Helical and Nonhelical Structures of Vinylene- and Azomethine

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Helical and Nonhelical Structures of Vinylene- and AzomethineLinked Heterocyclic Oligomers: A Computational Study of Conformation-Dependent Optoelectronic Properties Harikrishna Sahu and Aditya N. Panda* Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India S Supporting Information *

ABSTRACT: A systematic computational investigation has been carried out at the density functional level of theory to characterize various conformational isomers of furan, pyrrole, and thiophene based oligomers and consequently study their stabilities and electronic properties, especially in the cases of long oligomers. In these oligomers, adjacent heterocyclic rings are connected by either vinylene or azomethine linkages. B3LYP and B3LYP-D3 functionals are used to observe the effect of dispersion energy. Our results show that a combination of the B3LYPD3 functional and the 6-31G(d,p) basis set is suitable for ground-state studies of these systems. For long vinylene-linked oligomers, folding isomers are comparatively more stable than their respective linearly conjugated isomers, due to intramolecular noncovalent interactions. In the case of azomethine-linked oligomers, geometries and stabilities of conformers depend on the type of heterocyclic ring in the repeating unit. For vinylene-linked heterocyclic oligomers, first optically allowed electronic transitions of linearly conjugated oligomers have the largest oscillator strengths, and these absorption bands are dominated by HOMO to LUMO transitions. In the case of a few linear azomethine-linked oligomers, two major electronic transitions, S0 → S1 and S0 →S2, are noticed. However, transitions from S0 to higher electronic states are the most prominent transitions in cases of foldamers, except azomethine-linked thiophene foldamers. Major absorption bands of these helical oligomers are dominated by transitions from HOMO−N to LUMO+N orbitals. All the helical conformers are found to be circular dichroism active.

1. INTRODUCTION Conjugated polymers are potential candidates for a number of organic devices,1−12 including organic light-emitting diodes (OLEDs),13,14 organic field effect transistors (OFETs),15 and organic photovoltaics (OPVs),16,17 owing to their interesting optical, electronic, and electrochemical properties. Low manufacturing cost, flexibility, tunability, and ease of processing of these organic devices manifest their immense public demand.18−20 As a result of intensive research in this field, organic devices are steadily replacing their inorganic counterparts. However, organic devices are still in the early stage, and their performances are quite poor compared to the inorganic ones.13,14,16,21,22 The performance of these organic devices depends on many factors including absorption coefficient, band gap, ease of hole/electron injection and their mobilites, energies of frontier molecular orbitals, and molecular packing of polymers used as active materials.23−27 The above properties are largely dependent on the structure of a polymer. Configurational and conformational isomers of a molecule have different properties.28−32 Thus, understanding of the structure−property relationship is very important to develop an effective conjugated polymer. Recently, the interest in heterocyclic conjugated polymers has increased considerably.3−5,33−36 Chemical modification of these oligomers is of great interest for further development, © XXXX American Chemical Society

which may lead to novel conjugated oligomers having better performance in devices than the existing ones. One promising strategy to develop a suitable organic material is the introduction of vinylene and azomethine linkages in the main chain of an oligomer.37−42 Introduction of vinylene linkages in the main chain of a conjugated polymer reduces dihedral angles between the repeating units and increases the π-electron delocalization over the main chain.40,42 Azomethine-linked oligomers are isoelectronic with their vinylene-linked counterparts. Besides the ease of synthesis and modification of their structures, these azomethine-linked oligomers have numerous advantages in optical and hole/electron transport properties.37,38,43,44 Looking at the advantages of these linkages, a number of experimental and computational studies have been carried out to explore the structural details and usability of these heterocyclic conjugated polymers. These polymers have many different possible conformations.28,41 Stabilities of these conformations are discussed in the literature,28,41 but all the results are from studies on small (2 to 4 repeating units) linear oligomers. However, it is known that the intramolecular noncovalent interaction has a major impact on secondary structures like helices for a large π-conjugated oligomer. In Received: August 20, 2015

