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On the Way to Highly Emissive Materials: Increasing of Rigidity by Introduction of Furan Moiety in Co-Oligomers Igor P. Koskin, Evgeny A. Mostovich, Enrico Benassi, and Maxim S. Kazantsev J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08305 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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The Journal of Physical Chemistry

On the Way to Highly Emissive Materials: Increasing of Rigidity by Introduction of Furan Moiety in Co-Oligomers Igor P. Koskin,a,b Evgeny A. Mostovich,a,b Enrico Benassi*b,c and Maxim S. Kazantsev*a,b a

N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Lavrentieva 9, Novosibirsk, 630090, Russian Federation b

c

Novosibirsk State University, Pirogova 2, Novosibirsk, 630090, Russian Federation

School of Science and Technology, Nazarbayev University, 53 Kabanbay Batyr Ave, Astana, Republic of Kazakhstan, 010000

ABSTRACT

Rigid linear organic co-oligomers are prospective materials for organic optoelectronics. In this work, we explored intramolecular factors affecting the torsional rigidity and its influence on optoelectronic properties of the alternating furan/phenylene and thiophene/phenylene cooligomers in both ground and first singlet excited states. Furan/phenylene co-oligomer exhibits almost twice higher torsional rigidity than its thiophene analog. Effect of intramolecular O…H and S…H interactions on torsional barriers was found to be negligible as compared with the

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conjugation efficiency. The higher torsional rigidity of furan and thiophene co-oligomers has been proven to be reflected in the fine-structure of the UV-Vis absorption spectrum of the former. The increase of furan co-oligomer rigidity as compared with its thiophene analog lowers reorganization energy for hole, electron and exciton transfer. Remarkably the substitution of thiophene by furan almost 20 times lowers reorganization energy for exciton transfer. A noteworthy finding was also that in furan co-oligomer the higher rigidity was suggested to hinder “in molecule” photoluminescence quenching due to a possible conical intersection between bright state S1 and T3 excited state. Therefore, tuning of torsional rigidity impacts on emission and charge transport properties, being a very powerful tool on the way to high performance emissive organic semiconductors.

1. Introduction Linear conjugated

small

molecules

combining high

luminescence efficiency and

semiconducting properties are of great interest for organic optoelectronics:1–4 their single crystals are prospective materials for the light-emitting diodes,5–7 transistors,8,9 optically and possibly electrically-driven organic lasers.10–12 Generally, planar and rigid molecules (e.g., acenes13 or heteroacenes)14 demonstrate excellent charge mobility15,16 due to rigidity of their backbones coupled with close crystal packing leading to low reorganization energy and high transfer integrals.15 However, close crystal packing usually causes strong aggregation of transition dipole moments and hence photoluminescence (PL) quenching.17 Nevertheless, rod-like conjugated cooligomers based on alternating thiophene or vinylene and phenylene units were reported to break this trend: their single crystals demonstrated remarkable charge mobility and high PL quantum yields (QY).3,8,11,18 Moreover, easy synthetic tunability of small fluorophores allows controlling their optoelectronic and physical properties.1,3,19 For example, substitution of thiophene unit by


2

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furan was reported to increase the PL QY20 and solubility,21–23 which is highly favorable for solution-processed technologies. We have recently reported semiconducting and highly emissive furan/phenylene co-oligomer, 1,4-bis(5-phenylfuran-2-yl)benzene (BPFB),24 which exhibits higher PL QY both in solution and single crystal as compared with its thiophene analog, 1,4-bis(5-phenylthiophene-2-yl)benzene (BPTB, also reported as AC54; Chart 1). However, despite the higher performance of BPFB, intramolecular factors governing difference in optoelectronic properties between these systems (for instance torsional rigidity and torsional barriers) still remain poorly understood.24 Indeed, it was previously shown that conjugated backbone becomes more planar upon furan incorporation with respect to thiophene analogs for a few systems (bifurans,25 oligofurans26 and benzofurothiazoles27; no investigation was yet carried out for furan/phenylene co-oligomers) but connection between molecular planarity and torsional rigidity remains elusive since, generally speaking, planarity does not necessary mean rigidity and vice versa.30 Moreover, dispersion interactions affecting planarity of conjugated systems were suggested for a number of molecules (viz.,

substituted

bithiophenes,28

ethylenedioxythiophene-containing

molecules,29

2,2`-

bipyridines,30 and triphenylamines with different π-linkers).31 However, the exact nature of these phenomena, as well as the influence of dispersion interactions on torsional rigidity, still remain unexplored.


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Chart 1. Chemical structure of 1,4-bis(5-phenylfuran-2-yl)benzene (BPFB, X = O) and 1,4bis(5-phenylthiophene-2-yl)benzene (BPTB, X = S). Labels of dihedral angles (φ1, φ2), bond lengths (b1, b2) and hydrogen atoms (H1 – H6) of particular interest in this study are also depicted.

