Structures and Fluorescence Properties for the Crystals, Powders, and

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Structures and Fluorescence Properties for the Crystals, Powders, and Thin Films of Dithienylhexatrienes: Effects of Positional Isomerism Yoriko Sonoda, Norimitsu Tohnai, Ying Zhou, Yukihiro Shimoi, and Reiko Azumi Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00509 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 24, 2018

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Crystal Growth & Design

1

Structures and Fluorescence Properties for the Crystals, Powders, and Thin Films of Dithienylhexatrienes: Effects of Positional Isomerism Yoriko Sonoda,*† Norimitsu Tohnai,‡ Ying Zhou,† Yukihiro Shimoi,§ and Reiko Azumi† †

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST) ‡

Department of Material and Life Science, Graduate School of Engineering, Osaka University

§

Research Center for Computational Design of Advanced Functional Materials, AIST

ABSTRACT: and

thin

We investigated the structures and fluorescence properties for the crystals, powders films

of

the

two

positional

isomers

of

dithienylpolyene,

(E,E,E)-1,6-di(3-thienyl)hexa-1,3,5-triene (3T) and 1,6-di(2-thienyl)hexa-1,3,5-triene (2T). A combination of experimental single crystal X-ray and theoretical reduced density gradient (RDG)−atoms-in-molecule (AIM) analysis indicate the importance of CH/π and S/S interactions in 3T and CH/π and S/π interactions in 2T on their structure constructions. Although the molecules are arranged similarly in a herringbone fashion in the crystals, the total intermolecular interactions are considered to be significantly stronger in 3T than in 2T.

The X-ray diffraction measurements

suggest that the degree of structural order in powders and films is higher in 3T than in 2T. The spectroscopic differences for the crystal, powder, and film samples are larger in 3T. The absorption and fluorescence spectra for the film of 3T are blue- and red-shifted respectively from those of the crystal and powder, and the maximum fluorescence yields (0.23-0.27) are observed in powders. In 2T, both the absorption and fluorescence spectra of the film are slightly blue-shifted from those of the crystal and powder, and the fluorescence yields are similarly low (≤ 0.01) for all solids examined. We also find that the film orientation is substrate-dependent in 3T.

Corresponding author: Yoriko Sonoda †

Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan E-mail: [email protected] TEL: +81-29-861-6390 FAX: +81-29-861-6252

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Structures and Fluorescence Properties for the Crystals, Powders, and Thin Films of Dithienylhexatrienes: Effects of Positional Isomerism Yoriko Sonoda,*† Norimitsu Tohnai,‡ Ying Zhou,† Yukihiro Shimoi,§ and Reiko Azumi†

†

Electronics and Photonics Research Institute, National Institute of Advanced Industrial

Science and Technology (AIST), Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, Japan

‡

Department of Material and Life Science, Graduate School of Engineering, Osaka University,

2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

§

Research Center for Computational Design of Advanced Functional Materials, National

Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, Ibaraki 305-8568, Japan

Corresponding author: Yoriko Sonoda TEL: +81-29-861-6390 FAX: +81-29-861-6252 E-mail: [email protected]


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3

ABSTRACT We investigated the structures and fluorescence properties for the crystals, powders and thin films

of

the

two

positional

isomers

of

dithienylpolyene,

(E,E,E)-1,6-di(3-thienyl)hexa-1,3,5-triene (3T) and 1,6-di(2-thienyl)hexa-1,3,5-triene (2T). A combination of experimental single crystal X-ray and theoretical reduced density gradient (RDG)−atoms-in-molecule (AIM) analysis indicate the importance of CH/π and S/S interactions in 3T and CH/π and S/π interactions in 2T on their structure constructions. Although the molecules are arranged similarly in a herringbone fashion in the crystals, the total intermolecular interactions are considered to be significantly stronger in 3T than in 2T. The X-ray diffraction measurements suggest that the degree of structural order in powders and films is higher in 3T than in 2T. film samples are larger in 3T.

The spectroscopic differences for the crystal, powder, and

The absorption and fluorescence spectra for the film of 3T are

blue- and red-shifted respectively from those of the crystal and powder, and the maximum fluorescence yields (0.23-0.27) are observed in powders.

In 2T, both the absorption and

fluorescence spectra of the film are slightly blue-shifted from those of the crystal and powder, and the fluorescence yields are similarly low (≤ 0.01) for all solids examined. that the film orientation is substrate-dependent in 3T.


We also find


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INTRODUCTION The solid-state photophysical properties of linear π-conjugated molecules have been intensively and extensively studied because they have potential applications in optical and opto-electronic devices.1-8

The fluorescence emission properties of linear molecules often

depend strongly on the structural features such as molecular arrangements, orientation, and the degree of structural order in the solid state.9-12

For example, the fluorescence properties

of distyrylbenzenes are heavily affected by the molecular arrangements in the crystals.9,10

In

thiophene-based oligomers, the differences in the degree of structural order in crystals, powders, and films can be observed as significant differences in the solid-state emission properties.11,12

The establishment of clear structure-property relationships in linear

conjugated molecules is thus very important not only from scientific but also from industrial points of view.4,5 α,ω-Diphenylpolyenes are simple, linear conjugated molecules possessing interesting photophysical/chemical properties.13-15 Among them, 1,6-diphenylhexa-1,3,5-triene (DPH) and its ring-substituted derivatives have been shown to be highly fluorescent in solution14 and used as fluorescence probes in biomembrane studies due to their large fluorescence anisotropy.16,17

As for the solid-state fluorescence properties, we have systematically

investigated the crystal structures and photoproperties of the unsubstituted and ring-substituted DPHs and found the strong relationship between molecular arrangements and emission properties.18-23

The results can be understood in terms of strong excitonic

interaction in the crystals due to the large transition moments along the long molecular axis.22,23 For α,ω-dithienylpolyenes, on the other hand, the photophysical/chemical properties in solution were previously investigated and discussed in detail.24-26

Different from

diphenylpolyenes, dithienylpolyenes have two positional isomers with respect to the S atom of

the

thiophene

ring.

The

photophysical

properties

of

the

two

isomers

of

1,6-dithienylhexa-1,3,5-triene, 3T and 2T (Chart 1), are considerably different in solution; 3T


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exhibits strong and blue-shifted fluorescence relative to the emission from 2T.25 In addition, at least in low polar solvents such as methylcyclohexane/3-methylpentane, the fluorescence and excitation spectra of 3T depend clearly on the excitation (Ex) and emission (monitor) (Em) wavelengths, respectively.

These observations can be understood by assuming the

ground-state equilibrium of rotational isomers (rotamers) around the thienyl (Th)-CH (and/or other) single bond(s) (I-III in Chart S1(a) in Supporting Information (SI)), whose emission energies are slightly different each other (the NEER principle).25-28

In contrast, the shape of

the emission and excitation spectra of 2T is practically independent of Ex and Em.

This is

explained by the presence of an unique absorbing and emitting species and no intervention of rotational equilibrium in solution.25 The ground-state molecular geometry is predicted to be more twisted around the Th-CH bond in 3T than in 2T, resulting from larger steric hindrance due to the S-positional isomerism.25

It is probable that such geometrical difference between

3T and 2T leads to the difference in the ratio of rotamers at the equilibrium in solution. In contrast to the well-known photoproperties in solution, the solid-state photophysical properties of dithienylpolyenes are remaining unknown at present. We expect that solid-state structures would be significantly different in 3T and 2T as a result of the positional isomerism, and such structural differences will lead to different fluorescence properties

through

strong

excitonic

interactions.

For

oligothienyl

silanes,29

tetrathienoanthracenes,30,31 and other molecules with S-containing fused rings,32,33 interesting effects of S-positional isomerism on the solid-state structures and properties have recently been reported. In this study, the structures and fluorescence spectroscopic properties of 3T and 2T were investigated for the crystals, the powders obtained by different preparation procedures (crystal grinding and solvent evaporation), and the thin films prepared by vacuum deposition on different (quartz and glass) substrates.

The molecular arrangement, orientation, and the

degree of structural order in these solids were clarified by single crystal, powder, and thin film X-ray diffraction (XRD) measurements.34

The intermolecular interactions in the crystals



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were deeply analyzed by reduced density gradient (RDG)−atoms-in-molecule (AIM) analysis. Their spectroscopic properties were characterized by the measurements of absorption, fluorescence and excitation spectra, fluorescence decay and lifetimes, and emission quantum yields. Our study reveals that, although the molecules are arranged similarly in a herringbone fashion in both the crystals of 3T and 2T, the total intermolecular interactions are considered to be significantly stronger in 3T than in 2T as a result of the S-positional isomerism.

