Dual Character of Excited Radical Anions in ... - ACS Publications

Feb 7, 2017 - Citation data is made available by participants in Crossref's Cited-by Linking service. For a more comprehensive list of citations to th...
0 downloads 0 Views 2MB Size
Subscriber access provided by AUSTRALIAN NATIONAL UNIV

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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]

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

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.

ACS Paragon Plus Environment

2

Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

3

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

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

ACS Paragon Plus Environment

4

Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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).

ACS Paragon Plus Environment

5

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 24

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.

ACS Paragon Plus Environment

6

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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).

ACS Paragon Plus Environment

7

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

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

ACS Paragon Plus Environment

8

Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

9

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

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).

ACS Paragon Plus Environment

10

Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

11

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

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•−*

ACS Paragon Plus Environment

12

Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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•−*:

ACS Paragon Plus Environment

13

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

∆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’•−

ACS Paragon Plus Environment

14

Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

15

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

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.

ACS Paragon Plus Environment

16

Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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

ACS Paragon Plus Environment

17

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

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

ACS Paragon Plus Environment

18

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

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.

REFERENCES (1) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.; Müllen, K.; MacKenzie, J. D.; Silva, C.; Friend, R. H. Attaching Perylene Dyes to Polyfluorene: Three Simple, Efficient Methods for Facile Color Tuning of Light-Emitting Polymers. J. Am. Chem. Soc. 2003, 125, 437-443. (2) Briseno, A. L.; Mannsfeld, S. C. B.; Reese, C.; Hancock, J. M.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Perylenediimide Nanowires and Their Use in Fabricating Field-Effect Transistors and Complementary Inverters. Nano Lett. 2007, 7, 2847-2853. (3) Wang, S.; Pisula, W.; Müllen, K. Nanofiber Growth and Alignment in Solution Processed n-Type Naphthalene-Diimide-Based Polymeric Field-Effect Transistors. J. Mater. Chem. 2012, 22, 24827-24831. (4) Earmme, T.; Hwang, Y.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor. J. Am. Chem. Soc. 2013, 135, 14960-14963. (5) Rozanski, L. J.; Castaldelli, E.; Sam, F. L. M.; Mills, C. A.; Demets, G. J.; Silva, S. R. P. Solution Processed Naphthalene Diimide Derivative as Electron Transport Layers for Enhanced Brightness and Efficient Polymer Light Emitting Diodes. J. Mater. Chem. C 2013, 1, 3347-3352.

ACS Paragon Plus Environment

19

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 24

(6) Lin, Y.; Wang, Y.; Wang, J.; Hou, J.; Li, Y.; Zhu, D.; Zhan, X. A Star-Shaped Perylene Diimide Electron Acceptor for High-Performance Organic Solar Cells. Adv. Mater. 2014, 26, 5137-5142. (7) Kozycz, L. M.; Gao, D.; Tilley, A. J.; Seferos, D. S. One Donor-Two Acceptor (D-A1)-(DA2) Random Terpolymers Containing Perylene Diimide, Naphthalene Diimide, and Carbazole Units. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3337-3345. (8) Hwang, Y.; Earmme, T.; Courtright, B. A. E.; Eberle, F. N.; Jenekhe, S. A. n-Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend Morphology, and Compatibility Toward High-Performance All-Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 4424-4434. (9) Marcon, R. O.; Brochsztain, S. Aggregation of 3,4,9,10-Perylenediimide Radical Anions and Dianions Generated by Reduction with Dithionite in Aqueous Solutions. J. Phys. Chem. A 2009, 113, 1747-1752. (10) Debreczeny, M. P.; Svec, W. A.; Marsh, E. M.; Wasielewski, M. R. Femtosecond Optical Control of Charge Shift within Electron Donor-Acceptor Arrays: an Approach to Molecular Switches. J. Am. Chem. Soc. 1996, 118, 8174-8175. (11) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski, M. R. Excited Doublet States of Electrochemically Generated Aromatic Imide and Diimide Radical Anions. J. Phys. Chem. A 2000, 104, 6545-6551.

ACS Paragon Plus Environment

20

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

(12) Lukas, A. S.; Miller, S. E.; Wasielewski, M. R. Femtosecond Optical Switching of Electron Transport Direction in Branched Donor−Acceptor Arrays. J. Phys. Chem. B 2000, 104, 931-940. (13) Lukas, A. S.; Bushard, P. J.; Wasielewski, M. R. Ultrafast Molecular Logic Gate Based on Optical Switching between Two Long-Lived Radical Ion Pair States. J. Am. Chem. Soc. 2001, 123, 2440-2441. (14) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; König, B. Reduction of Aryl Halides by Consecutive Visible Light-Induced Electron Transfer Processes. Science 2014, 346, 725-728. (15) Fujitsuka, M.; Kim, S. S.; Lu, C.; Tojo, S.; Majima, T. Intermolecular and Intramolecular Electron Transfer Processes from Excited Naphthalene Diimide Radical Anions. J. Phys. Chem. B 2015, 119, 7275-7282. (16) Fujitsuka, M.; Ohsaka, T.; Majima, T. Dual Electron Transfer Pathways from the Excited C60 Radical Anion: Enhanced Reactivities Due to the Photoexcitation of Reaction Intermediates. Phys. Chem. Chem. Phys. 2015, 17, 31030-31038. (17) Lu, C.; Fujitsuka, M.; Sugimoto, A.; Majima, T. Unprecedented Intramolecular Electron Transfer from Excited Perylenediimide Radical Anion. J. Phys. Chem. C 2016, 120, 1273412741. (18) Zeng, L.; Liu, T.; He, C.; Shi, D.; Zhang, F.; Duan, C. Organized Aggregation Makes Insoluble Perylene Diimide Efficient for the Reduction of Aryl Halides via Consecutive Visible Light-Induced Electron-Transfer Processes. J. Am. Chem. Soc. 2016, 138, 3958-3961.

ACS Paragon Plus Environment

21

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

(19) Fujitsuka, M.; Cho, D. W.; Tojo, S.; Inoue, A.; Shiragami, T.; Yasuda, M.; Majima, T. Electron Transfer from Axial Ligand to S1- and S2-Excited Phosphorus Tetraphenylporphyrin. J. Phys. Chem. A 2007, 111, 10574-10579. (20) Fujitsuka, M.; Luo, C.; Ito, O. Electron-Transfer Reactions between Fullerenes (C60 and C70) and Tetrakis(dimethylamino)ethylene in the Ground and Excited States. J. Phys. Chem. B 1999, 103, 445-449. (21) Kavarnos, G. J.; Turro, N. J. Photosensitization by Reversible Electron Transfer: Theories, Experimental Evidence, and Examples. Chem. Rev. 1986, 86, 401-449. (22) Marcus, R. A. Chemical and Electrochemical Electron-Transfer Theory. Annu. Rev. Phys. