Chemical Physics Letters 550 (2012) 33–40

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Silole-based oligomers as electron transport materials

Huyen Thi Nguyen a, Vu Thi Thu Huong a, Minh Tho Nguyen a,b,⇑a Department of Chemistry, University of Leuven, B-3001 Leuven, Belgiumb Institute for Computational Science and Technology at HoChiMinh City (ICST), Quang Trung Software Park, HoChiMinh City, Viet Nam

a r t i c l e i n f o

Article history:Received 21 June 2012In final form 24 August 2012Available online 3 September 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.08.051

⇑ Corresponding author at: Department of Chemis3001 Leuven, Belgium. Fax: +32 16 32 79 92.

E-mail addresses: minh.nguyen@chem.kuleuven.beven.ac.be (M.T. Nguyen).

a b s t r a c t

The structure–property relationships of two series of silole-based oligomers including 1,1-substitutedoligo(2,5-siloles) and the compounds based on phenylethynylene moieties alternated with fused silolerings (PhEtS) are theoretically investigated using density functional theory. Their electronic, opticaland charge transport properties are analyzed in detail. All silole derivatives have typical low-lyingLUMOs. The reorganization energies and thereby the charge transport of these oligomers are evaluated.PhEtS molecules show reorganization energies comparable to those of thiophenes that are good for thisproperty. Calculated results thus suggested that silole-based oligomers can be considered as potentialcandidates to be used as materials for electron transport.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The first synthesis of siloles (silacyclopentadienes) was reportedin the late 1950s [1,2], but these five-membered rings have only re-cently attracted interest in part due to their LUMO levels. In fact,these frontier orbitals that arise from a r⁄–p⁄ combination be-tween the r⁄ silicon orbital and the p⁄ orbital of the butadienefragment, are energetically low-lying as compared with other cyc-lic compounds such as pyrrole, furan, thiophene, and pyridine, etc.[3,4]. This typical characteristic turns out to be relevant in thedevelopment of new optoelectronic devices.

It is well-known that the preparation of efficient electron trans-port materials are facing some difficulties such as their inherentinstability in air [5], electron trapping tendencies [6], matchingwork function of cathode metals with lowest unoccupied molecu-lar orbital (LUMO) of the materials, etc. [7]. In this context, withhigh capacity for accepting electron, fast electron mobility andhigh stability in air [8], siloles emerge among the most effectivecandidates for electron transport materials. Siloles are also efficientemitters with a very high brightness and external quantum yield[3,4]. Furthermore, the aggregation induced emission phenomenonwas observed in some silole derivatives in which the electrolumi-nescence (EL) performance increases dramatically going from solu-tion to thin film [9,10]. Thanks to these special properties, silolesand their derivatives have been used as light-emitting layers inEL devices [8,11–14].

ll rights reserved.

try, University of Leuven, B-

, minh.nguyen@chem.kuleu-

Many theoretical investigations have already been carried outon silole monomers with different substituents [15,16], siloles withfused thiophene or benzene rings [17–19], oligo(2,5-silole)s[20,21]. Recently, the structure property relationship of silolemonomers bearing symmetric electron donor–acceptors on 2,5-positions [22] and different substituents on silicon atom [23] haveexperimentally and theoretically been studied. In the latter Letter,the triple carbon–carbon bonds have been used as functionalbridging groups to link the silole ring and the triphenylsilyl groups.This type of silole-based materials bearing triple bond linkages be-tween aromatic rings has received more attention due to their spe-cial properties [4,24]. The silole-based acetylenes incorporate boththe rigidity of acetylenes and the low lying LUMOs of siloles result-ing in a system with low band gap and well-defined extended con-jugation [25]. Boydston and co-workers reported the synthesis andexperimental investigation on the electronic properties of oligo-mers based on phenylethynylene units linked to silole rings byacetylenes [26]. The thiophene analogs of these silole containingcompounds were also theoretically proven to be n-type semicon-ductor materials [27,28]. Accordingly, a detailed theoretical studyof the Boydston’s silole systems can reveal many new and interest-ing features that could be used for further experiment.

In this context, we set out to study the geometries, electronicstructures and charge transport properties of two different setsof silole-based oligomers, namely the 1,1-substituted oligo(2,5-sil-ole)s (referred to hereafter as siloles) and the compounds based onphenylethynylene moieties alternated with fused silole rings (de-noted hereafter as PhEtS, cf. Figure 1).

