Nonradiative Decay Mechanism of Fluoren-9-ylidene Malononitrile

Mar 1, 2011 - Considering the 0−0 singlet band gap of the donor (ES1 = 84 ... Frontier orbital maps for ambipolar FMDs computed at the B3LYP/6-31G* ...
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Nonradiative Decay Mechanism of Fluoren-9-ylidene Malononitrile Ambipolar Derivatives Leandro A. Estrada, Xichen Cai, and Douglas C. Neckers* Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States

bS Supporting Information ABSTRACT: We report recent results on the nonradiative decay (NRD) of fluoren-9ylidene malononitrile (FM) ambipolar derivatives (FMDs). 2,7- and 3,6-disubstituted FMDs present distinctive photophysics. Charge separation was found dominant for excited state relaxation. The radiative decay (RD) is sensitive to changes in temperature and solvent medium only for the case of 3,6-FMDs. Excited state deactivation of carbazole-containing 3,6-FMD (CPAFM36) was exclusively nonradiative in polar solvents with excited state lifetimes shorter than 10 ps. The charge separation/ recombination mechanism of the corresponding FMDs is suggested to fall in the inverted Marcus region of electron transfer. Given the electron-withdrawing properties of the FM unit, its ambipolar derivatives are suggested as potential candidates for airstable organic thin-film transistors and molecular organic photovoltaics.

1. INTRODUCTION Technical hurdles, such as low external quantum efficiencies for organic photovoltaics (OPVs) and poor stability in air and little n-channel conductivity for organic field effect transistors (OFETs), limit the infusion of organic compounds into devices otherwise marketable through the exploitation of properties that only organic semiconductors offer (i.e., flexible, large area displays).1,2 Most of the research on p-conducting materials for bulkheterojunction (BHJ) OPVs has been devoted to polymers3 where little progress has been attained through synthetic modification of either the donor or the fullerene acceptor.4 While the majority of recent advances in OPVs (external efficiencies of about 6%) have been accomplished via device engineering,5 molecular BHJ has been introduced as an alternative given the benefits obtained from the use of defined structures.6 The first reported examples of small molecule BHJ (SM-BHJ) cells used X-shaped oligothiophene derivatives.7 Despite the low efficiencies (∼0.30%), their lowest energy absorption maximum was near 390 nm. Further work led to the discovery of ambipolar materials as potential candidates for OPV cells. Triphenylamine derivatives were improved by the introduction of dicyanovinylene (DCV) groups, leading to efficiencies close to 2% in bilayer devices.8 Although the incompatibility of DCV with C60 derivatives represents an important hurdle for the realization of efficient BHJ devices with such a donor,9 the search for alternative solutions suggests further modification of novel ambipolar compounds. Last year a power conversion efficiency of 3% was achieved in a nonoptimized device with a mixture of a low band gap oligothiophene, 2,5-di-(2-ethylhexyl)-3,6-bis(500 -n-hexyl-[2,20 50 ,200 ]terthiophen-5-yl)-pyrrolo-[3,4c]-pyrrole-1,4-dione (SMDPPEH), as donor, and phenyl-C71-butyric acid methyl ester (C71-PCBM), as the acceptor.10 This efficiency set a new r 2011 American Chemical Society

benchmark in SM-BHJ cells, though there is still room for improvements. In spite of their lack of sufficient air stability under device operation conditions,11 n-type conducting materials have attracted much interest since the first report of n-channel OFETs by Guillaud et al.12 Recently, the first examples of air-stable molecular ambipolar semiconductors for TFTs were reported by Marks and Facchetti.19e Their rationale for molecular design was that stabilization of the LUMO can be achieved via derivatization of carbons 6 and 12 of indenofluorene, thus, allowing effective electron injection under ambient conditions. This raison d’^etre was first recognized in the 70s when fluorenone (FO) derivatives were incorporated as sensitizers in carbazole-containing polymers.13 The electron affinity of FO and FM derivatives was further studied by Perepichka’s and Bryce’s groups,14 whose studies realized attractively low band gap materials.15 Our group recently introduced ambipolar FMDs using donoracceptor-donor configuration (Figure 1).16 Studies on similar D-A-D triads have been reported based on dibenzothiophene-S,Sdioxide acceptor, where tuning of the emission properties was realized by incorporation of triarylamines donors in varied substitution patterns.17 Given that the photophysical properties of carbazoles (Cz) as electron donors have been previously studied in sufficient detail,1,2,18 incorporation to the FM core might prove interesting. Physical properties such as highly red-absorbing vertical transitions, high charge conductivity, appropriate bulk morphology, and superior thermal stability are within reach through this modification. Furthermore, ease of processing and efficient hole/ electron transport are characteristics of ambipolar compounds that Received: December 1, 2010 Revised: January 28, 2011 Published: March 01, 2011 2184

