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Photophysical and Electrochemical Characterization of BODIPYContaining Dyads Comparing the Influence of an A−D−A versus D− A Motif on Excited-State Photophysics Samuel J. Hendel,† Ambata M. Poe,‡ Piyachai Khomein,‡ Youngju Bae,‡ S. Thayumanavan,‡ and Elizabeth R. Young*,† †

Department of Chemistry, Amherst College, Amherst, Massachusetts 01002, United States Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003, United States

‡

S Supporting Information *

ABSTRACT: A complete photophysical characterization of organic molecules designed for use as molecular materials is critical in the design and construction of devices such as organic photovoltaics (OPV). The nature of a molecule’s excited state will be altered in molecules employing the same chromophoric units but possessing different molecular architectures. For this reason, we examine the photophysical reactions of two BODIPY-based D−A and A−D−A molecules, where D is the donor and A is the acceptor. A BODIPY (4,4′difluoro-4-bora-3a,4a-diaza-s-indacene) moiety serves as the A component and is connected through the meso position using a 3-hexylthiophene linker to a N-(2-ethylhexyl)dithieno[3,2b:2′,3′-d]pyrrole (DTP), which serves as the D component. An A−D−A motif is compared to its corresponding D−A dyad counterpart. We show a potential advantage to the A−D−A motif over the D−A motif in creating longer-lived excited states. Transient absorption (TA) spectroscopy is used to characterize the photophysical evolution of each molecule’s excited state. Global analysis of TA data using singular value decomposition and target analysis is performed to identify decay-associated difference spectra (DADS). The DADS reveal the spectral features associated with charge-transfer excited states that evolve with different dynamics. A−D−A possess slightly longer excited-state lifetimes, 42 ps nonradiative decay, and 4.64 ns radiative decay compared to those of D−A, 24 ps nonradiative decay, and 3.95 ns radiative decay. A longer lived A−D−A component is observed with microsecond lifetimes, representing a small fraction of the total photophyscial product. Steady-state and time-resolved photoluminescence augment the insights from TA, while electrochemistry and spectroelectrochemistry are employed to identify the nature of the excited state. Density functional theory supports the observed electronic and electrochemical properties of the D−A and A−D−A molecules. These results form a complete picture of the electronic and photophysical properties of D−A and A−D−A and provide contextualization for structure−function relationships between molecules and OPV devices.

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INTRODUCTION Global energy demands and environmental concerns over the last several decades have driven investment in alternative forms of energy harvesting. Solar energy has emerged as one appealing alternative source for electricity and chemical fuels.1 In recent years, organic photovoltaics (OPV) have shown promise owing to several favorable features including their use of low-cost materials, their ability to be fabricated using roll-toroll manufacturing techniques, and their potential to be constructed on flexible substrates.2 Advances in the field of OPV, particularly in the area of bulk heterojunction OPV, have been achieved through the design and characterization of small molecules and conjugated polymers. Blended films of poly(3hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are a well-established model system for this device architecture.3 Extensive work on the P3HT/PCBM © XXXX American Chemical Society

system has resulted in a wealth of knowledge regarding the function and efficient engineering of such devices.4−7 However, inherent limitations of the constituent components have spurred research into alternative molecular building blocks and OPV cell design. Small-molecule organics are attractive candidates in ongoing research efforts owing to several inherent advantages: (i) They are more reproducibly synthesized. (ii) They can be chemically tailored to control redox and absorption properties. (iii) They are less costly to produce and can be incorporated into new fabrication techniques for constructing OPV.2,8,9 Chemical and photostability, high extinction coefficients, and the ability to chemically modify Received: July 1, 2016 Revised: October 13, 2016