Each silole considered is labeled by a combination of letters andnumber. The letters indicate the two substituents at silicon atomwhere H denotes hydrogen, F fluorine, and Me methyl. The number

Figure 1. Structures of 1,1-substituted oligo(2,5-silole)s and PhEtS considered.

34 H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40

n followed the letters denotes the number of repeat units. For thePhEtS set of molecules, the suffix x–m will be added to PhEtS(PhEtSx-m) in which x indicates the number of fused silole rings,and m the number of repeat units. The basic properties investi-gated include their geometries, frontier orbitals gaps, electronaffinities (EAs), ionization (IEs), lowest allowed excitation andreorganization energies. The latter parameters are subsequentlyused to probe the ability of these silole-based oligomers as candi-dates for electron transport materials to be used for example inOLEDs.

2. Computational details

All quantum chemical calculations are performed using theGAUSSIAN-09 suite of programs. Geometries of different oligomersare optimized using density functional theory (DFT) methods withthe popular hybrid B3LYP functional in conjunction with the d-polarization 6-31G(d) basis set. Harmonic vibrational frequenciesare also calculated at the same level to confirm the nature of thestationary points. As a measurement of the degree of the delocal-ization, the A indices as defined by Julg [29] are calculated by usingthe following equation:

A ¼ 1� 225n

Xn

ðdi � davgÞdavg

� �2

ð1Þ

where n denotes the number of C–C bond, di and davg denote theindividual and average bond lengths, respectively. A large value of

A indicates a relatively small bond alternation and corresponds toa large delocalization.

The band gaps are determined by electronic excitation energycalculations using time-dependent density functional theory(TDDFT) with B3LYP functional. The IEs and EAs are calculated atthe B3LYP/6-31G(d) and B3LYP/6-31+G(d) levels, respectively(the + denotes the addition of a set of sp-diffuse functions). Totalintramolecular reorganization energies of hole- and electron-transport processes denoted as kþtot and k�tot , respectively, are calcu-lated according to the semi-classical Marcus theory [30] using theB3LYP/6-31G(d) adiabatic potential energy profiles as described inRef. [31]. They are sum of two relaxation energies as the systemgoes from neutral to charged states, k�C , and from charged statesback to neutral state, k�N

k�tot ¼ k�C þ k�N

The two relaxation energies are calculated as follow:

k�C ¼ ErelC � EC

k�N ¼ ErelN � EN

where ErelC is the energy of the charged state in the optimized geom-

etry of the neutral molecule, ErelN is the energy of the neutral state in

the optimized geometry of the charged molecule, EC and EN are theenergies in the optimized geometries of the charged and neutralmolecules, respectively.

H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40 35

3. Results and discussion

3.1. Molecular geometries

3.1.1. 1,1-Substituted oligo(2,5-silole)sThe selected optimized bond lengths and angles of neutral

monomer siloles along with the corresponding data for anionsand cations are presented in Table 1. The dihedral angles are notlisted here since all the optimized molecules possess a planarshape. In agreement with previous calculations on neutral HH-1,FF-1, and MeMe-1 [15], addition of electronegative 1,1-substitu-ents results in a shortening of the Si–C and C@C bond lengthsand an increase of C–C bond lengths and also C–Si–C bond angles.The selected bond lengths of other larger oligomers listed in TablesS1a–S1e of the Supplementary Information file (ESI) also show thesame trend as in the monomers.

In going from the terminal to the middle, the C@C bond lengthstend to increase while a decrease in C–C bond lengths of each silolering is observed, also for the C–C bond lengths between adjacentsilole units. This behavior which is similar to that of neutralMeMe-4 with electron-withdrawing groups –CF3 attached at theterminals [21], show that a stronger conjugation at the middle thanat the terminal of the molecules is a common characteristic of oli-go(2,5-silole)’s and its substituted derivatives. The C–Si–C bond an-gles of the oligomers are changed by less than a few tenths of adegree between different silole units (Table S2 of the ESI). Thebond angles in the middle of the molecules are slightly larger thanthose at the terminals.

The Julg’s indices A presented in Table S3 (ESI) agree well withthe above analysis of bond lengths and angles. In going from HH-nto HF-n and FF-n the A values decrease while addition of themethyl groups does not significantly affect the delocalization ofthe systems. Increase of the number of repeat units significantlyfacilitates the electron delocalization as manifested by the increas-ing A values from 0.25–0.40 in the monomers to 0.78–0.82 in thehexamers.

