Odd–Even Effect of Thiophene Chain Lengths on Excited State

Mar 24, 2017 - In a self-assembly material system, odd–even effects are manifested from long-range periodic packing motifs. However, in an amorphous...
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Odd−Even Effect of Thiophene Chain Lengths on Excited State Properties in Oligo(thienyl ethynylene)-Cored Chromophores Xian Wang,†,∥ Guiying He,†,∥ Yang Li,†,∥ Zhuoran Kuang,†,∥ Qianjin Guo,†,∥ Jin-Liang Wang,§ Jian Pei,*,‡ and Andong Xia*,†,∥ †

Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Key Laboratories of Bioorganic Chemistry and Molecular Engineering and of Polymer Chemistry and Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In a self-assembly material system, odd−even effects are manifested from long-range periodic packing motifs. However, in an amorphous material system, due to long-range disorder, such phenomena are less prone to appear. Here, we report the discovery of a remarkable odd−even effect on the excited state properties of a series of conjugated thienyl ethynylene (TE) oligomers with truxene as end-capping units, Tr(TE)nTr (n = 2−6), in solution. Using steady-state and time-resolved spectral measurements, we found the fluorescence quantum yield and excited state dynamics, both showing odd−even alternation with increasing thiophene−ethynylene chain lengths in apolar cyclohexane (CHX). It is found that the symmetry properties with different torsional modes dominate the excited state processes. In polar tetrahydrofuran (THF), solvation lowers the twisting barriers, leading to symmetry breaking without special odd−even alternation over structures. The results presented here will be helpful for understanding odd−even effects of conjugated polymers and designing novel photoelectric materials.

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INTRODUCTION Thiophene-based π-conjugated materials have been widely investigated for their application in optical and electronic devices such as molecular wires, light-emitting diodes, and solar cells, owing to their conformation-dependent optical and electronic properties.1−5 It has been investigated theoretically and experimentally that the electronic properties of conjugated systems are linearly correlated to chain lengths or, rather, delocalization lengths.6−8 However, it still remains unclear how specific structural or environmental parameters can be comprehensively manipulated to improve efficiencies in future organic photovoltaic devices or light-harvesting materials.9,10 For a light-harvesting system, higher electron delocalization over a larger number of repeating units is expected to possess larger transition moments, which means stronger absorption, and it can also lead to a larger exciton migration distance.2 Furthermore, introduction of vinylene or ethynylene linkages to a thiophene-based polymer chain, as slices for a “sandwich”, can reduce the dihedral angles between repeating units and increase the π-electron delocalization over the main chain.8,10,11 © XXXX American Chemical Society

However, accompanying structural defects in the one-dimensional π-conjugated chain may limit the extent of maximizing conjugation along the backbone, such as defects, broken chains, intrachain molecular coupling at unexpected sites, and interrupted conjugation due to large torsional angles, so that the “effective π-conjugation length” is shorter than expected.12,13 It has been demonstrated in detail that a saturation of the effective conjugation length will occur when the number of repeating units approaches a certain value; thus a concomitant deviation from the expected linear correlation versus chain length is intensively studied.8,12,14 There is an issue of the optimal number of repeating units through the backbone for macromolecules considering the deviation with direct structure relevance. Control and optimization of the chain lengths are effective strategies for light-harvesting efficiency improvement and economical synthesis design. Recently, apart Received: January 8, 2017 Revised: March 19, 2017 Published: March 24, 2017 A

DOI: 10.1021/acs.jpcc.7b00203 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

absorption (fs-TA) spectral measurements together with timedependent density functional theory (TD-DFT) calculations. Our work provides a systematic illustration of the specific odd− even effect during the excited state relaxation processes. The results presented here with these sensitivities of excited state properties over structures and solvents will be helpful for understanding the odd−even effects of novel conjugated polymers and the rational design of novel photoelectric materials.

