Photoinduced Electron Transfer in Naphthalene Diimide End-Capped

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Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers Austin L. Jones, Melissa K. Gish, Charles J. Zeman, John M. Papanikolas, and Kirk S. Schanze J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09095 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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The Journal of Physical Chemistry

Photoinduced Electron Transfer in Naphthalene Diimide End-Capped Thiophene Oligomers Austin L. Jones†, Melissa K. Gish‡, Charles J. Zeman IV†, John M. Papanikolas‡ and Kirk S. Schanze*≠#

† Department of Chemistry and Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida 32611-7200, United States. ‡ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North

Carolina 27599, United States ≠ Department of Chemistry, University of Texas at San Antonio, One UTSA Way, San Antonio, TX 78249 # Key Laboratory of Resource Chemistry, Shanghai Normal University, Shanghai 200234, China

KEYWORDS: Thiophene Oligomers, Donor-Acceptor Dyads, Photoinduced Electron Transfer, Ultrafast Spectroscopy

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ABSTRACT. A series of linear thiophene oligomers containing 4, 6, 8, 10 and 12 thienylene units were synthesized and end-capped with naphthalene diimide (NDI) acceptors with the objective to study the effect of oligomer length on the dynamics of photoinduced electron transfer and charge recombination. The synthetic work afforded a series of non-acceptor substituted thiophene oligomers, Tn, and corresponding NDI end-capped series, TnNDI2 (where n is the number of thienylene repeat units).

This paper reports a complete photophysical

characterization study of the Tn and TnNDI2 series by using steady-state absorption, fluorescence, singlet oxygen sensitized emission, 2-photon absorption, and nanosecondmicrosecond transient absorption spectroscopy. The thermodynamics of photoinduced electron transfer and charge recombination in the TnNDI2 oligomers were determined by analysis of photophysical and electrochemical data. Excitation of the Tn oligomers gives rise to efficient fluorescence and intersystem crossing to a triplet excited state that is easily observed by nanosecond transient absorption spectroscopy. Bimolecular photoinduced electron transfer from the triplet states, 3Tn*, to N,N-dimethylviologen (MV2+) occurs and by using microsecond transient absorption it is possible to assign the visible region absorption spectra for the one electron oxidized (polaron) states, Tn+.. The fluorescence of the TnNDI2 oligomers is quenched nearly quantitatively, and no long lived transients are observed by nanosecond transient absorption.

These findings suggest that rapid photoinduced electron transfer and charge

recombination occurs, NDI-1(Tn)*-NDI → NDI-(Tn)+.-NDI-. → NDI-Tn-NDI.

Preliminary

femtosecond-picosecond transient absorption studies on T4NDI2 reveal that both forward electron transfer and charge recombination occur with k > 1011 s-1, consistent with both reactions being nearly activationless. Analysis with semi-classical electron transfer theory suggests that both reactions occur at near the optimum driving force where ∆G ~ λ.


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Introduction Photoinduced electron transfer is a primary step in many biological processes including the life-sustaining process of photosynthesis.1 Electron transfer has applications in solar energy conversion and storage,2 molecular electronics,3 light-emitting diodes,4 and organic photovoltaics.5,6 Therefore, it is clear that understanding the mechanism of electron transfer in organic systems is essential for advancing solar conversion and organic electronics. A substantial effort has been put into understanding electron transfer mechanisms through creation of structures that incorporate electron donor, electron acceptor and bridge components to create molecular wire-like structures.7,8 Molecular wires have been synthesized through a variety of methods, including alteration of the donor and acceptor units,9 spacer unit,10 relative distances between donor and acceptor,11-13 and the orientation of donor/acceptor units.14 Oligo- and polythiophenes are versatile materials that are widely used in electronic, optoelectronic and sensory devices.15 Specifically, oligo- and polythiophenes are among the most widely used organic materials in field-effect transistors (OFET) and organic photovoltaic cells.16 Polythiophene is a preferred electron donor due to its relatively low oxidation potential (high HOMO energy), good chemical stability, high charge mobility, and tendency to form organized, semi-crystalline domains that facilitate charge transport.17 Given the importance of polythiophene in applications, oligothiophenes have also been explored extensively as donor molecules for studying light-induced charge transfer as well as active media in OFETs.18-20 Although several electron donor-acceptor molecules featuring oligothiophene motifs have been synthesized and investigated for their electron transfer kinetics, no report has revealed clear results regarding effect of thiophene oligomer length on the dynamics of photoinduced electron transfer and charge recombination.21 Perylenediimide (PDI) derivatives have been used as


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acceptors in oligothiophene-acceptor structures; however, these systems display complex transient absorption due to the overlapping excited state absorption and dynamics of the oligothiophene and PDI units.