The bond length alternation (BLA) of siloles is calculated as:

BLA ¼ 1=2ðdintra þ dinterÞ ð2Þ

where dintra denotes the bond length difference between C–C andC@C bonds of silole ring, and dinter the bond length difference

Table 1Selected bond lengths (Å) and angles (degree) of 1,1-substituted oligo(2,5-silole)scalculated at B3LYP/6-31G(d) level. Values in the parentheses correspond to HF/6-31G(d) calculation from Ref. [15].

d(C–Si) (Å) d(C@C) (Å) d(C–C) (Å) \ðC—Si—CÞ (degree)

NeutralHH-1 1.877

(1.877)1.350(1.331)

1.487(1.494)

92.5(92.0)

HF-1 1.871 1.348 1.495 93.4FF-1 1.862

(1.856)1.347(1.330)

1.501(1.505)

94.7(94.2)

HMe-1 1.881 1.350 1.488 92.1MeMe-1 1.886

(1.884)1.350(1.331)

1.488(1.494)

91.7(91.3)

AnionHH-1 1.847 1.405 1.427 93.6HF-1 1.836 1.400 1.435 94.1FF-1 1.823 1.401 1.436 95.7HMe-1 1.848 1.404 1.428 93.4MeMe-1 1.850 1.405 1.428 93.3

CationHH-1 1.906 1.399 1.427 88.3HF-1 1.905 1.395 1.434 88.8FF-1 1.894 1.395 1.438 90.4HMe-1 1.911 1.399 1.426 87.7MeMe-1 1.916 1.399 1.425 87.3

between C–C bond between adjacent rings and C@C bond of silolering. For HH-3 to HH-6, the BLAs are calculated using bond lengthsat the central silole rings. BLAs of neutral oligomers can be found inTable S4 (ESI). The BLAs of HH-2, HH-3, and HH-6 are close to valuesobtained from previous calculation [20,32]. A decrease in BLAs ingoing from dimers to hexamers again indicates an increase in p-conjugation of the systems as the chain length grows.

The optimized geometries of siloles are more strongly affectedby electron injection than hole creation. The anionic monomershave much shorter Si–C and C–C bond lengths, longer C@C bondlengths, and larger C–Si–C bond angles than the neutral counter-parts. Although the corresponding cations also have shorter C–Cand longer C@C bond lengths than the neutral species, the Si–Cbond lengths of the cations are longer. This elongation of Si–C bondresults in the narrow C–Si–C bond angles of cationic monomers.Except for HF-1 and FF-1, the effect of electron injection/hole cre-ation on the delocalization of the monomers are nearly the same.Both anions and cations have much larger Julg indices (0.94–0.98) than the neutral molecules (see Table S3, ESI). For HF-1 andFF-1, the A values of the cations are slightly lower than those ofthe anions which can be explained by the electron-withdrawing ef-fect of the F atoms resulting the lowering of electron density in therings. The effects of electron/hole injection are consistently re-duced when the size increases from monomers to hexamers. Dif-ferent from neutral species, the longer length in the chargedoligomers is less aromatic than the shorter one.

3.1.2. PhEtSThe PhEtS molecules as defined in Figure 1 have two different

arrangements, syn and anti, which are nearly equal in energy (seeillustration of these two conformations for PhEtS1-2 in Figure S2,ESI). Since the anti conformations are slightly more stable thanthe syn ones for most cases (Table S5, ESI), PhEtS in anti arrange-ments are chosen for further investigations.

Table 2 lists some selected bond lengths of PhEtS along with theavailable data of thiophene analogs [28] for the purpose of compar-ison. As x and m increase, the p delocalization within each silolering gets improved with the increase in CðSiloleÞ ¼ CðSiloleÞ bondlengths and the decrease in CðSiloleÞ � CðSiloleÞ bond lengths. The con-jugation of the CðSiloleÞ � Cð�CÞ � Cð�CÞ � CðPhÞ linking bonds betweensilole and phenyl rings depends on the number of repeat unit but isalmost invariant as the number of fused silole rings increase from 2to 4. Compared to thiophene analogs the silole derivatives haveapparently a better conjugation at the linking bonds representedin the shorter single bonds and longer triple bonds of the latter.

The local BLAs associated with the central single bonds,CðSiloleÞ � Cð�CÞ and Cð�CÞ � CðPhÞ, and the central triple bond, C � C,at three different positions I, II, and III (Figure 2) of PhEtS are cal-culated using the following formula [27,33]:

BLAPhEtS ¼dðCðSiloleÞ � Cð�CÞÞ þ dðCð�CÞ � CðPhÞÞ

2� dðC � CÞ ð3:3Þ

The average BLAs of PhEtS are also presented in Figure 2 forboth neutral and anionic molecules. For the neutrals, the averageBLAs decrease slightly with the increase of fused silole ring x andnumber of repeat units m. The differences between BLAs of thebonds at terminals of the molecules (position I) and the BLAs atpositions II and III are also small. Addition of one electron inducesan obvious decrease of average BLAs in which the main reductionstake place in the central positions of the molecules. Also PhEtShave some decreases of BLAs in going from monomers to trimerswhich amount to �0.004–0.006 Å.

