The optical gap of the organic semiconductor 1]benzothieno3,2-b]benzothiophene and its 2,7-dibrominated analogue is measured in solution and in the crystalline state by means of UV-vis and emission spectroscopy. Bromination leads to a change in molecular packing from herringbone to pi-stacked, resulting in a marked shift in the absorption and emission spectra which is found to be in accordance with TDDFT calculations

Gutzler, Rico

Kern, Klaus

Lotsch, Bettina V.

Nuss, Juergen

Vyas, Vijay S.

CrystEngComm

English

Open Access LMU

www.rsc.org/crystengcomm CrystEngCommCOMMUNICATIONBettina V. Lotsch et al.Optical gap in herringbone and π-stacked crystals of [1]benzothieno[3,2-b]benzothiophene and its brominated derivativeVolume 16 Number 32 28 August 2014 Pages 7359–7606CrystEngCommOpen Access Article. Published on 14 May 2014. Downloaded on 04/02/2015 07:29:57.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.COMMUNICATION View Article OnlineView Journal  | View IssueCrystEngCommThis journal is © The Royal Society of Chemistry 2014aMax Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart,Germany. E-mail: b.lotsch@fkf.mpg.deb Institut de Physique de la Matière Condensée, Ecole Polytechnique Fédérale deLausanne, CH-1015 Lausanne, Switzerlandc Department of Chemistry, Ludwig-Maximilans-Universität München,Butenandtstr. 5-13, 81377 München, GermanydNanosystems Initiative Munich (NIM) and Center for Nanoscience, Schellingstr. 4,80799 München, Germany† Electronic supplementary information (ESI) available: Synthesis andcharacterization of 2, crystal description of 1 and 2. CCDC 975935 and 975934for structures 1 and 2. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c4ce00752bFig. 1 (Top) Perspective view of the crystand (b) of C12H6S2Br2 (2), with unit celabelled with d between adjacent molecstructure of 1 perpendicular to bc planeperpendicular to bc plane and arbitrary dirCite this: CrystEngComm, 2014, 16,7389Received 9th April 2014,Accepted 14th May 2014DOI: 10.1039/c4ce00752bwww.rsc.org/crystengcommOptical gap in herringbone and π-stacked crystalsof [1]benzothieno[3,2-b]benzothiophene and itsbrominated derivative†Vijay S. Vyas,a Rico Gutzler,a Jürgen Nuss,a Klaus Kernab and Bettina V. Lotsch*acdThe optical gap of the organic semiconductor [1]benzothieno[3,2-b]benzothiophene and its 2,7-dibrominated analogue is measuredin solution and in the crystalline state by means of UV-vis andemission spectroscopy. Bromination leads to a change in molecularpacking from herringbone to π-stacked, resulting in a marked shiftin the absorption and emission spectra which is found to be inaccordance with TDDFT calculations.Organic semiconductors (OSCs) have found wide use inorganic field effect transistors,1,2 organic solar cells,3,4organic light-emitting diodes,5–7 etc. The performance ofthese devices crucially depends on the solid state packing ofthe OSCs1,8,9 and continuing efforts are underway to under-stand the effect of molecular structure on molecular packingin (hetero)acenes.1,2 Rational molecular design allows thetuning of intermolecular interactions, leading to differentcrystal packing motifs, and is used to optimize optical andelectronic properties in organic semiconductor devices.10Notably, halogenation of OSCs not only offers a viable path-way to enforce stacked crystal packing in OSCs that otherwisecrystallize in the herringbone arrangement, it also increasesenvironmental stability through larger ionization potentialsand reduces the HOMO/LUMO gap of the molecule.8 Theeffect of herringbone vs. π-stacked crystal structure is a topicof ongoing research, as the packing concomitantly affectselectronic and optical properties of the crystal.1,8,10–12 Thepreferred assembly of halogenated OSCs in π-stacked crystalsis used here to investigate the effect of molecular packing onthe electronic structure in the crystalline state. The knownOSC [1]benzothieno[3,2-b]benzothiophene (BTBT; 1, Fig. 1a)has been