22-32

Fullerene has also been utilized as an electron acceptor in

oligothiophene-fullerene dyads because it is widely used as the acceptor in organic photovoltaic materials, but competition between energy and electron transfer obscure the electron transfer dynamics in these molecules.

14,33-37

Other acceptors have also shown promise in conjunction

with oligothiophenes, including methyl viologen (MV2+) and various arylenes with electronwithdrawing substituents.38,39 The present investigation seeks to study the dynamics of photoinduced electron transfer and charge recombination in a series of naphthalene diimide (NDI) end-capped oligothiophenes (TnNDI2, Chart 1). In this study, NDI was selected as the acceptor for several reasons. First, NDI is a good electron acceptor with a relatively low reduction potential (-1.10 V vs. Fc/Fc+).40,41 Second, there is minimal ground state absorption overlap between NDI and the oligothiophenes, which allows selective excitation of the oligothiophene donor.

Third, the visible region

absorption spectrum of the NDI radical anion is well characterized, making it easy to identify its presence by visible region transient absorption spectroscopy.41 Finally, as discussed below, the phenylene spacer that is inserted between the oligothiophene (donor) and NDI (acceptor) units makes the donor-acceptor electronic coupling weak, ensuring that the forward (photoinduced) and return electron transfer reactions are in the non-adiabatic regime.


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Chart 1 The primary objective of this work is to study electron transfer between oligothiophenes with various lengths to an NDI acceptor as shown in equations 1 and 2, NDI-Tn-NDI + hν → NDI-1(Tn)*-NDI NDI-(Tn)+.-NDI-. →

→ NDI-(Tn)+.-NDI-.

NDI-Tn-NDI

(1) (2)

where 1Tn*NDI represents the singlet excited state of the Tn oligomer and Tn+ NDI- is the charge separated state where an electron is localized on one of the NDI acceptors, and the hole is on the Tn moiety. Thiophene oligomers of varying chain lengths from 4 to 12 units with and without NDI moiety end-caps were investigated. Variation of the length of the oligothiophene is expected to have several effects on the electron transfer processes. First, the energetics will change, due to the variation in first oxidation potential of the oligothophene with length. Second, and more


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interesting, is the possibility that the electronic structure of the charge separated state may change as the oligomer length increases. Specifically, as the thiophene oligomer length increases, the effects of localization/delocalization of the thiophene-localized cation (polaron) that is present in the charge-separated state may become apparent and may manifest by affecting the dynamics of charge recombination. This is the first of two papers that describe studies of the TnNDI2 and Tn oligomers. This paper outlines the photophysical and electrochemical properties, including, absorption, fluorescence, triplet excited state absorption/dynamics and energetic properties of the capped and non-capped oligomers. The subsequent paper will focus on application of ultrafast spectroscopy to study the dynamics of charge separation and recombination in the series including a detailed discussion of the effects of oligomer structure on the dynamics of charge separation and recombination.42,43 Experimental Characterization methods. NMR spectra were recorded using a Varian Inova 500 spectrometer operating at 500 MHz for 1H NMR and 125 MHz for

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C NMR. Mass spectral

analyses for newly synthesized compounds were recorded using an AB Sciex 5800 MALDI TOF/TOF by the Mass Spectrometry Services at the University of Florida. Electrochemistry. Cyclic and differential pulse voltammetry experiments were performed on a Bioanalytical Systems CV50W electrochemical analyzer at a sweep rate of 100 mV/s and 20 mV/s respectively. The setup consisted of a 2 mm platinum working electrode, a platinum wire as a counter electrode, and a silver wire as a pseudo-reference electrode. Sample solutions were prepared in methylene chloride with 0.1 M tetrabutylammonium hexafluorophosphate (NBu4PF6)