Comparison with the central BLAs of thiophene analogs at thesame level of calculation for monomers and trimers [27] alsoshows that silole compounds have lower BLAs and thus a higherdegree of electron delocalization over the backbone.

Table 2Selected bond lengths of side silole rings of PhEtSx-m (x = 1–4; m = 1–3) oligomers obtained at B3LYP/6-31G(d) level. Values in parentheses correspond to the thiophene analogstaken from Ref. [28].

PhEtSx-m x = 1 x = 2 x = 3 x = 4

m = 1 m = 2 m = 3 m = 1 m = 2 m = 3 m = 1 m = 2 m = 3 m = 1 m = 2 m = 3

CðSiloleÞ � Si 1.891 1.891 1.891 1.894 1.894 1.894 1.893 1.893 1.893 1.893 1.893 1.893CðSiloleÞ ¼ CðSiloleÞ 1.372 1.372 1.373 1.375 1.375 1.377 1.376 1.376 1.378 1.377 1.377 1.379CðSiloleÞ � CðSiloleÞ 1.456 1.455 1.454 1.454 1.453 1.451 1.452 1.451 1.449 1.450 1.450 1.448CðSiloleÞ � Cð�CÞ 1.401

(1.403)1.401 1.399 1.401

(1.402)1.401 1.399 1.400

(1.401)1.400 1.398 1.400

(1.402)1.400 1.397

C � C 1.221(1.219)

1.221 1.223 1.222(1.219)

1.222 1.223 1.222(1.218)

1.222 1.223 1.222(1.219)

1.222 1.224

Cð�CÞ � CðPhÞ 1.422(1.422)

1.421 1.417 1.421(1.422)

1.421 1.416 1.421(1.422)

1.420 1.416 1.421(1.422)

1.421 1.415

Figure 2. Bond length alternations of PhEtS (angstrom, B3LYP/6-31G(d)).

Table 3Lowest allowed vertical excitation energies (eV) of the 1,1-substituted oligo(2,5-silole)s and PhEtS calculated at TD-B3LYP/6-31G(d) level. The HOMO–LUMO energygaps (eV) obtained from B3LYP/6-31G(d) calculations are listed in parentheses.

n = 1 n = 2 n = 3 n = 4 n = 5 n = 6

HH-n 4.40(4.89)

3.21(3.36)

2.58(2.64)

2.18(2.22)

1.91(1.94)

1.72(1.75)

HF-n 4.09(4.69)

3.08(3.25)

2.49(2.55)

2.11(2.14)

1.85(1.87)

1.65(1.67)

FF-n 4.15(4.71)

3.13(3.27)

2.53(2.58)

2.15(2.17)

1.89(1.90)

1.69(1.71)

HMe-n 4.37(4.87)

3.20(3.36)

2.57(2.64)

2.18(2.22)

1.91(1.94)

1.71(1.75)

MeMe-n 4.38(4.88)

3.23(3.38)

2.59(2.66)

2.19(2.24)

1.92(1.96)

1.72(1.76)

m = 1 m = 2 m = 3

PhEtS1-m 2.65(2.85)

2.16(2.39)

1.95(2.22)

PhEtS2-m 2.19(2.41)

1.81(2.05)

1.64(1.91)

PhEtS3-m 1.91(2.12)

1.58(1.82)

1.44(1.69)

PhEtS4-m 1.72(1.91)

1.42(1.64)

1.29(1.53)

36 H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40

3.2. Molecular orbitals and excitation energies

3.2.1. Molecular orbitals3.2.1.1. 1,1-Substituted oligo(2,5-silole)s. The shapes of frontiermolecular orbitals of siloles are shown in Figure S3 (ESI). Similarto other related p-conjugated polymers such as polythiophenes,polyphospholes,. . . the HOMOs of siloles have anti-bonding charac-teristic while the LUMOs are rather bonding orbitals betweenrings. The 1,1-substituents do not alter the nature of the frontierorbitals but they induce a great effect on the energy levels.