This explains the observed higher degree of structure order in powders and films

and also the larger spectroscopic differences for the crystal, powder, and thin film samples in 3T than in 2T.

Due to the linear conjugated molecular structure, the slight differences in

structural order were able to be observed as clear spectroscopic differences in 3T.

Chart 1.

Chemical structures of 3T and 2T. (E,E,E)

S

S

3T

S

S

2T

EXPERIMENTAL SECTION Materials.

Trienes 3T and 2T were synthesized by the double Wittig reactions

of thiophenecarboxyaldehydes (Aldrich) and (E)-2-butene-1,4-bis(tributylphosphonium chloride).35

1

H NMR spectra were measured using a Bruker Avance 400 (400.03 MHz) with

tetramethylsilane (TMS) as internal reference.

J values are given in Hz.

GPC purification

was carried out using a Japan Analytical Industry LC-908. The sample purity was checked by a HITACHI ELITE LaChrom L-2455 HPLC system.


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7

(E,E,E)-1,6-Di(3-thienyl)hexa-1,3,5-triene

(3T).

To

a

mixture

of

the

tributylphosphonium salt (0.66 g, 1.25 mmol) and 3-thiophenecarboxyaldehyde (0.56 g, 5.0 mmol; 2 eq.) (in no solvent) was added a solution of sodium ethoxide in ethanol (0.83 M, 3 mL) at 45 oC under nitrogen atmosphere.

After stirring for 3-4 h, aqueous ethanol (60 % v/v,

35 mL) was added to the reaction mixture and the solution was vigorously stirred for 1 h. The resulting yellow precipitate was filtered off, washed with aqueous ethanol (60 % v/v, 25 mL) and water (20 mL), and dried under vacuum at room temperature (crude yield 56 %). The crude product was purified by GPC with chloroform eluent to remove a trace amount of high molecular weight byproducts. Multiple recrystallization from toluene (Tol) gave the single crystals of 3T suitable for X-ray analysis. 1

Mp 233 oC (506 K) (lit.36 228-230 oC).

H NMR (CDCl3) δ 7.24-7.30 (4H, m, thiophene), 7.17 (2H, bs, thiophene), 6.71 (2H, ddd, J

15.4, 6.8, and 3.0, triene), 6.60 (2H, d, J 15.4, triene), 6.43 (2H, dd, J 6.8 and 2.9, triene). (E,E,E)-1,6-Di(2-thienyl)hexa-1,3,5-triene

(2T).

To

a

solution

of

the

tributylphosphonium salt (1.32 g, 2.5 mmol) and 2-thiophenecarboxyaldehyde (0.56 g, 5.0 mmol) in ethanol (5 mL) was added a solution of sodium ethoxide in ethanol (0.63 M, 8 mL) at 45 oC under nitrogen atmosphere.

After stirring for 24 h, aqueous ethanol (60 % v/v, 70

mL) was added to the reaction mixture and the solution was vigorously stirred for 1 h. The resulting yellow precipitate was treated similarly as that described for 3T (crude yield 52 %). The crude product was purified by GPC and recrystallized twice from Tol to give the single crystals of 2T suitable for X-ray analysis.

Mp 218 oC (491 K) (lit.36 212-213 oC). 1H NMR

(CDCl3) δ 7.16-7.18 (2H, m, thiophene), 6.96-6.98 (4H, m, thiophene), 6.73 (2H, d, J 15.0, triene), 6.67 (2H, ddd, J 15.1, 5.9, and 2.9, triene), 6.42 (2H, dd, J 6.2 and 3.0, triene). Sample Preparation.

Crystal samples were obtained by recrystallization as

described above and used for X-ray structure analysis and spectroscopic measurements.

For

the XRD and spectroscopic measurements in powders, powder (gr) and powder (ev) samples were used.

The powders (gr) were obtained by manual grinding of the crystals.

The

powders (ev) were prepared by fast evaporation of chloroform solvent from a diluted solution



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of the crystals at 45 oC in the dark.

To complete the crystallization as fast as possible, we

used chloroform, which is a good solvent for 3T and 2T and has a lower bp than the recrystallization solvent Tol.

For the measurements in films, film (qu) and film (gl) were

vacuum evaporated on quartz (UV-grade fused silica, Tsukuba Hikari Kagaku) and glass (OA-10G alkali-free glass, Nippon Electric Glass) substrates (10 x 40 mm2), respectively. Both substrates were pretreated by UV/ozone cleaning for 30 min and set simultaneously for the deposition. The deposition pressure was 1 x 10-4 Pa and the growth rate was kept at around 0.1 nm s-1. Crystallographic Analysis of Single Crystals.

Diffraction data of the crystals

of 3T and 2T were collected at –60 oC (213 K) on a two-dimensional X-ray detector (PILATUS 200K/R) equipped in a Rigaku XtaLAB PRO diffractometer using thin multi-layer mirror monochromated Cu-Kα radiation (λ =1.54187 Å). The cell refinements were performed with software CrysAlisPro 1.171.39.5a.37

A direct method of SHELXT38 was

used for the structure solution of the crystals. All calculations were performed with the observed reflections [I > 2σ(I)] with the program CrystalStructure crystallographic software packages,39 except for refinement which was performed by SHELXL.40

All non-hydrogen

atoms were refined with anisotropic displacement parameters, and hydrogen atoms were refined using the riding model. Powder and Thin Film XRD Measurements.

The structures of powders and

films were characterized by XRD analysis using the Bragg-Brentano configuration with a 9 kW rotating anode generator (Rigaku, Smart-lab). Atomic Force Microscopy (AFM) Measurements.

The film morphology

was investigated by AFM, which was carried out on an SPA300 equipped with an SPI3800 controller (SII Nanotechnology Inc.) using the dynamic mode. Contact Angle Measurements.

The steady-state contact angles of the substrates

were determined by the liquid-droplet method using a Kyowa Kaimen Kagaku DropMaster DM-500.


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9

Measurements of Absorption, Fluorescence Emission, and Excitation Spectra. The absorption spectra of the crystals and powders were obtained by the Kubelka-Munk conversion of diffuse reflectance spectra.41 The reflectance spectra were measured using a Shimadzu UV-3150 spectrometer equipped with an integrating sphere accessory (ISR-3100). Fluorescence and excitation spectra were measured using a Hitachi F-7000 spectrometer with the front face geometry.

The wavelengths of Ex and Em in the measurements of

fluorescence and excitation spectra are shown in the captions of Figures and footnotes of Tables.

In all cases, no filter was used on the emission side during fluorescence monitoring. Measurements of Fluorescence Quantum Yields and Lifetimes.

quantum yields (φf) were measured using a Hamamatsu C11347.

Fluorescence

Fluorescence decay curve

and lifetime (τs) measurements were carried out by the time-correlated single-photon counting (TCSPC) method, using a Hamamatsu C11367 equipped with UV-vis LEDs (time resolution ≤ 100 ps).

In all measurements, Ex was set at 405 nm. The instrument response functions

(IRFs) had FWHM of 0.69 ns for 3T and 0.74 ns for 2T.

In the spectral and τs

measurements for the crystals and powders, the samples were placed between quartz plates (UV-grade fused silica, 20 x 20 mm2). The thicknesses were roughly estimated to be 0.025-0.030 mm and 0.010-0.015 mm for the crystals and powders, respectively. measurements of φf, the samples were encapsulated in quartz cells (φ15 mm x 5 mm).

In the The

film samples were covered with the plates of the same size and quality as those of the substrates. The film thicknesses were estimated from AFM to be about several hundred nm order (see the text). All spectroscopic and photophysical measurements were carried out at room temperature in air. RDG−AIM Analysis.

To obtain further insights on molecular packing form the

theoretical point of view, we performed the analyses of RDG42 and AIM43 for several molecular pairs in the crystal structures of 3T and 2T using the Multiwfn program.44



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RESULTS AND DISCUSSION 1. Structures 1.1. Crystals. Table 1 shows the single crystal X-ray data of 3T and 2T. contains two molecules, A and B, in a unit cell.

The crystal structure of 3T

Molecule B has a center of symmetry.

Molecule A is also basically symmetric, although it has no symmetry centers crystallographically.

The ORTEP drawings of A and B are shown in Figure S1(a) in SI.

Molecule 3T has three kinds of rotamers around the Th-CH single bond (I-III, Chart S1(a) in SI) as described above. the crystal structure.

Among them, only one kind of rotamer (I) (Chart 1) is observed in

In the structures of small molecules containing 3-Th groups such as

3,3'-bithiophene45,46 and others,29,47 rotational disorders are sometimes found around the Th-CH bonds.

In 3T, however, no such disorders are observed.

parameters of 3T are summarized in Table S1 in SI.

