Dual Character of Excited Radical Anions in Aromatic Diimide Bis

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Dual Character of Excited Radical Anions in Aromatic Diimide Bis(Radical Anion)s: Donor or Acceptor? Chao Lu, Mamoru Fujitsuka, Akira Sugimoto, and Tetsuro Majima J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00970 • Publication Date (Web): 07 Feb 2017 Downloaded from http://pubs.acs.org on February 13, 2017

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The Journal of Physical Chemistry

Dual Character of Excited Radical Anions in Aromatic Diimide Bis(Radical Anion)s: Donor or Acceptor? Chao Lu, Mamoru Fujitsuka,* Akira Sugimoto, and Tetsuro Majima* The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 81, Ibaraki, Osaka 567-0047, Japan AUTHOR INFORMATION Corresponding Authors *M. Fujitsuka. Tel: +81-6-6879-8496. Fax: +81-6-6879-8499. E-mail: [email protected] *T. Majima. Tel: +81-6-6879-8495. Fax: +81-6-6879-8499. E-mail: [email protected]

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ABSTRACT: Intramolecular electron transfer (ET) processes in the excited aromatic diimide bis(radical anion)s (ADI•−*-ADI’•−) were systematically investigated by applying femtosecond laser flash photolysis to bis(radical anion)s of naphthalenediimide (NDI) and perylenediimide (PDI), including NDI•−-m-NDI•−, NDI•−-p-NDI•−, PDI•−-m-PDI•−, and NDI•−-m-PDI•− (m and p indicate the substitution positions). The excitation of NDI•−-m-NDI•− and NDI•−-p-NDI•− initiated disproportionation reactions generating NDI and NDI2− with different ET rate constants. For the first time, the dual characteristics of ADI•−* were confirmed upon selective excitation of NDI•−-m-PDI•−: NDI•−* was unambiguously demonstrated to function as an electron donor in NDI•−*-m-PDI•−, whereas PDI•−* acted as an electron acceptor in NDI•−-m-PDI•−* because of the energetically preferable production of NDI-m-PDI2−. The relationship between the ET rate constants and driving forces in ADI•−*-ADI’•− can be reasonably analyzed by using the Marcus theory. The current findings provide a new viewpoint regarding the bipolaron-generating nature of ADI•−*-ADI’•− and facilitate simulating various types of photocarrier migration in the densely charged regions of homo- and heterogeneous n-type semiconductor materials upon irradiation.


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INTRODUCTION Aromatic diimides (ADIs), especially naphthalenediimide (NDI) and perylenediimide (PDI), are among the most widely explored components for n-type semiconductor materials in organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs).1-6 This is because of their remarkable properties such as ease of reduction, good charge mobility, excellent thermal stability, and high molar absorptivity. Recently, a new series of copolymers containing both NDI and PDI were designed as the electron acceptors for bulk heterojunction OSCs, and the efficiency (e.g., carrier mobility) of solar cells was found to vary substantially with the NDI/PDI composition.7,8 In addition, it is worth mentioning that NDI and PDI can be reduced stepwise in two phases: first producing radical anions, followed by dianions.9 Therefore, the reduced NDI and PDI as photocarriers including not only polarons (NDI•− and PDI•−) but also bipolarons (NDI2− and PDI2−) should be closely related to the electronic and optical performance of ADI-based materials, although studies focusing on bipolaron-generation mechanisms and dynamics are extremely limited. On the other hand, excited radical ions with powerful redox abilities have attracted increasing attention.10-18 NDI•− and PDI•− in the excited states (NDI•−* and PDI•−*) are stronger reductants than their ground-state counterparts. Wasielewski and his co-workers first reported the transient absorption features of excited ADI radical anions with lifetimes of up to a few hundreds of picoseconds, long enough to initiate various chemical reactions.10-13 Recently, our research group further elucidated the detailed characteristics of intramolecular electron transfer (ET) from NDI•−* and PDI•−* by applying femtosecond laser flash photolysis to several purposely reduced dyads.15,17 In particular, we examined photoinduced ET in a stepwise reduced PDI-PDI dimer (i.e., PDI•−-PDI or PDI•−-PDI•−); surprisingly, an intramolecular disproportionation producing