The electron withdrawing groups consistently lower the ener-gies of both HOMOs and LUMOs (Figure S4, ESI). Meanwhile, addi-tion of methyl groups causes an opposite effect with adestabilization of the frontier orbitals. However, the effects of all1,1-substituents on the HOMOs and LUMOs are nearly the samewhich results in a small variation in HOMO–LUMO gaps betweendifferent monomers and oligomers with the same length (Table 3).

The HOMO and LUMO energies of the monomers are re-calcu-lated using the slightly larger 6-31+G(d) basis set in order to checkthe effect of diffuse functions on the variations of frontier orbitalenergies as the 1,1-substituents change. Table S7 (ESI) shows thatall frontier orbitals obtained with the 6-31+G(d) basis set are morestable than those using the 6-31G(d) basis set. However, the effectson HOMOs and/or LUMOs of different monomers are nearly equalfor different molecules and thus the order of increasing energy ispreserved, namely: FF-1, HF-1, HH-1, HMe-1, MeMe-1. As the chainlengths increase, the HOMOs are destabilized while the LUMOsbecome stabilized and thus the HOMO–LUMO gaps invariablydecrease (see Table 3). Our results of HOMO and LUMO energies

of MeMe-1 are similar to those previously reported by Risko andco-workers [16].

3.2.1.2. PhEtS. The frontier orbitals of PhEtS also have the charac-teristics similar to those of siloles with anti-bonding HOMOs and

H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40 37

bonding LUMOs. The HOMOs and LUMOs of PhEtS are presented inFigure S5 (ESI) and Figure 3, respectively. Different from the LUMOsof siloles and thiophene analogs [27], those of PhEtS are not equallydelocalized between different fragments but more concentrated onthe fused silole rings than phenylene fragments. This might be ac-counted for the high electron acceptability of the fused silole rings.The PhEtS HOMO and LUMO energy levels listed in Table S8 (ESI)show clearly lower LUMO levels which is one of the advantagesof silole compounds. The increase of chain lengths and numberof fused silole rings both result in higher HOMO energies and lowerLUMO energies (Figure S6, ESI). The experimental band gaps ofPhEtS1-m analogs in which the 1,1-dimethyl-2,5-diphenyl substit-uents are attached to the silole rings converged at pentamer [26].However, the HOMO–LUMO gaps and excitation energies ofPhEtS1-m (m = 4–10) listed on Table S9 (ESI) show that the calcu-lated HOMO–LUMO gaps and the excitation energies are not con-verged yet even at 10 repeated units.

3.2.2. Excitation energies3.2.2.1. 1,1-Substituted oligo(2,5-silole)s. The 1,1-substituents andthe elongation of chain lengths induce similar effects on the lowestallowed excitation energies of siloles and the HOMO–LUMO gaps(see Table 3). One can see that the longer the chain lengths, the clo-ser the excitation energy and the HOMO–LUMO gap of each mole-cule. This suggests that with longer oligomers, the lowestexcitation energies can be approximated by the calculated valuesof the HOMO–LUMO gaps.

Calculations of 1,1-dimethylsilole oligomers with CF3 substitu-ents in terminals yielded the vertical excitation energies/HOMO–LUMO gaps of 3.2/3.3, 2.5/2.6, and 2.2/2.2 eV for dimer, trimer,and tetramer, respectively [21]. Comparison with MeMe-n (n = 2–4) shows that introduction of the CF3 substituents slightly lowersthe excitation energies and HOMO–LUMO gaps by 0.04–0.05 eV.The excitation energy of HH-2 calculated at TD-B3LYP/6-31+G(d)

Figure 3. LUMOs of PhEt

level was reported to be 3.1 eV by Zhang and co-workers [32]. Sim-ilar to HOMO–LUMO gaps, incorporation of diffuse functions yieldslower excitation energies.

3.2.2.2. PhEtS. The lowest excitation energies obtained from TD-B3LYP/6-31G(d) of PhEtS given in Table 3 also decrease as x andm increase. For the monomers, the excitation energies are in rangeof 1.7–2.7 eV, which are lower by 0.5–1.0 eV than those of corre-sponding thiophene analogs [27]. The effect of the number of fusedsilole rings on the excitation energies becomes greater which rep-resents in a decrease of 0.9 eV from PhEtS1-1 to PhEtS4-1 while forthiophene analogs, the decrease is only about 0.4 eV.