The major intramolecular

The lengths of the C–C single and C=C

double bonds, and the bond length alternation in the triene are all normal and similar in A and B.

Although the optimized structure of 3T in vacuum has twisted conformation around the

Th-CH bonds (see Introduction),25 the torsion angles around the Th-CH bonds are all less than 10 degree for both A and B in the crystal.

This is probably due to packing reason.

The crystal packing diagrams of 3T are displayed in Figure 1. The molecules are arranged in a herringbone fashion (Figure 1(a)). The dihedral angle formed by the least-square planes defined by all non-hydrogen atoms is 62 degree for the neighboring two molecules in the herringbone stack.

Along the b-axis, namely, the direction of the herringbone stacking,

several kinds of CH/π-type interaction48-51 are observed in the structure (Figure 1(b), blue dotted lines). In these interactions, the H atoms of the thiophene ring and the triene chain interact with the π-systems of the ring and the chain, respectively.

The distances of H···C(π)

are 2.8-3.0 Å and C···C(π) are 3.6-3.9 Å, which are the typical values in CH/π interactions.49 The angles of C–H···C(π) are 150-160 degree in most cases, showing the high linearity of these hydrogen bonds.51

These structural features suggest that the CH/π interactions in 3T


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11

Table 1.

Crystal Data of 3T and 2T 3T

2T

formula

C14H12S2

C14H12S2

formula weight

244.38

244.38

crystal color, habit

yellow, plate

yellow, needle

crystal size (mm3)

0.20 x 0.20 x 0.05

0.20 x 0.05 x 0.05

crystal system

monoclinic

triclinic

space group

P21/c

P-1

a (Å)

16.8719(3)

6.31326(19)

b (Å)

7.43294(11)

9.6669(5)

c (Å)

15.7629(3)

10.2038(4)

α (degree)

90

100.178(4)

β (degree)

111.873(2)

92.309(3)

γ (degree)

90

95.910(3)

V (Å3)

1834.49(6)

608.59(4)

Z

4

2

Dcalc (g/cm3)

1.327

1.333

T (K)

213

213

mp (K)

506

491

R1 (I>2σ(I))

0.0978

0.0383



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would be strong enough to play a decisive role in the construction of its crystal structure.

It

should be noted here that the H atoms involved in the CH/π interactions are all at the α-positions [CH(α)] of the thiophene rings in the molecule.

Along the long molecular axis,

the molecules interact each other via S/S contacts to form one-dimensional tapes of A···A and B···B in the structure (Figure 1(b), red dotted lines).

The S/S interaction is one of the σ-hole

based interactions,52-54 and often observed in S-containing compounds that are important in the field of molecular opto-electronics, such as fused thiophene30,31 and tetrathiafulvalene (TTF) derivatives,55,56 and others.29,57-59 The interaction plays an important role in constructing one- or two-dimensional networks in their structures.

In 3T, the distances of

S···S in the molecular tapes are 3.5-3.6 Å and the angles of C–S···S are 160-165 degree, which are similar to those in other examples for the S/S contacts.59 The observed high C–S···S linearity is in line with the σ-hole concept for the S/S interaction.52-54

Figure 1. Crystal packing diagrams of 3T. The CH/π and S/S contacts (< sum of van der Waals radii + 0.10 Å) are shown as blue and red dotted lines, respectively. Distances are in Å.

The crystal structure of 2T contains two molecules, A and B, in a unit cell. molecules have centers of symmetry.

Both

The ORTEP drawings are shown in Figure S1(b) in SI.

Among the possible three Th-CH rotamers (I-III, Chart S1(b) in SI), only one kind of rotamer (I) is observed in the structure (Chart 1).

We see no disorders in 2T, although the Th-CH

rotational disorder29 is reported for 2,2'-bithiophene.60,61


The major intramolecular

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13

parameters are summarized in Table S1 in SI.

The C–C single and C=C double bond lengths

in the triene are all similar and normal in A and B.

The Th-CH torsion angles are 5-10

degree, showing the high planarity of the molecules.

Figure 2.

Crystal packing diagrams of 2T.

The CH/π, S/π, and CH/CH contacts (< sum of

van der Waals radii + 0.10 Å) are shown as blue, red, and green dotted lines, respectively. Distances are in Å.

Figure 2 shows the crystal packing diagrams of 2T. a herringbone fashion (Figure 2(a)).

The molecules are arranged in

The dihedral angle formed by the least-square planes

defined by all non-hydrogen atoms is 70 degree.

Along the direction of the herringbone

stacking, we can see a few kinds of CH/π interactions (Figure 2(b), blue dotted lines).

The

respective distances of H···C(π) and C···C(π) are 2.9-3.0 Å and 3.7-3.9 Å, which are not very short compared with the typical values for the CH/π interaction.49

The angles of C–H···C(π)

are 130-170 degree, indicating that the hydrogen-bond linearity is relatively small in some cases.

These geometrical parameters suggest that the CH/π interactions are not very strong

in 2T, although they are probably one of the most important forces in the crystal.

We also

note that the H atoms involved in these CH/π interactions are all at the β-positions [CH(β)] of the thiophene ring. lines).

We can also see S/π contacts in the structure (Figure 2(b), red dotted

Like the S/S interaction, S/π is considered to be one of the σ-hole based interactions

often observed in S-containing compounds.29,53,62,63

Although the S/π interaction may



Page 14 of 52

14

considerably be weaker than the CH/π interaction,29 it can play some roles in the crystal structure construction of 2T.

The S/π interactions are similarly observed in the structure of

1,4-di(2-thienyl)buta-1,3-diene, a shorter analogue of 2T.34

As the S atoms in 2T are

positioned at the inner part of the molecule, no S/S contacts are found in the structure along the long molecular axis, and we only see CH/CH contacts in this direction (Figure 2(b), green dotted lines).

In this point, the crystal structure of 2T resembles to that of the unsubstituted

DPH.64 The magnitude of CH/π interaction in general correlates strongly with the acidity of CH.50,51

The acidity of CH(α) in a thiophene ring is stronger than that of CH(β), and the

CH/π interaction energy is calculated to be larger for CH(α)/π than for CH(β)/π for model thiophene dimers.29

Thus, in our case, the CH/π interactions in 3T would be significantly

stronger than those in 2T.

This expectation agrees with the geometrical observation that the

H···C(π) and C···C(π) distances are shorter and the C–H···C(π) linearity is higher in 3T than in 2T, as described above.

As for the intermolecular interactions along the herringbone

stacking, we can see a larger number of CH/π contacts in 3T than in 2T (Figures 1(b) and 2(b)).

For the intermolecular forces along the long molecular axis, on the other hand, the

S/S interaction, which is observed in 3T, would be much stronger than the CH/CH interaction in 2T.29 We can therefore expect that the total intermolecular interactions are significantly stronger in 3T than in 2T, although the molecules are arranged similarly in a herringbone pattern in the crystals.

It is interesting that this is purely a result of the S-positional

isomerism, the slight difference in the molecular structures of 3T and 2T.

In agreement with

the stronger intermolecular interactions in the crystal, mp of 3T (233 ℃) is measured to be somewhat higher than that of 2T (218 ℃).

Similar trends in mps for 3-Th and 2-Th

derivatives29 are reported for dithienylpolyenes having less than three double bonds.45,65 To further discuss the intermolecular interactions in crystals 3T and 2T, we performed RDG−AIM analysis. The results of the analysis are shown in Figures S2 and S3 in SI.

Consistent with the expectations from single crystal structure analysis, it is suggested


Page 15 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

that the S/S interaction in 3T is much more significant than the CH/CH interaction in 2T along the long molecular axis, and the CH(α)/π interaction in 3T is stronger than the CH(β)/π interaction in 2T along the direction of the herringbone stacking. interaction in 2T is also clearly indicated.

The presence of S/π

In addition, the RDG−AIM analysis reveals the

presence of CH/π isosurfaces involving the π-system of the triene chain.

Interestingly, it is

demonstrated that they are much larger in 3T than in 2T. Combining the above results, we will consider that the total intermolecular interactions are significantly stronger in crystal 3T than in 2T.

It should be noted here, however, that the RDG−AIM analysis suggests the

presence of significant CH(α)/π isosurface in 2T (Figure S3 (c), the large green area between the thiophene rings). The result is unexpected from crystal structure analysis, as the corresponding intermolecular distances are considerably large (> sum of van der Waals radii + 0.10 Å).

1.2. Powders. Figures 3(a) and (b) show the XRD patterns for the powder (gr) and powder (ev) of 3T, respectively.

The pattern of powder (gr) is in good agreement with that generated from the

single crystal XRD data.

This indicates that the grinding of crystal does not change the

crystal structure largely.