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as a supporting electrolyte. The methylene chloride was dried by refluxing over calcium hydride before use. A gentle flow of argon was maintained throughout the voltammetry experiments. Electrochemical potentials were calibrated against a ferrocene/ferrocenium internal standard in methylene chloride. Photophysical Analysis. All photophysical measurements were performed in spectroscopygrade chloroform in a 1 × 1 cm2 quartz cuvette unless otherwise noted. Steady state UV-visible absorption spectra were obtained on a Shimadzu UV-1800 dual beam spectrophotometer. Corrected steady-state emission measurements were performed on a Photon Technology International (PTI) spectrophotometer. All solutions used for measuring fluorescence quantum yield or molar extinction coefficient were kept at an optical density of 0.1 or lower. Refractive index corrections were accounted for when measuring the quantum yields of both the compound and standard. Fluorescence lifetimes were recorded with a PicoQuant FluoTime 100 Compact Fluorescence Lifetime Spectrophotometer using a time-correlated single photon counting (TCSPC) instrument, and were excited at 375 nm by a PDL-800B Picosecond Pulsed laser. NearIR emission measurements for singlet oxygen quantum yield experiments were conducted on a PTI Quantamaster near-IR spectrophotometer equipped with an InGaAs photodiode detector. All solutions used for singlet oxygen quantum yield experiments were oxygenated for 10 minutes before being measured. Nanosecond Transient-Absorption Spectroscopy. Measurements were performed on an inhouse built instrument using the third harmonic of a Continuum Surelite series Nd:YAG laser (λexc = 355 nm, 10 ns FWHM 4 mJ per pulse). A xenon flash lamp was used as the probe light for the pump-probe technique employed, and the signal was detected using a gated-intensified CCD mounted on a 0.18 M spectrograph (Princeton PiMax/Acton Pro 180). Experiments were


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conducted in HPLC grade tetrahydrofuran (THF) and 1:3 acetonitrile/THF mixtures for the intermolecular charge-transfer experiments. Oligomer concentrations were adjusted to give an optical density ≈ 0.7 at 355 nm, and all samples were deoxygenated for 30 minutes prior to measurement. Methyl viologen was used as the electron acceptor in the intermolecular charge transfer experiments at a concentration of 0.1 M. The solutions were continuously stirred throughout each experiment. A 1 cm path length flow cell (10 mL) was used for all transient absorption (TA) experiments. The TA spectra were collected from 350 nm to 850 nm with a 50 ns initial camera delay and a variable delay time increment depending on the triplet lifetime. Each time slice was averaged over 50 measurements. Femtosecond Transient-Absorption Spectroscopy. Femtosecond transient absorption spectroscopy was performed using a mode-locked Ti:Sapphire regenerative amplifer (ClarkMXR 2001) in a pump-probe configuration. The 775 nm (150 fs pulse width) generated light is split at the output where 90% of the beam is used to create the pump beam and 10% is used for the white light probe. The 425 nm pump is generated using the frequency doubled idler (λ = 945 nm) of a home-built optical parametric amplifier (OPA) mixed with residual 775 nm in a BBO crystal. The 10% output reserved for the probe is sent through a computer-controlled delay stage to control the time delay between the pump and probe at the sample. White light is then created via supercontinuum generation by focusing the 775 nm beam into a rotating CaF2 window. The polarization between the pump and probe is magic angle (54.7°). The two beams are focused and spatially overlapped into the sample and the changes in white light probe pulse are monitored by a CMOS sensor. Two-Photon Absorption (2PA). The second harmonic of an Nd;YAG Spectra-Physics Millenia eV laser (532 nm) was used to pump a mode-locked Spectra-Physics Tsunami


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femtosecond Ti:Sapphire laser. The Tsunami laser provides pulses of 80 MHz at 100 fs duration that can be tuned from 700-1000 nm. The 2PA excited fluorescence spectra were recorded on a SPEX Fluoromax spectrophotometer. The 2PA spectra and two-photon cross sections (σ2) were collected using the two-photon excited fluorescence method (2PEF), with rhodamine B as the reference compound. All solutions were prepared in spectral grade chloroform to have an optical density of less than 0.3 at the registration wavelength. Procedures for the 2PEF method and the standard cross-section were adopted from the literature.44 Results and Discussion Structures and Synthesis. In the present work, five oligothiophenes of varying length and the corresponding NDI end-capped oligomers were synthesized and characterized by electrochemistry and photophysics, with the goal of understanding the effect of oligomer length on photoinduced charge transfer. Each oligothiophene has a bithiophene core which excludes any alkyl solubilizing groups, installed to ensure a symmetrical oligomer. Extending from the bithiophene core, each of the additional thiophene units contains a 3-hexyl solubilizing group to afford solubility. The oligothiophenes were synthesized using