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DOI: 10.1021/acs.jpca.6b06590 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A physical properties via synthetic alteration are key design features for small-molecule OPV targets. While significant development of small molecule electron donors has received attention and success,2,8,10−12 far less work has been done to replace fullerenes and other fullerene derivatives that have served as the mainstay for electronaccepting materials in OPV. Fullerenes suffer from weak absorbance in the visible region,13,14 limited spectral breadth, instability in ambient conditions, relatively high production costs, and difficulty in tuning the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels.15−17 In recent years, design, synthesis, and testing of nonfullerene electron acceptors have gained traction.8,9,15−21 Several strategies have been undertaken in the development of small molecule acceptors including the addition of electron-withdrawing groups to appropriate chromophores and replacement atoms in extended π-systems with more electronegative atoms.9 Molecular electron acceptors have included systems based on vinazenes, fluoranthene-fused imides, fluorene, dithenylsilole, naphthalene diimide, diketopyrrolopyrrole, perylenediimide, pentacene, and benzothiadiazole, among others.2,8,9 Efforts to increase the molecular building blocks used for constructing electron acceptors are critical. Boron−dipyrromethane (BODIPY)-based dyes are excellent candidates. These dyes have been extensively studied since their first report in 196822 owing to their strong molecular absorption in the visible region, high fluorescence quantum yields, excellent stability, and rich redox chemistry.23 Various BODIPY moieties have been incorporated into molecular constructs in which they have served as facile charge-generation components.24−31 A recent report utilized BODIPY as the basis for designing a molecular OPV.32 To enhance the ability of BODIPY-based molecules to absorb in the low energy region of the spectrum, modulation of frontier orbitals is necessary.2,8 An extended, red-shifted spectrum is achieved through creation of intramolecular donor−acceptor interactions between BODIPY and another molecular building block that is either electron rich or electron poor relative to the BODIPY moiety.33 Lower lying LUMO levels formed from the donor−acceptor interaction modulate the electrochemical redox potentials of the dyads and generate red-shifted absorption.34 Construction of new OPV with small molecule organics will require acceptor and donor materials to be matched for optimal OPV performance. To enable optimal pairing, a fundamental electronic and photophysical picture of the constituent molecules is required. In this work, we examine an A−D−A molecule in which the BODIPY is connected through the meso position using a 3-hexylthiophene linker to an N-(2ethylhexyl)dithieno[3,2-b:2′,3′-d]pyrrole bridge (DTP) (Figure 1). This A−D−A molecule is compared to its corresponding D−A dyad counterpart. The bridging DTP unit, D, is used as a point of comparison for understanding the modulation of observed spectroscopic and electrochemical properties seen in the dyads. TA is employed to monitor the photophysical evolution of each molecule. Global analysis using singular value decomposition (SVD) and target analysis of TA data is performed to identify decay-associated difference spectra (DADS). The DADS reveal the spectral features associated with particular contributions to the kinetic evolution of the photogenerated species. Steady-state and time-resolved photoluminescence, electrochemistry, and spectroelectrochemistry are utilized to clarify the nature of the excited state.

Figure 1. Structures of D, D−A, and A−D−A presented in this work.

Spectroelectrochemistry yields the spectral features of the radical anion and radical cation of D−A and A−D−A, and the characteristic peaks serve to identify the origin of DADS for each species. Molecular orbitals, obtained using density functional theory calculations, support the observed experimental properties and behavior of the D−A and A−D−A molecules. Taken together, these results form a complete picture of the electronic and photophysical properties of these molecules.

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EXPERIMENTAL SECTION General Materials and Methods. Dichloromethane (DCM), sodium hydroxide (NaOH), and fluorescein were purchased from Sigma-Aldrich, and tetrabutylammonium hexafluorophosphate (Bu4NPF6) was purchased from Fluka. All reagents were used as received unless otherwise noted. Dichloromethane was used from a bottle of anhydrous, >99.8% dichloromethane stored in a glovebox or obtained from an inhouse solvent system (SciMatCo). The tetrabutylammonium hexafluorophosphate electrolyte was electrochemical analysis grade >99%. Samples for spectroscopy and electrochemistry were prepared in an MBraun Labmaster SP glovebox with anhydrous dichloromethane (Sigma-Aldrich) or degassed dichloromethane dispensed from a solvent system (SciMatCo). UV−vis Absorption and Steady-State Fluorescence Measurements. Absorption spectra were obtained with a PerkinElmer Lambda 9 spectrometer, and emission spectra were obtained with an ISS Chronos BH fluorimeter. For steadystate fluorescence measurements, D−A and A−D−A were excited at 470 nm and D was excited at 300 nm. Quantum yield measurements were recorded using fluorescein in 0.1 M NaOH as an emission reference (ϕ = 0.95). Quantum yields were calculated according to the equation ϕsample = ϕref

Isample A ref nsample 2 Iref A sample nref 2

where I is the integrated intensity of the fluorescence peak, A is the absorbance at 470 nm, and n is the refractive index of the solvent. B


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Figure 2. Synthetic scheme for synthesis of D (the bringing unit), D−A (the half D−A dyad), and A−D−A (the full A−D−A dyad).

Electrochemical Analysis. Electrochemistry and spectroelectrochemistry were performed in a glovebox. Electrochemical measurements were taken with a CH Instruments 760 potentiostat with a platinum working electrode, a silver wire quasi-reference electrode, and a platinum wire counter electrode. Voltammetric measurements were obtained using a 20 μM concentration in 100 mM Bu4NPF6 in dichloromethane. Cyclic voltammetry (CV) was recorded at a scan rate of 50 mV/s. After the CV was recorded, ferrocene was added, and an additional CV was recorded. The original CV was then corrected to the Fc/Fc+ redox couple. Differential pulse voltammetry (DPV) was recorded with an increment of 4 mV and a pulse period of 0.5 s. DPV pulses had an amplitude of 50 mV applied for 50 ms with a sampling width of 16.7 ms. In order to prevent fouling of the working electrode, reductive DPV scans were recorded before oxidative DPV scans. Square wave voltammetry (SWV) was recorded with an increment of 4 mV, a frequency of 15 Hz, and an amplitude of 25 mV. Reductive SWV scans were recorded before oxidative SWV scans. Spectroelectrochemical spectra were obtained with a USB2000+ Ocean Optics spectrometer. Concentrations were varied to create a steady-state absorbance measurement of 0.2− 0.5 AU at peak absorbance (at 511 and 512 nm for D−A and A−D−A, respectively) in a 2 mm glass cuvette. A platinum mesh working electrode, a platinum wire counter electrode, and a silver wire reference electrode were used. Each oxidative and reductive spectroelectrochemical experiment was run on a fresh sample. Electrical potentials were changed by 100 mV in the direction of a previously measured redox couple in order to generate the desired redox product. Exact potentials are not reported since a quasi-reference electrode does not accurately reproduce a specific potential. Time-Resolved Fluorescence. Time-resolved photoluminescence measurements were recorded in an ISS Chronos BH time-resolved fluorescence spectrometer. A light-emitting diode emitting 470 nm light with a ∼20 ns pulse duration was used as the excitation source. Samples were prepared in an inert glovebox environment in a 1 cm cuvette ensuring that absorption at the excitation wavelength was below 0.05 AU. The instrument response function was recorded using a dilute sample of coffee creamer in water. Time-resolved data were fit to a biexponential decay function against the instrument response. Although the shorter lifetime is too short to be accurately determined against the instrument response