The pattern of powder (ev) is not greatly different from that of

powder (gr), although the relative intensity of the peaks is somewhat different.

The results

show that the degree of structural order is considerably high for both powder (gr) and powder (ev), and that the molecular arrangements in the powders are fundamentally the same as that in the crystal.

As described in Introduction, the fluorescence and excitation spectra of 3T

are reported to be Ex- and Em-dependent in low polar solvents,25 and we have confirmed that the results are similar in methylcyclohexane and chloroform (Figure S4 in SI). Therefore, the molecules would be equilibrated as a Th-CH rotameric mixture (I-III in Chart S1(a) in SI) in chloroform before the solvent evaporation in the preparation procedure of powder (ev) (see



16

Experimental), and this could result in the disordered structure of powder (ev). however, its structure was shown to be considerably highly-ordered.

In fact,

The result indicates

that molecules 3T can fully crystallize from solution even in the fast evaporation of solvent.

Figure 3.

Powder and thin film XRD patterns of 3T. (a) Powder (gr), (b) powder (ev), (c)

film (qu), and (d) film (gl).

CuKα λ = 1.542 Å. 21.62

(a) 5.66 16.78

23.74 29.32

11.28

18.78 20.56 27.96

29.98

21.56

(b) Intensity (arb unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 52

23.66

5.60

(c)

11.24

16.70 18.74 20.56

29.26 29.90 27.92

5.66 11.30

23.68

(d)

0

5

10

15 20 2θ (degree)

25


30

Page 17 of 52

17

Figure 4.



CuKα λ = 1.542 Å.

21.84

(a)

23.32 29.90

14.00 16.42

27.32 27.80

18.12

21.86

(b) Intensity (arb unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23.24

14.04

27.84

29.80

(c) 17.72

(d) 21.84 23.24

10

15

20 25 2θ (degree)


30


Page 18 of 52

18

Figures 4(a) and (b) show the XRD patterns for the powder (gr) and powder (ev) of 2T.

In the pattern of powder (gr), the peaks are broad and the peak positions are not in very

good agreement with those in the pattern calculated from the single crystal data. show that some structural changes are induced by crystal grinding.

The results

The peak shapes and

positions in the pattern of powder (ev) are more significantly different from the calculated pattern than those in powder (gr), showing that the crystal structure is more heavily destroyed during solvent evaporation than on crystal grinding.

As reported previously,25 the

fluorescence and excitation spectra of 2T are only weakly dependent on Ex and Em in solution (Figure S5 in SI). The results are explained by assuming that an unique absorbing and emitting species is present in 2T, and the presence of less stable rotamers (Chart S1(b) in SI) is too small to be easily detected.

In powder (ev), however, the structural order of

molecules is considerably low as shown by XRD.

This can be attributed to the fact that

molecules 2T do not crystallize well in the fast evaporation of solvent.

1.3. Thin Films. The XRD patterns for the film (qu) and film (gl) of 3T are shown in Figures 3(c) and (d), respectively. The film thicknesses are 200-300 nm.

In the pattern of film (qu), sharp peaks

are observed at 2θ = 5.66 degree (n=1) and 11.3 degree (n=2).

The interlayer distance (d) is

calculated from the Bragg's equation (2dsinθ = nλ) to be 15.6 Å, which is near the lattice constant c = 15.8 Å in the single crystal data (Table 1).

This suggests that the orientation of

molecules on the quartz substrate is the edge-on type, where the short molecular axis is almost normal to the substrate.

The edge-on orientation is confirmed by the incident-angle

dependence of the polarized absorption spectrum (Figure S6 in SI).

In the pattern of film

(gl), a sharp peak is observed at a clearly different position of 2θ = 23.68 degree (n=2) from those in film (qu).

The calculated distance of d = 7.51 Å probably corresponds to the lattice

constant b = 7.43 Å (Table 1).

This suggests that the molecular orientation is the face-on

type on the glass substrate, where the molecular plane is near parallel to the substrate. As


Page 19 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19

shown, we can see clear orientation of molecules in both film (qu) and film (gl).

It has been

reported that the molecular orientation in thin films can be controlled not only by chemical modifications,66,67 but also by the experimental conditions in film preparation such as growth rates,66 temperatures68-70 including heating by light irradiation,71 and surface (pre)treatments of substrates.72-74

In the present case, however, all the experimental conditions in the

preparation procedure were the same for film (qu) and film (gl), and no pretreatments other than the UV-ozone cleaning were applied to the substrates (see Experimental).

It is

interesting that the molecular orientation in the film is clearly different depending on the substrate. The molecular density of the film is also substrate-dependent.

Figures 5 (a) and (b)

show the AFM images for the film (qu) and film (gl) of 3T, respectively. As seen, molecules are deposited more densely on the quartz substrate than on the glass one. The difference in molecular density for film (qu) and film (gl) can be attributed to the difference in the flatness and/or the surface free energy of substrate. to be similarly high for both substrates.

The flatness is shown from AFM measurements

On the other hand, the contact angles are measured

to be 25 degree and 36 degree for the quartz and glass substrates, respectively (Figure S7 in SI). The results indicate that the surface free energy is larger and the interaction between molecule and substrate is stronger for the quartz substrate than for the glass one.

This agrees

with the observed higher density of molecules for film (qu) than for film (gl). The difference in the magnitude of molecule-substrate interaction between quartz and glass substrates implies that the orientation of molecules in the first layer of the film is strongly influenced by the substrate.

Although the reason for the above-mentioned

substrate-dependence of the film orientation in 3T is unclear at present, it is possible that the first layer of molecules on the substrate plays an important role in determining the following growth of the film. In contrast to the case of 3T, we can see only very weak peaks in the XRD patterns for the films of 2T (Figures 4(c) and (d)).

The film thicknesses are estimated to be 400-500



Page 20 of 52

20

nm for film (qu) and 150-200 nm for film (gl), both of which are of the same order as those in 3T.

The observation of only weak XRD peaks should therefore be attributed to the low

degree of molecular orientation in the films, rather than the insufficient amounts of samples on the substrates. We note that the measurements of incident-angle dependence of the polarized absorption spectrum were carried out to determine the average molecular orientation in the films, but unsuccessful.

As in the case of 3T, on the other hand, the molecular density

is higher for the film (qu) than for the film (gl) (Figures 5 (c) and (d)). This shows that the film growth of 2T is also influenced by the substrate. Although the molecular arrangement patterns in the crystals are the herringbone-type in both 3T and 2T, the degree of structural order in powders and films is clearly higher in 3T than in 2T.

The results can be attributed to the strong intermolecular interaction in crystal

3T relative to that in 2T.

Figure 5.

AFM images for the thin films of 3T on (a) quartz and (b) glass substrates, and

2T on (c) quartz and (d) glass substrates.

Image area = 10 x 10 µm2


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21

2. Photophysical Properties 2.1.

3T.

Table 2 summarizes the photophysical data for the crystal, powders, and films of 3T.

Figure

6(a) shows the fluorescence and fluorescence excitation spectra for the crystal and powders of 3T.

The excitation spectra agree well with the absorption spectra. The fluorescence spectra

show no Ex-dependence and the excitation spectra show no Em-dependence.

The excitation

spectra of the crystal, powder (gr), and powder (ev) are fundamentally the same.

However,

the spectrum of the crystal is somewhat red-shifted relative to those of the powders. The maximum wavelength of the excitation spectrum (λex) is slightly longer for the crystal than for the powders.

The weak shoulders around 425 nm in the spectra of the powders are

broadened and not clearly seen in the spectrum of the crystal.

This suggests that the degree

of structural order is possibly higher in crystals than in powders, although the molecular arrangements are fundamentally the same for the crystal and powders as shown by XRD. Similarly, in a thiophene-benzene-thiophene oligomer, the absorption spectrum of the solution-grown large crystal is red-shifted with respect to that of the spin-coated polycrystalline film.12 The redshift in the crystal is explained by a slight change in the dielectric environment of the molecules.

The observation of more dense packing in the

crystal indicates increased nonresonant dispersive interactions between the molecules, which give rise to a spectral redshift relative to that in film aggregates with less dense packing. We observe weak vibrational structures in the fluorescence spectra for the crystal, powder (gr), and powder (ev).

However, the shoulders around 440 nm in the spectra of the

powders are absent in the spectrum of the crystal.