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Figure 1. Molecular structures of FMDs. Because all structures have the FM residue in common, their acronyms are based on their functionalities and the position of such in the FM ring. PA stands for phenylacetylene, while CPA stands for 1-(3,6-di-tert-butylcarbazol-9-yl)-4-ethynyl-benzene.

suggest important solutions for architectural improvement of OPVs and OFETs.19 Preliminary photophysical studies of FMDs point to optical absorptions modestly affected by changes in solvent polarity, contrary to the case of their fluorescence. In specific, 2,7-FMDs presented weakly allowed πAr f πDCV* transitions, superior fluorescence quantum yields (ΦF), and shorter emission lifetimes (τ) than the 3,6-variant. In contrast to all other cases, CPAFM36 evidenced complete emission quenching upon photoexcitation in solvents of high polarity at room temperature. This prompted us to study the dependence of their photophysical properties on solvent polarity via ultrafast transient absorption spectroscopy (UF-TAS). These studies were complemented with cyclic voltammetry and DFT computations to establish a clear picture of the excited state deactivation dynamics of the compounds of interest.

2. EXPERIMENTAL SECTION 2.1. Materials. 2,7-Bis(2-phenylethyn-1-yl)fluoren-9-ylidene malononotrile (PAFM27), 2,7-bis(2-(4-(3,6-di-tert-butylcarbazol-9-yl)phenylethyn-1-yl)fluoren-9-ylidene malononotrile (CPAFM27), 3,6-bis(2-phenylethyn-1-yl)fluoren-9-ylidene malononotrile (PAFM36), and 3,6-bis(2-(4-(3,6-di-tert-butylcarbazol-9-yl)phenylethyn-1-yl)fluoren-9-ylidene malononotrile (CPAFM36) were synthesized and characterized previously.16 All spectroscopic grade solvents used for optical measurements were purchased from commercial suppliers with the exception of THF and toluene. The latter solvents were purified through a solvent purification system using activated alumina. Anhydrous DMF and electrochemical grade ferrocene and tetra-n-butylammonium hexafluorophosphate (NBu4PF6) were purchased from commercial sources and used as received for electrochemical measurements. 2.2. Electrochemistry. Cyclic voltammetry (CV) experiments were performed at 298 K with a potentiostat/galvanostat in a three-electrode cell at a speed of 100 mV/s. An Ag/AgCl, a platinum disk (L = 2 mm, polished to a mirror finish), and a Pt wire were used as reference, working, and auxiliary electrodes, respectively. Ohmic drop compensation was not necessary for

the experiments described herein. Routine UV-vis spectroelectrochemistry (SEC) measurements were carried out at controlled potentials using thin layer conditions by substituting a Pt-disk for an Au-mesh as working electrode. For all experiments, the final solutions were prepared using degassed electrolytic solutions (0.1 M NBu4PF6 in DMF) and kept under argon for the duration of the SEC experiment. Ferrocene was used as internal standard for all measurements. Oxidation and reduction waves were measured separately. 2.3. Steady State Absorption and Photoluminescence Spectroscopy. Absorption spectra were recorded using a double-beam spectrophotometer, accurate to (1 nm at a concentration of approximately 1.0 μM in spectroscopic grade solvents. The absorption spectrum of anionic FM species was obtained by titration of an FM solution in THF (1.0 μM) with sodium benzophenone ketyl solution in THF (0.1 mM) under argon. Steady-state, time-resolved fluorescence measurements were performed on a single-photon-counting spectrofluorimeter equipped with pulsed NanoLEDs (tpulse ∼ 0.8 ns) for emission lifetime measurements. Solutions with optical densities below 0.1 at the wavelength of excitation were prepared from the corresponding spectroscopic grade solvents. For fluorescence studies at 77 K, the compounds were dissolved in spectroscopic grade 2-methyltetrahydrofuran (MeTHF). These solutions had optical densities ranging from 0.1 and 0.3 at the wavelength of excitation. A transparent quartz dewar containing liquid N2 was used to form glasses (in situ), and the dewar and sample were inserted in the fluorimeter for data collection. Fluorescence quantum yields of the luminophores were measured using quinine sulfate in 0.1 M H2SO4 as standard, once crosscorrelated with anthracene in absolute ethanol. The quantum yield of an unknown sample was calculated using the comparative method20 I Astd η2 Φ ¼ Φstd Istd 3 A 3 η2std where Φ is the quantum yield, I is the integrated intensity, A is the optical density, and η is the refractive index of the media. Temperature-dependence studies were carried out in a digitally controlled cryoprobe accurate to (1 C. 2185

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Figure 2. Frontier orbital maps for ambipolar FMDs computed at the B3LYP/6-31G* level of theory.