function, the longer lifetime could be accurately determined within an error of ∼10%. Transient Absorption (TA) Measurements. TA measurements were carried out using two different excitation wavelengths for each molecule (330 and 470 nm for D−A and 375 and 470 nm for A−D−A); both excitation wavelengths produced similar spectral features and lifetimes for each molecule. Excitation pulses for both femtosecond and nanosecond TA were generated by an optical parametric amplifier (TOPAS-C, Light Conversion) pumped by 96% of a Coherent Libra Ti:sapphire ∼1 W, 800 nm, 1 kHz laser with a pulse duration of 100 fs. The resulting pump beam was attenuated to 1−2 mW in order to prevent photobleaching. For femtosecond TA (fs-TA), the remaining 4% of the 800 nm laser was focused on a sapphire crystal to generate a white light probe spectrum ranging from 400 to 700 nm. The excitation pulse was chopped at 500 Hz in order to record a differential spectrum between the excited and steady-state sample. Two scans of 250 time points were used for each experiment, such that spectra were gathered at sequentially increasing and exponentially spaced time intervals. For nanosecond TA (ns-TA), a NKT Photonics supercontinuum white light generator was used to generate a probe spectrum ranging from 450 to 900 nm. Experiments were run for 15−25 min, such that spectra for 300 exponentially spaced time intervals were gathered randomly. For both femtosecond and nanosecond TA, the excitation pulse and the probe pulse were focused and overlapped onto the sample. Samples were prepared in dichloromethane a 2 mm cuvette in an inert glovebox environment with absorption intensities at the excitation wavelength ranging from 0.3 to 0.6 AU. The samples were stirred during measurements, and absorption spectra were taken before and after each TA measurement to ensure that no significant photobleaching occurred. Background and chirp corrections (when applicable) as well as Global Fitting of transient spectra were accomplished using the Surface Xplorer software from Ultrafast Systems. For femtosecond TA, four and three principal components were chosen via singular value decomposition for D−A and A−D−A, respectively. Global Fitting from these principal components yielded three-component spectra for D−A and two-component spectra for A−D−A. Computation. Density functional theory calculations were obtained by employing a 6-311G basis set and a Becke threeparameter hybrid exchange and a Lee−Yang−Parr (B3LYP) C


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The Journal of Physical Chemistry A correlation functional.35−37 Energy levels and images of frontier molecular orbitals were obtained. Orbitals were visualized using Gaussian 05.

Figure 4 shows the overlaid absorption spectra for D, D−A, and A−D−A. For each molecule, an absorption band occurs at

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EXPERIMENTAL RESULTS Synthesis. Synthesis of the BODIPY moiety and A−D−A core were adopted from previously published literature.32,38 The equivalent of n-BuLi was varied to control the stannylation of the A−D−A core, which was followed by a Stille crosscoupling reaction with the BODIPY unit to give D−A and A− D−A (Figure 2). Steady-State Properties. Figure 3 shows the absorption spectra for D−A and A−D−A. The absorption maxima for D−

Figure 4. Normalized and overlaid absorption spectra of A−D−A suite in dichloromethane.

wavelengths less than 400 nm that are attributed to the contribution of the bridging moiety. This peak occurs at increasingly lower energy wavelengths from D to D−A and A− D−A corresponding to the addition of the first and then the second BODIPY subunits. Additionally, vibrational features in the absorption band of D are absent in D−A and A−D−A. Figure 5 shows the cyclic voltammograms (CV) of D, D−A, and A−D−A. The CV of D shows an irreversible oxidation with

Figure 3. Steady-state absorption and emission spectra of (a) D−A and (b) A−D−A in dichloromethane. Emission spectra were excited at 470 nm. Figure 5. Cyclic voltammograms of A−D−A suite. Voltammograms were recorded at 50 mV/s in 100 mM Bu4NPF6 in dichloromethane using a platinum working electrode, Ag wire quasi-reference electrode, and a platinum wire counter electrode. Ferrocene was added as an internal standard.