reported as a prototypical p-type organic semiconductor,, 2014, 16, 7389–7392 | 7389al structure of (a) C12H8S2 (1)ll. (Bottom) Close contactsules in the crystals (packingand packing structure of 2ection).Fig. 2 (a) Absorption spectra of 1 (blue) and 2 (brown) recorded asdichloromethane solutions (solid lines) and in the crystalline state(dashed lines). (b) Emission spectra of 1 (blue) and 2 (brown) recordedas dichloromethane solutions (solid lines) and in crystalline state(dashed lines).CrystEngCommCommunicationOpen Access Article. Published on 14 May 2014. Downloaded on 04/02/2015 07:29:57.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlineas the 2,7-dialkyl substituted BTBTs show high charge carriermobility and good air stability.13,14 However, owing to theabsence of a viable direct synthetic route to functionalizeBTBT,15 its functionalization is achieved during the synthesisof the BTBT core, which makes the overall synthesis tedious.16,17Herein, we overcome this limitation and report a one-step directbromination of BTBT to form 2,7-dibromo[1]benzothieno[3,2-b]benzothiophene (2, Fig. 1b) and show that the bromination of1 significantly alters the packing in the crystal, resulting in atransition from a herringbone to a π-stacked structure.Optical spectra in solution and in the solid-state (UV-vis andemission) of 1 and 2 are measured to compare the optical gapof monomers and in the crystal.BTBT 1 was functionalized using bromine under ambientconditions to prepare the brominated BTBT 2 (see ESI†). Inorder to gain insight into the impact of halogenation onthe intermolecular packing, single crystals of 2 were obtainedby slow evaporation of its solution in chloroform. Theresulting needle shaped crystals were studied by single-crystal X-ray diffraction. Interestingly, while single crystals of2,7-substituted BTBT derivatives have been reported,18 we didnot find any literature evidence of the crystal structure of theparent BTBT. Therefore, single crystals of 1 were obtainedfrom a solution of dichloromethane and methanol. X-raydiffraction analyses indicate that 1 crystallizes in the well-known herringbone arrangement that many OSCs adopt(Fig. 1a). The brominated compound 2 crystallizes in aπ-stacked molecular arrangement with an interplanar distanceof ~3.5 Å (Fig. 1b), as has previously been observed in similarflat OSCs.8 Herringbone and coplanar crystal packing gohand-in-hand with a changing number of close contactsbetween atoms of adjacent molecules (14 contacts in 1 and10 contacts per molecule in 2), which include two S⋯S andfour S⋯C(sp2) short contacts per molecule in 1, and two S⋯Sand four Br⋯Br short contacts per molecule in 2 (see Fig. 1and ESI† for full list of close contacts).Absorption spectra of 1 and 2 in dichloromethane wererecorded under conditions of identical concentration (10−5 M)and temperature (22 °C) and are compiled in Fig. 2a. 2 showsstructured absorption bands that shift bathochromically(λedge = 3.64 eV in 1 to 3.58 eV in 2) due to the presence ofhalogens (see Table 1). This trend is in accordance with time-dependent density functional theory (TDDFT) calculations atthe M06-2X/def2-TZVP level of theory (Orca 3.0 (ref. 19)),which yield an S0–S1 transition energy of 4.44 eV for 1 and4.34 eV for 2. TDDFT with the M06-2X functional is reportedto produce reasonable results for excitation energies.20 In con-trast, the absorption spectra of the crystalline powderrecorded in diffuse reflectance mode (dashed lines, Fig. 2b)shows a marginal red shift in 2 (Eedge = 3.23 eV) when com-pared with 1 (Eedge = 3.25 eV). A comparison of solution andsolid-state absorption spectrum reflects the reduction of theoptical gap upon crystallization for both compounds. How-ever, in the herringbone structure of 1, the gap is reduced by0.39 eV (from 3.64 to 3.25 eV), while in π-stacked 2, the gap isreduced by 0.35 eV (from 3.58 to 3.23 eV). This trend is7390 | CrystEngComm, 2014, 16, 7389–7392opposite to what