2.4. Ultrafast Transient Absorption Spectroscopy. The instrumental setup for 90 fs laser pulses has been described previously.21 The sample was dissolved in 30 mL of spectroscopic grade toluene or DMF and transferred to a 50 mL flask, which was used as a reservoir for sample pumping through a flow quartz cell with an optical path of 2 mm. Steady state UV-vis absorption measurements were carried out before and after laser data acquisition to monitor any sample decomposition. All measurements were carried out at room temperature (20 ( 2 C). Signal decay/rise data analysis was carried out using (mathematical method) deconvolution of single exponential models. 2.5. Computational Methods. Gaussian03 was used to perform all calculations.22 The unconstrained geometries of the carbazole-containing compounds in the gas phase were optimized by Density Functional Theory (DFT) using Becke’s threeparameter functional23 hybridized with the Lee-Yang-Parr correlation functional24 and the 6-31G* split-valence basis set.25 The solubilizing tert-butyl groups from carbazole were removed to speed up the computations. TDDFT level of theory was used to calculate the vertical transitions of the first 10 states of each of the carbazole-containing compounds in the gas phase.26

3. RESULTS 3.1. Energetics. The optimized geometries (B3LYP/6-31G*) of the FMDs are fully planar structures in the PAFM skeletons with tilted carbazoles in the case of CPAFMs (dihedral angles ∼52, see Supporting Information for PAFMs structures). The frontier orbitals CPAFMs (Figure 2) show localization of the HOMO in the CPA units with major contribution from Cz, while the LUMO is localized in FM. This indicates that the HOMO f LUMO electron shift (e-shift) is of charge transfer (CT) type. A similar situation is found in the frontier orbital distribution of PAFMs. The calculated ground state dipole moment was higher for CPAFM27 (μg = 6.33 D) than for its 3,6-variant (μg = 1.34 D), while the difference between S1 and S0 state dipole moments are remarkably different for both compounds (Δμ = þ2.35 D for CPAFM27, and Δμ = þ35.1 D for CPAFM36).

TDDFT analysis reveals that the first vertical transition for all compounds involved major or exclusive contribution of HOMO f LUMO e-shift.16 In particular, CPAFM27 and PAFM36 presented mixing with another type of e-shift of minor contribution (HOMO-4 f LUMO for CPAFM27, HOMO-2 f LUMO for PAFM36) localized in the fluorene ring (Figures S4 and S6). Most low-energy vertical transitions involving LUMO present CT character. The ionization potential of Cz (IP = 5.3 V)27 and electron affinity of FM (EA = 3.9 eV)28 have been reported in polar medium (i.e., acetonitrile, DMF). Analysis of the driving force for electron transfer can be achieved through the Born dielectric continuum model, as proposed by Weller, assuming ion-pair solvent separation (IPSS),29 where ΔGSSIP ¼ E0 ðDþ =DÞ - E0 ðA=A- Þ     1 1 1 1 1 1 2 -e þ RDA εS 2 rD rA ε S εS 0 ΔGCS ¼ ΔGSSIP - E00

ð1Þ ð2Þ

ð3Þ ΔGCR ¼ - ΔGSSIP Considering the 0-0 singlet band gap of the donor (ES1 = 84 kcal/mol)30 and of the acceptor (ES1 = 66 kcal/mol),31 driving forces of -44.6 and -26.6 kcal/mol result for CS in the system, respectively, in a slightly polar medium such as THF. The centerto-center distance is assumed to be 1.2 nm, and the radius for Cz and FM are taken as 3.5 and 3.0 Å, correspondingly, upon evaluation of the optimized geometries.32 These numbers reflect a thermodynamically favored PET mechanism upon excitation of either donor of acceptor units within the assumed distance. Inasmuch as singlet energies around that of FMs are almost borderline cases, it is expected that SSIP formation might occur in the proposed FMDs if their singlet energies are high enough, the kinetic barriers are not sufficiently high, or the competing deactivation mechanisms (i.e., PL or ISC) are not significantly faster. PA connection should favor eT over higher distances through incoherent hopping or molecular wire behavior mechanism.33,34 2186

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Figure 3. Cyclic voltammogram of all FMDs at 298 K.