In the present work, we investigated the nature of the torsional rigidity and the impact of the rigidity on electronic properties for the alternating furan/ and thiophene/phenylene co-oligomers (Chart 1). BPFB shows almost twice higher torsional barriers as compared with BPTB. Furthermore, the Reduced Density Gradient (RDG) coupled with Non-Covalent Interaction Index (NCI) analyses32 proved that neither BPFB nor BPTB exhibit any specific dispersion interaction that can contribute to the torsional barriers. The higher torsional rigidity of BPFB as compared with BPTB mainly stems from the conjugation efficiency. The torsional rigidity of BPFB was demonstrated to be the main origin of the fine-structure of the S0 → S1 band of the UV-Vis absorption spectrum. Higher rigidity was also suggested to hinder intersystem crossing through the conical intersection of S1 and T3 excited states. Finally, we demonstrated that the higher the torsional rigidity the lower the reorganization energy for a charge as well as for an exciton transfers. The latter process was suggested to be significantly facilitated for the furan/phenylene co-oligomer as compared with the thiophene analog. 2. Computational details All calculations were performed using GAUSSIAN G09.D01 package.33 The post-processing electronic density topological analysis and the calculation of the RDG and electronic density Hessian calculation were performed using MultiWFN 3.3.9 package.34 Intermolecular interaction


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energies in BPFB and BPTB crystals were calculated using Crystal Explorer 17.35 Visualization was performed using GaussView (v. 5.0.8).36 Normal modes frequencies and displacements of S0 and S1r excited state were calculated employing homemade codes. For additional computational details see Supporting Information (SI). 2.1. Ground state geometry optimization and rigidity investigation The ground state geometry in the gas phase was fully optimized at Density Functional Theory (DFT), using B3LYP37 hybrid functional, M06-2X38 exchange-correlation functional and ωB97-X[D]39 long-range functional with empirical dispersion corrections coupled with 6-31+G*40, 6-311++G**41,42 and Def2-TZVP43,44 basis sets. These levels of theory were chosen since they are acknowledged amongst the DFT methods as “golden standards” for geometry optimization in terms of cost and performance.45-47 The D3 version of Grimme’s semi-empirical dispersion with Becke-Johnson damping GD3BJ48 was included in case of B3LYP to correct its underestimation of dispersion interactions. PES was explored via a relaxed scanning around the φ1 and φ2 dihedral angles (Chart 1) from the initial equilibrium geometry with eighteen 10-degree steps, employing B3LYP[GD3BJ], M06-2X, ω-B97-X[D] functionals coupled with 6-311++G** and Def2-TZVP basis sets. Molecular structures for both singly charged cations and anions were fully optimized at DFT UB3LYP[GD3BJ]/6-311++G** level of theory. PES of both singly charged cations and anions were explored via a relaxed scanning around the φ1 and φ2 dihedral angles (Chart 1) with eighteen 10-degree steps, employing UB3LYP[GD3BJ]/6-311++G** level of theory.


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2.2. ONIOM optimization The geometry of the central molecule (high-level layer) was taken from X-ray data (BPFB24, BPTB)49 and then replicated by translation and symmetry operations along three dimensions to form 1-layered box around it (middle-level layer). These operations were repeated for building the outer low-level layer (Fig. 1).

Figure 1. Schematic representation of layers treated at different levels of theory in ONIOM optimizations. Red, blue and green colors represent high-, middle- and low-level layers respectively. The high-level layer was described at DFT M06-2X/6-31+G*, the middle-level layer at semiempirical PM650 and the outer layer, consisting of constrained molecules, at molecular mechanics UFF51 levels of theory. The resulting structure was optimized via a three-layer ONIOM model.52,53 2.3. Excited states investigation To explore vertical excited states, UV-Vis absorption spectra were calculated at TimeDependent Density Functional Theory (TD-DFT). The nature of the vertical excited electronic state was analyzed. This exploration was performed employing two different hybrid functionals (B3LYP, PBE0)54 and their long-range corrected versions (CAM-B3LYP,55 LC-ωPBE)56 which are known to show good agreement with the experimental data in case of absence (the former) or