Stille coupling reactions as

reported by Koch and coworkers by using Pd(PPh3)4 catalyst, and they were purified via column chromatography.45 Each consecutive oligomer increased in length by two thiophene units, one on each side of the bithiophene core, and they are denoted by Tn (Chart 1), where n is the number of thiophene units in the overall structure. The donor-acceptor compounds, referred to as TnNDI2, feature NDI acceptor units on both termini of the oligomer. The symmetry afforded by bis-substitution of the NDI moieties lead to facile synthetic routes and good yields of the pure compounds. Note that the TnNDI2


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compounds contain a phenylene ring between the oligothiophene and diimide nitrogen of the NDI units; this moderates electronic coupling between the oligothiophene donor and the NDI acceptor. As discussed below, this weak coupling means that electron transfer is likely nonadiabatic, involving a locally excited state of the donor oligothiophene giving rise to a charge separated state, where the hole is localized on the oligothiophene segment and the electron is localized on the NDI. Complete synthetic schemes, procedures, and structural characterization for all of the compounds are located in the Supporting Information. Electrochemistry. Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were performed on all oligothiophenes (i.e, T4 through T12) to identify the effect oligomer length has on the oxidation potentials. Differential pulse voltammetry was used to obtain accurate oxidation potentials due to the sensitivity of the oligothiophenes to electropolymerization which gives rise to irreversible waves in the CV.46 The electrochemical results were complementary to those in the literature and are located in the supporting information (Fig. S23), revealing the oxidation potential decreasing with increased oligomer length.18,47,48 Two reversible waves appear for oligomer T6 at 330 mV and 470 mV referenced to Fc/Fc+ and the waves become closely spaced as conjugation increases from T6 to T12. This is due to the formation of a dication state which becomes increasingly stabilized as conjugation increases to the point where the T12 waves converge and appear as a single wave. There is a new oxidation peak visible in the cyclic votammogram of T4 on the reverse scan that appears around 200 mV, attributed to oxidative α,α’ homocoupling of two oxidized oligomers.49 For the T4NDI2-T12NDI2 series two reversible cathodic peaks were observed in the cyclic voltammograms, corresponding to sequential one-electron reductions of the NDI unit.50,51 The half-wave potential for the first reduction of the TnNDI2 oligomers was observed at -1.06 V with


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respect to ferrocene (Fc/Fc+). This reduction potential agrees well with previous reports for reduction of the NDI moiety.40,51 The CV of T10NDI2 is shown in the Supporting Information (Fig. S24). ). In a later section, we will consider the relationship between the redox potentials for the oligomers with the energies of the charge-separated states. Absorption and Fluorescence Spectroscopy. The absorption and emission spectra of the Tn and TnNDI2 series were measured in chloroform and are shown in Figure 1. The Tn model oligomers display a strong π-π* absorption band in the UV-visible absorption spectra with a maximum absorbance at 378 nm for T4, shifting incrementally to 440 nm for T12.36 The red-shift in the absorption spectrum and increase of the molar extinction coefficient (Table 1) is consistent with increasing conjugation length in the oligomers. The molar absorptivity per thiophene repeat unit is estimated to be between 6500-8000 L mol-1 cm-1 which is consistent with previous literature.47 The broad, structureless absorption bands are likely due to the presence of several nonplanar conformers in the electronic ground state with a minimal difference in their energy giving conformational freedom in solution.52 Steady-state emission spectra of the Tn oligomers were recorded by exciting at the corresponding absorption maximum of each oligomer. The fluorescence of the Tn oligomers shows distinct vibronic structure due to the rigid, quinoidal-like planar structure adopted in the excited state.53 Again, the fluorescence spectra red shift with increased oligomer length due to the increased stabilization of the singlet excited state by increased conjugation. Fluorescence quantum yields (ϕfl) were determined for the Tn series using 9,10-diphenylanthracene (DPA, ϕfl = 0.90) in cyclohexane as a standard.54 The fluorescence quantum yield increases from 0.23 for T4 to 0.42 for T6, and then remains constant at ~0.38 for the oligomers with n > 6 thiophenes. The increase in quantum yield from T4 to T6 is due to a decrease in the efficiency of singlet-