García and co-workers also proved that the excitation energiesobtained from TD-CAM-B3LYP/6-31G(d) were closer to the exper-imental optical band gaps than the TD-B3LYP/6-31G(d) results[27]. For PhEtS, the excitation energies are also calculated usingthe CAM-B3LYP functional, and the obtained results are listed inTable S11 (ESI). The long range exchange functional thus yields lar-ger excitation energies as compared with the standard B3LYP func-tional. For monomers, the differences of these values obtained bytwo functionals are only 0.06–0.09 eV, whereas for dimers and tri-mers, the variations become larger which are in range of 0.19–0.25 eV. Accordingly, the longer the chain lengths, the larger thedeviations between results obtained by long range and B3LYP func-tionals becomes.

3.3. Ionization energies and electron affinities

3.3.1. 1,1-Substituted oligo(2,5-silole)sIonization energies (IEs) and electron affinities (EAs) of siloles

and PhEtS are calculated, and they are also used to evaluate thecharge injection capacities. These values are listed in Table 4. Thedifference between the IE and the work function of anode materialis called hole injection barrier and the difference between the EA

S (B3LYP/6-31G(d)).

38 H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40

and work function of cathode material is called electron injectionbarrier.

The injection barriers of less than 0.3–0.4 eV were proposed tobe necessary for having high net injected charge [34,35]. Comparedto the normally used anode material indium tin oxide (ITO) withwork function U of �4.8 eV [36], the IEs of siloles are systemati-cally higher, but the differences become smaller as the chainlengths increase. The hole injection barriers decrease from veryhigh values for monomers (maximum barrier �4.3 eV) to quitesmall barriers in hexamers (minimum barrier �0.1 eV). High holeinjection barriers come from electron-withdrawing substituted sil-oles in which FF-1 has the highest IE of 9.11 eV.

The EAs change rapidly following the elongation of chainlengths. Hexamers with vertical and adiabatic EAs in range of1.9–3.0 eV and 2.2–3.4 eV, respectively, have quite small electroninjection barriers when some low work function cathodes suchas Sm (U = �2.7 eV), Ba (U � �2.7 eV), and Ca (U = �2.9 eV) areused. However, one disadvantage of these cathodes is their highreactivity and thus low stability in air. The very high EA of FF-6facilitates the usage of other cathodes such as Mg (U � �3.7 eV)or air-stable Al (U � �4.3 eV). Longer FF-n oligomers are expectedto have higher EAs which increase the probability of electron injec-tions to the empty LUMOs and reduce the electron injection barri-ers when stable cathodes are used.

3.3.2. PhEtSThe PhEtS are characterized by IEs and EAs in the range of 5.1–

6.5 eV and 1.4–3.1 eV, respectively (Table 5). Clearly, with an ITOanode, the increase of chain lengths m and number of fused silole

Table 4IEs, EAs, and reorganization energies (all given in eV) of 1,1-substituted oligo(2,5-silole)s.

n Ionization energies Electron affinities

IEv IEa EAv EAa

HH-n1 8.55 8.28 �0.26 0.132 7.13 6.93 0.82 1.133 6.45 6.27 1.39 1.694 6.04 5.87 1.77 2.055 5.75 5.58 2.03 2.326 5.54 5.36 2.23 2.53

HF-n1 8.83 8.52 0.21 0.632 7.44 7.22 1.22 1.573 6.78 6.58 1.81 2.144 6.37 6.18 2.19 2.515 6.09 5.89 2.47 2.796 5.88 5.67 2.67 3.01

FF-n1 9.11 8.80 0.38 0.862 7.77 7.54 1.48 1.863 7.13 6.92 2.10 2.464 6.73 6.53 2.50 2.855 6.46 6.25 2.79 3.146 6.26 6.03 3.00 3.37

HMe-n1 8.32 8.04 �0.19 0.152 6.89 6.69 0.71 1.033 6.21 6.03 1.25 1.564 5.80 5.62 1.61 1.915 5.51 5.34 1.86 2.166 5.30 5.12 2.05 2.36

MeMe-n1 8.13 7.84 �0.30 0.072 6.70 6.49 0.58 0.923 6.02 5.83 1.12 1.434 5.61 5.43 1.47 1.775 5.32 5.14 1.71 2.016 5.11 4.93 1.89 2.20

rings x result in lower hole injection barriers. The same effect canbe observed for the electron injection barrier in which increasing xresults in a raise in adiabatic EAs of the monomers from 1.7 to2.4 eV, and an increase of m induces an average change in adiabaticEA of 0.8 eV for the whole set of PhEtS compounds.