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presence of charge separation (the latter).57 The aforementioned functionals were coupled with the 6-31+G*, 6-311++G**, and Def2-TZVP basis sets. S0 → Sn (n = 1 to 10) transitions were calculated; the energy and the nature of the first 10 triplet states were also computed. Performance of TD-DFT combined with Tamm-Dankoff approximation (TDA) at B3LYP/6311++G** and PBE0/6-311++G** levels of theory was also tested. The molecular geometry of S1r state was fully optimized at TD-DFT B3LYP[GD3BJ]/6311++G** and TD-DFT/PBE0/6-311++G** levels. Transition energies and oscillator strengths of the first 5 singlet excited states and energies of the first 5 triplet states were evaluated at TD(A)-DFT, employing the PBE0 and B3LYP functionals combined with 6-311++G** basis set. The vibronic progression of the S0→S1 electronic transition was simulated at B3LYP[GD3BJ]/6311++G** level of theory including Duschinsky and Herzberg-Teller effects in the gas phase.5860

Additionally transition energies of the first 5 singlet and triplet excited states and energies were

evaluated along the relaxed PES of φ1 and φ2 dihedral angles with nine 10-degree steps at TDDFT level, employing the PBE0 and B3LYP functionals combined with 6-311++G** basis set. 2.4. Dispersion interactions Two approaches were used to evaluate regions of possible dispersion interactions for both S0 and S1r states: the topological analysis based on Bader’s AIM theory61 and the NCI index (s) along with RDG32 (Equation 1). Hessian of electron density was additionally calculated along RDG isosurface (|isovalue| = 0.5 a.u.).

= ( ) / |∇|// ,

(1)

where is the electron density.


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2.5. Reorganization energy Two types of reorganization energy were calculated, viz., involved in charge mobility and exciton transfer processes. Reorganization energy for charge transfer was calculated from potential energy surfaces62 (Equations 2 and 3). Electronic energy calculations were made at DFT (U)B3LYP[GD3BJ]/6-311++G** level of theory. " " λ = () − () # + %&'() − %&'() #

(2)

" , , " λ()*+ = () − () # + &'() − &'() #

(3)

± - electronic energy of cation/anion at equilibrium geometry of the neutral where () " form; () - electronic energy of the equilibrium geometry of the neutral form; ± %&'/&'() - electronic energy of the equilibrium geometry of the cationic/anionic form; " %&'/&'() - electronic energy of neutral molecule at the equilibrium geometry of the

cationic/anionic form. Reorganization energy for exciton transfer (λexc) was calculated from the displacements along normal coordinates according to equations (4) and (5). ./ = 122 3/ 4/

(4)

.5( = ∑ ./

(5)

where λi indicates contribution of i-th normal mode to overall reorganization energy, νi is the i-th normal mode frequency, and ∆i represents the dimensionless displacement. Required normal modes frequencies and displacements of S0 and S1r excited states were calculated at TD-DFT B3LYP[GD3BJ]/6-311++G** level of theory.


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3. Results and discussion 3.1. Ground State 3.1.1. Geometry optimization The geometries of the studied molecules were optimized at the DFT level (see SI, Table S1 and S2 for additional levels of theory) and found to be almost planar for BPFB (Fig. 2a) and twisted for BPTB (Fig. 2b). This could be a result of sterical repulsion between the sulfur atom and the H1 hydrogen atom of phenyl ring (Chart 1). BPFB exhibits smaller dihedral angles in the gas phase probably due to the absence of such repulsion and higher torsional barriers for the rotation around the corresponding C-C bonds (vide infra). The effect of crystal surroundings commonly causes sufficiently strong intermolecular interactions capable to overcome torsional barriers. This is the reason for the poor correspondence between computed torsional angles and X-Ray data (see Fig. S1 and Fig. S2 for interaction energies in BPFB and BPTB crystals). To verify the influence of crystal surroundings on the molecular geometries, three-layer ONIOM optimizations were performed. As a result, planar geometry of central BPFB molecule distorts (Fig. 2a), showing dihedral angles to be very close to the X-ray data.18 BPTB demonstrates (Fig. 2b) a tendency to planarization under influence of crystal environment with the dihedral angles φ1 and φ2 equal to 22.5 and 22.6 degrees, respectively (φ1 = 30.0 degrees, φ2 = 28.0 degrees at the same level of theory but without crystal environment). Although ONIOM optimized geometries are not in a perfect match with the X-Ray data, we may outline the trend: BPFB crystal surroundings distort the molecular geometry, whereas in the case of BPTB it tends to get planar. The molecular geometries of single charged anionic and cationic forms of BPFB and BPTB were also optimized (Table S3 and S4) at the DFT level. BPFB retains planar geometry in both


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forms (Fig. 2c), whereas only anionic form of BPTB is almost planar and cationic form of BPTB (Fig. 2d) shows deviation from planarity.