from a universal linear relation, the chain length dependence of optoelectronic materials was investigated, which shows profound odd−even effects in power conversion efficiency or hole mobility.15−20 These illustrate that a subtle change (the number of repeating thiophene units) can have a significant impact on the molecular packing motif in thin film as evidenced by circular dichroism or X-ray diffraction technique.21−23 The reported odd−even effects with chain lengths in various film systems are normally related to the packing form because of long-range periodic packing motifs.18,24 Such an effect on excited state properties is rarely observed in solution due to flexible conformation and the lack of long-range order.25 In this work, we address the odd−even effect of thiophene− ethynylene chain lengths on the excited state properties of a set of molecular wires, Tr(TE)nTr (n denotes the number of thiophene rings, n = 2−6), with linear structures connecting truxene (Tr) as end-capping units (see structures in Figure 1).26 The well-designed oligomers have been synthesized

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RESULTS AND DISCUSSION Steady-State Measurements and TD-DFT Calculations. Figure 2a shows the absorption spectra of Tr(TE)nTr (n = 2−6) in nonpolar cyclohexane (CHX). It is found that all the compounds show intense absorption in the region 350−500 nm with a peak around 410 nm. A bathochromic shift up to the maximum wavelength at ∼486 nm (n = 6) with an increase of the number of (TE)n units in the molecular backbone is observed.27 The continuous red shift is due to the increase of effective conjugation lengths, which is confirmed by a concomitant decrease in bond length alternations (BLA) and an increase in oscillator strengths by density functional theory (DFT) calculations (see Table S1).9,11 The primary absorption peaks of Tr(TE)nTr (n = 2−6) around 410 nm are typically attributed to intramolecular charge transfer (ICT) dependent π−π* interaction between terminal truxene (Tr) and central (TE) n units, which is identified by a small positive solvatochromic absorption in polar THF solvent (see comparative data in Table 1) as reported in other quadrupolar D−π−D systems.9,28 A secondary peak around 309 nm appearing in the absorption spectra, which exhibits no obvious solvatochromism, is assigned to the S0 → S1 transition of truxene units.26 Compared to the experimental results, it is found that the corresponding absorption wavelengths obtained

Figure 1. Molecular structures of Tr(TE)nTr (n = 2−6) compounds.

previously,27 and their electronic properties are expected to show intensive sensitivity to structure in both the ground states and the excited states. As a result, instead of a linear structure− function relationship, a significant odd−even alternation of fluorescence quantum yields and the dynamics of excited state structural relaxation of Tr(TE)nTr chromophores in nonpolar cyclohexane but not in polar tetrahydrofuran are clearly observed by using steady state and femtosecond transient

Figure 2. (a) Absorption spectra of Tr(TE)nTr (n = 2−6) in CHX. All absorption spectra shown are normalized at 309 nm for better comparison. (b) Calculated molecular orbitals (MOs) of TrTE2Tr at the B3LYP/6-31G* level. (c, d) Normalized fluorescence spectra of Tr(TE)nTr (n = 2−6) in (c) CHX and (d) THF. Fluorescence spectra were recorded under 400 nm excitation. (e, f) Distinct odd−even trends with increasing number of thiophene−ethynylene units were observed in fluorescence quantum yields (e), calculated (gas-phase) ground state dipole moments (f, solid line), and dihedral angle differences between S1 and S0 at the B3LYP/6-31G* level (f, dashed lines, detailed in Figure S5). TU is short for thiophene− ethynylene unit. B

DOI: 10.1021/acs.jpcc.7b00203 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Photophysical Parameters for Tr(TE)nTr (n = 2−6) in CHX (2C−6C) and THF (2T−6T) λabsa [nm] 2C 3C 4C 5C 6C 2T 3T 4T 5T 6T

309, 309, 309, 309, 309, 309, 309, 309, 309, 309,

392 404 409 411 415 394 409 417 423 424

λemb [nm] 458 472 479 484 487 460 476 486 491 493

(487) (503) (510) (514) (518) (490) (506) (514) (520) (521)

Stokes shift [cm−1]