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triplet intersystem crossing (see below). Fluorescence lifetimes were also recorded in chloroform and followed a similar trend as the quantum yield data suggesting that the radiative decay rate is relatively constant across the series (Table 1). The absorption spectra of the TnNDI2 oligomers are essentially a superposition of the individual absorption spectra of the oligothiophene and NDI components. The absorption and emission spectra of NDI are found in the SI (Figure S26) for comparison. This finding suggests that there is relatively weak electronic interaction between the oligothiophene segment and the NDI acceptor units. The weak interaction is likely due to the phenylene unit between the donor and acceptor which is twisted relative to the plane defined by the NDI moiety mitigating π overlap between the Tn and NDI units.55 On the other hand, the phenylene group appears to be more strongly coupled to the Tn unit, as evidence by a small red-shift in the π-π* absorption band in the absorption spectra of the TnNDI2 oligomers relative to the corresponding Tn oligomer. The NDI absorption bands, which are clearly resolved in the TnNDI2 spectra at 361 and 381 nm, show no shift as the length of the Tn unit is increased. Fluorescence from the TnNDI2 compounds was strongly quenched, with quantum yields reduced by 50- to 100-fold compared to the Tn models (see ϕfl values in Table1). The fluorescence quenching is attributed to very rapid photoinduced electron transfer from the excited Tn segment to an NDI acceptor (eq. 1), as discussed in more length below. Due to the strong quenching of the fluorescence, the vibronic shape that was present in the model oligomers is lost (see insets in the fluorescence panels, right hand side of Fig. 1). The fluorescence quantum yields were measured for the NDI substituted oligomers using Ru(bpy)3Cl2 (ϕ = 0.04) in aerated water as a standard.56 The quantum yield of T4NDI2 (ϕfl =0.0004), when compared to T4 (ϕfl =0.23) provides clear evidence that photoinduced electron transfer occurs. Also, it is apparent


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that the fluorescence quantum yields increase slightly as the length of the Tn segments, suggesting that photoinduced electron transfer becomes slightly less efficient as conjugation increases.

Figure 1: Absorption (left) and normalized fluorescence (right) spectra of Tn and TnNDI2 oligomers. Absorption is plotted as a function of molar extinction coefficient (). The relative fluorescence intensity of corresponding Tn and TnNDI2 oligomers (black solid lines and red dashed lines, respectively) reflects the relative quantum yields. The insets in the right panels are the normalized fluorescence spectra of the corresponding TnNDI2 oligomers.


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Table 1: Photophysical Properties of Tn and TnNDI2 Oligomers

Compound

λmax(abs)/nma

/M-1cm-1b

σ2/GMc

ϕfl/%d

τf/nse

ϕ∆/%f

τT/µsg

T4

378

27,000

2

23

0.3

72

1.5

T4NDI2

409

32,000

--

0.04

h

i

g

T6

412

37,000

14

42

0.7

66

18

T6NDI2

428

41,000

--

0.1

h

i

g

T8

429

52,000

39

38

0.6

63

13

T8NDI2

433

60,000

--

0.14

h

i

g

T10

436

76,000

83

38

0.64

54

12

T10NDI2

439

84,000

--

0.47

h

i

g

T12

440

95,000

186

38

0.6

49

11

T12NDI2

440

101,000

--

0.94

h

i

g

a,b

Measured in chloroform and recorded as thiophene π-π* transition at room temperature. cRecorded at 800 nm in chloroform. dMeasured in chloroform; oligothiophenes were measured with respect to 9,10-diphenylanthracene (ϕ = 0.90) in cyclohexane and the DA compounds with respect to Ru(bpy)3Cl2 (ϕ = 0.04) in aerated D.I. water. e Measured by TCSPC in chloroform, TnNDI2 oligomers lifetimes were faster than the instrument response time ~100 ps. f Measured in CDCl3 using terthiophene in CDCl3 (ϕ = 0.84) as an actinometer. gMeasured in deoxygenated THF, TnNDI2 oligomers exhibited no triplet absorption. h Lifetime is less than instrument response which is ~100 ps. i Singlet oxygen yield is < 1%.

Two-Photon Absorption Spectroscopy. Thiophene has also been used in many donoracceptor oligomers and polymers for the study of two-photon absorption for applications in photovoltaics and organic light emitting diodes (OLEDS).26,57-59 Therefore, it was of interest to