Figure 2. Molecular geometries of neutral and charged forms of BPFB (a, c) and BPTB (b, d) respectively. Values of dihedral angles (φ1 and φ2) and bonds length (b1 and b2) are shown for (a), (b): gas phase optimized geometry (bold), Optimized geometry of high-level layer of ONIOM calculations (Italics), and X-Ray analysis (plane style)18,45 and for (c),(d): cationic state (Italics) and anionic state (plain style). Level of theory: (U)B3LYP[GD3BJ]/6-311++G** for the gas phase optimization and M06-2X/6-31+G* for ONIOM high-level molecule. 3.1.2. Torsional rigidity investigation We quantify internal molecular torsional rigidity as the electronic energy barrier of conjugated molecule subjected to (forced) rotation around an interring bond. To compare the torsional rigidity of BPFB and BPTB, we performed ground state PES analyses of BPFB and BPTB as a function of dihedral angles.


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Torsional Energy (kJ/mol)

a) 24

BPFB BPTB

20

0.025 0.020

16 0.015

12

0.010

8

∆b1 (Å)

0.005

4

0.000

0 0

30

60

90

120

150

180

ϕ1 (degs)

Torsional Energy (kJ/mol)

b) 60

BPFB BPTB

50

0.04 0.03

40

0.02

30 20

∆b1 (Å)

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


0.01

10 0.00 0 0

30

60

90

120

150

180

ϕ1 (degs)

Figure 3. Relaxed scans of the ground state PES of neutral (a) and cationic (b) BPFB (red circles) and BPTB (blue squares) forms upon rotation around φ1 dihedral angle. Changes in b1 bond length of BPFB (red dashed line) and BPTB (blue dashed line) due to rotation around φ1 dihedral angles are also depicted. Level of theory: B3LYP[GD3BJ]/6-311++G** (see Table S5 for additional levels of theory). BPFB PES scans for dihedral angles φ1 (Fig. 3a) and φ2 (Fig. S3a) show the same trend with two energy minima and one energy maximum whereas BPTB φ1 and φ2 PES both have two energy minima and maxima. Overall BPFB exhibits almost twice higher torsional barrier than BPTB for both dihedral angles (Table 1). Note that the torsional freedom of BPFB is comparable with that previously reported for α,α`-oligofurans26 and twice lower than that for BPTB which implies thiophene co-oligomer higher flexibility around ground state geometry. Rotation around


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dihedral angles accompanied by the increase of the corresponding bonds length and hence the loss of conjugation. The higher bonds length changes upon rotation for BPFB as compared with BPTB are a sign of more efficient conjugation and higher contribution of conjugation breaking to the torsional barrier. Therefore, not only the oligofurans22,23 possess the higher torsional rigidity as compared with oligothiophenes but also the introduction of furan moiety increases cooligomer torsional rigidity. Table 1. Stationary points, values of torsional barriers and torsional freedom (at the room temperature) for BPFB and BPTB PES of φ1 and φ2 dihedral angles. Level of theory: B3LYP[GD3BJ]/6-311++G**

cation

neutral

BPFB

anion

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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BPTB

φ1

φ2

φ1

φ2

Stationary points (max) / degs

90

90

0; 90; 180

0; 90; 180

Stationary points (min) / degs Energy of torsional barrier / kJmol-1 Torsional freedom / degs

0; 180

0; 180

30; 150

30; 150

20

23

2; 10

1; 12

25

25

50

50


90

90

0; 90; 180

90


0; 180

0; 180

20; 160

0; 180

30

60

0.5; 22

50

20

10

35

15


90

90

90

90


0; 180

0; 180

0; 180

0; 180

42

50

31

42

25

20

25

25

In order to obtain deeper insight on the impact of torsional rigidity on the charge transport properties (vide postea Par. 3.3), we explored PES of singly charged cationic (Fig. 3b and Fig. S3b) and anionic forms (Fig S3c and Fig. S3d) of BPFB and BPTB as a function of dihedral


12

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angles. Overall BPFB exhibits higher torsional barrier than BPTB in both dihedral angles of cationic and anionic forms (Table 1). As in the case of neutral species, the torsional freedom is stiffer for BPFB as compared with BPTB. However, exploration of PES as a function of torsional angle does not allow identifying the nature of the different PES shapes and torsional barriers. It is heuristically reasonable to propose two main factors as responsible for torsional rigidity: conjugation breaking of the heterocyclephenyl bond or the intramolecular dispersion interactions (Chart 2).22,29 Chart 2. Possible intramolecular interactions in BPFB and BPTB molecules. The numeration is assigned to the only hydrogen atoms that may participate in dispersion interactions between conjugated rings.

To evaluate the relative efficiency of conjugation in the investigated co-oligomers, we compared their overall Bond Length Alternation (BLA); bond length and Wiberg’s bond order between conjugated heterocycle and phenyl rings (Table 2). Smaller BLA and shorter b1 and b2 bonds length of BPFB as compared with BPTB mean that furan-containing co-oligomer exhibits more efficient conjugation in the neutral form. Additionally, BLA of charged forms of both BPFB and BPTB is lower than BLA of neutral forms, which is a sign of more efficient conjugation in charged forms as compared with neutral ones.