Φfc

3676 3566 3573 3670 3562 3641 3441 3405 3274 3301

0.43 0.28 0.33 0.29 0.36 0.25 0.22 0.20 0.19 0.18

τFd [ns] 0.51 0.33 0.34 0.32 0.31 0.47 0.34 0.35 0.34 0.33

± ± ± ± ± ± ± ± ± ±

0.04 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.01 0.01

(93 (98 (93 (94 (98

± ± ± ± ±

3%), 1%), 5%), 4%), 2%),

0.98 1.02 1.24 1.30 0.91

± ± ± ± ±

0.03 0.03 0.04 0.04 0.02

(7 (2 (7 (6 (2

± ± ± ± ±

3%) 1%) 5%) 4%) 2%)

τTe [ns]

Φ1O2f

231 174 159 146 138 258 221 201 183 163

0.184 0.180 0.152 0.155 0.154 0.116 0.102 0.098 0.098 0.100

a

Maximum absorption wavelength. bMaximum emission wavelength. The shoulder emission peaks are shown in parentheses. cFluorescence quantum yields obtained using 9,10-diphenylanthracene as standard. dBoth τF values denote the fluorescence lifetimes with two exponentials obtained from the reconvolution fits. The percentage indicates the contribution of that component. eτT values denote the triplet lifetimes with single exponential fits. fSinglet oxygen quantum yields obtained using benzophenone as standard.

Tr(TE)nTr (n = 3, 5) compounds which have an axial symmetry, μg is slightly larger than 0.2. Higher planarity and linearity of the conjugated backbones are observed in optimized S1 geometries (Figure S3) in contrast to S0 geometries (Figure S2a). Quinoidal (TE)n backbone (Figure S4) is expected to form in the S1 state, because the carbon−carbon single bonds with bond length of ∼1.41 Å in the S0 state are changed to carbon−carbon double bonds with bond length of ∼1.38 Å in the S1 state for all derivatives (Table S2). Quinoid structure for the S1 state increases the rigidity of the backbone and limits the rotation around C−C single bond between thiophene rings; thus the vibronic structures of thiophene−ethynylene chains are able to be observed in the emission spectra (Figure 2c,d).33 In addition, the dihedral angles between adjacent thiophene−ethynylene units (TUs) (Table S3, Figure S5b) also reveal an odd−even alternation in the values of 1TU − 2TU and (n − 1)TU − nTU (n = 3−6), illustrating that the compounds with even numbers of TUs have larger ground-state distortion at both ends of the thiophene−ethynylene chain than those with odd ones. Upon excitation, the internal inter-ring twisting is obvious for the even-numbered due to relative larger difference of the inter-ring torsion angles between S0 and S1 (Figure 2f and Figure S5c), resulting in a higher energy barrier for the geometry change from Franck−Condon state relaxing to S1. Although the optimized geometries for all the compounds in CHX and THF obtained (Table S4, Table S5) by the polarizable continuum model (PCM)34 show similar odd−even trends (Figure S6) to the gas-phase results, the absence of the odd−even alternation observed experimentally in polar THF mainly results from a solvation-induced symmetry breaking, which takes effect by quickly lowering the polar excited state potential energy surface and twisting barrier.28,32,35,36 To determine intrinsic causes for these specific excited state properties in different solvents, fluorescence lifetime measurements were further performed, and the fitted results are listed in Table 1. These lifetimes probed at emission maxima are found to be single and two exponential in CHX and THF, respectively. The half-lives within a few hundred picoseconds were roughly in agreement with those in oligo(thienyl ethynylene)s (∼0.31 ns),37 suggesting that this part of emission is mainly contributed by the (TE) n chromophore. The long-lifetime emission only observed in THF is assumed to derive from a symmetry-broken excited state because of the fast solvation effect in polar THF, whereas there almost no solvation occurred in nonpolar CHX.9,28 The