The effect of m on the adiabatic EA is most pronounced forPhEtS1-m systems whereas for other systems with higher numberof fused rings, the effects of m are less important. The vertical andadiabatic EAs of thiophene analogs (monomers, dimers, and tri-mers) were reported to be in the range of 0.6–1.9 eV and 0.8–1.9 eV, respectively [27]. These values are obviously much smallerthan those of the PhEtS studied here. The largest adiabatic EA of3.1 eV belongs to PhEtS4-3 molecule, which is close to the workfunctions of Ba, and Ca. . . but is still quite far from those of Mgand Al. Longer chain lengths and higher number of fused silolerings are thus needed to obtain higher EA which can match withthe high work function cathodes.

The adiabatic EA of PhEtP1-1, a phospholenium cation analogof PhEtS1-1 is also calculated for comparison. Similar to a previ-ously studied phospholenium cation [37], PhEtP1-1 has a veryhigh adiabatic EA of 5.6 eV which is significantly higher thanthe adiabatic EAs of all systems studied here, presumably dueto the positive charge (this corresponds to the IE of the corre-sponding neutral). However, it should be noted that the high EAis not a sufficient condition to conclude the n-type semiconduct-ing property of a material. The very high EA of PhEtP1-1 results ina very high electron injection barrier of 1.3 eV when the highwork function Al cathode is used, which is too large for efficientcharge generation process.

Hole transport Electron transport

kþC kþN kþ k�C k�N k�

0.26 0.26 0.52 0.30 0.29 0.590.21 0.20 0.41 0.22 0.21 0.430.19 0.19 0.38 0.20 0.19 0.390.19 0.19 0.38 0.19 0.19 0.380.19 0.19 0.38 0.19 0.19 0.380.20 0.20 0.40 0.20 0.20 0.40

0.29 0.30 0.59 0.33 0.32 0.650.22 0.22 0.44 0.25 0.25 0.500.20 0.20 0.40 0.22 0.22 0.440.20 0.20 0.40 0.21 0.21 0.420.21 0.21 0.42 0.21 0.22 0.430.22 0.22 0.44 0.22 0.23 0.45

0.29 0.30 0.59 0.37 0.36 0.730.23 0.23 0.46 0.26 0.26 0.520.21 0.21 0.42 0.23 0.23 0.460.21 0.21 0.42 0.22 0.22 0.440.21 0.21 0.42 0.22 0.22 0.440.22 0.23 0.45 0.22 0.23 0.45

0.27 0.27 0.54 0.31 0.29 0.600.21 0.21 0.42 0.22 0.22 0.440.20 0.19 0.39 0.20 0.20 0.400.19 0.19 0.38 0.19 0.19 0.380.19 0.19 0.38 0.19 0.19 0.380.20 0.20 0.40 0.20 0.20 0.40

0.28 0.28 0.56 0.31 0.30 0.610.22 0.22 0.44 0.23 0.23 0.460.20 0.20 0.40 0.20 0.20 0.400.20 0.19 0.39 0.20 0.19 0.390.19 0.19 0.38 0.19 0.19 0.380.20 0.20 0.40 0.20 0.20 0.40

Table 5IEs, EAs, and reorganization energies (all in eV) of 1,1-substituted oligo(2,5-silole)s and PhEtS.

m Ionization energies Electron affinities Hole transport Electron transport

IEv IEa EAv EAa kþC kþN kþ k�C k�N k�

PhEtS1-m1 6.46 6.33 1.40 1.65 0.15 0.15 0.29 0.17 0.16 0.332 5.94 5.84 2.02 2.22 0.11 0.11 0.22 0.11 0.12 0.233 5.70 5.63 2.33 2.51 0.08 0.08 0.16 0.08 0.09 0.17

PhEtS2-m1 6.22 6.07 1.72 1.98 0.17 0.17 0.33 0.18 0.17 0.342 5.73 5.61 2.29 2.49 0.12 0.13 0.25 0.12 0.13 0.243 5.51 5.42 2.53 2.72 0.09 0.10 0.18 0.09 0.09 0.18

PhEtS3-m1 6.03 5.87 1.95 2.21 0.18 0.18 0.36 0.18 0.17 0.352 5.56 5.42 2.49 2.71 0.13 0.15 0.29 0.13 0.14 0.273 5.36 5.24 2.68 2.93 0.10 0.12 0.21 0.09 0.11 0.20

PhEtS4-m1 5.87 5.69 2.14 2.39 0.20 0.19 0.39 0.18 0.17 0.352 5.42 5.25 2.65 2.90 0.16 0.20 0.36 0.15 0.20 0.353 5.23 5.06 2.80 3.12 0.12 0.18 0.30 0.11 0.19 0.30

H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40 39

Nevertheless, it should be stressed that the EAs are usuallyunderestimated by DFT calculations, in particular when small basissets are used. Due to the fact that no experimental EA values for sil-oles are available, a proper calibration for this parameter cannot becarried out yet. Although a quantitative evaluation of the injectionbarriers is dependent on the accuracy of the IE and EA values, acomparison of the relative trend is expected to be qualitativelycorrect.