Table 1. Redox Potentials in DMF vs Fc/Fcþ compound

E1red0 (V)

E2red0 (V)

PAFM27

-0.89

-1.61

CPAFM27

-0.82

-1.55

PAFM36

-0.88

-1.54

CPAFM36

-0.81

-1.39

FM

-1.00

-1.75

Epa(V)

IP (eV) >5.7

4.0

þ0.71

5.6

4.1

>5.7

4.0

þ0.71

5.6

4.1

EA (eV)

3.9

3.2. Cyclic Voltammetry. CV experiments were carried out in DMF solution containing 0.1 M NBu4PF6 at room temperature against Fc/Fcþ in order to characterize the charged species that could be potentially produced after photoexcitation. Results are outlined in Figure 3. All compounds showed multiredox behavior consisting of two single-electron reduction waves localized exclusively in the FM unit (E(FM/FM•-) = -1.00 V; E(FM•-/ FM2-) = -1.75 V)28 and one reversible oxidation wave for Czcontaining FMDs within the scanned region. The oxidation from Cz-substituted FMDs has about twice the current of that from the cathodic reductions, as expected given the D-A stoichiometry of these compounds. Extension of the π-system through functionalization on the FM core via 2,7 and 3,6 disubstitution similarly stabilizes the LUMO energy of the overall molecule. This is reflected in the comparability of first and second reduction potentials of all FMDs. Such potentials are smaller than those belonging to parent FM (Table 1). Furthermore, there is only a slight influence of Cz in the electron affinity of all FMDs signifying that the LUMO of the system must be localized exclusively in the FM unit. Finally, smaller anodic peak potentials (Epa) were found for all Cz-substituted FMDs. The ionization potentials and electron affinities were calculated using the following formulas: EA(eV) = Eonsetred þ 4.9; IP(eV) = Eonsetox þ 4.9. These formulas are based on the assumptions that the energy level of SCE relative to vacuum is 4.4 eV as suggested by Jenekhe,35 and SCE reference with respect to Fc/Fcþ is þ0.5 V. Assuming Koopmans’ theorem is valid,36 these values could be taken as reference for the HOMO and LUMO energy levels for all FMDs. Table 1 summarizes the pertinent values to this discussion.

The second reduction potential of CPAFM36 presented irreversible coalescence of bands at about -1.5 V versus Fc/ Fcþ (ca. -1.0 vs Ag/AgCl, see Figure S9 in Supporting Information). While the origin of this phenomenon is still unknown, it can be speculated to be either one of the following: a comproportionation reaction taking place near the working electrode whereby the doubly reduced acceptor exchanges an electron with the neutral reactant to form two molecules of singly reduced specie (i.e., A2- þ A f 2A•-),37 or an intramolecular electron transfer from the other Cz donor to form the specie (D•þ-A2--D•þ). Finally, the values for the electron affinity were about the same for all FMDs (∼4.0 eV), which highlights a very stable LUMO and the consideration that these materials could be stable upon thin-film OFET operating conditions. This characteristic makes them attractive as semiconductor candidates for such an end given the similarities of the LUMO energy values with those reported by Facchetti and Marks for ambipolar indenofluorene derivatives.19e 3.3. Steady State Absorption and Photoluminescence. We have previously studied the absorption and photoluminescence properties of ambipolar FMDs and refer the reader to ref 16 and the Supporting Information for details (section S3). The nonlinear dependence of the emission maxima and Stoke’s shifts of such compounds with the Onsager solvent parameter (f(ε,n)) highlights specific solvent effects in the excited state.38 Nevertheless, the Lippert-Mataga plot (Figure 4A) indicates small differences between ground and excited state dipole moment, in line with the TDDFT calculations for CPAFM27 (vide supra). Our group has previously reported this kind of behavior for other types of Cz-based D-A compounds in which multiple excited states operate in solutions of high polarity such as solvent nonrelaxed and relaxed ICT states.39 Contrarily, the high difference between ground and excited state dipole moments exhibited by CPAFM36 implies an important role of the solvent reorganization. The fluorescence redshift observed from cyclohexane to toluene is important enough to be considered (Figure S12). In principle, such a shift can be attributed to conformational changes taking place in the excited state. To verify this, PL measurements were carried out in 10 K increments by warming CPAFM36 in the MeTHF matrix from 80 K (Figure 4B). From 80 to 90 K the emission intensity is lowered while the spectral shape remains unaltered, whereas increasing the temperature from 90 K promotes a bathochromic shift almost in parallel to the decrease of emission intensity. Such PL reduction is more apparent at T > 100 K, although the signal intensifies while hypsochromically shifting in the 110-130 K range prior to quenching. This is consistent with a constraint in molecular motion from the chromophore in the different glass transition temperatures exhibited by the MeTHF matrix (i.e., R-glass morphology at 91 K and metastable crystalline phase at 103 K) as this unsuccessfully accommodates to align its dipole with that of the crystalline phase of MeTHF.40 These observations, plus the fact that all FMDs possess low reorganization energies (λv ∼2-5 kcal/mol), imply that the solvent reorganization is likely to have a very active role in the NRD of CPAFM36. Low PL quantum yields (ΦPL) in toluene highlight an efficient NRD. Most FMD emission lifetimes decreased upon increasing solvent polarity, evidencing NRD is more effective in solvents of high polarity. A charge separation/recombination mechanism in which the geometry changes of ground and excited state, induced by changes in solvent polarity, might play a very significant role is proposed. 2187

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Figure 4. (A) Lippert-Mataga plot for FMDs, yielding the dipole moment difference, Δμ, from the slopes. The solvents are from left to right cyclohexane, toluene, ethyl acetate, DCM, and DMF. (B) PL spectra of CPAFM36 in MeTHF at variable temperatures in steps of 10 K, starting from 80 K.