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PDI and PDI2− was observed upon excitation of PDI•−-PDI•−.17 Considering the roles of the densely populated charges in homo- and heterogeneous organic conductors, the generation of bipolarons via the irradiation of concentrated polarons should be regarded as an important process. Nevertheless, several puzzles (e.g., the driving force, spatial distance, and excitation wavelength dependences) remain to be solved for understanding the related processes in polymeric and crystalline structures. Thus, in this study, NDI, PDI, NDI-m-NDI, NDI-p-NDI, PDI-m-PDI, and NDI-m-PDI (m and p indicate the substitution positions) were prepared as target molecules (Scheme 1). 2-Ethylhexyl and tridecan-7-yl groups were introduced to ensure substantial solubility in organic solvents. Additionally, a phenyl (or 2,5-dimethylphenyl) group was used as a spacer to realize a fixed donor-acceptor distance and minimize the π-conjugation with NDI or PDI due to the perpendicular conformation caused by steric effects. A systematic investigation was conducted to clarify the bipolaron-generating nature of ADI•−*-ADI’•− in various organic molecular devices.

Scheme 1. Chemical Structures of NDI, PDI, NDI-m-NDI, NDI-p-NDI, PDI-m-PDI, and NDIm-PDI


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EXPERIMENTAL SECTION Materials. PDI, PDI-m-PDI, and NDI-m-PDI were synthesized as reported in our previous work.17 The synthesis procedures for NDI, NDI-m-NDI, and NDI-p-NDI are summarized in the Supporting Information. In the present study, N,N-dimethylformamide (DMF) was used as the solvent for all spectroscopic measurements. Tetrakis(dimethylamino)ethylene (TDAE) was purchased from Tokyo Chemical Industry. Apparatus. Steady-state absorption spectra were measured using a Shimadzu UV-3600 UVvis-NIR spectrometer. Transient absorption spectra during femtosecond laser flash photolysis were measured as described previously.19 In this study, the samples were excited by a 475 or 700 nm femtosecond laser pulse (~130 fs fwhm, ~5 µJ per pulse).


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RESULTS AND DISCUSSION Figure 1 shows the steady-state absorption spectra of NDI-m-PDI in DMF with varying concentrations of TDAE. The features of neutral NDI and PDI were simultaneously observed in the absorption spectrum of NDI-m-PDI. When TDAE was added as a reducing agent,20 the absorbance of NDI and PDI decreased and a new set of bands attributable to PDI•− appeared with the maximum occurring at 700 nm. As the TDAE concentration increased, a growing peak was clearly observed at 475 nm. This can be attributed to the generation of NDI•− because of the similar reduction potentials of NDI and PDI (−0.48 and −0.43 V vs. SCE, respectively).11 The absorbance of NDI•− and PDI•− reached their maxima with a two-equivalent addition of TDAE, indicating the quantitative reduction of NDI-m-PDI. The band intensity of NDI•−-m-PDI•− was maintained for several hours after removing the oxygen from the solvent by Ar bubbling. We noted that the spectral features of NDI•− and PDI•− in NDI•−-m-PDI•− were essentially the same as those of the monomeric species,11 suggesting that negligible interactions existed. Most importantly, the selective excitation of the NDI•− and PDI•− moieties in this heterodimer could be achieved at 475 and 700 nm, respectively. As for the other target molecules including NDI, PDI, NDI-m-NDI, NDI-p-NDI, and PDI-m-PDI, one or two equivalents of TDAE were added to prepare the radical anion or bis(radical anion), respectively.


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Figure 1. Steady-state absorption spectra of NDI-m-PDI (75 µM) in DMF with varied concentrations of TDAE (0−150 µM).

Figure 2 shows the transient absorption spectra of NDI•− measured during laser flash photolysis using a 475 nm femtosecond laser. The spectrum taken 6 ps after laser excitation showed both positive and negative signals, indicating the generation of NDI•−* and the bleaching of NDI•−, respectively. With an increase in the delay time, the positive signals showed a decrease whereas the negative signals showed a recovery. This phenomenon is attributed to the D1 → D0 deactivation process of NDI•− with a lifetime of 112 ps (8.9 × 109 s-1), which can be obtained by fitting a single exponential function to the decay kinetics of ∆O.D. at 650 nm (Figure S1).


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Figure 2. Transient absorption spectra of NDI (0.20 mM) in DMF in the presence of TDAE (0.20 mM) during a 475 nm femtosecond laser excitation.