A and A−D−A occur at similar wavelengths, 511 and 512 nm, respectively, stemming from the parent BODIPY moiety. Local maxima also occur at 594 and 638 nm for D−A and A−D−A that are indicative of charge-transfer bands. With this extended, red-shifted absorption, A−D−A can absorb 744 nm light at 10% of the molecule’s peak absorbance, whereas D−A can only absorb 689 nm light at the same relative absorbance intensity. The extended absorption in A−D−A significantly increases the amount of solar photon flux A−D−A can absorb. Figure 3 also shows the emission spectra for D−A and A−D−A. Both molecules were excited at 470 nm in order to reduce spectral overlap of excitation and emission. While the emission spectrum of A−D−A demonstrates a relatively small Stokes shift of 19 nm, the emission spectrum of D−A demonstrates a much larger Stokes shift of 125 nm. Steady-state photophysical data are summarized in Table 1. Fluorescence is heavily quenched in D−A and A−D−A compared to parent BODIPY moieties, with quantum yields below 1% (see Table 3).

a half-wave potential of 0.821 V vs Fc/Fc+, confirming the bridging substituent’s electron-donating ability, and no reductive peak. Both D−A and A−D−A undergo two oxidative events and one reductive event. For D−A, all three redox events are irreversible. Conversely, for A−D−A, all three redox events are reversible, as demonstrated by the peak current ratios approaching unity (Table 2). Upon performing squarewave voltammetry (Figure S1), the peak height for the reductive event of A−D−A was found to be twice the peak height for both oxidative events (Table S1). Additionally, the Table 2. Electrochemical Data of the A−D−A Suitea V, vs Fc/Fc+ molecule D

Table 1. Steady-State Photophysical Data of the A−D−A Suitea molecule

λmax(ab) (nm)

ε (M−1 cm−1)

λmax(em) (nm)

D D−A A−D−A

299 333, 511, 594 384, 512, 638

18295 41649 67278

399 559, 636 531

E1/2 red

E1/2 ox 1

E1/2 ox 2

0.821 −1.194

D−A

0.059 0.375 −1.092

A−D−A

0.401 0.842

a

peak current ratio 0.552 3.591 2.287 1.016 1.076 0.901

a

Bold indicates peak absorption or emission wavelength. All data were recorded in dichloromethane. Emission spectra of D were collected under 300 nm excitation, and D−A and A−D−A were excited at 470 nm.

All data were recorded in 100 mM Bu4NPF6 in dichloromethane using a platinum working electrode, Ag wire quasi-reference electrode, and a platinum wire counter electrode. Ferrocene was added as an internal standard. D


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The Journal of Physical Chemistry A peak height for the reductive event of D−A was found to be roughly equal to the height for the first oxidative event. Furthermore, performing differential pulse voltammetry (Figure S2) on A−D−A revealed that the reductive peak integrates to twice the value of either oxidative peak, which each demonstrated equal integrated areas (Table S2). Additionally, the differential pulse voltammogram of D−A shows that the reductive peak integrates to roughly the same value as the first oxidative peak. Figure 6 shows the oxidative and reductive spectroelectrochemistry absorbance spectra of D−A and A−D−A. The

Figure 7. Transient absorption spectral evolution of (a) D−A and (b) A−D−A over 3 and 4.5 ns, respectively. Samples were prepared in dichloromethane in an inert glovebox environment and were excited at 330 and 375 nm for D−A and A−D−A, respectively. Samples excited at 470 nm produced the same transient spectra and lifetimes.

performed on D−A and A−D−A using a 470 nm pulsed LED in a time-correlated single-photon counting spectrometer. Kinetic traces of the sample and IRF were recorded for each sample (Figure S4), and fitting was carried out by deconvolving the contribution of the instrument response function (IRF) with the signal to gain photoluminescence lifetimes. Fits obtained yielded two lifetimes, one of which was too short to be reliably resolved by the instrument. The other lifetime is reported in Table 3. Lifetimes determined by time-resolved photoluminescence and TA kinetic fitting are tabulated in Table 3.

Figure 6. Oxidative and reductive spectroelectrochemistry of (a) D−A and (b) A−D−A overlaid with the steady-state absorption for comparison of peak positions. All data were recorded in dichloromethane. Spectroelectrochemistry spectra were recorded in 100 mM Bu4NPF6 electrolyte with a platinum mesh working electrode, a platinum wire counter electrode, and a silver wire reference electrode.