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measure the 2PA cross sections (σ2) of the thiophene oligomers that were the subject of the present study. 2PA was probed in the Tn series using the 2-photon excited fluorescence (2PEF) method with near-infrared excitation from a Ti:sapphire oscillator (100 fs, ~100 MHz).60,61 The σ2 values were calculated and plotted using the 2PEF method for all members of the Tn series. The cross-section at 800 nm is presented in Table 2, and the spectra are located in the supporting information (Fig. S24). Two-photon absorption studies were not performed on the NDI substituted compounds due to their low fluorescence yields. 2PA cross sections and spectra have been previously reported for dendritic and macrocyclic thiophene oligomers, but to the best of our knowledge have not been reported for the linear oligomers.62-64 Each model oligomer has the same 2PA spectral shape with a maximum absorption around 725 nm (Fig. S24). The σ2 values increase with conjugation length and molar absorptivity, which both increase as a function of oligomer length.65 The increase in σ2 is nonlinear when comparing the variation of σ2/n across the series. T4 has a 2 GM absorption per thiophene repeat unit, whereas T12 features has σ2/n = 15.5 GM per repeat unit cross section, suggesting the enhanced σ2 of the longer oligomers reflects a synergistic effect arising from the increased conjugation and consequent electronic polarizability. In general, it appears that the linear compounds absorb two photons more efficiently than their dendritic analogues, but are less adept than macrocyclic thiophene oligomers.63,64 This is due to the conjugation length systematically increasing with the linear and macrocyclic oligothiophenes, whereas thiophene dendrimers inherently have a disrupted conjugated backbone. A dendrimer consisting of 42 repeat units has σ2/n = 14.7 GM per repeat unit, whereas a macrocyclic thiophene consisting of 15 repeat units has σ2/n = 98 GM per repeat unit.63,64


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Nanosecond-Microsecond Transient Absorption Spectroscopy: Triplet States and Bimolecular Electron Transfer. To better understand the nature of the excited states of T4-T12, transient absorption spectra of the compounds were measured using nanosecond-microsecond pump-probe spectroscopy for deoxygenated THF solutions (Fig. 2, left). In all oligomers a strong transient absorption is observed, characterized by a negative feature due to ground state bleaching and a positive absorption in the visible and near-infrared. The spectra are similar to those previously reported for the triplet state in oligothiophenes, and on this basis we attribute the spectra triplet-triplet (TT) absorption of the Tn oligomers.66 Note that the TT absorption band red shifts as the oligomer length increases. This trend has been seen before in oligomers of 1 to 7 repeat units in work by de Melo and coworkers.67 In agreement with their report, the T4 oligomer shows a vibronic structure, whereas all the other spectra are structureless. The TnNDI2 oligomers featured no signal in the nanosecond-microsecond transient absorption measurements; this is attributed to the rates of the excited state electron transfer and recombination exceeding the detection limit of the instrument (∼15 ns, see below). Lifetimes of the excited triplet states for the model oligomers increases from T4 (1.5 µs) to T6 (18 µs) and decreased as the length increased further (Table 1). In order to characterize the absorption of the radical cation (positive polaron) states of the Tn oligomers, bimolecular photoinduced electron transfer experiments were performed using methyl viologen (MV2+) as an electron acceptor. Photoinduced electron transfer occurs from the triplet state of the oligothiophene to MV2+, eg. 3

Tn* + MV2+ → Tn•+ + MV•+

(3)

Experiments were performed in deoxygenated 1:3 acetonitrile:THF solution with an oligomer concentration such that the optical density was 0.7 at λexc = 355 nm and [MV2+] = 0.1 mM. The


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spectra show generally similar features and temporal evolution (Fig. 2). At early times after the laser flash, the triplet absorption is observed and then the spectrum evolves on the time scale 100 to 1000 ns to display a red-shifted absorption due to the Tn•+ cation radical (positive polaron) as well as narrow band at 390 nm which is assigned to the MV+ cation.68 Interestingly, the absorption of the polaron states for T4 and T6 are distinct and red-shifted relative to the triplet absorptions; however, for the longer oligomers the polaron absorption broadens and does not display a distinct red-shift from the triplet absorption. The T4 cation absorption has been reported previously, and recently exploited in donor-acceptor peptide nanostructures.14,69

Figure 2: (Left) Transient absorption difference spectra of Tn oligomers in deoxygenated THF (λexc = 355 nm, 10 mJ/pulse, 50 ns initial delay). (Right) Transient absorption spectra of Tn oligomers with 0.1 mM MV2+ in deoxygenated 1:3 acetonitrile/ THF (λexc = 355 nm, 10 mJ/pulse, 50 ns initial delay, 100 ns delay increment).