This probably results from the fact that

the blue-edge region of the spectrum is reduced in intensity due to the reabsorption effect in crystals.11,75

As the reabsorption is expected to be more efficient in larger crystals,75 the

observation that the 440 nm-shoulder is more evident for powder (gr) than for powder (ev) probably indicates that the crystal size is larger for powder (ev) than for powder (gr). maximum wavelength of the fluorescence spectrum (λf) is slightly longer and the spectral


The


Page 22 of 52

22

Table 2. Photophysical Data for the Crystal, Powders, and Thin Films of 3T. crystal

powder (gr)

powder (ev)

film (qu)

film (gl)

410,

425, 404,

426, 404,

399, 374,

401, 377,

375

375

376

353

357

467,

440, 463,

441, 464,

447, 481,

450, 480,

496

496

496

518

520

φf c

0.18

0.23

0.27

0.13

0.17

〈τs〉 (ns)d

1.70

1.40

1.68

1.68

2.06

kf (s-1)e

1.1 x 108

1.6 x 108

1.6 x 108

7.7 x 107

8.3 x 107

knr (s-1)f

4.8 x 108

5.5 x 108

4.3 x 108

5.2 x 108

4.0 x 108

λexa (nm)

λf b (nm)

a

Fluorescence excitation maxima.

Emission (monitor) wavelength: Em = 490 nm (for

crystal and powders) and 480 nm (for films). wavelength: Ex = 375 nm. mean lifetimes.

c

b

Fluorescence emission maxima.

Fluorescence quantum yields. ±10%.

Excitation wavelength: Ex = 405 nm.

Em = the underlined λf.

e

d

Excitation

Intensity weighted

Emission (monitor) wavelength:

Radiative rate constants: kf = φf/〈τs〉.

knr = (1–φf)/〈τs〉.


f

Nonradiative rate constants:

Page 23 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23

Figure 6.

Fluorescence emission and excitation spectra of 3T.

(gr) (blue), and powder (ev) (red). Excitation wavelength: Ex = 375 nm.

(a) Crystal (black), powder

(b) film (qu) (solid line) and film (gl) (dotted line). Emission wavelength: Em = (a) 490 nm and (b) 480

nm.

red-region is somewhat higher in intensity for the crystal than for the powders (Table 2 and Figure 6(a)).

Although this is probably an apparent spectral shift due to reabsorption, it may

also be possible that the fluorescence redshift corresponds to the absorption redshift in crystals with respect to those in powders.

For the thiophene-benzene-thiophene oligomer

mentioned above, the decrease in fluorescence intensity of the blue-region and the total



Page 24 of 52

24

redshift of the spectrum of the crystal relative to that of the spin-coated film have been reported.12

In this case, the decreased intensity of the spectral blue-region for the crystal is

attributed to an improved structural order of molecules in crystals with respect to the aggregates in films.

It is considered that the higher structural order gives rise to stronger

interchain interactions and to a different dielectric environment in the crystal, which results in the spectral redshift in the crystal with respect to that in the film.

As both the absorption and

fluorescence spectra are similar for the crystal and powders in 3T, the Stokes shifts (∆Ess) calculated from λex and λf are similar for all. The results suggest that the magnitudes of intermolecular excitonic interaction are not very different in these solids.23 This is consistent with the powder XRD data, which show that the molecular arrangements in the powders are basically the same as that in the crystal. The fluorescence and excitation spectra are similar in shape (Figure 6(b)) and the positions of the peaks and shoulders are almost the same in the spectra of the film (qu) and film (gl) (Table 2).

Considering the thicknesses of 200-300 nm, the reabsorption effect

would be negligible on the fluorescence spectra for the films.

Although the molecular

orientations to the substrates are shown by XRD to be clearly different for the film (qu) (edge-on) and the film (gl) (face-on), their spectral features are similar. This suggests that the pattern of molecular arrangement and therefore the magnitude of excitonic interaction are not very different in these films. Interestingly, the absorption spectra of the films are blue-shifted whereas the fluorescence spectra are red-shifted from those of the crystal and powders (Figure 6).

As a

result of this, absorption-fluorescence overlaps are smaller and ∆Ess are larger for the films than for the crystal and powders. We therefore consider that the magnitude of H-type excitonic interaction is larger in films than in crystals and powders.9,76

It is possible that the

molecular arrangements in the films can be somewhat different from those in the crystal and powders. In 3T, φf are in the range of 0.1-0.3 (Table 2).


The values are relatively high

Page 25 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25

compared with those of DPHs, which are less than 0.1 in most cases.22,23

The low φf for the

crystals of DPHs are at least partially due to the efficient singlet fission in the solids.77-79 Although the errors in the solid-state φf measurements were relatively large, the maximum values (0.23-0.27) were obtained for the powders in 3T.

The higher φf in powders than in

crystals would be due to the less efficient reabsorption in powders.

The increase in φf by

crystal grinding is similarly observed in p-terphenyl75 and a phenylene-thiophene co-oligomer.11

The fluorescence decay curves are multi-exponential for all samples,

suggesting various deactivation routes in these solids. As Em goes to longer wavelengths, the intensity-weighted mean lifetime (〈τs〉) becomes longer for all the samples examined (Table S2 in SI).

The formation of excimeric species and/or energy migration to structural defects

will be one of the possible deactivation routes.

Although 〈τs〉 are also shown to be dependent

on the crystal size in phenylene vinylene oligomers,80 the values are not greatly different in crystals and powders in 3T.

As a result of smaller φf and longer 〈τs〉, the fluorescence rate

constants (kf = φf/〈τs〉) are calculated to be somewhat smaller in films than in crystals and powders (Table 2).

Considering the optically forbidden nature of the fluorescence transition

in H-type aggregates, the smaller kf for the films are consistent with the observation of blueand red-shifts in the absorption and fluorescence spectra for the films from those for the crystal and powders.

On the other hand, the rate constants for the nonradiative decay [knr =

(1–φf)/〈τs〉] are not very different for all solids examined.

2.2.

2T.

Table 3 summarizes the photophysical data of 2T in the solid state.

Figure 7(a) shows the

absorption and fluorescence spectra for the crystal and powders of 2T.

Although the

fluorescence excitation spectra are fundamentally the same as the absorption spectra, they are very noisy probably due to very small φf (Table 3).

In all cases, no Ex-dependence of the

fluorescence spectrum or Em-dependence of the excitation spectrum is observed.

Both the

absorption and fluorescence spectra are similar in shape, and the wavelength of the absorption



Page 26 of 52

26

Table 3. Photophysical Data for the Crystal, Powders, and Thin Films of 2T.

a

crystal

powder (gr)

powder (ev)

film (qu)

film (gl)

λa a/λexb (nm)

450

454

451

441

440

λf c (nm)

496, 520

492, 522

496, 522

490, 518

489, 518

φf d

0.008

0.012

0.011

0.008

0.008

〈τs〉 (ns)e

0.47

0.37

0.48

0.37

0.33

kf (s-1)f

1.7 x 107

3.2 x 107

2.3 x 107

2.2 x 107

2.4 x 107

knr (s-1)g

2.1 x 109

2.7 x 109

2.1 x 109

2.7 x 109

3.0 x 109

Absorption maxima (for crystal and powders).

b

Fluorescence excitation maxima (for films).

Emission (monitor) wavelength: Em = 520 nm. Excitation wavelength: Ex = 405 nm. weighted mean lifetimes.

d

c

Fluorescence emission maxima.

Fluorescence quantum yields. ±10%.


wavelength: Em = the underlined λf.

f


Intensity

Emission (monitor)


rate constants: knr = (1–φf)/〈τs〉.

e

g

Nonradiative

Page 27 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27

Figure 7.

Absorption, fluorescence emission and excitation spectra of 2T.

(a) Crystal

(black), powder (gr) (blue), and powder (ev) (red).

(b) film (qu) (solid line) and film (gl)

(dotted line).

Emission wavelength: Em = 520 nm.


maximum (λa) and λf are only slightly different for the crystal and powders (Table 3). The relative intensity of the fluorescence peak around 495 nm to that of the peak around 520 nm is smaller in the crystal than in the powders.

The result would be due to the decreased intensity

of the 495 nm-band due to reabsorption, as in the case of the 440 nm-band in 3T.

The

relative intensity of 495 nm-band to that of 520 nm-band is the largest for the powder (gr) and the smallest for the crystal, probably indicating that the crystal size is also the largest for the



Page 28 of 52

28

crystal and the smallest for the powder (gr).

The trend is similarly seen in 3T.

Although

the XRD data show that the degree of structure order is lower in powders than in crystals, the similar spectral data suggest that the molecular arrangements and excitonic interactions are similar in these solids. As for the films, the XRD data give almost no structural information; however, from the similar excitation and fluorescence spectra (Figure 7(b)), we can expect that the molecular arrangements would not be greatly different in the film (qu) and film (gl). In contrast to the case of 3T, both λex (λa) and λf for the films are slightly blue-shifted from those for the crystal and powders in 2T (Figure 7).