by computations are significantly red shifted (Table S1). This overestimation of the calculated absorption is well-known to be related to the use of B3LYP/6-31G* level of theory.29 For ethynylene-linked heterocyclic oligomers, as shown in Figure 2b, Figure S1, and Table S1, the first optically allowed electronic transitions have the largest oscillator strengths, and the absorption bands are dominated by highest occupied to lowest unoccupied molecular orbital (HOMO to LUMO) transitions. All chromophores exhibit similar electronic distribution in the first excited states (Figure S1). Calculated HOMO coefficients show a delocalization over the whole molecular backbone, whereas LUMOs show less extensive electronic distribution on the truxene moieties, indicating that LUMOs are dominated by the electronic nature of thienyl ethynylene (TE) units. Furthermore, emission spectra with clear vibronic structures are observed (see Figure 2c,d). The vibronic structures are assigned to 0−0 and 0−1 transitions of oligo(thienylene ethynylene)s as reported in earlier works,30,31 whereas no obvious vibronic structures are observed in the absorption spectra. The lack of obvious mirror-image symmetry between absorption and emission spectra indicates different absorption (Franck−Condon) and fluorescence (relaxed) configurations. The large apparent Stokes shift (about 1 eV) between the emission and absorption maxima for all the compounds also reflect this disorder (Table 1). The mirrorless image signature takes place for the conformational relaxation which mainly relates to the torsional modes of TE units that have a soft ground-state potential but a stiffer excited-state potential.31 The vibronic structure observed in nonpolar CHX is progressively smeared out with increasing chain lengths, indicating that longer oligomers have higher degrees of freedom in their excited state structures. This effect manifests especially in polar THF because of more disorder factors such as solvation and conformation twisting.32 In fluorescence quantum yield (Φf) measurements, a pronounced odd−even alternation is clearly observed for all the compounds in CHX (Figure 2e), showing distinct emission probabilities which are mainly influenced by excited state conformational relaxation. Ground state and excited state geometry optimizations further highlight this odd−even effect. Gas-phase calculated dipole moments (μg) of the ground states (Figure 2f) are small, suggesting that these chromophores have nearly nonpolar ground states. For Tr(TE)nTr (n = 2, 4, 6) compounds, μg is close to 0 as a result of an inversion center (see Figure S2b), while for C

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100 ps in CHX exhibit a broad-band spectral shape, while in THF a weak double-peak-like spectrum is observed. After 100 ps, the broad ESA signals in CHX gradually develop to narrowband spectra around 700−750 nm (Figure 3), suggesting the formation of triplet state in nonpolar CHX. This can be identified by comparing the spectra at 1 ns from fs-TA measurement and at 200 ns from nanosecond laser flash photolysis in degassed solutions (see section S3 in Supporting Information),38 where two ESA spectra match well with each other (Figure 4). However, for the compounds in THF (Figure 4 and Figure S7), a mismatch in ESA between the TA spectra at 1 ns and at 200 ns in the range 600−800 nm is observed. The blue-side ESA with a peak around 650 nm at 1 ns relative to the long-wavelength triplet state absorption around 750 nm is typically attributed to the absorption of the long-lived conformational relaxed S1 state, resulting from fast solvationinduced twisting in polar THF, where the energy level of the excited state conformational relaxed states could be equal to or slightly lower than that of the triplet state (see Figure 5a, right panel). Furthermore, the quantum yield of 1O2 production was used as an approximation for estimating the quantum yield of these triplet states. As shown in Table 1, the quantum yield of 1 O2 production observed in THF is much lower in contrast to that in CHX, verifying the above-mentioned less formation of triplet states in THF due to solvation-induced competition between conformational twisting and intersystem crossing (ISC). This is also why the relative long fluorescence lifetime (∼1 ns) is observed in polar THF as mentioned in Table 1. Evidence is provided for the existence of several intermediate states that could be attributed to the conformational relaxed state and the triplet state. A kinetic model favors well a sequential mechanism on the picosecond time scale (see section S3 in Supporting Information), and the mechanism is proposed to describe the excited state geometrical relaxation and intersystem crossing of Tr(TE)nTr (n = 2−6) in both nonpolar CHX and polar THF (Figure 5a). The three- and four-exponential fittings of decay traces of intermediate states in CHX and THF are required to give the best global fitting for all the compounds, where the obtained evolution-associated difference spectra (EADS) and concentrations are shown in Figure 5c−f (for TrTE2Tr as examples). All the fitted kinetic parameters are listed in Table 2. The time constants τ2 and τ3, which represent the excited state conformational relaxation (twisting) and intersystem crossing (ISC) processes, respectively, exhibit obvious odd−even alternation (Figure 5b) for all the compounds in apolar CHX. This odd−even trend of the rates for conformational twisting (τ2) and ISC (τ3) obtained by global analysis are consistent with the changes of observed fluorescence quantum yields mentioned above. To interpret the odd−even effects occurring in CHX, it is found from the DFT calculations mentioned above that there are two main internal rotation ways. One is end-group rotation (Figure S5a) that occurs intensely in both odd- and even-numbered compounds, and the other is inter-ring twisting (Figure S5c) that manifests more in the even-numbered ones. Recent work on oligothiophenes reported that inter-ring twisting undergoes conformational relaxation on a time scale of tens of picoseconds.39 We found that the conformational relaxation processes in CHX for the even-numbered compounds is slower (∼10 ps) than that for the odd-numbered compounds (∼2 ps), emphasizing that the twisting barriers for the even compounds are higher compared to the odd ones. This leads to the large red shifts of the simulated emission (shown in Figure S12)