is observed in emission spectra (λexc =300 nm) of 1 and 2 recorded under dilute conditions (10−6 Msolution in dichloromethane) at room temperature (22 °C). Asshown in Fig. 2b, the structured emission bands at 341 and356 nm (λmax) in 1 are replaced by one broad spectrum of 2 at363 nm (λmax). The emission spectra of 1 and 2 recorded inthe crystalline state exhibited marked bathochromic shifts assimilarly observed in the UV-vis spectra. Thus, the solid stateemission maximum of 2 is observed at 407 nm, therebyexhibiting a red shift of 44 nm (0.37 eV) in comparison to theemission maximum in solution. The crystalline state emissionspectrum of 1 was observed at 376 nm and hence shows asmaller red shift of only 20 nm (0.19 eV) from that insolution.To validate the influence of molecular packing on excita-tion energies extending on the commonly discussed dimermodel,11,21,22 time-dependent DFT calculations (M06-2X/def2-TZVP) were carried out on clusters of different sizes whosegeometry was taken from the experimental crystal data of 1and 2. TDDFT gives insight into relative excitation energiesand transition moments of molecular clusters even thoughthe absolute values of these quantities do not necessarilycoincide with experimental values. Fig. 3 shows the oscillatorstrength of the lowest lying excitation energies for hexamersand the monomers 1 and 2. The cluster geometries were cho-sen by taking into account different close contacts in thecrystal structure. For 1, the clusters were defined by the crys-tallographic axes c and b, respectively, both involving S⋯Sand S⋯C contacts (Fig. 3b and c). In case of 2, cluster (f)involves Br⋯Br contacts between stacks (also Fig. 1. bottomright), while cluster (g) is a stack of six molecules. Similarly,cluster (h) includes short S⋯S contacts between two π-stacks.As shown in Fig. 3, the stacked arrangement of 2 has thelowest excitation energy just above 4.0 eV (red solid lines; g)while its oscillator strength is negligible. For hexamers of 2,the first transition with appreciable oscillator strength isfound at ~4.20 eV (f–h; blue, red and green solid lines), thatcorresponds to the fourth lowest transition (S0 to S4). AllThis journal is © The Royal Society of Chemistry 2014Table 1 Photophysical properties for 1 and 2Eedge absa (eV) Eedge absb (eV) λmax PLa ,c (nm) λmax PLb ,c (nm) Eonsetd (eV) Eexcitatione (eV)1 3.64 3.25 356 376 1.38 4.442 3.58 3.23 363 407 1.49 4.34a In dichloromethane. b In solid state. c Upon excitation at 300 nm. d V vs. Ag/AgCl. e TDDFT/M06-2X/def2-TZVP on optimized structure.Fig. 3 TDDFT excitation energies of different clusters of 1 (dottedbars, (a) to (d)) and 2 (solid bars, (e) to (h)). Geometric structures aretaken from the experimental crystal data.CrystEngComm CommunicationOpen Access Article. Published on 14 May 2014. Downloaded on 04/02/2015 07:29:57.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinelower-lying transitions have small oscillator strengths due tothe cancellation of transition dipole moments within thestack. On the other hand, for the typical arrangement ofmolecules in the herringbone structure (c; green dotted line)we observe a transition at 4.18 eV, which is lower whencompared with the stacked clusters (f–h). Calculations on aheptamer cluster of 1 (violet dotted line d) results in evenlower transition energy of 4.12 eV.‡ Thus, in the herringbonearrangement, the transition with largest oscillator strength isthe one with lowest excitation energy due to constructiveinteractions of transition dipole moments. We infer that thistrend extends to the crystals and hence small transitiondipole moments for the lowest excitation energies in theThis journal is © The Royal Society of Chemistry 2014stacked arrangement in 2 are responsible for the smallerobserved shift in the solid state absorption spectra whencompared to 1, in which dipole moments do not cancel eachother. The calculations also explain the observance of smalleremission energy for 2, as compared to 1 in the emission spec-tra, asserting internal conversion (Sn to S1) in 2 before emis-sion. The smaller emission energy of 2 can be explained byinvoking Kasha's rule: internal conversion from Sn to S1 in 2occurs before emission due to vibronic coupling.23 A full listof excitation energies and dipole moments of all clusters isgiven in ESI.