Figure 5. UF-TAS evolution from 1 to 1000 ps for PAFM27 with corresponding kinetics traces for wavelengths of interest. (Top) Data acquired in toluene. (Bottom) Data acquired in DMF.

3.4. Ultrafast Transient Absorption Spectroscopy. The ultrafast transient absorption spectra (UF-TAS) of all FMDs within the 150 fs instrument response function at room temperature after excitation with 90 fs train laser pulses (LP) at the excitation wavelength in toluene and DMF solutions are outlined below. A signal evolution analysis in toluene solutions is described in the Supporting Information. 2,7-Fluoren-9-ylidene Malononitrile Derivatives (2,7-FMDs). The PAFM27 solution in toluene after irradiation at 350 nm with 90 fs LPs (Figure 5) presents negative absorption at λ < 350 nm (Figure S15) and two positive absorbing bands with maxima centered at 400 and 660 nm (broad), respectively. The broad

positive band exhibits a shoulder centered at 505 nm. The negative feature is composed of destructive interference between the GS and the excited state absorption (ESA), where the contribution of the former is more significant than the latter. The bleach band amplifies with τ(d) ∼ 0.4 ps, while reducing with τ(g) = 117 ps (Figure S17). All positive ESA bands rise constantly with similar kinetics in both maxima (τrise ∼ 0.5 ps at 505 nm, τrise ∼ 0.63 ps at 660 nm). At times later than 6 ps, these bands decay following the sum of a monoexponential component plus a constant (τ1(d) = 125-145 ps, τ2(d) > 1.1 ns). The shortest component is comparable to that of the bleach recovery, while the latter is consistent with the long PL lifetime found for 2188

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Figure 6. UF-TAS spectra of PAFM27 after excitation at 350 and 533 nm in DMF at room temperature after 10 ps delay time, along with the differential absorption spectra of parent FM at -600 and -1200 mV, respectively.

this compound (Table S2).16 If assumed that the bleach recovers up to a residual signal where the destructive interference between GS and ESA is such that the signal is null at this range of wavelengths, then all the ESA bands must come from the same state.41 The long-lived component is in agreement with the emission lifetime of PAFM27 (τF ∼ 10 ns).16 In a parallel way, excitation of the low energy absorption band (λexc = 533 nm, 120 fs laser pulses, see Figure S18) shows similar spectral features to the ones observed when exciting the higher energy absorption band (λexc = 350 nm). Furthermore, these signals decay monoexponentially (τ(d) = 121-131 ps) with comparable kinetics to the shortest component of the ESA decay after photoexcitation at 350 nm (Figure S19). While the operative excited state is the same for both excitations in accordance with Kasha’s rules,42 there seems to be an upper potential energy surface (PES) crossing that withdraws part of the ES population toward a different photoproduct. PAFM27 solution in DMF was irradiated with 90 fs LPs at 350 nm to test the effect of solvent polarity on the kinetics of signal decay (Figure 5). The spectral shapes of the ESA are identical to those acquired in toluene, yet their dynamics after 6 ps were significantly faster (τ1(d) = 31-43 ps, τ2(d) > 1.1 ns). This observation highlights a charge separation (CS) mechanism taking place. To probe this, the GS spectroelectrochemical (SEC) absorption spectrum of PAFM27 in DMF was taken at -600 and -1200 mV. Characterization of the absorption features of [PAFM27]•- and [PAFM27]2- was achieved after subtraction of the GS FM absorption at 0 V from that obtained from the UV-vis SEC measurements. To avoid solvent effects, measurements were carried out in DMF at room temperature under inert atmosphere (Figure 6). The differential spectra of the singly and doubled reduced PAFM27 species presented interesting similarities. These similarities were also apparent in the doubly reduced parent FM (Figure S10) and the photoexcited PAFM27 with low and high energy LP (Figure 6). Two electron transfers seem possible after PAFM27 photoexcitation (i.e., generation of D•þ-A2--D•þ) wherein the kinetics of the second electron injection could dominate over a thermodynamically less favored center. A similar case has been reported by Bryce and Perepichka when observing, for the first time, the formation of a covalent D2þ-σ-A•- redox state using, precisely, an FMD as electron accepting unit: 2[10-(4,5-dimethyl-[1,3]dithiol-2-ylidene)-9,10-dihydroanthracen-9ylidene]-5-methyl-[1,3]dithiol-4-ylmethyl ester.43 The solubility of

