly Fluorescent Dithieno[3,2-b:20,30-d]pyrrole-based Oligomers: Effects of N-Functionalization and Terminal Aryl Unit. Phys. Chem. Chem. Phys. 2012, 14, 6101–6111. [CrossRef] [PubMed] Molecules 2018, 23, 2279 13 of 14

17. Amb, C.M.; Chen, S.; Graham, K.R.; Subbiah, J.; Small, C.E.; So, F.; Reynolds, J.R. Dithienogermole As a Fused Electron Donor in Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2011, 133, 10062–10065. [CrossRef] [PubMed] 18. Fei, Z.; Kim, J.S.; Smith, J.; Domingo, E.B.; Anthopoulos, T.D.; Stingelin, N.; Watkins, S.E.; Kim, J.-S.; Heeney, M. A low band gap co-polymer of dithienogermole and 2,1,3-benzothiadiazole by Suzuki polycondensation and its application in transistor and photovoltaic cells. J. Mater. Chem. 2011, 21, 16257–16263. [CrossRef] 19. Gendron, D.; Morin, P.-O.; Berrouard, P.; Allard, N.; Aich, B.R.; Garon, C.N.; Tao, Y.; Leclerc, M. Synthesis and Photovoltaic Properties of Poly(dithieno[3,2-b:20,30-d]germole) Derivatives. Macromolecules 2011, 44, 7188–7193. [CrossRef] 20. Green, J.P.; Han, Y.; Kilmurray, R.; McLachlan, M.A.; Anthopoulos, T.D.; Heeney, M. An Air-Stable Semiconducting Polymer Containing Dithieno[3,2-b:20,30-d]arsole. Angew. Chem. Int. Ed. 2016, 25, 7148–7151. [CrossRef][PubMed] 21. Kato, T.; Imoto, H.; Tanaka, S.; Ishidoshiro, M.; Naka, K. Facile synthesis and properties of dithieno- [3,2-b:20,30-d]arsoles. Dalton Trans. 2016, 45, 11338–11345. [CrossRef][PubMed] 22. Green, J.P.; Cryer, S.J.; Marafie, J.; White, A.J.P.; Heeney, M. Synthesis of a Luminescent Arsolo-[2,3-d:5,4- d0]bis(thiazole) Building Block and Comparison to Its Phosphole Analogue. Organometallics 2017, 36, 2632–2636. [CrossRef] 23. Qi, T.; Guo, Y.; Liu, Y.; Xi, H.; Zhang, H.; Gao, X.; Liu, Y.; Lu, K.; Du, C.; Yu, G.; Zhu, D. Synthesis and properties of the anti and syn isomers of dibenzothieno[b,d]pyrrole. Chem. Commun. 2008, 6227–6229. [CrossRef][PubMed] 24. Balaji, G.; Valiyaveettil, S. Synthesis and Properties of Symmetric and Unsymmetric Dibenzothieno- pyrroles. Org. Lett. 2009, 11, 3358–3361. [CrossRef][PubMed] 25. Balaji, G.; Della Pelle, A.M.; Popere, B.C.; Chandrasekaran, A.; Thayumanavan, S. Synthesis and properties of thienopyrrole based heteroacenes – indolodibenzothienopyrrole and dicarbazolodithienopyrrole. Org. Biomol. Chem. 2012, 10, 3455–3462. [CrossRef][PubMed] 26. Jung, I.H.; Kim, J.-H.; Nam, S.Y.; Lee, C.; Hwang, D.-H.; Yoon, S.C. Development of New Photovoltaic Conjugated Polymers Based on Di(1-benzothieno)[3,2-b:20,30-d]pyrrole: Benzene Ring Extension Strategy for Improving Open-Circuit Voltage. Macromolecules 2015, 48, 5213–5221. [CrossRef] 27. Gupta, A.; Flynn, B.L. Linear and Angular Heteroacenes from Double-Electrophilic Cyclization (DEC) and DEC-Reductive Elimination of Diynes. Org. Lett. 2017, 19, 1939–1941. [CrossRef][PubMed] 28. Gao, P.; Cho, D.; Yang, X.; Enkelmann, V.; Baumgarten, M.; Müllen, K. Heteroheptacenes with Fused Thiophene and Pyrrole Rings. Chem. Eur. J. 2010, 16, 5119–5128. [CrossRef][PubMed] 29. Balaji, G.; Phua, D.I.; Shim, W.L.; Valiyaveettil, S. Synthesis and Characterization of Unsymmetric Indolodithienopyrrole and Extended Diindolodithienopyrrole. Org. Lett. 2010, 12, 232–235. [CrossRef] [PubMed] 30. Mitsudo, K.; Shimohara, S.; Mizoguchi, J.; Mandai, H.; Suga, S. Synthesis of Nitrogen-Bridged Terthiophenes by Tandem Buchwald-Hartwig Coupling and Their Properties. Org. Lett. 2012, 14, 2702–2705. [CrossRef] [PubMed] 31. Wetzel, C.; Mishra, A.; Mena-Osteritz, E.; Liess, A.; Stolte, M.; Würthner, F.; Bäuerle, P. Synthesis and Structural Analysis of Thiophene-Pyrrole-Based S,N-Heteroacenes. Org. Lett. 2014, 16, 362–365. [CrossRef] [PubMed] 32. Qin, P.; Kast, H.; Nazeeruddin, M.K.; Zakeeruddin, S.M.; Mishra, A.; Bäuerle, P.; Grätzel, M. Low band gap S,N-heteroacene-based oligothiophenes as hole-transporting and light absorbing materials for efficient perovskite-based solar cells. Energy Environ. Sci. 2014, 7, 2981–2985. [CrossRef] 33. Rasmussen, S.C. The Nomenclature of Fused-ring Arenes and Heterocycles: A Guide to an Increasingly Important Dialect of Organic Chemistry. Chem. Texts 2016, 2, 16. [CrossRef] 34. Jung, I.H.; Kim, J.-H.; Nam, S.Y.; Lee, C.; Hwang, D.-H.; Yoon, S.C. A di(1-benzothieno)-[3,2-b:20,30-d]pyrrole and isoindigo-based electron donating conjugated polymer for efficient organic photovoltaics. J. Mater. Chem. C 2016, 4, 663–667. [CrossRef] 35. Dahlmann, U.; Neidlein, R. The Diyne Reaction of 3,30-Bis(phenylethynyl)-2,20-bithiophene Derivatives via Rhodium Complexes: A Novel Approach to Condensed Benzo[2,l-b:3,4-b’]dithiophenes. Helv. Chim. Acta 1997, 80, 111–120. [CrossRef] Molecules 2018, 23, 2279 14 of 14