†ConclusionsThe observance of a large bathochromic shift in the solid-state vs. solution absorption spectra of the BTBT-type organicsemiconductors 1 and 2 is a crystal packing effect, in whichthe interaction of molecular orbitals leads to a reduction ofthe optical gap for large cluster sizes (cf. Fig. 3). The herring-bone arrangement of 1 allows for efficient contraction of theoptical gap due to constructive interactions of transitiondipole moments. In the stacked crystal of 2, however, thelowest excitation energies are accompanied by vanishing tran-sition dipole moments, which do not significantly contributeto the experimental absorption spectra. The generation ofexcitons thus requires less energy in the parent compoundBTBT compared to its brominated counterpart. How freeelectron–hole pairs in herringbone vs. stacked crystals behavein transport experiments is yet another interesting topic,which shall be addressed in future experiments.Notes and references‡ In the π-stacked heptamer, the lowest excitation energy is 4.00 eV with negli-gible oscillator strength. The first transition with appreciable oscillatorstrength is found at 4.20 eV, which is the same value as in the π-stackedhexamer and larger than in the herringbone heptamer (d).1 K. Takimiya, S. Shinamura, I. Osaka and E. Miyazaki, Adv.Mater., 2011, 23, 4347–4370.2 W. Jiang, Y. Li and Z. Wang, Chem. Soc. Rev., 2013, 42,6113–6127.3 Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012, 41, 4245–4272.4 A. Mishra and P. Bäuerle, Angew. Chem., Int. Ed., 2012, 51,2020–2067.5 M. O'Neill and S. M. Kelly, Adv. Mater., 2011, 23, 566–584.6 H. Ma, H. L. Yip, F. Huang and A. K. Y. Jen, Adv. Funct.Mater., 2010, 20, 1371–1388.CrystEngComm, 2014, 16, 7389–7392 | 7391CrystEngCommCommunicationOpen Access Article. Published on 14 May 2014. Downloaded on 04/02/2015 07:29:57.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online7 A. Y. Amin, A. Khassanov, K. Reuter, T. Meyer-Friedrichsenand M. Halik, J. Am. Chem. Soc., 2012, 134, 16548–16550.8 M. L. Tang and Z. Bao, Chem. Mater., 2011, 23, 446–455.9 G. Heimel, I. Salzmann, S. Duhm and N. Koch, Chem.Mater., 2011, 23, 359–377.10 S. Varghese and S. Das, J. Phys. Chem. Lett., 2011, 2,863–873.11 J. Cornil, D. Beljonne, J. P. Calbert and J. L. Brédas, Adv.Mater., 2001, 13, 1053–1067.12 M. D. Curtis, J. Cao and J. W. Kampf, J. Am. Chem. Soc.,2004, 126, 4318–4328.13 C. Liu, T. Minari, X. Lu, A. Kumatani, K. Takimiya andK. Tsukagoshi, Adv. Mater., 2011, 23, 523–526.14 K. Takimiya, H. Ebata, K. Sakamoto, T. Izawa, T. Otsubo andY. Kunugi, J. Am. Chem. Soc., 2006, 128, 12604–12605.15 B. Košata, V. Kozmík and J. Svoboda, Collect. Czech. Chem.Commun., 2002, 67, 645–664.7392 | CrystEngComm, 2014, 16, 7389–739216 M.-C. Um, J. Kwak, J.-P. Hong, J. Kang, D. Y. Yoon, S. H. Lee,C. Lee and J.-I. Hong, J. Mater. Chem., 2008, 18, 4698–4703.17 C. Ruzié, J. Karpinska, A. R. Kennedy and Y. H. Geerts,J. Org. Chem., 2013, 78, 7741–7748.18 T. Izawa, E. Miyazaki and K. Takimiya, Adv. Mater., 2008, 20,3388–3392.19 F. Neese, WIREs Comput. Mol. Sci., 2012, 2, 73–78.20 D. Jacquemin, E. A. Perpète, I. Ciofini, C. Adamo, R. Valero,Y. Zhao and D. G. Truhlar, J. Chem. Theory Comput., 2010, 6,2071–2085.21 J. Cornil, D. A. dos Santos, X. Crispin, R. Silbey andJ. L. Brédas, J. Am. Chem. Soc., 1998, 120, 1289–1299.22 J. Gierschner, M. Ehni, H. J. Egelhaaf, B. Milián Medina,D. Beljonne, H. Benmansour and G. C. Bazan, J. Chem. Phys.,2005, 123, 144914.23 M. Kasha, H. R. Rawls and M. A. El-Bayoumi, Pure Appl.Chem., 1965, 11, 371–392.This journal is © The Royal Society of Chemistry 2014