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DOI: 10.1021/acs.jpcc.5b08100 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C addition to other helical polymers,45−47 heterocyclic conjugated helical polymers like substituted polythiophene and polypyrrole have already been synthesized.48−50 These helices are either in the form of nonaggregated intrachain helical conformations or in the aggregated form of planar oligomers. Few studies show that helices of substituted polythiophenes are formed due to intermolecular helical packing of planar polythiophene chains in aggregated states.51−53 On the other hand, many researchers have synthesized polythiophene derivatives having nonaggregating folded structures.54,55 Computational studies based on the semiempirical method32 and density functional theory30 also support intrachain helical conformations of these polymers. In cases of fluorinated thienylfurans56 and phenylene-ethylene foldamers,57 computational investigations show that hydrogen bonding and π−π stacking interactions between adjacent helical turns lead to folding structures. In our previous work58 on helical conformers of pyridine-containing oligomers, the importance of noncovalent interactions in stabilizing the helix-like structures was discussed. These studies and their results raise the possibilities of formation of similar looking folding structures for vinylene- and azomethine-linked heterocyclic oligomers. As far as we know, this issue has not been addressed yet, and the structural aspects of these type of polymers are yet to be revealed. Indeed, helical polymers are interesting compounds.46,59 In addition to being of immense biological interest, helices are progressively seeking attention for applications in materials science. Their unique helical structures fulfill many essential needs. If one of the two helices (i.e., right and left handed helices) can be selectively synthesized, all helical oligomers will behave as optically active material due to their inherent chiral structures.46 For conjugated polymers with helical structures, good intramolecular charge transfer properties are expected, in addition to having small band gaps.60,61 Intramolecular delocalization of π-electrons along the twist axis in helical polymers may result in strong coupling between neighboring helical turns in a helical polymer, and this raises the possibility of facilitation of intramolecular charge transfer processes. A helical π-conjugated polymer having high conductivity is also suitable for applications like a molecular solenoid.30,62−65 In this article, we present and discuss the conformationdependent optoelectronic properties of heterocyclic π-conjugated oligomers. As it is known, optoelectronic properties of a molecule are dependent on its conformation. Stabilities of different conformers may vary with an increase in chain length, as for larger oligomers intramolecular noncovalent interactions affect the geometries of conformers. As a result, optoelectronic properties of a molecule may also change with an increase in chain length. However, in the literature, optoelectronic properties of long oligomers have been calculated mostly either by applying the periodic boundary condition or by increasing the chain length of the most stable conformer out of the various possible conformations for a small oligomer. These methods may miss the picture of the conformation−electronic properties relationship for large chain oligomers. Hence, systematic studies on conformational analyses, starting from small to large oligomers, are necessary to be carried out, and subsequently their optoelectronic properties need to be explored. In cases of heterocyclic ring-vinylene/azomethine polymers, the absence of information on the structural, energetics, and absorption properties of different conformers, especially for long chains, in the literature motivates us to investigate these types of oligomers. Our studied heterocyclic

oligomers are made up of thiophene, pyrrole, and furan rings connected through two types of linkages, i.e., vinylene and azomethine, which form a complete set enabling a systematic investigation. Three different types of conformational isomers for each of the six studied heterocyclic oligomers are chosen. Out of the three, one forms a folded structure after addition of a certain number of repeating units, while the other two isomers are linear. B3LYP and dispersion-corrected B3LYP functionals were used to obtain ground-state optimized geometries. To compare the three conformational isomers of each heterocyclic oligomer, ground-state optimized geometries, vertical ionization potentials, HOMO−LUMO gaps, electronic transitions, and oscillator strengths of these transitions are taken into consideration. We have analyzed the effects of dispersion force on the structures of these species and stabilization of helical polymers. In Section 2, computational aspects are briefly discussed. This is followed by results and discussion in Section 3 and conclusions in Section 4.

2. COMPUTATIONAL DETAILS All calculations are carried out with the Orca 3.0 program.66 B3LYP and dispersion-corrected B3LYP (B3LYP-D3) functionals67 with the 6-31G(d,p) basis set are used to optimize the ground-state geometry of all the studied compounds. In B3LYP-D3, the Becke−Johnson damping68 was used with the D3 version of dispersion. No constraint is applied during the above optimization processes. To calculate vertical excitation energies and oscillator strengths, single-point time-dependent DFT (TDDFT) calculations are performed using the B3LYP functional with the same basis set at the optimized ground-state geometries obtained by the B3LYP-D3/6-31G(d,p) level. To comment on the optical activity of the folded conformers, electronic circular dichroism (CD) spectra have also been computed. A maximum number of repeating units considered varied from system to system, and the largest oligomer in this study consists of 14 units to keep the computational cost under control. In fact, we stopped the optimizations once a helix/ foldamer is formed and did not increase the size further. Vertical ionization potential (IP) values are calculated for the longest chain compounds of all the conformers at the B3LYPD3/6-31G(d,p) level using the following formula: IP = E+(M0) − E0(M0) . Here EX(MY) represents the energy of M with charge “X” at “Y” charged geometry of a molecule. 3. RESULTS AND DISCUSSION Sketch map representations of all the studied compounds are shown in Figure 1. Furan, pyrrole, and thiophene rings substituted at their “2” positions by either a vinylene or azomethine linkage are taken as repeating units. In each of these oligomers, two adjacent heterocyclic rings are trans to each other with reference to either CC or CN linkages. Three different types of conformational isomers, A, B, and C, of each oligomer are considered in this work. In A, hetero atoms of adjacent heterocyclic rings are in the same side, and in B and C, these are in the opposite sides of the linkages. However, the difference between B and C is that the heterocyclic rings are attached at two possible different trans positions of the linkage. Denoting furan, pyrrole, and thiophene by F, P, and T and vinyl and azomethine as V and Am, respectively, the studied oligomers can be identified by the following abbreviation: conformation-(heterocyclic ring linkage type)n, where n is the number of repeating units. All the studied compounds were B