3.4. Intramolecular reorganization energy

3.4.1. 1,1-Substituted oligo(2,5-silole)sThe reorganization energies are important parameters to evalu-

ate the charge transport properties [27]. Table 4 lists the reorgani-zation energies for hole transport, kþ, and electron transport, k�,processes of all studied molecules. In general, a low reorganizationenergy implies a better charge transport through the molecules. Allsiloles have quite large reorganization energies at short chainlengths. In going from monomers to pentamers, both kþ and k�

are getting reduced, but the hexamers do not follow such trendand have instead higher reorganization energies than the penta-mers. Since the reorganization energies are obtained from the adi-abatic potential energy surface (PES) calculated using B3LYP/6-31G(d) level, the errors might inherently come from the accuracyof the predicted PES. Therefore, higher level of geometry optimiza-tion is needed to examine the reason for this case. All the 1,1-sub-stituents induce an increase in the reorganization energies of thesystems in which the withdrawing substituents yield highest kþ

and k� while addition of methyl groups leave kþ and k� almostunchanged.

At short chain lengths, siloles have larger k� than kþ but the dif-ferences between them actually vanish as the number of repeatedunits increase. This suggests a more balance between hole- andelectron-transport abilities of longer siloles.

3.4.2. PhEtSThe reorganization energies of PhEtS are also listed on Table 5.

Compared to siloles, PhEtS exhibit lower reorganization energies inboth hole- and electron-transport processes. The increasing chainlength reduces both the kþ and k� values, but the effect turns outto decrease as the number of fused silole rings increases. ThePhEtS1-m have thus the lowest values of kþ and k� (0.16–0.33 eV) and the PhEtS4-m have the highest values of 0.30–0.39 eV. The enhance of the values kþ and k� as x goes from 1 to

4 can be explained by the distribution of LUMO of the systems(Figure 3).

Higher number of fused silole rings attracts more electrons tothe fused silole ring fragments and leave the phenylene fragmentswith very small portion. The localization of electrons on fused sil-ole rings affects the geometries and thereby reduces the electrontransport efficiency through the molecules. This also explains thelarger kþ and k� values of PhEtS as compared to the thiophene ana-logs [27]. Similar to siloles, higher chain lengths have a better bal-ance in hole- and electron-transport reorganization energies withthe differences being less than 0.02 eV for dimers and trimers.

4. Conclusions

In this theoretical study, a number of interesting results emergeas follows:

(i) All studied molecules have planar shape. Addition of elec-tron withdrawing 1,1-substituents invariably decreases theconjugation of the molecules whereas the methyl (or alkyl)groups exert a very small effect.

(ii) The electron injections induce a sharp increase in the delo-calization of the systems which represented in the loweringof Julg’s indices as compared with neutral counterparts.

(iii) The LUMOs of PhEtS do not equally spread over the wholemolecules but more concentrated on the fused silole rings.Increase of the number of fused silole rings results in a moreobvious electron localization.

(iv) The siloles and PhEtS have quite low excitation energies thattend to decrease as a function of chain lengths (in both sil-oles and PhEtS) and number of fused silole rings (in case ofPhEtS).

(v) The obtained IEs and EAs of siloles and PhEtS suggest that asthe chain lengths increase, both hole and electron injectionbarriers become lower.

(vi) All the monomers have relatively large reorganization ener-gies but as the chain lengths increase, the reorganizationenergies become smaller. For PhEtS, the increase of the num-ber of fused silole rings also generates a similar effect on thereorganization energies.

(vii) Although the higher reorganization energies of PhEtS deriv-atives as compared with the well-known thiophene analogsmight result in lower charge transport ability, the longerchain oligomers are expected to further lower the reorgani-

40 H.T. Nguyen et al. / Chemical Physics Letters 550 (2012) 33–40

zation energies in such a way that they become comparablewith those of thiophene analogs, and accordingly, thesePhEtS compounds can be considered as promising candi-dates for electron transport materials.

Acknowledgements

The authors are indebted to the KULeuven Research Council forcontinuing support via the GOA, IDO and IUAP programs. MTNthanks the ICST for supporting his stays in Viet Nam.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cplett.2012.08.051.

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