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the FMDs studied herein in DMF represented a limiting factor for acquisition of detailed data (i.e., low temperature SEC). Therefore, until derivatization to more soluble products is realized, the possibility of generating a D•þ-A2--D•þ species remains purely speculative. All the same, photoinduced CS implies that the observed emission for this compound must be fluorescence from the ICT state and the fastest decay component must account for internal reorganization of the molecule (i.e., planarization of Cz unit with respect to the PA plane). The spectral shape and kinetics of CPAFM27 is similar to those of parent PAFM27 in toluene when excited at 350 nm (Figure 7). This is indicative of ICT mechanism, wherein the spectral broadening may be explained through the contribution of the Cz•þ absorption.44 The broad signal at 395-770 nm develops an intensity maximum around 690 nm at ca. 1 ps, redshifting ca. 10 nm afterward to finally decay biexponentially (Figure S22). The bleach signal observed in the 350-395 nm range presents a slightly different recovery (τ1(d) = 107 ps, τ2(d) > 1.1 ns) than most positive absorbing bands (τ1(d) = 126 ps, τ2(d) > 1.1 ns at 400 nm; τ1(d) = 130 ps, τ2(d) > 1.1 ns at 505 nm; τ1(d) = 138 ps, τ2(d) > 1.1 ns at 660 nm). The transient spectrum after photoexcitation at 533 nm presents comparable features to those from PAFM27 excited at the same wavelength (Figures S23,24). While the ESA bands showed slight changes from those after photoexcitation at 350 nm (i.e., the red-absorbing maxima at 700 nm lowers its intensity), their decay kinetics are quite similar (τ410 nm = 122 ps, τ500 nm = 108 ps, τ630 nm = 117 ps). Judging by the differences in spectral shape, a potential energy surface (PES) crossing from an upper state with that of the lower lying CS state for this compound is of consideration (i.e., generation of D•þ-A2--D•þ species). The effect of solvent polarity on the kinetics of CPAFM27 signal decay is more noteworthy than in the PAFM27 case (Figure 7). The spectral features of the ESA bands are similar to those acquired in toluene after excitation at 350 nm. Also, the first decay component of the biexponential kinetic model for all ESA bands is much faster in DMF than in toluene (τ1(d) e 5 ps, τ2(d) > 1.1 ns). UF-TAS acquisition after pumping at the lowest energy absorption band (λexc = 533 nm, 120 fs LPs) for comparison purposes was problematic given the limited solubility of such compounds in DMF at the required concentrations (see that ε350 nm . ε533 nm). Nevertheless, given spectral similarities with PAFM27 and the negligible solvatochromism in the GS absorption spectra, it seems that the same features should be reproduced independently of solvent polarity. 3,6-Fluoren-9-ylidene Malononitrile Derivatives (3,6-FMDs). The UF-TAS evolution of PAFM36 after excitation with 400 nm/90 fs LPs was distinctive from all other FMDs in toluene and DMF (Figure 8). In toluene, the spectral evolution can be divided into two sections: a high energy (330-500 nm) sigmoidal signal of big amplitude and a low energy (500-780 nm) sigmoidal signal of smaller amplitude. The GS bleach predominates over the excited state absorption at high energies (λ < 450 nm) and vice versa at low energies (Figure S21). While at times later than 6 ps, the GS bleach signal remains somewhat steady, the pump light bandwidth in such range of wavelengths kept us from monitoring the signal progression. All other positive ΔOD signals decay with similar kinetics (τdecay = 0.78-1.5 ns) resembling the PL lifetime of this compound in toluene (τF = 1.0 ns). The UF-TAS spectrum after pumping at the lowest energy absorption band (λexc = 500 nm, 120 fs LPs) lack of some 2189

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Figure 7. UF-TAS evolution from 1 to 1000 ps for CPAFM27 with corresponding kinetic traces for 360, 400, 505, and 660 nm probe wavelengths. (Top) Data acquired in toluene. (Bottom) Data acquired in DMF.