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Table 2. Values of BLA (at optimized geometry and in the PES maxima), b1 and b2 bond lengths and Wilberg’s Bond Orders (WBO). Level of theory: B3LYP[GD3BJ]/6-311++G**. BPFB State Neutral Cationic Anionic

State Neutral Cationic Anionic

Optimized 0.064 0.038 0.041

BLA / Å φ1 = 90 degs 0.092 0.077 0.078

Optimized 0.076 0.042 0.041

BLA / Å φ1 = 90 degs 0.095 0.074 0.075

φ2 = 90 degs 0.088 0.080 0.082 BPTB φ2 = 90 degs 0.093 0.076 0.078

Bond length / Å b1 b2 1.455 1.452 1.441 1.421 1.434 1.424 Bond length / Å b1 b2 1.461 1.463 1.451 1.431 1.444 1.432

WBO b1 1.0975 1.2107 1.1642

b2 1.1071 1.1472 1.2019

WBO b1 1.0872 1.2183 1.1640

b2 1.0959 1.1467 1.2085

Concerning the intramolecular interactions, one may suggest two types of possible dispersion interactions hypothetically present in the investigated co-oligomers (Chart 2): attraction between H1 or H5 and heteroatom X, and repulsion between H2 and H3; H4 and H6. To investigate whether they exist or not we applied the AIM theory, which correlates electron density critical points (CPs) and interactions between atoms.63 AIM theory applied to ground states of BPFB and BPTB demonstrated the absence of any CPs in the regions of possible hydrogen bonds (Fig. 4a and 4c). However, a forced planarization of phenyl and thiophene rings (which is accessible via thermal vibration of these conjugated cycles) leads to a formation of two additional CPs: bond CP and ring CP (Fig. 4e) between H2 and H3 rather than expected bonding between H1…S.


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Figure 4. AIM and RDG analyses, based on the ground state electron density. Critical points (CPs) of BPFB (a), BPTB (b) and BPTB with planarized phenyl and thiophen cycles (e). Plots of the NCI isosurfaces (s = 0.5 a.u. and a red-to-blue color scale from -0.015 a.u. < sign(λ2) ρ(r) < +0.015 a.u.) calculated for BPFB (b) and BPTB (d) optimized geometries and for BPTB geometry with planarized phenyl and thiophene cycles (f). Blue color stands for repulsion, yellow/orange for attraction. However, AIM theory was shown to be possibly misleading in some cases, especially when weak repulsion occurs.64 It is also impossible to investigate repulsion regions employing only


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AIM theory. To further explore possible dispersion interactions and their nature, NCI index (s) and RDG were analyzed.32 RDG analysis of both BPFB and BPTB ground state optimized geometry shows the presence of dispersion interaction regions between H1…X (where X = O, S) and between H2 and H3 (Fig. 4b, 4d and 4f). Nevertheless, isosurfaces between H1…X and between H2…H3 depict a combination of bonding and antibonding. Bonding interactions between these pairs of atoms are, therefore, counterbalanced by repulsion due to sterical hindrance. The attraction between H1…X or between H2…H3 does not benefit ground state torsional rigidity and higher rigidity of BPFB. Taking into account BLA analysis, the higher torsional rigidity of furan/phenylene co-oligomer as compared with its thiophene analog mainly stems from the more efficient conjugation. 3.2. Electronic excited states We used PBE0 functional coupled with 6-311++G** basis set and Tamm-Dancoff approximation for analysis of vertical excitations as it shows the best agreement with experimental data (see Supplementary Note 1 and Table S6). Analysis of vertical excitations from the ground state to singlet excited states shows that S0 → S1 transition has the largest oscillator strength and, subsequently, intensity and probability of population (Table S8). This absorption band was found to be mainly HOMO→LUMO (Fig. S4). Concerning the fluorescence, we focused our attention on investigation of S1 because it is the emissive excited state and thus its torsional rigidity may also play a role in thermal PL quenching.65 3.2.1. S1 state optimization and rigidity investigation Planar structures were found for both BPFB and BPTB in relaxed S1 states (Fig. 5). The πsystems of the molecular backbone of both co-oligomers were found to adopt a quinoid structure.


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The shorter bond lengths and smaller dihedral angles as compared with that at relaxed S0 state are the signs of more efficient conjugation in the the S1r relaxed state.