This can be understood

by assuming lower degree of structural order (looser packing) and weaker intermolecular (excitonic) interaction in the films.11,12 In 2T, φf are around 0.01 and similarly low for all the samples examined, although the values for the powders are measured to be slightly higher than those for the crystal and films, as in 3T (Table 3). The fluorescence decay curves are multi-exponential in all cases. The values of 〈τs〉 are considerably short and strongly Em-dependent (Table S3 in SI). Although the time-resolution of our TCSPC apparatus is rather limited (about 100 ps), we can see that 〈τs〉 becomes longer as Em goes to longer wavelengths. In consistent with the small spectral differences, kf and knr are calculated to be similar for all solids.

CONCLUSIONS In this study, we have investigated the structures and fluorescence properties for the crystals, powders,

and

thin

films

of

3T

and

2T,

the

two

positional

1,6-dithienylhexa-1,3,5-triene with respect to the S atom of the ring.

isomers

of

Combination of

experimental single crystal X-ray analysis and theoretical RDG−AIM analysis indicate the importance of CH/π and S/S interactions in 3T and CH/π and S/π interactions in 2T on their structure constructions. Although the molecules are arranged similarly in a herringbone fashion in both of the crystals, the total intermolecular interactions are considered to be


Page 29 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29

significantly stronger in 3T than in 2T due to the S-positional isomerism.

This can explain

the XRD results which show that the degree of structural order in powders and films is higher in 3T than in 2T. We also find that the molecular orientation in the film (qu) of 3T is the edge-on type whereas that in the film (gl) is the face-on type. molecular orientation is clearly different depending on the substrate.

It is interesting that the The absorption spectra

for the films of 3T are blue-shifted whereas the fluorescence spectra are red-shifted relative to those for the crystal and powders.

The resulting smaller absorption-fluorescence overlaps

and larger ∆Ess for the films suggest that the magnitude of H-type excitonic interaction is larger in the films than in the crystal and powders.

In contrast, both the absorption and

fluorescence spectra for the films of 2T are slightly blue-shifted from those for the crystal and powders.

The maximum φf (0.23-0.27) are observed for the powders in 3T, whereas the

values are similarly low (≤ 0.01) in 2T for all the samples examined.

It is very probable that

the higher degree of structure order in powders and films leads to the observation of larger spectroscopic differences for the crystal, powder, and film samples in 3T than in 2T.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.xxxxxxx. Possible rotamers around the Th-CH single bonds, major intramolecular parameters for the X-ray structures, results of RDG−AIM analysis, fluorescence lifetimes at different Em in the solid state, ORTEP drawings, Ex- and Em-dependences of the fluorescence and emission spectra in solution, incident-angle dependence of the polarized UV-vis absorption spectrum in films, photographs showing contact angles of substrates (PDF). Accession Codes CCDC 1567123 and 1567124 contain the supplementary crystallographic data for this paper.



Page 30 of 52

30

These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

ACKNOWLEDGMENTS We gratefully acknowledge Electronics and Photonics Research Institute and National Institute of Advanced Industrial Science and Technology (AIST) for financial support.

This

study was partially supported by JSPS KAKENHI Grant Number JP16H05371 (Coordination Asymmetry).

We thank Dr. K. Suzuki (Hamamatsu Photonics) for valuable discussions on

the reabsorption effects on the fluorescence spectra and φf of organic crystals, and Dr. F. Sasaki (AIST) for useful suggestions on the molecular orientation in thin films. also due to Mrs. S. Manaka (AIST) for the contact angle measurements.


Thanks are

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31

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(18) Sonoda, Y.; Kawanishi, Y.; Ikeda, T.; Goto, M.; Hayashi, S.; Yoshida, Y.; Tanigaki, N.; Yase, K. Fluorescence Spectra for the Microcrystals and Thin Films of trans,trans,trans-1,6-Diphenyl-1,3,5-hexatrienes. J. Phys. Chem. B 2003, 107, 3376−3383. (19) Sonoda, Y.; Goto, M.; Tsuzuki, S.; Tamaoki, N. Fluorescence Spectroscopic Properties and Crystal Structure of a Series of Donor-Acceptor Diphenylpolyenes. J. Phys. Chem. A 2006, 110, 13379−13387. (20) Sonoda, Y.; Goto, M.; Tsuzuki, S.; Tamaoki, N. Fluorinated Diphenylpolyenes: Crystal Structures and Emission Properties. J. Phys. Chem. A 2007, 111, 13441−13451. (21) Sonoda, Y.; Tsuzuki, S.; Goto, M.; Tohnai, N.; Yoshida, M. Fluorescence Spectroscopic Properties of Nitro-Substituted Diphenylpolyenes: Effects of Intramolecular Planarization and Intermolecular Interactions in Crystals. J. Phys. Chem. A 2010, 114, 172−182. (22) Sonoda, Y.; Goto, M.; Norikane, Y.; Azumi, R. Crystal Structures and Fluorescence Spectroscopic Properties of Cyano-Substituted Diphenylhexatrienes. Cryst. Growth Des. 2014, 14, 4781−4789. (23) Sonoda, Y.; Goto, M.; Matsumoto, Y.; Shimoi, Y.; Sasaki, F.; Furube, A. Halogenated (F, Cl, Br, or I) Diphenylhexatrienes: Crystal Structures, Fluorescence Spectroscopic Properties, and Quantum Chemical Calculations. Cryst. Growth Des. 2016, 16, 4060−4071. (24) Cooper, T. M.; Natarajan, L. V.; Sowards, L. A.; Spangler, C. W. Investigation of Solvatochromism in the Low-Lying Singlet States of Dithienyl Polyenes. Chem. Phys. Lett. 1999, 310, 508−514. (25) Bartocci, G.; Spalletti, A.; Becker, R. S.; Elisei, F.; Floridi, S.; Mazzucato, U. Excited-State Behavior of Some all-trans-α,ω-Dithienylpolyenes. J. Am. Chem. Soc. 1999, 121, 1065−1075. (26) Bartocci, G.; Galiazzo, G.; Ginocchietti, G.; Mazzucato, U.; Spalletti, A. Effect of Thienyl Groups on the Photoisomerization and Rotamerism of Symmetric and Asymmetric Diaryl-Ethenes and Diaryl-Butadienes. Photochem. Photobiol. Sci. 2004, 3, 870−877.



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(27) Mazzucato, U.; Momicchioli, F. Rotational Isomerism in trans-1,2-Diarylethylenes. Chem. Rev. 1991, 91, 1679−1719. (28)

We

reported

a

similar

wavelength-dependent

fluorescence

spectrum

for

(E,E,E)-1,6-di(2-naphthyl)hexa-1,3,5-triene; Sonoda, Y.; Shimoi, Y.; Goto, M.; Tohnai, N.; Kanesato, M. Fluorescence Properties of (E,E,E)-1,6-Di(n-naphthyl)-1,3,5-hexatriene (n = 1, 2): Effects of Internal Rotation. J. Phys. Chem. A 2013, 117, 566−578. (29) Durka, K.; Gontarczyk, K.; Luliński, S.; Serwatowski, J.; Woźniak, K. Isomeric and Isostructural Oligothienylsilanes−Structurally Similar, Physicochemically Different: The Effect of Interplay between C−H···C(π), S···C(π), and Chalcogen S···S Interactions. Cryst. Growth Des. 2016, 16, 4292−4308. (30) Brusso, J. L.; Hirst, O. D.; Dadvand, A.; Ganesan, S.; Cicoira, F.; Robertson, C. M.: Oakley, R. T.; Rosei, F.; Perepichka, D. F. Two-Dimensional Structural Motif in Thienoacene Semiconductors: Synthesis, Structure, and Properties of Tetrathienoanthracene Isomers. Chem. Mater. 2008, 20, 2484–2494. (31) Duan, Y.-A.; Li, H.-B.; Geng, Y.; Wu, Y.; Wang, G.-Y.; Su, Z.-M. Theoretical Studies on the Hole Transport Property of Tetrathienoarene Derivatives: The Influence of the Position of Sulfur Atom, Substituent and π-Conjugated Core. Org. Electron. 2014, 15, 602–613. (32) Yi, W.; Zhao, S.; Sun, H.; Kan, Y.; Shi, J.; Wan, S.; Li, C.; Wang, H. Isomers of Organic Semiconductors Based on Dithienothiophenes: The Effect of Sulphur Atoms Positions on the Intermolecular Interactions and Field-Effect Performances. J. Mater. Chem. C 2015, 3, 10856−10861. (33) Li, J.; Shan, T.; Yao, M.; Gao, Y.; Han, X.; Yang, B.; Lu, P. The Effect of Different Binding Sites on the Optical and Electronic Properties of Tetraphenylethylene-Substituted Thiophene Isomers. J. Mater. Chem. C 2017, 5, 2552−2558. (34) The crystal structure of 3T was newly determined in this study.