features such as the bleach signal at 400 nm and the peaks located at 660 and 750 nm compared to that excited at 350 nm (Figure S25). Apparently, the region from 380 to 440 nm presents destructive interference of GS and ESA where the extinction coefficients of both transitions appear to be nearly identical for this case. Likewise, the loss of vibrational modes in the low energy region could be ascribed to destructive interference with stimulated emission (SE). All the same, the decay profiles at 345, 450, 560, 650, and 750 nm are similar to those found for PA36 when excited at 400 nm (τ345 nm = 0.97 ns, τ450 nm = τ560 nm = τ750 nm = 1.1 ns, τ660 nm = 1.2 ns). UF-TAS of PAFM36 solutions in DMF were acquired after pumping with 90 fs laser pulses at 400 nm to test the validity of the observed Sn r S1 transitions (Figure 8). The effects of increasing solvent polarity for this 3,6-FMD were minimal on the spectral shape (small blue-shift in the overall signal) and kinetics (τdecay = 0.5-0.9 ns, compare with τF in ref 16), respectively, if compared to those of 2,7-FMDs. The spectral changes of CPAFM36 in toluene after excitation with 90 fs LPs at 400 nm embodied the destructive interaction between positive excited state absorption and negative GS bleach (Figures 9 and S24). At early times, the high energy (350500 nm) sigmoidal signal increases its amplitude reaching plateau at around 6 ps; the low energy sigmoidal (500-780 nm) presents a series of shifts while increasing its intensity. The shape of the overall spectrum at early times (t ∼ 6 ps) resembles that of parent PAFM36 for λ g 500 nm. After 6 ps, the bands initially centered at 560 and 660 nm coalesce up to 200 ps, in which they reach a maximum ΔOD signal (Figure S25). Afterward, both bands dissociate while decaying with the lowest energy band slightly red-shifted. The bleach bands remained steady after reaching a plateau at about 50 ps, similar to parent PAFM36. Conversely,

spectral evolution in DMF after irradiation with 90 fs pulses at 400 nm was remarkably fast, with all signal progressions achieved during the 10 ps time window (Figure 9). The spectral shape of CPAFM36 evolution in DMF resembles that acquired for uncoupled FM•- (Figure B4.28) and Cz•þ (λabs = 750-800 nm).45 This implies complete charge separation as DMF intervenes to solvate the charges (i.e., TICT). It is apparent that the spectrum in toluene at 1 ns is the convolution of the spectra in the same solvent after 6 ps plus that of DMF solution after 1 ps (Figure 10). The bands centered at 360, 430, and 750 nm seem to belong to a common parent, where the kinetics complexity revealed the presence of another specie (360 nm: τ1 = 8 ps, τ2 = 0.18 ns; 430 nm: τdecay > 1.1 ns; 750 nm: τ1 = 6 ps, τ2 = 17 ps, τ3 > 1.1 ns). This observation highlights a slower transition between ICT and LE states in solution. To probe this, the UF-TAS was acquired after pumping the low energy GS absorption band using 120 fs laser pulses at 500 nm (Figure S28). Interestingly, the initial spectral features are maintained throughout the course of the signal evolution within the time frame of the experiment (6-1500 ps). Moreover, all signals decay consistently with similar rate constants (τdecay = 1.6-2.5 ns), which in turn were comparable to those obtained by photoluminescence (τPL = 3.18 ns).16 This experiment allows for assertion that there is, indeed, surface crossing of an upper singlet state with that of the CS specie in toluene, where a kinetic barrier is sufficiently large to slow down the CS process. The CS specie is infinitely long-lived within the window of the experiment (τ > 1 ns) in toluene in accordance with its PL lifetime. The barrier between S1 and CS state is proven to be solvent dependent, given that the spectral features and kinetics are indistinguishable when exciting the CPAFM36 solution in DMF with 350 and 500 nm LPs. 2190

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Figure 8. UF-TAS evolution from 1 to 1000 ps for PAFM36 with corresponding kinetic traces for 350, 380, 440, 550, 645, and 750 nm probe wavelengths. (Top) Data acquired in toluene. (Bottom) Data acquired in DMF.

4. DISCUSSION The redox potentials for one electron oxidation and reduction, outlined in Table 1, are independent of the expansion of the π-electronic density after incorporation of Cz units; hence, they must be governed by the FM unit. Conversely, oxidation potentials were ruled by the donor on the FM skeleton and independent of substitution. Both suggest weak D-A electronic interaction in the ground state, thus, reinforcing our conclusions on the absorption spectra of FMDs and the interpretation of the TDDFT calculations.16 The mapping of optimized geometries for both HOMOs and LUMOs for all FMDs corroborated their poor constructive overlap (Figure 2). In the case of 2,7-FMDs, the former orbital is mainly localized in the donor residue and the latter in the FM unit. Overall, the bridge orbitals give an important contribution to those from Cz while destructively overlapping the FM unit. The observed oscillator strength can be approximated as the product of individual “forbiddenness factors” that reduce the values of the maximum possible oscillator strength fmax as follows:46 fobs ¼ ðfe  fv  fs Þfmax