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Singlet oxygen quantum yield experiments were performed on the Tn oligomers to determine the lower limit for singlet-triplet intersystem crossing yield. These measurements were conducted in CDCl3 after 10 minutes of purging with oxygen using terthiophene (ϕ∆ = 0.84) as a standard.70 Tetrathiophene sensitized singlet oxygen most efficiently (ϕ∆ = 0.72) and the yields decreased with increasing oligomer length to 0.49 for T12 (Table 2). Not surprisingly, the TnNDI2 oligomers exhibited negligible singlet oxygen quantum yields (ϕ∆ < .01), consistent with the strong quenching of the singlet state noted above. For the Tn series, when the fluorescence and singlet oxygen quantum yields are summed, the total is in the range of 0.9 – 1.0, indicating that the singlet state decays predominantly via radiative decay (fluorescence) and intersystem crossing. The same effect was previously observed in a study of thiophene oligomers with n = 1 – 5.67 Ultrafast Transient Absorption Spectroscopy. The fluorescence of the TnNDI2 oligomers is quenched very efficiently, providing compelling evidence that the predominant decay pathway is via a sequence of photoinduced forward and return electron transfers, eq. 1 and 2 (see above). A complete study of the dynamics of these process will be the subject of a subsequent full paper;43 however, herein we provide the results of a transient absorption study of T4NDI2 on timescale from 100 fs – 50 ps. These results give clear evidence for the occurrence of ultrafast electron transfer and charge recombination in the TnNDI2 series. The ultrafast time evolving transient absorption difference spectra for T4NDI2 are shown in Figure 3. Immediately following excitation, the difference spectrum features a strong ground state bleach located from 350 to 470 nm, weak stimulated emission at 500 – 575 nm, and broad absorption longer wavelengths. The most significant feature to emerge is a sharp absorption feature with λmax ~ 475 nm which is present even at the earliest times, but its intensity increases


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(concomitant with decay of the stimulated emission) over a timescale of 100 fs – 3.2 ps. This absorption feature is clearly due to NDI-, which is produced by photoinduced intramolecular oligo(thiophene) to NDI electron transfer, eq. 1.50 After the rapid spectral evolution that occurs in the first 3 ps, the transient absorption spectrum (which is attributed to the charge separated state) decays uniformly in less than 30 ps. Analysis of the spectral evolution dynamics affords estimates for the lifetime of forward electron transfer and charge recombination as kFET = 6.6 x 1011 s-1 and kCR = 1.7 x 1011 s-1. Clearly both forward and return electron transfer occur at nearly the rates expected for activationless, non-adiabatic electron transfer processes.

Figure 3: Ultrafast transient absorption spectra of T4NDI2 from 350 nm to 750 nm at the indicated time delays following a 420 nm laser excitation pulse in DCM.

Energetics and General Discussion. As seen in the previous section, the dynamics of forward electron transfer and charge recombination in T4NDI2 occur on an ultrafast timescale. In order to understand the thermodynamics of the electron transfer reactions for all members of the series, we utilized the electrochemical and spectroscopic data to estimate the driving force for


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the two electron transfer reactions (eq. 1 and 2). The energetics of electron transfer process are related to the energy of the charge separated state (ECS), ECS = E1/2(ox) - E1/2(red) - ∆EC

(4)

where E1/2(ox) is the first one-electron oxidation potential of the Tn donor, E1/2(red) is the first one-electron reduction potential of the NDI acceptor, and ∆EC is the Coulombic interaction energy within the ion-pair state at a specific distance.11 The Coulombic term is quantified using the expression e2/4πɛ0ɛr, where r is the center-to-center distance of the ions, e is the charge of an electron, 0 is the vacuum permittivity constant and ɛ is the static dielectric constant of the solvent. Using the data gathered from electrochemistry (SI, Table S1), steady-state emission and molecular modeling (to estimate the donor-acceptor separation distance), ECS values were calculated and are presented in Table 2, along with the energies of the singlet and triplet excited state (ES and ET, respectively). Finally, the driving force for photoinduced electron transfer and charge recombination (∆GFET and ∆GCR, respectively) are computed by using the following expressions, ∆GFET = ECS – ES

(5)

∆GCR = -ECS

(6)

the values for the TnNDI2 series are also collated in Table 2. Table 2: Energetics for Photoinduced Electron Transfer and Charge Recombination


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Compound

ES/eVa

ET/eVb

ECS/eVc

∆GFET/eVd

∆GCR/eVe

T4NDI2

2.72

1.79

1.45

-1.27

-1.45

T6NDI2

2.41

1.61

1.31

-1.1

-1.31

T8NDI2

2.28

1.51

1.26

-1.02

-1.26

T10NDI2

2.26

1.46

1.22

-1.04

-1.22

T12NDI2

2.19

1.42

1.18

-1.01

-1.18

a

Singlet energies were determined by taking the 0-0 band in the fluorescence spectrum of the corresponding model oligothiophene. bTriplet energies were estimated by using an expression reported elsewhere.66 cCharge-separated energies were calculated using Equation 4. d∆GFET = ECS – ES. e ∆GCR = -ECT.