Table 3. Time-Resolved Photophysical Data of D−A and A− D−Aa

differential spectrum of oxidized D−A shows negative peaks at 510 and 586 nm and a broad positive peak at 875 nm. The baseline was not recovered upon reduction of the oxidized species, indicating irreversible chemical processes occurred during bulk electrolysis. The differential spectrum of reduced D−A shows negative peaks at 511 and 606 nm as well as a growth at 543 nm. After bulk reduction of D−A, the initial baseline was recovered upon regeneration of the neutral species. The differential spectrum of oxidized A−D−A shows a negative peak at 510 nm and two growths at 683 and 947 nm. The differential spectrum of reduced A−D−A shows negative peaks at 511 and 645 nm as well as growths at 554 and 834 nm. The initial baseline was observed upon the regeneration of the neutral species for both oxidized and reduced A−D−A. Transient Absorption and Time-Resolved Photoluminescence. Figure 7 shows the femtosecond transient absorption spectra for D−A and A−D−A. The TA results were consistent regardless of exciting into the bridge or the BODIPY absorbance bands. Therefore, D−A and A−D−A were excited into the bridge absorption bands at 330 and 375 nm, respectively, in order to prevent excitation light from obscuring with the spectral window for the TA measurement. Absorption spectra taken before and after TA demonstrate that no photodegradation occurred (Figure S3). The TA data in Figure 7 for both D−A and A−D−A showed a very tiny residual spectrum that is difficult to see on the scale of the represented spectra. For this reason, nanosecond transient absorption spectroscopy was performed on both D−A and A−D−A. Only A−D−A demonstrated a measurable signal on the nanosecond scale; however, this signal represents a virtually insignificant fraction of the photophsical product and the results are not presented in this work. Time-resolved photoluminescence was

transient absorption

time-resolved PL

molecule

QY

τ500 nm bleach recovery (ps)

τ700 nm growth decay (ps)

τfl (ns)

D−A A−D−A

0.0016 0.0036

23.6 42.4

23.2 45.7

3.95 4.64

a

Samples were prepared in dichloromethane in an inert glovebox environment. Transient absorption samples were excited at 330 and 375 nm for D−A and A−D−A, respectively. Time-resolve PL samples were excited with a 470 nm pulsed nanoLED.

Density Functional Theory (DFT). Figure 8 shows molecular orbitals for D−A and A−D−A obtained from DFT calculations. Hexyl and 2-ethylhexyl groups are truncated for simplicity. For both D−A and A−D−A, electron density is localized on the BODIPY moieties in the LUMO. For A−D−A, there is little electronic communication between the two BODIPY moieties in both the LUMO and the LUMO+1. Electron density is localized on the bridge moiety of the HOMO for both D−A and A−D−A, extending through the thienyl linkers but not to the BODIPY moieties. In contrast, the HOMO−1 for D−A and A−D−A are localized entirely on one BODIPY moiety.

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DISCUSSION Comparing the photophysical properties of two molecular architectures demonstrates that an A−D−A design leads to longer lived excited states than a D−A design, which may be an E


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moiety, e.g., one BODIPY in D−A and both BODIPY moieties in A−D−A. Further, the first oxidation event corresponds to oxidation of the A−D−A bridging unit and the second oxidation event corresponds to the oxidation of one BODIPY moiety, e.g., one BODIPY in D−A and only one of the BODIPY moieties in A−D−A. Using square wave voltammetry (SWV) and differential pulse voltammetry (DPV), the number of electrons attributed to each electrochemical event can be characterized. For A−D−A, the height of the reductive SWV peak is twice the height of either of the oxidative peaks (Figure S1), and the reductive DPV peak integrates to twice the value as either of the oxidative peaks (Figure S2), providing experimental evidence that the reduction is a two-electron event and the oxidations are each one-electron events. For D−A, one clear reductive and one clear oxidative event are observed in SWV and DPV. The reductive and oxidative peaks show the same peak height in SWV (Figure S1) and integrate to the same area in DPV scans (Figure S2), indicating that they both involve the transfer of the same number of electrons. For the reasons outlined below, the reductive and oxidative events are ascribed to one-electron events. By further considering the number of electrons transferred and electrochemical shifts observed in the series, the origin of each event can be understood. In reduction, electrons are added to the LUMO; therefore, the reduction potential reflects the energy level of the LUMO. The reduction potentials for D−A and A−D−A are similar to the reduction potential of the 2-thienyl-BODIPY moiety, which does show a significant shift compared to the parent BODIPY. (Unpublished data from our laboratory. Continuing work on the 2-thienyl-BODIPY derivatives is being pursued and will be the subject of a forthcoming publication.) The addition of the bridging substituent onto the 2-thienyl-BODIPY moiety does not further affect the reduction potential for either D−A or A− D−A, indicating that the bridging DTP substituent does not contribute electron density to the LUMO. DFT confirms that electron density in the LUMO is localized only on the 2thienyl-BODIPY moieties for both D−A and A−D−A (Figure 8). Therefore, the reduction is attributed to reduction of the BODIPY moieties. Further, because there is no electronic communication between the two BODIPY moieties in A−D− A, they exist in identical and distinct electronic environments. Therefore, the reduction potential for both BODIPY moieties is the same, leading to a two-electron reductive event in A−D−A. For these reasons, the reduction events are ascribed to reductions of the BODIPY moieties and the two-electron reduction event in A−D−A provides additional validation of a lack of electronic communication between BODIPY moieties across the bridging substituent. Upon oxidation, electrons are removed from the HOMO; therefore, the oxidation potential reflects the energy level of the HOMO. The first oxidation potential of A−D−A in the CV scans is higher than the first oxidation potential of D−A (0.401 V vs Fc/Fc+ for A−D−A and 0.059 V vs Fc/Fc+ for D−A, Figure 5 and Table 2), meaning electrochemistry reveals that the HOMO of A−D−A is stabilized compared the HOMO of D−A. DFT confirms that increased conjugation of the bridge with the thiophene linkers lowers the HOMO energy level in A−D−A (Figure 8). The lower HOMO level in A−D−A arises from the extended conjugation of the bridging unit and confirms the assignment of the first oxidation event to the oxidation of the bridging unit. The second oxidation event originates from oxidation of one of the BODIPY moieties. In