Optical gap in herringbone and pi-stacked crystals of 1]benzothieno3,2-b]benzothiophene and its brominated derivative

Universität München: Elektronischen Publikationen

www.rsc.org/crystengcomm
 
CrystEngComm
COMMUNICATION
Bettina V. Lotsch et al.
Optical gap in herringbone and π-stacked crystals of [1]benzothieno[3,2-b]
benzothiophene and its brominated derivative
Volume 16 Number 32 28 August 2014 Pages 7359–7606
CrystEngComm
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COMMUNICATION View Article OnlineView Journal  | View IssueCrystEngCommThis journal is © The Royal Society of Chemistry 2014
aMax Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart,
Germany. E-mail: b.lotsch@fkf.mpg.de
b Institut de Physique de la Matière Condensée, Ecole Polytechnique Fédérale de
Lausanne, CH-1015 Lausanne, Switzerland
c Department of Chemistry, Ludwig-Maximilans-Universität München,
Butenandtstr. 5-13, 81377 München, Germany
dNanosystems Initiative Munich (NIM) and Center for Nanoscience, Schellingstr. 4,
80799 München, Germany
† Electronic supplementary information (ESI) available: Synthesis and
characterization of 2, crystal description of 1 and 2. CCDC 975935 and 975934
for structures 1 and 2. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ce00752b
Fig. 1 (Top) Perspective view of the cryst
and (b) of C12H6S2Br2 (2), with unit ce
labelled with d between adjacent molec
structure of 1 perpendicular to bc plane
perpendicular to bc plane and arbitrary dirCite this: CrystEngComm, 2014, 16,
7389
Received 9th April 2014,
Accepted 14th May 2014
DOI: 10.1039/c4ce00752b
www.rsc.org/crystengcommOptical gap in herringbone and π-stacked crystals
of [1]benzothieno[3,2-b]benzothiophene and its
brominated derivative†
Vijay S. Vyas,a Rico Gutzler,a Jürgen Nuss,a Klaus Kernab and Bettina V. Lotsch*acdThe optical gap of the organic semiconductor [1]benzothieno[3,2-
b]benzothiophene and its 2,7-dibrominated analogue is measured
in solution and in the crystalline state by means of UV-vis and
emission spectroscopy. Bromination leads to a change in molecular
packing from herringbone to π-stacked, resulting in a marked shift
in the absorption and emission spectra which is found to be in
accordance with TDDFT calculations.
Organic semiconductors (OSCs) have found wide use in
organic field effect transistors,1,2 organic solar cells,3,4
organic light-emitting diodes,5–7 etc. The performance of
these devices crucially depends on the solid state packing of
the OSCs1,8,9 and continuing efforts are underway to under-
stand the effect of molecular structure on molecular packing
in (hetero)acenes.1,2 Rational molecular design allows the
tuning of intermolecular interactions, leading to different
crystal packing motifs, and is used to optimize optical and
electronic properties in organic semiconductor devices.10
Notably, halogenation of OSCs not only offers a viable path-
way to enforce stacked crystal packing in OSCs that otherwise
crystallize in the herringbone arrangement, it also increases
environmental stability through larger ionization potentials
and reduces the HOMO/LUMO gap of the molecule.8 The
effect of herringbone vs. π-stacked crystal structure is a topic
of ongoing research, as the packing concomitantly affects
electronic and optical properties of the crystal.1,8,10–12 The
preferred assembly of halogenated OSCs in π-stacked crystalsis used here to investigate the effect of molecular packing on
the electronic structure in the crystalline state. The known
OSC [1]benzothieno[3,2-b]benzothiophene (BTBT; 1, Fig. 1a)