Figure 3a shows the transient absorption spectra of NDI•−-m-NDI•− measured during laser flash photolysis using a 475 nm femtosecond laser. The spectrum taken 4 ps after laser excitation indicated the generation of NDI•−*-m-NDI•−. Meanwhile, sharp absorption peaks appeared at 385 nm (NDI) and 425 nm (NDI2−) due to the generation of NDI-m-NDI2−, suggesting an intramolecular ET process, as described in eq 1. kintraET

NDI•−*-m-NDI•− NDI-m-NDI2−

(1)

Thus, a disproportionation reaction was confirmed to occur upon excitation of NDI•−-m-NDI•−. By analyzing the kinetic trace of ∆O.D. at 650 nm (Figure S2) and taking the D1 → D0 deactivation process into account, the intramolecular ET rate constant (kintraET) was calculated to


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be 1.3 × 1011 s-1. With further increase in the delay time, the positive bands decreased continuously, whereas the negative bands showed a recovery, suggesting a back ET (BET) process, as described in eq 2. kintraBET

NDI-m-NDI2− NDI•−-m-NDI•−

(2)

The intramolecular BET rate constant (kintraBET) was estimated to be 1.1 × 1010 s-1 based on the decay of NDI-m-NDI2− (Figure S3). Similar phenomena were also observed upon excitation of NDI•−-p-NDI•− (Figure 3b). Compared to NDI•−*-m-NDI•−, NDI•−*-p-NDI•− exhibited a slower ET with a kintraET of 8.3 × 1010 s-1 (Figure S4), and the generated NDI-p-NDI2− showed BET that did not complete within the instrumental time window.

Figure 3. (a) Transient absorption spectra of NDI-m-NDI (0.20 mM) in DMF in the presence of TDAE (0.40 mM) during a 475 nm femtosecond laser excitation. (b) Transient absorption spectra of NDI-p-NDI (0.20 mM) in DMF in the presence of TDAE (0.40 mM) during a 475 nm


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femtosecond laser excitation. (Blue and red arrows indicate the peak positions of NDI and NDI2−, respectively.)

Figure 4a shows the transient absorption spectra of NDI•−-m-PDI•− measured during laser flash photolysis using a 475 nm femtosecond laser. The spectrum taken 2 ps after laser excitation indicated the generation of NDI•−*-m-PDI•−. The most significant positive peak due to PDI2− was detected at 575 nm with a kintraET of 3.3 × 1011 s-1 (Figure S5), suggesting a process indicated in eq 3. kintraET

NDI•−*-m-PDI•− NDI-m-PDI2−

(3)

Unambiguously, NDI•−* acted as an electron donor in this reaction. On the other hand, Figure 4b shows the transient absorption spectra of NDI•−-m-PDI•− upon 700 nm laser excitation. The spectrum taken 2 ps after the laser excitation exhibited positive bands with the greatest intensity at 460 nm, which are attributable to PDI•−*. By increasing the delay time, the 575 nmmaximized signals appeared in the spectra, suggesting the production of NDI-m-PDI2− indicated in eq 4. kintraET

NDI•−-m-PDI•−* NDI-m-PDI2−

(4)

Surprisingly, here, the excited radical anion (PDI•−*) was found to be an electron acceptor. The kintraET of this process was calculated to be 1.8 × 109 s-1 by considering the D1 → D0 deactivation of PDI•−* at 460 nm (Figure S6).17 The entire BET processes could not be traced for either NDI•−*-m-PDI•− or NDI•−-m-PDI•−*, owing to instrumental limitation. Moreover, kinetics of individual intermediates were confirmed in species-associated spectra obtained by global analysis (Figure S7).


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Figure 4. (a) Transient absorption spectra of NDI-m-PDI (0.20 mM) in DMF in the presence of TDAE (0.40 mM) during a 475 nm femtosecond laser excitation. (b) Transient absorption spectra of NDI-m-PDI (0.12 mM) in DMF in the presence of TDAE (0.24 mM) during a 700 nm femtosecond laser excitation. (Blue and red arrows indicate the peak positions of NDI and PDI2−, respectively.)