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The Journal of Physical Chemistry C

Figure 1. Sketch map representations of the repeating units of the studied heterocyclic oligomers, where n starts at 1 and goes up to 14. Three different conformations (A, B, and C) of oligomers are shown in the figure. Furan, pyrrole, and thiophene rings are represented by F, P, and T, and vinylene and azomethine linkages are represented by V and Am, respectively. The dihedral angle (ϕ) between two repeating units of these conformations is also depicted as ϕ = ∠a−b−c−d. Figure 2. Optimized structures of A (a: optimized at the B3LYP/631G(d,p) level, b: optimized at the B3LYP-D3/6-31G(d,p) level), B (c), and C (d) isomers of (FV)9. Distances between the atoms of two adjacent helical turns of conformation A are given in Å.

fully optimized at B3LYP/6-31G(d,p) and B3LYP-D3/631G(d,p) levels. Frequency calculations were carried out for all A, B, and C isomers of a few selected small oligomers. The absence of any imaginary frequency showed that all are minima in the potential energy hypersurface. 3.1. Ground State: Geometries, Diameter of Helices, Bond Length Alternation, and Dipole Moment. Monomers of all the compounds obtained using both the functionals are found to be planar. Bond lengths of CC linkages of optimized structures of dimers for the C conformation obtained at these levels are in good agreement with the previous work.41 Angles between heterocyclic rings and these linkages obtained at the B3LYP/6-31G(d,p) level are the same as the results of the previous work,41 while little deviations (≤0.5°) are noticed for the results obtained at the B3LYP-D3/6-31G(d,p) level. All these dimers are planar, which matches with the previous results.41 Keeping in mind the above results for dimers, these combinations of functionals and basis sets have been chosen to study the structures of large oligomers (up to 14-mer). Optimized structures of isomer A of (FV)9, (PV)9, and (TV)14 oligomers are shown in Figures 2 and 3 and Figure S1, respectively. As shown in these figures, all are helix-like structures. The π-stacking distance between two adjacent helical turns of all these helical oligomers is shown in the same figure. As shown, the optimized structure of A-(FV)9 obtained using the B3LYP functional is slightly bent at the terminal units of the oligomer, while two adjacent helical turns reside parallel to each other for the B3LYP-D3 functional. For all these vinylene-linked helical oligomers, distances between the adjacent helical turns obtained by the B3LYP functional are comparatively larger than the distances obtained by the B3LYPD3 functional. In A-(PV)9, shown in Figure 3, hydrogen bonds are formed between the nitrogen and hydrogen atoms of two adjacent helical turns, and this results in a bent structure in the case of B3LYP-D3. Ground-state structures of A-(FAm)9, A(PAm)9, and A-(TAm)10 obtained by the two different functionals are shown in Figures S2, S3, and S4, respectively. None of the structures for A-(FAm)9 and A-(PAm)9 obtained

Figure 3. Ground-state structures of A-(PV)9 optimized at B3LYP/631G(d,p) (a) and B3LYP-D3/6-31G(d,p) (b) levels. Distances between the atoms of two adjacent helical turns of conformation A are given in Å.

by either of the functionals are helical, unlike the vinylenelinked compounds. This is due to noncovalent interactions between different parts of these oligomers. A representation of the interunit dihedral angle of conformer A is shown in Figure 1. For A-(TAm)2, the dihedral angle (ϕ) between the two monomers is ∼−25.7° for both the functionals. As a result, successive addition of repeating units leads to a helix-like structure, similar to the helical conformer of polythiophene.32 C


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Stabilities of different conformations were calculated with respect to the conformer A, and the results obtained for vinylene- and azomethine-linked oligomers are depicted in Figures 4 and 5, respectively. In Figure 4, B3LYP results show that C is the most preferred conformation for all the vinylenelinked oligomers. The stability of this conformation gradually increases with the increase in chain length. For B3LYP-D3, similar results are obtained for (FV)1−7, (PV)1−8, and (TV)1−12. However, unlike the B3LYP results, the helical structures of (FV)8−9, (PV)9, and (TV)14 are found to be the most stable conformations. Figure 5 shows that conformations C and A of FAm and PAm, respectively, are the most stable conformations for both the functionals. Stabilities of these conformations do not vary regularly with the size of the oligomers. For TAm oligomers, results are similar for both the functionals. Conformer C is the most stable structure, and its stability gradually increases with the increase of chain length. Noncovalent interactions play an important role in stabilizing the folding of oligomers.56,58 To check the performance of the two DFT functionals considered in this work, interaction energies between the overlapping regions of the helix are calculated at B3LYP-D3 optimized geometries for all the studied helical oligomers. The rest of the oligomer was ignored during this calculation. Single point calculations were carried out for this overlapping region which can be considered as a “πstacked” system, in addition to computing the single point energies of the individual units. Interaction energies were calculated using the supermolecular approach as Eint = EAB AB − (EAA + EBB), and these were later corrected for the basis set AB AB AB Y superposition error (BSSE) to give ECP int = EAB − (EA + EB ). EX is the energy of fragment X with basis set of Y. These energies calculated using both the B3LYP and B3LYP-D3 functionals with the 6-31G(d,p) basis set are reported in Table 2. Negative and positive values of Eint and ECP int indicate stabilization and destabilization of this π-stacked system in comparison to the individual entities, respectively. This, as a result, points at the effect of noncovalent interaction between adjacent helical turns of a helix. Results of the B3LYP-D3 functional for all the systems, except A-(TAm)10, exhibit the fact that the π−π interaction between adjacent helical turns helps in stabilizing the systems. In the case of A-(TAm)10, there is no interaction between the adjacent helical turn due to its large pitch (32.714 Å). It is to be mentioned that the basis set used in this study is not very large, and uncorrected results show strong non-