has been reported as a prototypical p-type organic semiconductor,, 2014, 16, 7389–7392 | 7389
al structure of (a) C12H8S2 (1)
ll. (Bottom) Close contacts
ules in the crystals (packing
and packing structure of 2
ection).
Fig. 2 (a) Absorption spectra of 1 (blue) and 2 (brown) recorded as
dichloromethane solutions (solid lines) and in the crystalline state
(dashed lines). (b) Emission spectra of 1 (blue) and 2 (brown) recorded
as dichloromethane solutions (solid lines) and in crystalline state
(dashed lines).
CrystEngCommCommunication
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View Article Onlineas the 2,7-dialkyl substituted BTBTs show high charge carrier
mobility and good air stability.13,14 However, owing to the
absence of a viable direct synthetic route to functionalize
BTBT,15 its functionalization is achieved during the synthesis
of the BTBT core, which makes the overall synthesis tedious.16,17
Herein, we overcome this limitation and report a one-step direct
bromination of BTBT to form 2,7-dibromo[1]benzothieno[3,2-b]
benzothiophene (2, Fig. 1b) and show that the bromination of
1 significantly alters the packing in the crystal, resulting in a
transition from a herringbone to a π-stacked structure.
Optical spectra in solution and in the solid-state (UV-vis and
emission) of 1 and 2 are measured to compare the optical gap
of monomers and in the crystal.
BTBT 1 was functionalized using bromine under ambient
conditions to prepare the brominated BTBT 2 (see ESI†). In
order to gain insight into the impact of halogenation on
the intermolecular packing, single crystals of 2 were obtained
by slow evaporation of its solution in chloroform. The
resulting needle shaped crystals were studied by single-
crystal X-ray diffraction. Interestingly, while single crystals of
2,7-substituted BTBT derivatives have been reported,18 we did
not find any literature evidence of the crystal structure of the
parent BTBT. Therefore, single crystals of 1 were obtained
from a solution of dichloromethane and methanol. X-ray
diffraction analyses indicate that 1 crystallizes in the well-
known herringbone arrangement that many OSCs adopt
(Fig. 1a). The brominated compound 2 crystallizes in a
π-stacked molecular arrangement with an interplanar distance
of ~3.5 Å (Fig. 1b), as has previously been observed in similar
flat OSCs.8 Herringbone and coplanar crystal packing go
hand-in-hand with a changing number of close contacts
between atoms of adjacent molecules (14 contacts in 1 and
10 contacts per molecule in 2), which include two S⋯S and
four S⋯C(sp2) short contacts per molecule in 1, and two S⋯S
and four Br⋯Br short contacts per molecule in 2 (see Fig. 1
and ESI† for full list of close contacts).
Absorption spectra of 1 and 2 in dichloromethane were
recorded under conditions of identical concentration (10−5 M)
and temperature (22 °C) and are compiled in Fig. 2a. 2 shows
structured absorption bands that shift bathochromically
(λedge = 3.64 eV in 1 to 3.58 eV in 2) due to the presence of
halogens (see Table 1). This trend is in accordance with time-
dependent density functional theory (TDDFT) calculations at
the M06-2X/def2-TZVP level of theory (Orca 3.0 (ref. 19)),
which yield an S0–S1 transition energy of 4.44 eV for 1 and
4.34 eV for 2. TDDFT with the M06-2X functional is reported
to produce reasonable results for excitation energies.20 In con-
trast, the absorption spectra of the crystalline powder
recorded in diffuse reflectance mode (dashed lines, Fig. 2b)
shows a marginal red shift in 2 (Eedge = 3.23 eV) when com-
pared with 1 (Eedge = 3.25 eV). A comparison of solution and