following femtosecond time-resolved transient absorption measurements further help to identify the solvent-dependent excited state dynamical behaviors. Transient Absorption (TA) Measurements. To selectively excite the π−π* absorption band for all the compounds, the 400 nm pump wavelength was chosen for fs-TA measurements. The time-resolved difference absorption spectra of Tr(TE)nTr (n = 2−6) in apolar CHX and polar THF at various delay times from 0.4 to 1000 ps in the visible spectral range are shown in Figure 3. All spectra are composed of a

Figure 3. (a−j) Transient absorption spectra at different time delays for Tr(TE)nTr (n = 2−6) in CHX (a−e) and THF (f−j) with 400 nm excitation.

negative signal in the 425−600 nm range and a positive signal in the 600−780 nm range. In apolar CHX, the negative signals with fine structures, which are consistent with the steady state emission spectra of Tr(TE)nTr, are ascribed to the stimulated emission (SE). However, in polar THF, structureless SE bands appear as the thiophene−ethynylene chain lengthened, similar to the changes observed in the steady-state emission spectra (Figure 2d). In addition, the intensities of SE signals between 0.4 and 6 ps slightly increase in CHX but apparently decrease in THF, suggesting that there is a fast (within 6 ps) solvation process in polar THF (∼0.94 ps for 22 °C as reported in ref 36) before the structural relaxation from the vertical excited state (Franck−Condon state) to stable emission state. Meanwhile, the positive excited-state absorption (ESA) signals within D

DOI: 10.1021/acs.jpcc.7b00203 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Spectra at 1 ns from fs-TA under 400 nm excitation compared to that at 200 ns from ns-TA under 355 nm excitation: (a, b) TrTE2Tr, (c, d) TrTE3Tr, and (e, f) TrTE4Tr in CHX and THF. Spectra comparisons for Tr(TE)nTr (n = 5−6) are shown in Figure S7.

Figure 5. (a) Proposed excited state relaxation mechanism of Tr(TE)nTr (n = 2−6) in aerated CHX and THF. (b) Distinct odd−even effects of time constants for excited state conformational relaxation and intersystem crossing. (c−f) Global analysis results (EADS and concentrations) of transient absorption spectra of TrTE2Tr in (c, d) CHX and (e, f) THF, where the black line stands for the instrument response. Insets of parts c and e represent the kinetics at selected wavelengths for showing the quality of global fitting. The red asterisk in the inset of part c refers to the nonresonant coherent signal from pure cyclohexane.40 Time-resolved TA analysis results for Tr(TE)nTr (n = 3−6) are shown in Figures S8−S11.