Figure S4 shows that helical structures of A-(TAm)10 obtained by both the functionals are similar. Interunit dihedral angles of A-(TAm)5 and A-(TAm)10 obtained at the B3LYP-D3/631G(d,p) level are listed in Table 1. As reported in the table, Table 1. Calculated Interunit Dihedral Angles (ϕ1−9) of A(TAm)n and B-(TAm)n at the B3LYP-D3/6-31G(d,p) Level, Where n = 5 and 10a

a

dihedral angles

A-(TAm)5

A-(TAm)10

B-(TAm)5

B-(TAm)10

ϕ1 ϕ2 ϕ3 ϕ4 ϕ5 ϕ6 ϕ7 ϕ8 ϕ9

−27.341 −25.512 −25.422 −25.034

−27.615 −25.938 −26.155 −26.333 −26.304 −26.096 −26.011 −25.764 −25.200

23.667 −20.827 21.018 −23.161

24.273 −21.554 21.583 −21.515 21.755 −21.516 21.474 −21.612 23.406

All angles are in degrees.

interunit dihedral angles of these two oligomers are ∼25−27°, which are close to the value for A-(TAm)2. Although not shown, dihedral angles of B3LYP optimized structures are similar to those for B3LYP-D3 results. From the above, except TAm oligomers, it is clear that B3LYP-D3 and B3LYP functionals behave very differently from each other while predicting the structures for conformation A, and this may be a result of noncovalent interactions69,70 that is taken care of in B3LYP-D3. All the linearly conjugated oligomers obtained by B3LYP and B3LYP-D3 functionals are planar, except those of B-TAm oligomers. Structures of planar oligomers are similar to those for B-(FV)9 and C-(FV)9 shown in Figure 2. The structure of B-(TAm)10 obtained at the B3LYP-D3/6-31G(d,p) level is shown in Figure S5. For B-TAm oligomers, noticeable interunit dihedral angles are observed. The dihedral angle between two adjacent repeating units for B-(TAm)2 is 24.9°, for both the functionals. The existence of similar dihedral angles in the structures of other B-TAm oligomers makes these nonplanar. A careful inspection of interunit dihedral angles of B-(TAm)5 and B-(TAm)10 in Table 1 shows that terminal dihedral angles are comparatively larger than other dihedral angles.

Figure 4. Energies of different conformations with respect to that of conformation “A”. (a), (b), and (c) show the results for (FV)n, (PV)n, and (TV)n, respectively. Results are shown for B3LYP and B3LYP-D3 functionals. Here, n is the number of repeating units. D


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Figure 5. Energies of different conformations with respect to that of conformation “A”. (a), (b), and (c) show the results for (FAm)n, (PAm)n, and (TAm)n, respectively. Results are shown for B3LYP and B3LYP-D3 functionals. Here, n is the number of repeating units.

Table 2. Interaction Energies between the Overlapping Regions and Geometrical Parameters of Helical Oligomersa Eint/ECP int (kcal/mol) helices

B3LYP-D3

B3LYP

diameter (Å)

pitch (Å)

u

d = (p/u)

A-(FV)9 A-(PV)9 A-(TV)14 A-(TAm)10

−34.87/−11.41 −25.97/−11.43 −46.41/−24.77 −0.01/−0.07

−9.81/13.81 −12.28/2.37 1.44/23.21 −0.01/−0.07

16.473 17.636 25.554 11.179

3.510 3.992 3.438 32.714

7 7 11 8

0.501 0.570 0.313 4.089

a

Eint and ECP int represent the uncorrected and BSSE corrected interaction energies, respectively. Values of interaction energies calculated at the B3LYPD3/6-31G(d,p) and B3LYP/6-31G(d,p) levels are shown. p, u, and d represent pitch, number of repeating units per turn, and rise per repeating unit, respectively.