solid-state absorption spectrum reflects the reduction of the
optical gap upon crystallization for both compounds. How-
ever, in the herringbone structure of 1, the gap is reduced by
0.39 eV (from 3.64 to 3.25 eV), while in π-stacked 2, the gap is
reduced by 0.35 eV (from 3.58 to 3.23 eV). This trend is7390 | CrystEngComm, 2014, 16, 7389–7392opposite to what is observed in emission spectra (λexc =
300 nm) of 1 and 2 recorded under dilute conditions (10−6 M
solution in dichloromethane) at room temperature (22 °C). As
shown in Fig. 2b, the structured emission bands at 341 and
356 nm (λmax) in 1 are replaced by one broad spectrum of 2 at
363 nm (λmax). The emission spectra of 1 and 2 recorded in
the crystalline state exhibited marked bathochromic shifts as
similarly observed in the UV-vis spectra. Thus, the solid state
emission maximum of 2 is observed at 407 nm, thereby
exhibiting a red shift of 44 nm (0.37 eV) in comparison to the
emission maximum in solution. The crystalline state emission
spectrum of 1 was observed at 376 nm and hence shows a
smaller red shift of only 20 nm (0.19 eV) from that in
solution.
To validate the influence of molecular packing on excita-
tion energies extending on the commonly discussed dimer
model,11,21,22 time-dependent DFT calculations (M06-2X/def2-
TZVP) were carried out on clusters of different sizes whose
geometry was taken from the experimental crystal data of 1
and 2. TDDFT gives insight into relative excitation energies
and transition moments of molecular clusters even though
the absolute values of these quantities do not necessarily
coincide with experimental values. Fig. 3 shows the oscillator
strength of the lowest lying excitation energies for hexamers
and the monomers 1 and 2. The cluster geometries were cho-
sen by taking into account different close contacts in the
crystal structure. For 1, the clusters were defined by the crys-
tallographic axes c and b, respectively, both involving S⋯S
and S⋯C contacts (Fig. 3b and c). In case of 2, cluster (f)
involves Br⋯Br contacts between stacks (also Fig. 1. bottom
right), while cluster (g) is a stack of six molecules. Similarly,
cluster (h) includes short S⋯S contacts between two π-stacks.
As shown in Fig. 3, the stacked arrangement of 2 has the
lowest excitation energy just above 4.0 eV (red solid lines; g)
while its oscillator strength is negligible. For hexamers of 2,
the first transition with appreciable oscillator strength is
found at ~4.20 eV (f–h; blue, red and green solid lines), that
corresponds to the fourth lowest transition (S0 to S4). AllThis journal is © The Royal Society of Chemistry 2014
Table 1 Photophysical properties for 1 and 2
Eedge abs
a (eV) Eedge abs
b (eV) λmax PL
a ,c (nm) λmax PL
b ,c (nm) Eonset
d (eV) Eexcitation
e (eV)
1 3.64 3.25 356 376 1.38 4.44
2 3.58 3.23 363 407 1.49 4.34
a In dichloromethane. b In solid state. c Upon excitation at 300 nm. d V vs. Ag/AgCl. e TDDFT/M06-2X/def2-TZVP on optimized structure.
Fig. 3 TDDFT excitation energies of different clusters of 1 (dotted
bars, (a) to (d)) and 2 (solid bars, (e) to (h)). Geometric structures are
taken from the experimental crystal data.
CrystEngComm Communication
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View Article Onlinelower-lying transitions have small oscillator strengths due to
the cancellation of transition dipole moments within the
stack. On the other hand, for the typical arrangement of
molecules in the herringbone structure (c; green dotted line)
we observe a transition at 4.18 eV, which is lower when
compared with the stacked clusters (f–h). Calculations on a