Figure 5. S1 State optimized geometries of BPFB (a) and BPTB (b). Level of theory: TD-DFT PBE0/6-311++G** (see Fig. S4 for additional level of theory). In order to study the influence of torsional rigidity on the intersystem crossing, we performed a PES scanning around dihedral angles for S1r state of both BPFB and BPTB (Fig. S3e and S3f). Two stationary points observed on the S1r torsional PES for φ1 and φ2 with BPFB having ~1.5 and ~1.2 times higher torsional barriers respectively as compared with BPTB (Table 4). The difference between torsional barriers of BPFB and BPTB always remains ~10 kJ mol-1 for all forms (neutral, cationic, anionic and S1r) and dihedral angles (φ1 and φ2). The higher rotational barrier found for BPFB with respect to BPTB implies higher torsional rigidity of S1 state; the torsional freedom is also stiffer for BPFB than for BPTB due to a more efficient conjugation. Additionally to the higher torsional barriers BPFB also possesses lower BLA at S1r state as compared with BPTB. Moreover BLA (Table 4) as well as the conjugation efficiency undertake a remarkable changings upon rotation around φ1 and φ2 dihedral angles. These findings suggest more efficient conjugation of BPFB as compared with BPTB. Thereby we conclude that S1r state of furan co-oligomer may be less affected by thermal fluorescence quenching as compared with its thiophene analog.


17


Table 4. Stationary points, values of torsional barriers, torsional freedom (at the room temperature) and BLA data for S1r state BPFB and BPTB PES of φ1 and φ2 dihedral angles. Level of theory: TD-DFT/PBE0/6-311++G** BPFB

S1r state

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 18 of 41

BPTB

φ1

φ2

φ1

φ2


90

90

90

90


0

0

0

0

41

66

28

56

20

10

30

25

BLA / Å Difference in BLA between min and max / Å

0.044 0.036

0.054 0.041

0.034

0.038

AIM theory and NCI index with RDG are normally applied to the investigation of interactions that present in molecules in their ground state. However to the best of our knowledge the analysis of RDG isosurface has never been reported before for excited states. In this work, we applied both Bader’s theory and RDG analysis also to the electron density of the relaxed first excited state. Figure 6c, shows the formation of additional ring and bond CPs between each H2 and H3 pair of atoms for BPTB, whereas for BPFB no additional CPs were observed. This may be assigned to possible intramolecular interaction stabilizing planar geometry of BPTB. However, NCI analysis (Fig. 6b and 6d) reveals that RGD isosurfaces between H1…X and between H2…H3 depict a combination of bonding and antibonding interactions which means that attraction is counterbalanced by repulsion, as it was shown for S0 state. Therefore attraction between H1…X and repulsion between H2…H3 do not play any significant contribution to PES of S1r state and higher torsional barrier of BPFB is only due to more efficient conjugation.


18

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Figure 6. AIM analysis, based on the ground state electron density critical points (CPs) of optimized S1 excited state of BPFB (a) and BPTB (c). Plots of the NCI isosurfaces (s = 0.5 a.u. and a red-to-blue color scale from -0.015 a.u. < sign(λ2) ρ(r) < +0.015 a.u.) calculated for optimized S1 excited states of BPFB (b) and BPTB (d). Blue color stands for repulsion, yellow/orange for attraction. 3.2.2. Conical intersections As the torsional rigidity may affect intersystem crossing, being one of the possible PL quenching channels, we explored the possibility of excited electronic states degeneration along the torsional φ1 and φ2 coordinates. Upon rotation around φ1, S1 and T3 excited states of both BPFB (Fig. 7a) and BPTB (Fig. 7c) are almost degenerate. The rotation around φ2 implies intersections between S1 and T3 excited states only in the case of BPTB (Fig. 7d) at


19


φ2 ~ 60 degrees. Additionally S1 state proximate to T4 state near φ1 and φ2 ~ 90 degrees for both BPFB and BPTB. The conical intersection shown for BPTB at φ2 ~ 60 degrees may be an additional channel of PL quenching. Although we cannot deduce any quantitative conclusions about energetics of conical intersections from TD-DFT calculations, our results suggest that this process is less favorable for BPFB than for BPTB, because its conjugated backbone is more rigid both in the ground and excited states.

b)

BPFB ϕ1 rotation 3.6

T4

3.2 2.8

T3 S1

2.4

T2

2.0

T1 0

Relative Energy (eV)


a)

15

30

45

60

75

BPFB ϕ2 rotation 3.6

T4

3.2 2.8

S1 T3

2.4

T2

2.0

T1

90

0

15

30

ϕ1 (degs)

c)

d)

BPTB ϕ1 rotation 3.6 3.2 2.8

T4 T3 S1

2.4

T2

2.0

T1 15

30

45

60

75

90

BPTB ϕ2 rotation 3.6

T4

3.2 2.8

T3 S1

2.4

T2

2.0

T1 0

0

45

ϕ2 (degs)



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 20 of 41

60

75

15

90

ϕ1 (degs)

30

45

60

75

90

ϕ2 (degs)

Figure 7. Relative energy (with respect to energy of ground state) of T1, T2, T3, T4 (red squares) and S1v vertical excited levels (blue circles) evaluated along a) φ1 PES scan of BPFB; b) φ2 PES scan of BPFB; c) φ1 PES scan of BPTB; d) φ2 PES scan of BPTB. Level of theory: (TD-)DFT PBE0/6-311++G** (see Fig. S5 for additional (TD-)DFT B3LYP/6-311++G** level of theory).