azomethine-linked oligomers have smaller Δr values compared to those for their respective vinylene-linked oligomers. Performance of an organic device depends on the groundstate dipole moments (DM) of organic compounds.29,73−75 Large DM of a molecule can cause strong intermolecular interactions, which can have a profound effect in the solid state. It is known that DM varies with the change of conformational isomer of a compound.29,75 To check the dependency of DM on three different conformations considered in this work, values of ground-state DM calculated at the B3LYP-D3/6-31G(d,p) level for the longest chain of all the studied compounds are listed in Table 3. In the case of our studied systems,

covalent interactions between helical turns for all the systems. Compared to the B3LYP-D3 results, interaction energies obtained by the B3LYP functional are either very small or even positive. The poor performance of the B3LYP functional on the prediction of the dispersion interaction is already mentioned in the literature.56,71,72 From now onward, all the discussion will be based on the optimized structures predicted at the B3LYP-D3/6-31G(d,p) level. Diameter, pitch (p), number of repeating units needed to complete one helical turn (u), and rise per repeating unit (d) of the largest helical oligomer of each compound are listed in Table 2. Diameters of A-(FV)9 and A-(PV)9 are nearly the same, i.e., 16.473 and 17.636 Å, respectively. For this reason, both have the same number of repeating units in one helical turn. A-(TV)14 and A-(TAm)10 oligomers have the largest and the smallest diameters having values of 25.554 and 11.179 Å, respectively. Requirement of 11 repeating units to complete one helical turn leads to a large diameter for A-(TV)14. The helix pitch is the largest for A-(TAm)10 and is the smallest for A-(TV)14. Accordingly, rise per repeating unit (d), defined as p/u, is the largest for A-(TAm)10 and is the smallest for A(TV)14. Bond length alternation (Δr) values are calculated for all the studied compounds and plotted in Figure S6. Δr is the difference between the average of carbon−carbon single and double bond lengths along the π-conjugation path.37,43 Pyrrolebased oligomers connected by different linkages have smaller Δr values compared to the values obtained for respective furan and thiophene carrying oligomers. Δr values for furan- and thiophene-based oligomers are nearly the same. Further, differences in Δr values between different conformational isomers are not significant. It is worth mentioning that

Table 3. Dipole Moments of the Largest Oligomers of All the Studied Compounds Calculated at the B3LYP-D3/631G(d,p) Levela conformations

(FV)9

(PV)9

(TV)14

(FAm)9

(PAm)9

(TAm)10

A B C

1.643 0.785 0.950

2.535 2.164 1.846

2.559 0.321 0.485

6.786 27.998 14.457

1.616 28.381 8.654

19.839 30.748 20.510

a

All values are in Debye.

heteroatoms of adjacent rings are either on the same side (as in A) or on the opposite sides (as in B and C) of the linkage. Hence, it is expected that B and C isomers would have smaller dipole moments as a result of cancellation of oppositely oriented dipole moments than that of A. The same is observed in Table 3 for vinylene-linked conformers, although the differences between the DM values are small; however, as has been discussed, A adopts a folded form after a certain chain length, and this structure may decide the trend of variations of DM between A, B, and C. Apparently, a clear correlation is E


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the figure that for the vinylene-linked compounds, the vertical IPs of the helical conformations are larger compared to the other conformations. The IP increases in the order: B < C < A. But the differences between the IP values are very small in cases of (FV)9 and (TV)14. However, the IP of (PV)9 seems to depend strongly upon the conformation, and comparatively larger differences between the IPs of different conformations are noticed. As shown, azomethine-linked compounds do not have any general trend. C, A and A have the largest, and A, B and C have the smallest IP values for (FAm)9, (PAm)9 and (TAm)10, respectively. 3.2. Frontier orbitals, excitation energies, absorption and CD spectra. HOMO−LUMO gaps (ΔH−L) and first excitation energies (E1g) of vinylene- and azomethine-linked oligomers are plotted in Figure 7 and Figures S7−S11. Figures

obtained between the chain length and DM values for A conformers as shown in Table S1. In cases of A oligomers, the DM value increases up to certain n, and then it starts to decrease. The dipole moment becomes the smallest once a folded structure is formed. This decrease is certainly the result of cancellation of individual bond dipole moments because of the adoption of folded structure. After completion of a helical turn, DM values of these oligomers again start to increase as is observed in cases of (FV)9, (PV)9, and (TV)14 in Table 3. In these systems, there are more repeating units than needed to complete one helical turn. In cases of B and C conformers reported in Table S1, DM values for oligomers with odd n are larger compared to that for the oligomer with even n. The presence of extra rings and extra heteroatoms in the case of odd n structures explains this trend. Table 3 also shows the DM values for the largest of azomethine-linked oligomers. It is observed that folding conformers have the smallest DM values, while the DM values for B and C are very large in these cases. The presence of azomethine linkages in the main chain results in large dipole moment values in the case of linearly conjugated oligomers, while cancellation of bond dipole moments in cases of folded structures leads to smaller DM values than those for B and C. It is worth mentioning that the same trend is seen in cases of (FAm)9 and (PAm)9, although A conformers of these compounds are not strictly helical as mentioned before. Variations of the DM as a function of chain length for these oligomers are reported in Table S2. In cases of FAm and PAm oligomers, results similar to those for vinylene-linked species are observed for the A isomers. However, in contrast to other helical oligomers, the DM value of TAm oligomers increases with increasing n. As a consequence of larger pitch, the cancellation of individual bond dipole moments effectively gets reduced, and this leads to an increase of the DM value per addition of a repeating unit. For B and C oligomers, DM values increase with an increase in chain length in most of the cases. Figure 6 shows the variation of vertical IP with the change of conformation for the longest chain of the studied vinylene- and azomethine-linked heterocyclic oligomers. It can be seen from