Although the

structure of 2T had been already solved at room temperature [Buschmann, J. F.; Ruban, G.


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35

The Crystal Structures of Some Oligomeric Conjugated Dithienylalkapolyenes. Acta Cryst. B 1978, 34, 1923−1927], we performed the single crystal XRD analysis of 2T at –60 oC (213 K) to compare its structure with that of 3T under the same experimental conditions.

Out result

is fundamentally the same as that reported previously. (35) Li, Y.; Josowicz, M.; Tolbert, L. M. Diferrocenyl Molecular Wires. The Role of Heteroatom Linkers. J. Am. Chem. Soc. 2010, 132, 10374–10382. (36) Spangler, C. W.; Liu, P.-K.; Dembek, A. A.; Havelka, K. O. Preparation and Oxidative Doping of α,ω-Dithienyl Polyenes. J. Chem. Soc. Perkin Trans. 1 1991, 799–802. (37) Rigaku Oxford Diffraction (2015), Software CrysAlisPro 1.171.39.5a Rigaku Corporation, Tokyo, Japan. (38) SHELXT Version 2014/5. Sheldrick, G. M. Acta Cryst. A 2014, 70, C1437. (39) Rigaku (2015). CrystalStructure. Versions 4.2. Rigaku Corporation, Tokyo, Japan. (40) SHELXL Version 2014/7. Sheldrick, G.M. Acta Cryst. A 2008, 64, 112–122. (41) Vargas, W. E.; Niklasson, G. A. Applicability Conditions of the Kubelka-Munk Theory. Appl. Opt. 1997, 36, 5580–5586. (42) Johnson, E. R.; Keinan, S.; Mori-Sánchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W. Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498−6506. (43) Bader, R. F. W. Atoms in Molecules A Quantum Theory; Clarendon Press: Oxford, 1995. (44) Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580−592. (45) Visser, G. J.; Heeres, G. J.; Wolters, J.; Vos, A. Disorder in Crystals of the Dithienyls and β-Thiophenic Acid. Acta Cryst. B 1968, 24, 467–473. (46) Costa, J. C. S.; Gomes, L. R.; Santos, L. M. N. B. F.; Low, J. N. 3,3’-Bithiophene. Acta Cryst. E 2010, 66, o916. (47) Hisaki, I.; Osaka, K.; Ikenaka, N.; Saeki, A.; Tohnai, N.; Seki, S.; Miyata, M. Arrangement

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of

π‑Stacked

Columnar


Assemblies

of


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Octadehydrodibenzo[12]annulene: Substituent Effects of Peripheral Thienyl and Phenyl Rings. Cryst. Growth Des. 2016, 16, 714−721. (48) Desiraju, G. R. Hydrogen Bridges in Crystal Engineering: Interactions without Borders. Acc. Chem. Res. 2002, 35, 565−573. (49) Nishio, M. CH/π Hydrogen Bonds in Crystals. CrystEngComm 2004, 6, 130–158. (50) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J. Defining the Hydrogen Bond: An Account (IUPAC Technical Report). Pure Appl. Chem. 2011, 83, 1619–1636. (51) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637–1641. (52) Politzer, P.; Murray, J. S.; Clark, T. Halogen Bonding and Other σ-Hole Interactions: A Perspective. Phys. Chem. Chem. Phys. 2013, 15, 11178−11189. (53) Beno, B. R.; Yeung, K.-S.; Bartberger, M. D.;. Pennington, L. D.; Meanwell, N. A. A Survey of the Role of Noncovalent Sulfur Interactions in Drug Design. J. Med. Chem. 2015, 58, 4383−4438. (54) Pascoe, D. J.; Ling, K. B.; Cockroft, S. L. The Origin of Chalcogen-Bonding Interactions. J. Am. Chem. Soc. 2017, 139, 15160–15167. (55) Novoa, J. J.; Rovira, M. C.; Rovira, C.; Veciana, J.; Tarres, J. C–H···S and S···S: Two Major Forces in Organic Conductors. Adv. Mater. 1995, 7, 233–237. (56) Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Ribas, X.; Tarrés, J.; Mas, M.; Molins, E.; Veciana, J.; Rovira, C. Correlation between Crystal Structure and Mobility in Organic Field-Effect Transistors Based on Single Crystals of Tetrathiafulvalene Derivatives. J. Am. Chem. Soc. 2004, 126, 8546–8553. (57) Kobayashi, K.; Masu, H.; Shuto, A.; Yamaguchi, K. Control of Face-to-Face π-π


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Stacked Packing Arrangement of Anthracene Rings via Chalcogen-Chalcogen Interaction: 9,10-Bis(methylchalcogeno)anthracenes. Chem. Mater. 2005, 17, 6666–6673. (58) Hao, Z.; Wu, X.; Sun, R.; Ma, C.; Zhang, X. Morphology Controlled Nanostructures Self-Assembled from Phthalocyanine Derivatives Bearing Alkylthio Moieties: Effect of Sulfur–Sulfur and Metal–Ligand Coordination on Intermolecular Stacking. ChemPhysChem 2012, 13, 267–273. (59) Bai, M.; Thomas, S. P.; Kottokkaran, R.; Nayak, S. K.; Ramamurthy, P. C.; Guru Row, T. N. A Donor−Acceptor−Donor Structured Organic Conductor with S···S Chalcogen Bonding. Cryst. Growth Des. 2014, 14, 459−466. (60) Chaloner, P. A.; Gunatunga, S. R.; Hitchcock, P. B. Redetermination of 2,2'-Bithiophene. Acta Cryst. C 1994, 50, 1941–1942. (61) Pelletier, M.; Brisse, F. Bithiophene at 133 K. Acta Cryst. C 1994, 50, 1942–1945. (62) Esparza-Ruiz, A.; Peña-Hueso, A.; Hernández-Díaz, J.; Flores-Parra, A.; Contreras, R. Effect of Weak Sulfur Interactions and Hydrogen Bonds in the Folded or Unfolded Conformation of bis[2-(1H-Benzimidazol-2-yl)phenyl]disulfide Derivatives. Cryst. Growth Des. 2007, 7, 2031−2040. (63) Rajagopal, S. K.; Salini, P. S.; Hariharan, M. S···π, π−π, and C−H···π Contacts Regulate Solid State Fluorescence in Regioisomeric Bisthiazolylpyrenes. Cryst. Growth Des. 2016, 16, 4567−4573. (64) DPH crystallizes in two polymorphic (orthorhombic and monoclinic) forms. The structure of 2T more resembles to the monoclinic one of DPH. See: Harada, J.; Harakawa, M.; Ogawa, K. Conformational Change of all-trans-1,6-Diphenyl-1,3,5-hexatriene in Two Crystalline Forms. CrystEngComm 2008, 10, 1777–1781. (65) Kellogg, R. M.; Groen, M. B.; Wynberg, H. Photochemically Induced Cyclization of Some Furyl- and Thienylethenes. J. Org. Chem. 1967, 32, 3093–3100. (66) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de



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Leeuw D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685–688. (67) Osaka, I.; Saito, M.; Koganezawa, T.; Takimiya, K. Thiophene–Thiazolothiazole Copolymers: Significant Impact of Side Chain Composition on Backbone Orientation and Solar Cell Performances. Adv. Mater. 2014, 26, 331–338. (68) Sun, X.; Ren, Z.; Liu, J.; Takahashi, I, Yan, S. Structure Evolution of Poly(3-hexylthiophene) on Si Wafer and Poly(vinylphenol) Sublayer. Langmuir 2014, 30, 7585−7592. (69) Naboka, M.; Soubatch, S.; Nefedov, A.; Tautz, F. S.; Wöll, C. Direct Evidence of the Temperature-Induced Molecular Reorientation in Tetracene Thin Films on AlOx/Ni3Al(111). J. Phys. Chem. C 2014, 118, 22678−22682. (70) Truger, M.; Roscioni, O. M.; Röthel, C.; Kriegner, D.; Simbrunner, C.; Ahmed, R.; Głowacki, E. D.; Simbrunner, J.; Salzmann, I.; Coclite, A. M.; Jones, A. O. F.; Resel, R. Surface-Induced Phase of Tyrian Purple (6,6'-Dibromoindigo): Thin Film Formation and Stability. Cryst. Growth Des. 2016, 16, 3647−3655. (71) Pithan, L.; Cocchi, C.; Zschiesche, H.; Weber, C.; Zykov, A.; Bommel, S.; Leake, S. J.; Schäfer, P.; Draxl, C.; Kowarik, S. Light Controls Polymorphism in Thin Films of Sexithiophene. Cryst. Growth Des. 2015, 15, 1319−1324. (72) Dhagat, P.; Haverinen, H. M.; Kline, R. J.; Jung, Y.; Fischer, D. A.; DeLongchamp, D. M.; Jabbour, G. E. Influence of Dielectric Surface Chemistry on the Microstructure and Carrier Mobility of an n-Type Organic Semiconductor. Adv. Funct. Mater. 2009, 19, 2365–2372. (73) Zhou, Y.; Taima, T.; Miyadera, T.; Yamanari, T.; Kitamura, M.; Nakatsu, K.; Yoshida, Y. Glancing Angle Deposition of Copper Iodide Nanocrystals for Efficient Organic Photovoltaics. Nano Lett. 2012, 12, 4146−4152. (74) Taima, T.; Shahiduzzaman, Md.; Ishizeki, T.; Yamamoto, K.; Karakawa, M.; Kuwabara, T.; Takahashi, K. Sexithiophene-Based Photovoltaic Cells with High Light