20

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Another possible channel of PL quenching could be a spin-orbit coupling (SOC), which facilitates prohibited singlet-triplet crossing and therefore allows a system to relax in a nonradiative way. The presence of heavy-atom (such as sulfur, for instance) in an organic molecule may favor stronger SOC and proximity of S1 and triplet states.66 As previously reported for oligothiophenes this phenomenon causes very high triplet quantum yields which strongly quenches PL via intersystem crossing.67 It is reasonable to hypothesize that the heavy-atom effect could also be one of possible origins of PL quenching for BPTB, however, investigation of the causes of PL quenching goes beyond the main scope of this work. 3.2.3 Vibronic structure of absorption spectra According to the previously reported absorption spectra BPFB demonstrates fine-structured spectrum24 whereas BPTB does not.68 A reasonable explanation may evoke the higher torsional rigidity of BPFB in both S0 and S1r states with respect to BPTB (vide supra). To quantummechanically confirm this hypothesis, we computationally predicted the vibronic structure of the S0 → S1 band of absorption spectra of BPFB and BPTB (Fig. 8a and 8b). The results of theoretical prediction are in excellent agreement with experimental data. Minor discrepancies in intensities and width arise from the different temperature (calculations were carried out for 0 K at which the high vibrational levels are less populated, whereas experimental spectra were reported for the room temperature24,68). Fine structure of vibrationally resolved absorption bands suggests that 0→0` transition is quenched in BPTB due to more floppy and subsequently nonplanar molecular geometry in comparison with BPFB which shows more rigid and planar molecular geometry.


21


a)

BPFB

Absorbance (a.u.)

1.0 0.8 0.6 0.4 0.2 0.0 300

325

350

375

400

425

450

475

425

450

475

λ (nm)

b)

BPTB 1.0

Absorbance (a.u.)

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 22 of 41

0.8 0.6 0.4 0.2 0.0 300

325

350

375

400

λ (nm)

Figure 8. Sticks of vibronic progression and their convolution of the S0 → S1 band of absorption spectra of BPTB (a) and BPFB (b). Experimental spectra (black lines) for co-oligomers THF solutions were adapted from refs.24,69 with permission of The Royal Society of Chemistry. Convolution was done assuming Gaussian band shape, with (arbitrary) σ = 0.1 eV. 3.3. Reorganization energy According to the Marcus theory,70,71 charge and energy transfer efficiencies are affected by several factors; amongst them is the reorganization energy as an energetic barrier for the transport due to electron and geometrical rearrangements. In order to compare transfer properties


22

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of investigated compounds and understand how does torsional rigidity affect them, hole, electron and exciton reorganization energies were calculated. BPFB

exhibits

almost

1.5 times

lower

reorganization

energy

for

hole

transfer

(.7897 = 217 meV) as compared with BPTB (.78:7 = 347 meV). The reorganization energy for = 226 meV, electron transfer is also lower (1.8 times) for furan co-oligomer (.7897 .78:7 = 416 meV). The exciton reorganization energy is a one of the main parameters of interest since it is involved in the kinetics of exciton transport.72 The reorganization energy for exciton transfer for BPFB (.7897 = 38 meV) was found to be ~ 20 times lower than that for BPTB 5( (.78:7 5( = 775 meV). Due to its higher torsional rigidity in all investigated states (neutral, anionic, cationic, S1r), BPFB stays planar which significantly lowers contribution of geometric rearrangements to reorganization energy. On the other hand, BPTB is rigid and hence planar only in charged and excited states making geometric rearrangements a significant contributor to the barrier for charge and energy transfer. Therefore the higher torsional rigidity should be considered as a factor that dramatically lowers the reorganization energy of furan/phenylene cooligomer for hole, electron and exciton transfer due to a small geometry and electron rearrangements. 4. Conclusions We provided a deep theoretical exploration of torsional rigidity as well as its origins for the alternating heteroaryl/phenylene co-oligomers outstanding among the most promising materials for organic optoelectronics. The higher rigidity of furan co-oligomer as compared with its thiophene analog both in S0 and S1r states was demonstrated by means of the state-of-the-art computational methods such as Bader’s AIM theory and RDG analysis. For the first time, AIM theory and RDG analysis were applied to excited states. In contrast to the previous reports