Figure 7. ΔH−L (a) and E1g (b) of FV oligomers as a function of the reciprocal number of the double bonds (x) in oligomers.

show that there is a gradual decrement in the values of ΔH−L and E1g with increase in chain length for all the isomers, except for E1g of A-(FV)8 and ΔH−L of A-(TV)14. For almost all A conformers, it is noticed that deviations in slopes of E1g and ΔH−L occur when an oligomer starts to form a folding structure. To elucidate this interruption in HOMO−LUMO gaps, HOMO and LUMO energies of (FV)6−9 are plotted in Figure S12. It is observed that HOMO and LUMO energies for B and C isomers follow a certain trend: HOMO energy increases and LUMO energy decreases with increase in the size of chain. However, for A, sudden changes in the trends of HOMO and LUMO energies occur as the oligomer starts to complete one helical turn. This is a consequence of loss of planarity of the backbone chain as the helix formation starts and this results in breaking the general trend observed for B and C. Absorption spectra of A, B and C conformers for the longest oligomer of the studied compounds are plotted in Figure 8. For each compound, the 25 lowest singlet transitions are considered. In these spectra, full-width at half-maximum height is 1500 cm−1. The main feature of these simulated spectra is that the spectra of the isomer A are always blue-shifted compared to those of isomers B and C, in each case. For all the folding structures, contributions of more than one electronic transitions are significant. In absorption spectra of isomers B and C, only one electronic transition has the significant

Figure 6. IPs of the longest oligomers of studied vinylene- and azomethine-linked compounds calculated at the B3LYP-D3/6-31G(d,p) level. F


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Figure 8. Simulated UV−vis spectra of A, B and C isomers for (FV)9 (a), (PV)9 (b), (TV)14 (c), (FAm)9 (d), (PAm)9 (e), (TAm)10 (f). The fwhm is 1500 cm−1.

being HOMO → LUMO type excitations, these transitions involve occupied orbitals other than HOMO and unoccupied orbitals other than LUMO. After completion of a helical turn, S0 → S1 transitions have negligible oscillator strengths for isomer A of vinylene-linked compounds. The main absorption peaks shown in Figure 8(a), (b), and (c) for A-(FV)9, A-(PV)9, and A-(TV)14 originate from S0 to Sm=16,17,19, Sm=5,11,13,19, and Sm=14,15,18,19 transitions, respectively. Eg, fosc’s, and configurations for all the important electronic transitions for (FAm)n=4,5,9, (PAm)n=4,5,9, and (TAm)n=4,5,10 are presented in Tables S7, S8, and S9, respectively. Results for other azomethine-linked compounds are listed in Tables S10−S15. As is the case with vinylene-linked oligomers, S0 → S1 transitions in cases of C(FAm), B/C-(PAm), and B/C-(TAm) oligomers have the largest oscillator strengths. These transitions are assigned to HOMO → LUMO excitations predominantly. Along with the S0 → S1 transition, S0 →S2 transitions have noticeable oscillator strengths for few of these oligomers. For example, for C(FAm)9, in addition to the major absorption peak at 770.9 nm ( fosc = 3.71), another peak of medium intensity appears at 667.9 nm ( fosc = 1.70). This secondary peak arises due to HOMO → LUMO+1 and HOMO−1 → LUMO transitions. However, transitions from S0 to higher excited states are major transitions for A-(FAm)n≥5, B-(FAm)n≥7, and A-(PAm)n≥5. In these cases, mainly HOMO−N and LUMO+N orbitals are involved. In the case of A-TAm oligomers, unlike other folding oligomers, first electronic excitation is one of the major electronic transitions along with a few other important higher excitations. The major absorption peaks for A-(FAm)9, A-(PAm)9, and A-(TAm)10 shown in Figure 8(d), (e), and (f) are due to S0 to Sm=5,18,19, S5, and Sm=1,2,5,10 transitions, respectively. The magnitude of the oscillator strength of an electronic transition is directly proportional to the degree of spatial overlap of orbitals involved in that transition.76−80 Thus, it is instructive to look at the electron density in molecular orbitals. Electron distributions in HOMOs and LUMOs of few selected vinylene-linked A oligomers are shown in Figure 9, and those of B and C isomers are shown in Figure S13. As can be seen, in HOMOs and LUMOs of B and C isomers of (FV)9, π electrons are spread over the entire chain having large electron density in the middle of the oligomer. Similar results are noticed for B and C isomers of (PV)9 and (TV)14. Thus, overlapping of electron