Table 2. Global Fitting Parameters Obtained from Femtosecond Transient Absorption Spectra of Tr(TE)nTr (n = 2−6) in Two Solvents 2C 3C 4C 5C 6C

τ1 [ps]

τ2 [ps]

− − − − −

13.2 ± 2.0 2.5 ± 0.3 7.8 ± 1.3 1.3 ± 0.2 10.1 ± 1.6

τ3 [ps]

τ4 [ns]

± ± ± ± ±

>1 >1 >1 >1 >1

436 270 347 247 303

30 15 20 13 20

τ1 [ps] 2T 3T 4T 5T 6T

0.9 0.9 1.2 1.0 0.7

± ± ± ± ±

0.1 0.1 0.2 0.1 0.1

τ2 [ps] 3.5 4.7 5.0 4.8 4.9

± ± ± ± ±

0.3 0.5 0.6 0.5 0.5

τ3 [ps]

τ4 [ns]

± ± ± ± ±

>1 >1 >1 >1 >1

373 234 232 221 293

20 10 10 10 16

reported in the literature,8 the fast solvation largely lowers the excited state potential energy surface and then sequentially leads to the fast conformational twisting across such a low rotation barrier. The final energy level of the excited state conformational relaxed states could be equal to or slightly lower than that of the triplet state (see Figure 5a). This could also help to explain why there is a long fluorescence lifetime observed in polar THF rather than in nonpolar CHX as seen in Table 1.

from the conformational relaxed state for the odd-numbered chromophores. In THF, the special odd−even effect vanishes as a result of symmetry breaking.9,28 As shown in Figure 5e,f and Table 2, it is found that the global fitted time constants from the solvation induced symmetry-broken ICT′ state to a stable conformational relaxed state are almost the same for all the compounds in THF. Considering the structureless emission bands observed in Figure 3f−j, the first decay within 1 ps which only presented in THF is assigned to fast solvation of THF and electron-vibration coupling process.32 Since the rotational barrier about the triple bond is very low, ∼0.9 kcal/mol as E

DOI: 10.1021/acs.jpcc.7b00203 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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elsewhere.44−46 Briefly, fs-TA spectra were measured with pump pulse of 400 nm and probing with white light generated from the fundamental output of the Ti:sapphire regenerative amplifier (Coherent Legend Elite, 1 mJ, 50 fs, 500 Hz, 800 nm).47 The pump energy at samples with 400 nm is about ∼65 μJ with ∼125 μm spot size. Magic angle (54.7°) pump−probe polarization was set using a half-wave plate to eliminate anisotropic signals. The samples were prepared in solutions of ∼1 × 10−5 M in 1 mm path length quartz cuvettes to minimize reabsorption. Global analysis of the dispersion-corrected data were done using the Glotaran software.48,49 Nanosecond Laser Flash Photolysis System. Nanosecond transient absorption spectra (ns-TA) were obtained on an LP920 laser flash photolysis spectrometer (Edinburgh Instruments Ltd.) with excitation at 355 nm with laser pulses of ∼8 ns duration (a Nd:YAG laser Surelite II, Continuum Inc., 10 Hz).50 Samples of ∼1 × 10−6 M aerated or degassed solutions (purged with N2 for 10 min) were prepared in 1 cm path length quartz cuvettes. Singlet Oxygen Quantum Yields. The singlet oxygen emissions were collected by a focusing lens and directed to a photomultiplier tube (Hamamatsu Photonics, R5509-72), which was placed in a chamber cooled to −80 °C by utilizing liquid nitrogen as the system coolant. The 1O2 quantum yields were monitored by its emission at around 1270 nm following 355 nm excitation, and were calculated by comparing the luminescence intensities toward that of the standard photosensitizer benzophenone (BP) (ΦBP = 0.35, in benzene).51 In detail