Scheme 2 shows the molecular orbital diagrams for the intramolecular ET processes upon excitation of NDI•−-electron acceptor (A), NDI•−-NDI•−, and NDI•−-PDI•−. Generally, a radical anion has a single electron in its highest occupied molecular orbital (HOMO = SOMO), corresponding to the lowest unoccupied molecular orbital (LUMO) of its neutral state. In the case of NDI•−*-A, one electron is excited from the HOMO-1 to HOMO (SOMO) of NDI•−, and then moves to the LUMO of A (Scheme 2a). Here, the excited radical anion acts solely as an electron donor, which is consistent with all the previous reports.10-18 However, when it comes to


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the case of NDI•−*-NDI•−, the role of the excited radical anion (electron donor or acceptor) is difficult to be determined because injecting the excited electron of NDI•−* into the SOMO of the counter NDI•− and receiving the SOMO electron from NDI•− at the HOMO-1 of NDI•−* are both energetically possible pathways (Scheme 2b). Such duality was clearly confirmed upon selective NDI•− and PDI•− excitation of NDI•−-PDI•− (Scheme 2c). In the particular case of NDI•−-PDI•−*, PDI•−* worked as an electron acceptor and NDI-PDI2− was preferably generated because of the lower second reduction potential of PDI (−0.70 V vs. SCE) than that of NDI (−0.99 V vs. SCE).11 Thus, for the first time, excited radical anion was proved to be not only an electron donor but also an electron acceptor by applying femtosecond laser flash photolysis to the bis(radical anion) of an ADI heterodimer.

Scheme 2. Molecular Orbital Diagrams for the ET Processes in (a) NDI•−*-A, (b) NDI•−*-NDI•−, and (c) NDI•−*-PDI•− and NDI•−-PDI•−*


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The rate constants (kET) and driving forces (−∆GET) for the ET processes in ADI•−*-ADI’•− are summarized in Table 1. The −∆GET values for intramolecular ET and BET (−∆GintraET and −∆GintraBET, respectively) were calculated according to eqs 5-10.21 In the case of NDI•−*-NDI•− or PDI•−*-PDI•−: ∆GintraET = e[E(ADI•−/ADI) − E(ADI2−/ADI•−)] − ED1(ADI•−*)

(5)

∆GintraBET = e[E(ADI2−/ADI•−) − E(ADI•−/ADI)]

(6)

In the case of NDI•−*-PDI•−: ∆GintraET = e[E(NDI•−/NDI) − E(PDI2−/PDI•−)] − ED1(NDI•−*)

(7)

∆GintraBET = e[E(PDI2−/PDI•−) − E(NDI•−/NDI)]

(8)

In the case of NDI•−-PDI•−*:


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∆GintraET = e[E(PDI•−/PDI) − E(PDI2−/PDI•−)] − ED1(PDI•−*)

(9)

∆GintraBET = e[E(PDI2−/PDI•−) − E(PDI•−/PDI)]

(10)

where E(ADI•−/ADI), E(ADI2−/ADI•−), and ED1(ADI•−*) represent the reduction potentials of ADI and ADI•− and the D1 state energy of ADI•−*, respectively. In particular, the values of E(NDI•−/NDI), E(NDI2−/NDI•−), E(PDI•−/PDI), and E(PDI2−/PDI•−) were applied as −0.48, −0.99, −0.43, and −0.70 V vs. SCE, respectively.11 Due to the D1 ← D0 absorption bands of NDI•− and PDI•− at 770 and 955 nm, respectively,11 the following values were used for ED1(NDI•−*) and ED1(PDI•−*): 1.61 and 1.30 eV, respectively. As shown in Table 1, the values of −∆GintraET for NDI•−*-m-NDI•−, PDI•−*-m-PDI•−, NDI•−*-m-PDI•−, and NDI•−-m-PDI•−* were calculated to be 1.10, 1.03, 1.39, and 1.03 eV, respectively. In these dyads, efficient ET processes were detected, and the kintraET values tended to increase as the −∆GintraET values increased. Figure 5 shows the relationship between the −∆GET and kET in ADI•−*-A determined previously using the Marcus theory (weak-colored squares).17 We further plotted the data (strong-colored circles) obtained from the ET processes initiated by ADI•−*-ADI’•−. The −∆GET dependence of kET observed in the present study can be reasonably explained by the same Marcus parabola in the normal region.22-24 The striking difference between the kET values from NDI•−* and PDI•−* is attributable to the energy required to form the reduced spacer and the distance between the electron donor and acceptor.17

Table 1. Estimated kET and −∆GET for Intramolecular ET Processes in ADI•−*-ADI’•−


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kintraETa (s-1)

−∆GintraET (eV)

kintraBETa (s-1)

−∆GintraBET (eV)

NDI•−*-m-NDI•−

1.3 × 1011

1.10

1.1 × 1010

0.51

PDI•−*-m-PDI•−

2.4 × 109

1.03

< 1 × 109

0.27

NDI•−*-m-PDI•−

3.3 × 1011

1.39

< 1 × 109

0.22

NDI•−-m-PDI•−*

1.8 × 109

1.03

< 1 × 109

0.27

a

Estimation error: < 10%.