Figure 8. Density functional theory molecular orbital images and energy level diagram calculated by DFT employing a 6-311G basis set and a Becke three-parameter hybrid exchange and a Lee−Yang−Parr (B3LYP) correlation functional.

advantage in designing such systems for photovoltaics applications. Steady-State Properties. The steady-state absorption spectra of D−A and A−D−A provide evidence that the BODIPY moiety and bridging moiety are electronically decoupled in the ground state. Although the peak absorption for BODIPY without any aryl substituents at the meso position occurs at 497 nm, the peak absorption for the meso-linked 2thienyl-BODIPY derivative occurs at 510 nm in dichloromethane.39,40 The absorption spectra of D−A and A−D−A demonstrate similar peak wavelengths to the meso-linked 2thienyl-BODIPY at 511 and 512 nm, respectively (Figure 3). DFT calculations confirm that the bridging moiety does not communicate with the BODIPY moieties in any of the frontier molecular orbitals (Figure 8). The lack of a bathochromic shift from the meso-linked 2-thienyl-BODIPY derivative to D−A to A−D−A further supports that there is little ground-state electronic communication between BODIPY and the bridging moiety. While the bridging moiety does not communicate electronically with BODIPY, the bridge does communicate with the thiophene linkers. A bathochromic shift of the bridging subunit’s absorbance peak occurs when two 2-thienyl-BODIPY substituents are successively attached to the bridging subunit, thereby extending the conjugation of that component. The absorbance peaks red-shift from 299 nm for D to 333 nm for D−A to 384 nm for A−D−A (Figure 4 and Table 1). Density functional theory supports this observation (Figure 8). Thus, absorption spectroscopy shows that although there is electronic communication between the bridge and the thiophene linkers, there is no electronic communication between the bridge and the BODIPY subunits. Detailed electrochemical analysis of D−A and A−D−A furnishes the chemical origins of each electrochemical event within the D−A and A−D−A molecules. Specifically, the reduction event corresponds to the reduction of each BODIPY F


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3). At the end of the temporal experimental window for our TA system (5 ns), a very small residual excited-state growth remains that cannot be characterized by fs-TA. Time-resolved photoluminescence is used to characterize this longer-lived excited-state residual. No signal could be found using ns-TA on the D−A moiety. Fs-TA was also performed on A−D−A. Upon excitation, a broad growth at wavelengths greater than 540 nm convolves with a ground-state bleach. Although it is represented with a positive absorption, a charge-transfer band bleach (at 640 nm) is present in the transient spectra convolved with excited-state absorption in that region. The excited-state spectrum of A−D− A is characterized by a ground-state bleach at 510 nm and excited-state absorption bands at 450 nm, 550 nm, and at wavelengths longer than 710 nm. The growths at 450 and 550 nm indicate the presence of a reduced BODIPY moiety (Figure 6b), and the broad growth above 710 nm suggests the presence of oxidized bridge moiety (Figure 6b). In similar fashion to the D−A, the A−D−A excited state exhibits features of a chargetransfer state with a partial positive charge on the bridge and a partial negative charge on the BODIPY that relaxes with a lifetime of 46 ps (Table 3). The 510 nm bleach corresponding to the BODIPY moiety ground state and the growth at 700 nm remains as a very tiny fraction in the residual fs-TA spectrum. Time-resolved photoluminescence is presented to resolve a portion the evolution of the long-lived species (vide infra). Additionally, ns-TA was carried out. Ns-TA does reveals a growth at around 850 nm concomitant with the relaxation of the 510 nm bleach and 700 nm growth from the residual fs-TA spectrum. The growth and relaxation of these species occur on the microsecond time scale and show a complex set of kinetics. Because the residual fs-TA spectrum that leads to these microsecond kinetics represents an insignificant fraction of the fs-TA photophysics, the characterization of this long-lived component is beyond the scope of this manuscript, which is focused on describing the ultrafast photophysics of these moieties, and is left as the focus of future work. Time-Resolved Photoluminescence. Time-resolved photoluminescence (TRPL) was carried out to measure fluorescent lifetimes of D−A and A−D−A. For both D−A and A−D−A, the fluorescence kinetics fit to two lifetimes against the instrument response function. While the shorter lifetime could not be accurately determined though TRPL because of experimental limitations of our time-correlated single photon counting spectrometer, it may correspond to the short lifetimes determined by fs-TA. The longer lifetime provides insight into the lifetime of the residual unresolved spectrum in fs-TA. Therefore, the longer lifetimes determined by TRPL are attributed to the residual spectra for D−A and, in part, for A− D−A, given in Table 3. These lifetimes are similar to the longlived excited state of the 2-thienyl-BODIPY derivative.39,40 Global Analysis and Decomposition of Decay Associated Differential Spectra. Global analysis separates TA spectra into independent transient spectra, according to an individual spectrum’s lifetime. To determine the number of independent transient spectra, single value decomposition is first used to determine a basis set for global fitting. For D−A, four spectra were assigned through SVD analysis. Subsequent global fitting of the D−A fs-TA spectra yields three contributing spectra, two with finite lifetimes and one with a lifetime that could not be resolved within the 5 ns experimental window (Figure 9a). Lifetimes associated with each spectrum are tabulated in Table 4. The spectrum associated with the