36. Gao, J.; Li, R.; Li, L.; Meng, Q.; Jiang, H.; Li, H.; Hu, W. High-Performance Field-Effect Transistor Based on Dibenzo[d,d0]thieno[3,2-b;4,5-b0]dithiophene, an Easily Synthesized Semiconductor with High Ionization Potential. Adv. Mater. 2007, 19, 3008–3011. [CrossRef] 37. Dienes, Y.; Eggenstein, M.; Kárpáti, T.; Sutherland, T.C.; Nyulászi, L.; Baumgartner, T. Phosphorus-Based Heteropentacenes: Efﬁciently Tunable Materials for Organic n-Type Semiconductors. Chem.–Eur. J. 2008, 14, 9878–9889. [CrossRef][PubMed] 38. Alessandrini, L.; Braga, D.; Jaafari, A.; Miozzo, L.; Mora, S.; Silvestri, L.; Tavazzi, S.; Yassar, A. Optical Properties of Dibenzo[d,d0]thieno[3,2-b;4,5-b0]dithiophene Monocrystals: The Effect of Intermolecular Interactions. J. Phys. Chem. A 2011, 115, 225–231. [CrossRef][PubMed] 39. Zhao, Y.; Hao, W.; Ma, W.; Zang, Z.; Zhang, H.; Liu, X.; Zou, S.; Zhang, H.; Liu, W.; Gao, J. Easily-soluble heteroacene bis(benzothieno)silole derivatives for sensing of nitro explosives. New J. Chem. 2014, 38, 5754–5760. [CrossRef] 40. Oechsle, P.; Paradies, J. Ambidextrous Catalytic Access to Dithieno[3,2-b:20,30-d]thiophene (DTT) Derivatives by Both Palladium-Catalyzed C−S and Oxidative Dehydro C−H Coupling. Org. Lett. 2014, 16, 4086–4089. [CrossRef][PubMed] 41. Kabir, S.M.H.; Miura, M.; Sasaki, S.; Harada, G.; Kuwatani, Y.; Yoshida, M.; Iyoda, M. New Syntheses of Tricyclic Thiophenes and Cyclic Tetrathiophenes Using Transition-Metal-catalyzed Cyclization. Heterocycles 2000, 52, 761–774. 42. Katritzky, A.R.; Pozharskii, A.F. Handbook of Heterocyclic Chemistry, 2nd ed.; Pergamon Press: New York, NY, USA, 2000; p. 61, ISBN 0-08-042989-0. 43. Katritzky, A.R.; Pozharskii, A.F. Handbook of Heterocyclic Chemistry, 2nd ed.; Pergamon Press: New York, NY, USA, 2000; p. 81, ISBN 0-08-042989-0. 44. Barbarella, G.; Zambianchi, M.; Antolini, L.; Folli, U.; Goldoni, F.; Iarossi, D.; Schenetti, L.; Bongini, A. Conformational properties of 3,30-, 3,40- and 4,40-dimethyl- and -bis(methylsulfanyl)-2,20-bithiophenes. J. Chem. Soc. Penkin Trans. 2 1995, 10, 1869–1873. [CrossRef] 45. Katritzky, A.R.; Pozharskii, A.F. Handbook of Heterocyclic Chemistry, 2nd ed.; Pergamon Press: New York, NY, USA, 2000; p. 71, ISBN 0-08-042989-0. 46. Cardona, C.M.; Li, W.; Kaifer, A.E.; Stockdale, D.; Bazan, G.C. Electrochemical Considerations for Determining Absolute Frontier Orbital Energy Levels of Conjugated Polymers for Solar Cell Applications. Adv. Mater. 2011, 23, 2367–2371. [CrossRef][PubMed] 47. Evenson, S.J.; Mumm, M.J.; Pokhodnya, K.I.; Rasmussen, S.C. Highly Fluorescent Dithieno[3,2-b:20,30- d]pyrrole-based Materials: Synthesis, Characterization and OLED Device Applications. Macromolecules 2011, 44, 835–841. [CrossRef] 48. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, Revision A.02. Gaussian, Inc.: Wallingford, CT, USA, 2009. 49. Becke, A.D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648–5652. [CrossRef] 50. Lee, C.T.; Yang, W.T.; Parr, R.G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 1988, 37, 785–789. [CrossRef] 51. Larson, R.C.; Iwamoto, R.T.; Adams, R.N. Reference electrodes for voltammetry in acetonitrile. Anal. Chim. Acta 1961, 25, 371–374.

Sample Availability: Samples of the compounds are not available from the authors.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).