heptamer cluster of 1 (violet dotted line d) results in even
lower transition energy of 4.12 eV.‡ Thus, in the herringbone
arrangement, the transition with largest oscillator strength is
the one with lowest excitation energy due to constructive
interactions of transition dipole moments. We infer that this
trend extends to the crystals and hence small transition
dipole moments for the lowest excitation energies in theThis journal is © The Royal Society of Chemistry 2014stacked arrangement in 2 are responsible for the smaller
observed shift in the solid state absorption spectra when
compared to 1, in which dipole moments do not cancel each
other. The calculations also explain the observance of smaller
emission energy for 2, as compared to 1 in the emission spec-
tra, asserting internal conversion (Sn to S1) in 2 before emis-
sion. The smaller emission energy of 2 can be explained by
invoking Kasha's rule: internal conversion from Sn to S1 in 2
occurs before emission due to vibronic coupling.23 A full list
of excitation energies and dipole moments of all clusters is
given in ESI.†
Conclusions
The observance of a large bathochromic shift in the solid-
state vs. solution absorption spectra of the BTBT-type organic
semiconductors 1 and 2 is a crystal packing effect, in which
the interaction of molecular orbitals leads to a reduction of
the optical gap for large cluster sizes (cf. Fig. 3). The herring-
bone arrangement of 1 allows for efficient contraction of the
optical gap due to constructive interactions of transition
dipole moments. In the stacked crystal of 2, however, the
lowest excitation energies are accompanied by vanishing tran-
sition dipole moments, which do not significantly contribute
to the experimental absorption spectra. The generation of
excitons thus requires less energy in the parent compound
BTBT compared to its brominated counterpart. How free
electron–hole pairs in herringbone vs. stacked crystals behave
in transport experiments is yet another interesting topic,
which shall be addressed in future experiments.
Notes and references
‡ In the π-stacked heptamer, the lowest excitation energy is 4.00 eV with negli-
gible oscillator strength. The first transition with appreciable oscillator
strength is found at 4.20 eV, which is the same value as in the π-stacked
hexamer and larger than in the herringbone heptamer (d).
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The optical gap of the organic semiconductor [1]benzothieno[3,2-b]benzothiophene and its 2,7-dibrominated analogue is measured in solution and in the crystalline state by means of UV-vis and emission spectroscopy. Bromination leads to a change in molecular packing from herringbone to pi-stacked, resulting in a marked shift in the absorption and emission spectra which is found to be in accordance with TDDFT calculations

Infoscience - École polytechnique fédérale de Lausanne

Optical gap in herringbone and pi-stacked crystals of [1]benzothieno[3,2-b]benzothiophene and its brominated derivative

Vijay S. Vyas

Rico Gutzler

Jürgen Nuss

Klaus Kern

Bettina V. Lotsch

Crossref

Optical gap in herringbone and π-stacked crystals of [1]benzothieno[3,2-b]benzothiophene and its brominated derivative

https://epub.ub.uni-muenchen.de/22619/1/oa_RSC_22619.pdf

Optical gap in herringbone and pi-stacked crystals of 1]benzothieno3,2-b]benzothiophene and its brominated derivative

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Infoscience - École polytechnique fédérale de Lausanne

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