Using all of the thermodynamic information collected in Table 2, we are able to construct an energy level diagram (Fig. 4), which illustrates the relative energies of all important states for the TnNDI2 series. There are several important features that emerge from inspection of this diagram. First, for all of the oligomers, the energy of the charge-separated state (ECS) lies roughly ½ way between the first singlet state and the ground state. This means that the driving force for both photoinduced electron transfer and charge recombination lies in the strongly exothermic region (∆G < -1.0 eV). Second, the triplet state lies above the charge-separated state, and therefore, the triplet is not believed to be involved in the photophysics of the TnNDI2 oligomers - forward electron transfer occurs from the singlet state, and charge recombination proceeds directly to the ground state (i.e., the triplet is not formed by charge recombination). This premise is supported by the observation that for the TnNDI2 oligomers the triplet state is not observed by nanosecond transient absorption and singlet oxygen sensitization is also not observed.


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Figure 4: Energy diagram with a range of energies extending from T4NDI2-T12NDI2. Singlet state energies were calculated from the first emission band maxima of the corresponding model oligothiophene. State energies estimated as described in text and Table 2.

An important consideration is to understand why the forward and return electron transfer reactions are so fast in T4NDI2. In order to place the reactions into context, we have applied Marcus semi-classical electron transfer theory to analyze the observed dynamics.

First, we

estimated the total reorganization energy (λ) for T4NDI2 by using eqs. 7 and 8, λ = λi + λo λs = e2/4π0[1/2rD + 1/2rA – 1/rDA][1/n2 -1/s]

(7) (8)

where λi is the internal reorganization energy, λs is the solvent reorganization energy, rD and rA are the donor and acceptor radii respectively, rDA is the total center-to-center distance between the donor and acceptor, 0 is the permittivity of vacuum, n is the refractive index of the solvent, and s is the static dielectric constant. The internal reorganization energy (λi) was estimated as 0.3 eV based on work previously established using a diethylaniline/C60 donor-acceptor molecule.71 30,35,72-74 The outer sphere reorganization energy (λo) was estimated as 0.95 eV using


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eq. 8 (see Supporting Information for details). The total reorganization energy for T4NDI2 was calculated to be ~1.25 eV. Comparison of the estimated total reorganization energy for T4NDI2 with the energetics in Table 2 reveal that forward electron transfer should be activationlesss (-∆G ~ λ), while charge recombination is slightly in the Marcus inverted region (-∆G > λ).75 In view of this fact, the observation that both reactions are ultrafast (k > 1011 s-1) is not at all surprising. Further confirmation that the forward and return reactions can be described as nearly activationless, nonadiabatic electron transfer reactions comes from an analysis where we apply the semi-classical expression applying the electron-transfer rate equation (eq. 9) to a range of ∆Get while simultaneously scaling the system with the coupling constant (HAB) to produce the Marcus plot shown in Figure S25.76

=

∗ ∗ −

( ) #

(9)

The electronic coupling term (HAB) was adjusted to .006 eV (48 cm-1) to afford a maximum electron transfer rate of 5.4 x 1011 s-1 at ∆G = -1.27 eV. This electronic coupling is consistent with a weakly coupled donor-acceptor system, and given the close match of the driving force for forward and return electron transfer with the total reorganization energy, the fast observed dynamics may not be surprising. Furthermore, temperature-dependent transient absorption studies were performed on T4NDI2, with temperatures ranging from -2 °C to 22 °C (Figure S27, A). The range was limited to a low of -2 °C due to instrument limitations, and the high was limited to 22 °C due to the boiling point of the solvent (DCM). An Arrhenius plot was constructed using the temperature dependent charge recombination lifetimes to reveal an activation energy of 1011 s-1. A complete report of the spectroscopy and dynamics of the excited and charge separated states in the Tn and TnNDI2 series will be reported in a forthcoming paper. Supporting Information Available Complete experimental details concerning the synthesis and spectroscopic characterization of Tn and TnNDI2. Complete set of 1H and 13C NMR spectra for all new compounds. Electrochemical data, 2-photon absorption spectra, and semi-classical (Marcus) theory calculation for T4NDI2. Information is available free via the internet at http://pubs.acs.org. Acknowledgement We acknowledge the Welch Foundation for support through the Welch Chair (Grant No. AX0045-20110629).


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Photoinduced Electron Transfer in Naphthalene Diimide End-Capped

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