contribution in the case of vinylene-linked oligomers. But for most of the azomethine-linked oligomers, contribution of a second electronic transition can not be neglected. The most intense bands of B and C are very close and in few cases, they overlap. Excitation energies (Eg), oscillator strengths ( fosc), and configurations involved in all the important transitions for B and C isomers of (FV)9, (PV)9, and (TV)14 are listed in Table 4. Similarly, the results for isomers A of (FV)n=4,5,9, (PV)n=5,6,9, Table 4. Electronic Transition Data for the Lowest Excitation (S0 to S1) Obtained by the TDDFT Method for B and C Isomers of FV, PV, and TV Oligomers at the B3LYP/ 6-31G(d,p) Levela conformations

oligomers

Eg (eV)

fosc

B

(FV)9 (PV)9 (TV)14 (FV)9 (PV)9 (TV)14

1.782 1.901 1.563 1.766 1.834 1.532

6.699 6.855 10.162 4.534 5.097 8.407

C

a

configurations H H H H H H

→ → → → → →

L(97%) L(96%) L(89%) L(97%) L(96%) L(89%)

The HOMO and LUMO are indicated by H and L, respectively.

and (TV)n=8,14 are listed in Table 5. This information for other vinylene-linked oligomers is presented in Tables S3−S6. As reported in Table 4 and Table S3, for B and C isomers of vinylene-linked oligomers, the lowest optically allowed electronic transition has the largest oscillator strength. As an example, absorption peaks at 695.9 nm (fosc = 6.70) and 702.1 nm (fosc = 4.53) for B and C isomers of (FV)9, respectively, originate from S0 → S1 transitions. All these transitions are dominated by HOMO → LUMO transitions. Eg and fosc values decrease and increase, respectively, with an increase in chain length. Additionally, for B and C isomers of the same chain length, Eg follows the order: (TV)n < (FV)n < (PV)n. The results are distinctly different for A, as seen in Figure 8 and discussed in the previous paragraph. For isomer A of (FV)n=1−4, (PV)n=1−5, and (TV)n=1−8, the electronic transition S0 → S1 has the largest oscillator strength. On further addition of repeating units to these oligomers, transitions from the ground state to higher excited states become prominent transitions. Instead of G


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Table 5. Electronic Transition Data Obtained by the TDDFT Method for Isomer A of FV, PV, and TV Oligomers at the B3LYP/6-31G(d,p) Levela oligomers

m

Eg

fosc

(FV)4

1 5

2.482 3.727

1.894 1.831

(FV)5

1 5

2.196 3.264

1.458 2.512

(FV)9

1

1.738

0.065

16

3.210

1.675

17

3.266

1.783

19

3.321

1.130

m

Eg

fosc

configurations

(PV)5

1 5

2.274 3.292

2.204 2.078

(PV)6

1 5

2.092 2.968

1.784 2.632

8

3.494

1.041

H-2 → L+4(21%) H-1 → L+5(22%) H-3 → L(24%) H → L+3(18%) H-4 → L+2(23%) H-1 → L+5(22%)

(PV)9

1

1.663

0.077

H → L(98%) H-1 → L(42%) H → L+1(38%) H → L(98%) H-1 → L(40%), H → L+1(37%) H-2 → L+1(38%) H-3 → L(39%) H → L(97%)

5

2.490

1.597

H → L+2(39%)

11

2.947

1.503

3.031 3.383

1.294 1.319

configurations

oligomers

m

Eg

13 19 fosc

(TV)8

1 5

1.746 2.379

2.778 2.504

8

2.759

1.217

1

1.505

0.007

14 15 18 19

2.288 2.351 2.460 2.513

1.844 1.977 2.077 1.331

(TV)14

a

oligomers

H → L(97%) H-1 → L(44%) H → L+1(42%) H → L(98%) H-1 → L(42%) H → L+1(42%) H → L(86%)

H-4 → L(22%) H → L+3(16%) H-4 → L(25%) H-4 → L+1(39%) configurations H → L(97%) H → L+1(30%) H-1 → L(28%) H-2 → L+1(32%) H-1 → L+2(28%) H-1 → L(54%) H → L+1(43%) H-4 → L+1(56%) H-2 → L+4(29%) H-4 → L+2(26%) H-4 → L+2(48%) H-2 → L+4(26%)

The HOMO and LUMO are indicated by H and L, respectively. Electronic transitions are from S0 to Sm. Transition energies (Eg) are in eV.