23


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 24 of 41

dealing with the planarity caused by intramolecular dispersion interactions the higher torsional rigidity of BPFB solely stems from efficient conjugation, being the effect of dispersion S…H and O…H intramolecular interactions negligible. Taking into account possibility of intersystem crossing upon cycle rotation, we concluded that higher torsional rigidity is favorable for hindering PL quenching. The rigidity of BPFB was demonstrated to be the main origin of the fine-structure of the S0 → S1 band of the UV-Vis absorption spectrum. Crystal surroundings of furan co-oligomer distorts the molecular geometry, whereas in thiophene/phenylene one it is induced to be planar. We demonstrated that the higher the torsional rigidity the lower the reorganization energy especially for exciton transport which confirms that BPFB may be a promising candidate for optoelectronic applications. The obtained data significantly supplement the previously reported experimental findings serving also as a basis for the molecular design of rigid high performance emissive organic semiconductors. AUTHOR INFORMATION Corresponding Authors Dr. M.S. Kazantsev, e-mail: [email protected] Prof. E. Benassi, e-mail: [email protected] Funding sources Russian Foundation for Basic Research project 16-33-60011 mol_a_dk Notes Authors declare no conflict of interest.


24

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ACKNOWLEDGMENTS This work was supported by Russian Foundation for Basic Research project 16-33-60011 mol_a_dk. Authors acknowledge the Novosibirsk State University program “5-100” and Siberian Supercomputing Center (http://www2.sscc.ru/). E.B. expresses his gratitude to the Rector of the Novosibirsk State University, the Corresponding Member of the Russian Academy of Science, Prof. M.P. Fedoruk, for his invitation to visit the Novosibirsk State University.

ASSOCIATED CONTENT Additional computational details, geometry optimizations at different levels of theory, coordinates of optimized ground states, intermolecular interaction energies in crystals, coordinates of single charged states, values of torsional barrier at different levels of theory, relaxed scans of PES, brightest absorption band transition energy deviation, deviation of optical gaps, properties of S0 → S1 transition, plots of frontier molecular orbitals, S1 state optimized geometries (B3LYP), relative energy of vertical excited states evaluated along PES scan (B3LYP). This material is available free of charge via the Internet at http://pubs.acs.org.

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TABLE OF CONTENTS


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Figure 1. Schematic representation of layers treated at different levels of theory in ONIOM optimizations. Red, blue and green colors represent high-, middle- and low-level layers respectively. 72x21mm (300 x 300 DPI)


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Figure 2. Molecular geometries of neutral and charged forms of BPFB (a, c) and BPTB (b, d) respectively. Values of dihedral angles (φ1 and φ2) and bonds length (b1 and b2) are shown for (a), (b): gas phase optimized geometry (bold), Optimized geometry of high-level layer of ONIOM calculations (Italics), and XRay analysis (plane style)18,45 and for (c),(d): cationic state (Italics) and anionic state (plain style). Level of theory: (U)B3LYP[GD3BJ]/6-311++G** for the gas phase optimization and M06-2X/6-31+G* for ONIOM high-level molecule. 190x142mm (300 x 300 DPI)



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Figure 4. AIM and RDG analyses, based on the ground state electron density. Critical points (CPs) of BPFB (a), BPTB (b) and BPTB with planarized phenyl and thiophen cycles (e). Plots of the NCI isosurfaces (s = 0.5 a.u. and a red-to-blue color scale from -0.015 a.u. < sign(λ2) ρ(r) < +0.015 a.u.) calculated for BPFB (b) and BPTB (d) optimized geometries and for BPTB geometry with planarized phenyl and thiophene cycles (f). Blue color stands for repulsion, yellow/orange for attraction. 220x191mm (300 x 300 DPI)


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Figure 5. S1 State optimized geometries of BPFB (a) and BPTB (b). Level of theory: TD-DFT PBE0/6311++G** (see Fig. S4 for additional level of theory). 63x15mm (300 x 300 DPI)



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Figure 6. AIM analysis, based on the ground state electron density critical points (CPs) of optimized S1 excited state of BPFB (a) and BPTB (c). Plots of the NCI isosurfaces (s = 0.5 a.u. and a red-to-blue color scale from -0.015 a.u. < sign(λ2) ρ(r) < +0.015 a.u.) calculated for optimized S1 excited states of BPFB (b) and BPTB (d). Blue color stands for repulsion, yellow/orange for attraction. 190x142mm (300 x 300 DPI)


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Table of content 13x7mm (600 x 600 DPI)


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