A−D−A, a clear second one-electron oxidation is observed as described above. DFT shows that electron density resides on only one of the BODIPY moieties in the HOMO−1. In similar fashion, the DFT of D−A shows electron density on the BODIPY moiety. Electrochemistry and DFT results combine to identify the second oxidation event as the oxidation of only one of the BODIPY moieties in A−D−A. Taken together, these results confirm that A−D−A undergoes a two-electron reduction of the BODIPY moieties and two one-electron oxidations, the first being the bridge and the second being one of the BODIPYs, while D−A undergoes a one-electron reduction of the BODIPY moiety and one one-electron oxidation of the bridge. Spectroelectrochemistry allows for the identification of a charge-transfer state in the excited-state evolution of these species by presenting spectroscopic handles indicative of charged D or A moieties. To determine the spectroscopic markers of oxidized and reduced species, spectroelectrochemistry was carried out on D−A (Figure 6a) and A−D−A (Figure 6b). Both show absorption features that appear around 550 nm indicative of reduced BODIPY moieties. For A−D−A, the absorption spectra of the doubly reduced species produced an additional absorption at 834 nm, indicating that there may be some form of electronic communication between the BODIPY moieties in the two electron reduced state. By contrast, D−A, which possesses only one BODIPY moiety, does not demonstrate any such low energy growth upon reduction. While the singly oxidized A−D−A demonstrates a narrow absorption band at 683 nm and a broad growth at wavelengths longer than 800 nm, the singly oxidized D−A produces a broad growth at wavelengths longer than 700 nm. The red-shifting in the >700 nm peak in D−A to wavelengths longer than 800 nm in A−D−A likely arises from the increased conjugation of DTP bridging unit in the A−D−A system. The narrow absorption band at 683 nm in A−D−A may arise from interactions between the two BODIPY subunits in the oxidized A−D−A species. Chemical degradation of D−A occurs as the steadystate absorption spectrum was not recovered upon rereducing the oxidized D−A. The degradation likely corresponding to the dimerization of two D−A molecules upon oxidation and for this reason spectroelectrochemisty was run under very dilute conditions for very short durations to avoid contribution of dimerized D−A in the recorded spectra. Femtosecond-Transient Absorption (fs-TA) Spectroscopy. As molecular excited states evolve on the femtosecond time scale, TA spectroscopy is a valuable tool to determine excited-state dynamics. Fs-TA spectroscopy was performed on D−A. Upon excitation, a broad growth at 575 nm appears concurrent with a bleach at 510 nm. The growth blue-shifts and narrows to form a peak at 550 nm. This blue-shift represents vibrational cooling, which resolves on a time scale of ∼1 ps into the excited-state absorbance spectrum. The vibrationally relaxed excited-state spectrum of D−A is characterized by a ground-state bleach at 510 nm and excited-state absorption bands below 460 nm, at 560 nm and at 710 nm. The growths at 460 and 560 nm indicate the presence of a reduced BODIPY moiety according to reductive spectroelectrochemistry (Figure 6a). The broad growth at 710 nm indicates the presence of oxidized bridge moiety according to oxidative spectroelectrochemistry (Figure 6a). Therefore, this excited state exhibits features of a charge-transfer state with a partial positive charge on the bridge and a partial negative charge on the BODIPY. This charge-transfer state relaxes with a lifetime of 23 ps (Table G


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Figure 10. Reaction scheme for deactivation of the D−A and A−D−A excited states. Details of the analysis and explanation of the scheme are provided in the text.