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Absorption Coefficient via Crystalline Polymorph Control. J. Phys. Chem. C 2017, 121, 19699−19704. (75) Katoh, R.; Suzuki, K.; Furube, A.; Kotani, M.; Tokumaru, K. Fluorescence Quantum Yield of Aromatic Hydrocarbon Crystals. J. Phys. Chem. C 2009, 113, 2961−2965. (76) Mu, S.; Oniwa, K.; Jin, T.; Asao, N.; Yamashita, M.; Takaishi, S. A Highly Emissive Distyrylthieno[3,2-b]thiophene Based Red Luminescent Organic Single Crystal: Aggregation Induced Emission, Optical Waveguide Edge Emission, and Balanced Ambipolar Carrier Transport. Org. Electron. 2016, 34, 23–27. (77) Dillon, R. J.; Piland, G. B.; Bardeen, C. J. Different Rates of Singlet Fission in Monoclinic versus Orthorhombic Crystal Forms of Diphenylhexatriene. J. Am. Chem. Soc. 2013, 135, 17278−17281. (78) Katoh, R.; Hashimoto, M.: Takahashi, A.; Sonoda, Y.; Yago, T.; Wakasa, M. Singlet Fission in Fluorinated Diphenylhexatrienes. J. Phys. Chem. C 2017, 121, 25666−25671. (79) Wakasa, M.; Yago, T.; Sonoda, Y.; Katoh, R. Structure and Dynamics of Triplet-Exciton Pairs Generated from Singlet Fission Studied via Magnetic Field Effects. Communications Chemistry 2018, 1, Article No.9. (80) Gierschner, J.; Ehni, M.; Egelhaaf, H.-J.; Milián Medina, B.; Beljonne, D.; Benmansour, H.; Bazan, G. C. Solid-State Optical Properties of Linear Polyconjugated Molecules: π-Stack contra Herringbone. J. Chem. Phys. 2005, 123, 144914.



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40

Chart 1.

Chemical structures of 3T and 2T. (E,E,E)

S

S

3T

S

S

2T


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41

Table 1.

Crystal Data of 3T and 2T 3T

2T

formula

C14H12S2

C14H12S2

formula weight crystal color, habit

244.38 yellow, plate

244.38 yellow, needle

crystal size (mm3) crystal system

0.20 x 0.20 x 0.05 monoclinic

0.20 x 0.05 x 0.05 triclinic

space group

P21/c

a (Å)

16.8719(3)

P-1 6.31326(19)

b (Å) c (Å)

7.43294(11) 15.7629(3) 90 111.873(2) 90

9.6669(5) 10.2038(4) 100.178(4) 92.309(3) 95.910(3)

1834.49(6) 4

608.59(4) 2

T (K)

1.327 213

1.333 213

mp (K)

506

491

R1 (I>2σ(I))

0.0978

0.0383

α (degree) β (degree) γ (degree) V (Å3) Z Dcalc (g/cm3)



Page 42 of 52

42

Table 2. Photophysical Data for the Crystal, Powders, and Thin Films of 3T. λexa (nm)

crystal

powder (gr)

powder (ev)

film (qu)

film (gl)

410,

425, 404, 375

426, 404, 376

399, 374, 353

401, 377, 357

496

440, 463, 496

441, 464, 496

447, 481, 518

450, 480, 520

0.18

0.23

0.27

0.13

0.17

375 b

λf (nm) φf

c d

467,

〈τs〉 (ns) kf (s-1)e

1.70

1.40

1.1 x 10

1.6 x 10

knr (s-1)f

4.8 x 108

5.5 x 108

8

1.68 8

1.68 8

2.06

1.6 x 10

7

7.7 x 10

8.3 x 107

4.3 x 108

5.2 x 108

4.0 x 108

a

Fluorescence excitation maxima. Emission (monitor) wavelength: Em = 490 nm (for crystal and powders) and 480 nm (for films). bFluorescence emission maxima. Excitation c wavelength: Ex = 375 nm. Fluorescence quantum yields. ±10%. dIntensity weighted mean lifetimes. Excitation wavelength: Ex = 405 nm. Emission (monitor) wavelength: Em = the underlined λf.

e


knr = (1–φf)/〈τs〉.


f

Nonradiative rate constants:

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43

Table 3. Photophysical Data for the Crystal, Powders, and Thin Films of 2T. λa a/λexb (nm)

crystal

powder (gr)

powder (ev)

film (qu)

film (gl)

450

454

451

441

440

c

496, 520

492, 522

496, 522

490, 518

489, 518

d

0.008

0.012

0.011

0.008

0.008

λf (nm) φf 〈τs〉 (ns)e kf (s-1)f knr (s-1)g

0.47

0.37 7

0.48 7

0.37 7

0.33 7

1.7 x 10

3.2 x 10

2.3 x 10

2.2 x 10

2.4 x 107

2.1 x 109

2.7 x 109

2.1 x 109

2.7 x 109

3.0 x 109

a

Absorption maxima (for crystal and powders). bFluorescence excitation maxima (for films). c Emission (monitor) wavelength: Em = 520 nm. Fluorescence emission maxima. Fluorescence quantum yields. ±10%. eIntensity Excitation wavelength: Ex = 405 nm. Emission (monitor)

Excitation wavelength: Ex = 405 nm. weighted mean lifetimes.

wavelength: Em = the underlined λf.

f

d


rate constants: knr = (1–φf)/〈τs〉.


g

Nonradiative


Page 44 of 52

44

Figure 1. Crystal packing diagrams of 3T. The CH/π and S/S contacts (< sum of van der Waals radii + 0.10 Å) are shown as blue and red dotted lines, respectively. Distances are in Å.


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45

Figure 2. Crystal packing diagrams of 2T. The CH/π, S/π, and CH/CH contacts (< sum of van der Waals radii + 0.10 Å) are shown as blue, red, and green dotted lines, respectively. Distances are in Å.



Page 46 of 52

46

Figure 3.



CuKα λ = 1.542 Å.


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47

Figure 4.



CuKα λ = 1.542 Å.



Page 48 of 52

48

Figure 5.

AFM images for the thin films of 3T on (a) quartz and (b) glass substrates, and

2T on (c) quartz and (d) glass substrates.

Image area = 10 x 10 µm2


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49

Figure 6. Fluorescence emission and excitation spectra of 3T. (a) Crystal (black), powder (gr) (blue), and powder (ev) (red). (b) film (qu) (solid line) and film (gl) (dotted line). Excitation wavelength: Ex = 375 nm. nm.

Emission wavelength: Em = (a) 490 nm and (b) 480



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50

Figure 7. Absorption, fluorescence emission and excitation spectra of 2T. (a) Crystal (black), powder (gr) (blue), and powder (ev) (red). (b) film (qu) (solid line) and film (gl) (dotted line).


Emission wavelength: Em = 520 nm.


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51

For Table of Contents Use Only Structures and Fluorescence Properties for the Crystals, Powders, and Thin Films of Dithienylhexatrienes: Effects of Positional Isomerism Yoriko Sonoda,* Norimitsu Tohnai, Ying Zhou, Yukihiro Shimoi, and Reiko Azumi

The structures and fluorescence properties were investigated for the crystals, powders, and thin films of the two S-positional isomers of dithienylpolyene, (E,E,E)-1,6-di(3-thienyl)hexa-1,3,5-triene (3T) and 1,6-di(2-thienyl)hexa-1,3,5-triene (2T). The effects of positional isomerism on the pattern of intermolecular interaction, the degree of structural order, and the spectroscopic differences in these solids are discussed.


Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

3T

Page 52 of 52

CH/ S/S

2T

CH/ S/ CH/CH


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