Figure 5. Relationship between −∆GET and kET in the dyads of ADI•−*. The weak-colored solid and hollow squares correspond to ET and BET, respectively in ADI•−*-A. The strong-colored solid and hollow circles correspond to ET and BET, respectively in ADI•−*-ADI’•−.

Since the ADI dyads can be treated as the simplest units of related polymeric or crystalline materials, it is worth noting that the ET processes exhibited by ADI•−*-ADI’•− provide valuable


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information relating to the generation of bipolarons in the densely charged regions of organic semiconductors upon irradiation.25-27 Considering the intramolecular ET and subsequent BET detected in NDI•−*-NDI•−, PDI•−*-PDI•−, NDI•−*-PDI•−, and NDI•−-PDI•−*, several migration ET

ET

ET

ET

ET

routes of photocarriers (e.g., NDI•−* → NDI•− → NDI, NDI•−* → PDI•− → PDI, and NDI•− → ET

PDI•−* → PDI) can be proposed for ADI-based polymers and crystals, which are much more complex than the dyad systems (Scheme 3). The different kintraET observed in NDI•−*-m-NDI•− and NDI•−*-p-NDI•− reflected the effect of spatial distance in a multi-molecular structure, revealing the possibility of additional charge transport. According to the present results, the kintraET obtained from NDI•−*-m-NDI•− (1.3 × 1011 s-1) or NDI•−*-m-PDI•− (3.3 × 1011 s-1) was 1-2 orders of magnitude faster than the D1 → D0 deactivation of NDI•−* (8.9 × 109 s-1), indicating the facile formation of a bipolaronic state. Moreover, the photochemical performance of an interface is always an attractive point to be clarified when utilizing co-polymers.28,29 In the current study, the generation of the same ET products from NDI•−*-m-PDI•− and NDI•−-m-PDI•−* suggests that a directional charge transport (from NDI•− to PDI•−) occurs at the interface between the charged NDI and PDI regions under photoirradiation. Thus, the above-mentioned findings successfully simulated the unique ET behaviors of the homogeneous NDI/NDI and PDI/PDI, and heterogeneous NDI/PDI components in various organic molecular devices and demonstrated the importance of the pathways from excited bis(radical anion)s. The detailed study on the environment effect (e.g., solvent polarities) is expected to be carried out in the future and will perhaps provide further information for the present ET processes.


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Scheme 3. Cartoon for Photocarrier Migration in the Densely-Charged ADI Components upon Irradiation

CONCLUSIONS The excited ET in densely charged ADIs was systematically investigated using a series of dyad molecules: NDI•−-m-NDI•−, NDI•−-p-NDI•−, PDI•−-m-PDI•−, and NDI•−-m-PDI•−. Different kintraET were observed for the generation of NDI and NDI2− in NDI•−*-m-NDI•− and NDI•−*-pNDI•−. Interestingly, the duality of the excited radical anion, which could act as both an electron


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donor and acceptor, was clearly revealed upon excitation of the NDI•− and PDI•− moieties in NDI•−-PDI•−, respectively. By applying the Marcus theory, the relationship between the kET and −∆GET in ADI•−*-ADI’•− can be reasonably explained. The results presented here provide valuable insights into the unique characteristics of ADI•−*-ADI’•− as effective initiators for bipolaron generation in related OFETs, OLEDs, and OSCs.

ASSOCIATED CONTENT Supporting Information. Synthesis procedures, kinetic traces, and global analysis. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *M. Fujitsuka. E-mail: [email protected] *T. Majima. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work has been partly supported by a Grant-in-Aid for Scientific Research (Projects 25220806 and others) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.

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TOC Graphic


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Dual Character of Excited Radical Anions in Aromatic Diimide Bis

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