ð4Þ

This implies that the electronic transitions between frontier orbitals have low oscillator strengths since the value of the overlap integral ÆπAr|πDCVæ must lay close to zero (thus fe ∼ 0) regardless of the contribution of the Franck-Condon factors (fv) and the fact that the S1 r S0 transition is spin allowed (fs ∼ 1). TDDFT calculations in vacuum (B3LYP/6-31G*) predicted that all transitions involving LUMO have small

oscillator strengths. Consequently, the experimental rates of radiative decay are slow16 in accordance with what could be predicted through the Strickler-Berg relation (i.e., kS-B 8  10-6 s-1 and krad = 6.0  10-4 s-1 for PAFM27, Table S1 and ref 16), notwithstanding known limitations.47 Therefore, previous results obtained from CV and steady state UV-vis absorption spectra where it was found that the electronic communication between frontier orbitals is poor in the ground state16 are well supported by theory. Assuming the model for solvent separated ion pair (SSIP) holds valid for this case, calculation of the energies of the solvated ion pairs was achieved via the Born dielectric continuum model.29 DFT-optimized geometries of FMDs allowed access to electronic parameters such as the D-A center-to-center intramolecular distance (rDA) and the ionic radii of the individual species (rD and rA). The free energies of CS and CR, determined using the singlet energies derived from the average energies of the lowest energy absorption and highest energy emission maxima,48 are outlined in Table 2, along with the lifetimes of CS and CR obtained by UF-TAS for most compounds. In toluene, the calculated driving force for CS is unfavorable for both Cz-containing compounds. The opposite trend is found for CR in the same solvent as expected. In DMF, the CS driving force is somewhat similar for most compounds (from -14 to -18 kcal/ mol) as well as those for CR (from -34 to -41 kcal/mol). Therefore, the solvent medium seems to dominate the overall process for eT from a thermodynamic point of view. This is expected given the distance between D and A units (914 Å) and the poor electronic communication in the ground state. However, it is interesting to see that CS still occurs in 2191

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Figure 9. UF-TAS evolution from 1 to 1000 ps for CPAFM36 with corresponding selective kinetic traces from 350 to 750 nm probe wavelengths. (Top) Data acquired in toluene. (Bottom) Data acquired in DMF.

Figure 10. UF-TAS spectra of CPAFM36 in toluene solution after 6 ps and 1 ns delays and in DMF solution after 1 ps delay, after excitation with 90 fs pulses of 400 nm light.

toluene. This implies that the SSIP condition is invalid when treating this type of eT in such nonpolar medium as evidenced by the lower energy of the CT specie with respect to that of S1 when comparing the CV and steady-state absorption and PL results (E(Dþ-A-) ∼ 37 kcal/mol, E(S1) ∼ 52 kcal/mol).32,16 The position of the substituents in the FM skeleton significantly affects the rates of CS and CR processes in toluene. Lifetimes for CS and CR in toluene were found similar for both 2,7-FMDs and different of that of CPAFM36, where in all cases 2,7-FMDs separated charges more rapidly while recombination occurred more slowly (Table 2). This result is consistent with the

initial observations of Martínez and Bardeen where meta-conjugation in phenylacetylene dendrimers increases the excited state electronic coupling.49 The same groups studied this phenomenon using ab initio methods for theoretical computations and concluded that excitonic coupling between meta-linked PA derivatives was enhanced with at least a factor of 5 with respect of para-linked counterparts in the first singlet excited state.49b Previously, Zimmermann used advanced computational tools to study the effects of meta-substitutions in photochemical hydrolyses when treating meta- and para-methoxybenzoyl cations and radicals at the CASSCF(8,8)/6-31G* level in 1995.50 Zimmerman’s work strongly supported primary suggestions of Havinga 30 years prior where, based on simple H€uckel calculations, the higher reactivity of meta-nitrophenyl phosphate ester over its para-isomer toward photoinduced hydrolysis was explained in terms of the contrasted electron donation and withdrawal in the first singlet excited state of aromatic compounds with that commonly used for the description of their ground state chemistry.51 Further exploitation of this phenomenon has been recently achieved by many when engineering self-assembling molecules for diverse purposes such as long-lived charge separation,52 controlled spin-dynamics,53 and efficient light emission.54 Therefore, this trend determines the molecular design of FMDs, especially if further exploration as ambipolar materials for OPVs and OTFTs is to be accomplished. Within a semiclassical treatment for long-range eT, the rate constant can be described in terms of Fermi’s golden rule for radiationless decay. Generally, the vibronic Franck-Condon factor (FC) depends exponentially on the driving force for eT and other important parameters such as the internal reorganization energy of the DA chromophore (λv), the solvent 2192

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Table 2. Properties of CS and CR for FMDs in Toluene and DMF toluene cmp

τCS (ps)

ΔGCS (kcal/mol)

τCR (ns)

DMF ΔGCR (kcal/mol)

τCS (ps)

10.0

ΔGCS (kcal/mol)