Figure 9. Decay-associated differential spectra of D−A and A−D−A resulting from global fitting of fs-TA data on samples prepared in dichloromethane in a glovebox and excited at 330 nm (D−A) and 375 nm (A−D−A).

cannot be determined through ns-TA. The residual may correspond exclusively to the emissive species measured in TRPL to have a 4.0 ns lifetime. The fs-TA obtained for A−D−A provides a picture of the dynamics for this molecule. From fs-TA of A−D−A, a broad growth at wavelengths greater than 550 nm is observed. While this growth is convolved with the steady-state bleach of the charge-transfer band, the growth overlaps with the two growths at 550 and 830 nm as seen in the reductive spectroelectrochemistry. Additionally, absorbance at 680 nm as seen in the oxidative spectroelectrochemisty of A−D−A is apparent. The three peaks indicate the presence of a charge-transfer excited state. The charge-transfer excited state relaxes with a time constant of 42 ps. As this state relaxes, a residual spectrum is revealed that resembles a BODIPY bleach and a growth at wavelengths greater than 700 nm, indicating the presences of an excited BODIPY species. We posit that this represents a BODIPY-based excited state that is also formed directly upon excitation. The BODIPY-based excited state decays radiatively with a time constant of 4.6 ns. The tiny residual remaining on the fs-TA time scale is considered insignificant and is not summarized in the reaction scheme at this time. Thus, the photophysical picture reveals that there are modestly longer lived excited states in the A−D−A architecture than in the D− A architecture. When designing new molecules for photovoltaic applications, an A−D−A architecture may result in longer lived excited states than a D−A architecture.

Table 4. Global Fitting Kinetic Tracesa lifetimes (ps) molecule

t1

t2

D−A A−D−A

1.07 41.7

19.9

a Global fitting lifetimes were obtained from samples prepared in dichloromethane and excited at 330 nm (D−A) and 375 nm (A−D− A).

shortest lifetime (1.07 ps) represents D−A undergoing vibrational cooling. This vibrational cooling characteristically relaxes into a narrowed blue-shifted absorbance spectrum, which is associated with the second shortest lifetime (19.0 ps). The residual spectrum is characterized by a broad growth at 475 nm that is convolved with a 510 nm steady-state bleach. Three spectra were used in SVD analysis of A−D−A fs-TA. Subsequent global fitting of A−D−A fs-TA spectra yields two spectra, one with a finite lifetime and one with a lifetime unable to be resolved within the 5 ns experimental window (Figure 9b). The finite lifetime is characterized by a steady-state bleach at 510 nm and a broad growth at 730 nm. The residual spectrum demonstrates a bleach at 510 nm and a slight growth at 700 nm. Formation of Charge-Transfer States upon Photoexcitation. The transient absorption spectra of both D−A and A−D−A demonstrate signatures of charge-transfer excited states that evolve with different kinetics and through intermediates. A schematic diagram is presented in Figure 10 to summarize the photophysics of each molecule. Upon initial excitation, a vibrationally excited state of D−A is formed that undergoes vibrational cooling with a time constant of ∼1 ps. From fs-TA of D−A (Figure 7a), the relaxed excited state shows a growth at 550 nm corresponds to the BODIPY radical anion and the growth at wavelengths greater than 700 nm corresponds to the oxidized bridge, as demonstrated by reductive and oxidative spectroelectrochemistry of D−A (Figure 6a). Thus, the excited state of D−A demonstrated charge-transfer character. This excited states relaxes either radiative decay with a time constant of 4.0 ns or thermally through internal conversion with a 20 ps time constant. D−A shows a residual broad growth in fs-TA at wavelengths less than 500 nm, indicating a long-lived BODIPY excited state. This contribution is insignificant and the lifetime of this excited state

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CONCLUSION In this work, we provide a detailed electronic and spectroscopic examination of two molecules, one with a common D−A motif and one possessing an A−D−A motif. Both species form charge-transfer excited states that evolve with different photophysics. Transient absorption spectroscopy and timeresolved photoluminescence show that these molecules undergo nonradiative relaxation with time constants on the order of 20−40 ps and radiative decay with time constants of ∼4 ns. Of the two species, A−D−A possesses slightly longer excited-state lifetimes, 42 ps for nonradiative decay compared to 24 ps for D−A, and 4.64 ns radiative decay compared to 3.95 ns for D− A. A longer lived species with microsecond lifetimes is observed with ns-TA for A−D−A but represents only a small fraction of the photophyscial product. The origins and kinetics of this residual product are minor in the context of this ultrafast characterization and appear relatively complex, and they are, H


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therefore, left as the focus of future work. Electrochemistry, spectroelectrochemistry, and DFT are used to characterize the influence of the second BODIPY unit in the A−D−A motif and identify the nature of the excited states of D−A and A−D−A. Although the two BODIPY moieties do not demonstrate electronic communication in the ground state of A−D−A, the presence of the second BODIPY has a profound influence on the photophysical behavior of the A−D−A molecule. Longer lived excited states accessible to the A−D−A architecture may be an advantage when designing new molecules for photovoltaic applications.

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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.jpca.6b06590. Additional electrochemical analysis (SWV, DPV), UV− vis data on TA samples, and time-resolved photoluminescence data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.R.Y. thanks the NSF Major Research Instrumentation program for funds that established the laser facility at Amherst College under Grant No. CHE-1428633. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for support of this research. S.T. acknowledges partial support from the NSF (CHE-1307118).

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