HOMO compared to that in the LUMO of the oligomer. In A(PV)9, the HOMO is localized at one end of the helix, while the LUMO is localized at the other end of the helix. In the HOMO of A-(TV)14, the overlapping region has larger electron density than the nonoverlapping region, and in the LUMO, π electrons are mainly accumulated over the nonoverlapping region. Thus, the degree of electron density overlapping between HOMO and LUMO for A isomers of these compounds is quite small, in comparison to those for the B and C isomers. This results in decreasing the involvement of HOMOs and LUMOs toward major absorption peaks in cases of A isomers, and contributions of HOMO−N and LUMO+N orbitals become prominent for these transitions. CD spectra of A, B, and C conformers for the longest oligomer of each of our studied compounds are depicted in Figure 10. It is observed that all four helical oligomers A-(FV)9, A-(PV)9, A-(TV)14, and A-(TAm)10 are CD active having strong positive and negative Cotton effects. In addition, A(PAm)9 with an irregular folding structure is also optically active. As expected, all linearly conjugated conformers of the studied oligomers are found to be CD inactive. It is worth mentioning that CD spectra are sensitive to basis sets and DFT functionals, and the present work only highlights the fact that few of the studied oligomers are optically active.

Figure 9. Electron distributions in HOMOs and LUMOs of A isomers of (FV)9, (PV)9, and (TV)14.

density between HOMO and LUMO is quite large for these oligomers, and the main absorption peaks are dominated by HOMO → LUMO transitions. However, a much different organization is observed for isomer A of these oligomers. Although π electrons in HOMO and LUMO of A-(FV)9 are spread over the entire helical chain, larger electron density is found at the overlapping region of adjacent helical turns for the

4. CONCLUSION In this work, structural and optoelectronic properties of vinylene- and azomethine-linked furan-, pyrrole-, and thioH


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significant oscillator strengths in addition to other important electronic transitions, and in most of the cases, it is the most important electronic transition. This study suggests that helical π-conjugated heterocyclic oligomers, especially vinylene-linked oligomers, are interesting systems with quite strong CD signals for chiral conjugated materials. We hope that this computational work will stimulate experimental investigations of these helical systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08100. Figures: Geometries of few selected oligomers, HOMO and LUMO energies of (FV)4−9, bond length alternation values, ΔH−L and E1g of the oligomers. Tables: Dipole moments and electronic transition data of the oligomers (PDF)

■

Figure 10. CD spectra of A, B, and C isomers for (FV)9, (PV)9, (TV)14, (FAm)9, (PAm)9, and (TAm)10. Results are obtained at the TDDFT/B3LYP/6-31G(d,p) level.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

phene-based oligomers have been studied at the B3LYP/631G(d,p) and B3LYP-D3/6-31G(d,p) levels. Three different types of conformational isomers of each oligomer are considered. Out of these, one is expected to form folding structures after addition of a certain number of repeating units, and the other two have linear ground-state geometries. Our results show that the B3LYP functional fails to predict the ground-state geometries of studied foldamers, except in the case of TAm oligomers. However, the B3LYP-D3 functional is suitable for this purpose as it takes care of the dispersion interactions, while the B3LYP does not. As a result of these interactions, the order of stabilities of conformers changes with increasing chain length, and for long oligomers, helices are more stable than the corresponding linear isomers. However, in the case of TAm oligomers, a linear conformer is a more stable structure than the helical one due to the lack of π−π interaction between adjacent helical turns of their helical geometries having a large pitch. In FAm and PAm, the formation of hydrogen bonds between two terminal units results in nonhelical structures. The number of repeating units per helical turn (u), diameter, and pitch of the studied helical oligomers depend on the types of heterocyclic ring and linkage present in their repeating units. For helical oligomers, u is 7 for FV and PV, 8 for TAm, and 11 for TV. TAm and TV have the smallest and the largest diameters, respectively. Optoelectronic properties of the studied foldamers are quite different from those for linear isomers. Absorption bands of these foldamers are blue-shifted with respect to the absorption bands of the corresponding linear isomers. In the case of linear isomers, the first allowed electronic transition is the prominent transition, and in some cases of azomethine-linked oligomers, transitions other than the S0 to S1 transition have significant oscillator strengths. In most of the transitions, HOMO and LUMO orbitals are mainly involved. However, in foldamers, more than one electronic transition has significant contributions in absorption spectra. These electronic transitions are transitions from S0 to higher electronic excited states. Instead of HOMO and LUMO orbitals, HOMO−N and LUMO+N orbitals are mainly involved in these major electronic transitions. Unlike other foldamers, for TAm, the first electronic transitions have

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS H.S. acknowledges the University Grants Commission (UGC), New Delhi, for a senior research fellowship. This study was supported in part by a research grant from the Department of Science and Technology, New Delhi, India (DST Project No. SB/S1/PC-035/2013). We acknowledge the Center for Development of Advanced Computing (CDAC), Pune, for providing us with the high performance computing facility.

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