CONCLUSIONS A series of Tr(TE)nTr (n = 2−6) compounds were designed to explore how the excited state properties are influenced by the number of repeating building blocks and the solvent environments. The chromophores with even and odd numbers of thiophene rings adopt central and axial symmetry, respectively. All compounds favor more rigid planar structures after excitation simulated by TD-DFT analysis. The differences in symmetries lead to a significant odd−even effect on the excited state conformational relaxation processes in apolar solvent (i.e., cyclohexane). For centrosymmetric chromophores (n = 2, 4, 6), both end-group rotation and inter-ring twisting occur in the excited state conformational relaxation. For axisymmetric compounds (n = 3, 5), this relaxation is dominated by just end-group rotation without regard to long-lived inter-ring twisting. Furthermore, in polar tetrahydrofuran, the solvation quickly lowers the excited state twisting barriers, leading to the symmetry breaking without such odd−even alternating trends over structures. All results presented here demonstrate the intriguing odd−even effects caused by the backbone lengths of Tr(TE)nTr (n = 2−6) compounds, which also inspire us to study if the vested properties of other similar copolymers can be altered nonlinearly by a subtle change (changing the number of repeating units along backbones). Unfortunately, these structural adjustments frequently go unnoticed when researchers focus their entire attention on expanding derivatives by adding large functional groups. Our results will be a clue to designing better structure-dependent photovoltaic materials for showing how many repeating thiophene units should be introduced into the backbone, and how the operating surrounding environment should be adapted for the future development of the next generation of functional organic lightharvesting materials.

ϕ In2τ = ϕBP IBPn ph 2τBP

where n is the refractive index, τ is the phosphorescence lifetime, and I and IBP mean the luminesence peak intensities of 1 O2 produced by Tr(TE)nTr oligomers and BP, respectively.52 Computational Details. All chromophores were modeled on the Gaussian 09 package53 and performed in a vacuum by the use of B3LYP functional.54 Analogous ground-state structures in combination with the polarizable continuum model (PCM)34 by means of CHX and THF with dielectric constants 2.0255 and 7.58,56 respectively, are shown in Tables S4 and S5. To reduce the computational cost, n-alkyl side chains that are largely uninvolved in the electronic processes were replaced by methyl groups. For 6-31G* no symmetry constraints were imposed on the ground-state geometry. Excited state structures with related photophysical properties were calculated in a vacuum at the TD-B3LYP/6-31G* level.9 In this study, the calculations were done for 10 excited states (sufficient for the visible absorption spectrum analysis in this system) and optimized for the first excited state. Since recent calculations generally confirmed that the transoid conformers are energetically more stable than the cisoid conformers,29,57 discussions here were based on all-trans conformations.

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EXPERIMENTAL SECTION Sample Preparation. All the compounds of Tr(TE)nTr (n = 2−6) were obtained through iterative Pd-mediated Sonogashira coupling reactions.27,41,42 Details of the synthesis and characterization of these compounds have been reported elsewhere.27 All compounds were identified by 1H and 13C NMR spectroscopies and MALDI-TOF mass spectroscopy, respectively. Purified chromophores stored in powder form were dissolved in proper solvents. Both CHX and THF were AR grade and used as received without further purification at ambient temperature. UV−Vis Absorption and Fluorescence Spectral Measurements. Absorption and fluorescence spectra were measured in 1 × 1 cm quartz cuvettes using a UV−vis spectrometer (Model U-3010, Hitachi, Japan) and a fluorescence spectrometer (F-4600, Hitachi, Japan), respectively.43 Fluorescence quantum yields were estimated with 9,10-diphenylanthracene as standard (Φf = 0.97) under the same excitation wavelengths. Fluorescence Lifetimes. The fluorescence lifetimes were performed in dilute solutions by a time-correlated singlephoton counting (TCSPC) spectrometer (F900, Edinburgh Instrument) under 370 nm excitation using an 80 ps laser diode (PicoQuant PDL 808). The instrument response function was ∼300 ps. Femtosecond Pump−Probe Absorption Spectroscopy. Femtosecond transient absorption (fs-TA) was measured using a home-built femtosecond broad-band pump−probe setup with a time resolution of ∼100 fs as described

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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.7b00203. Additional results, such as detailed DFT simulated structures and MO orbitals in a vacuum and in two solvents, and comparison of fs-TA and ns-TA data in two solvents (PDF) F

DOI: 10.1021/acs.jpcc.7b00203 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xian Wang: 0000-0003-3520-7397 Jin-Liang Wang: 0000-0001-5726-3336 Jian Pei: 0000-0002-2222-5361 Andong Xia: 0000-0002-2325-3110 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (Nos. 2013CB834604, 2013CB933501, and 2015CB856505), NSFCs (Nos. 21333012, 21673252, 21672023, and 21373232), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB12020200).

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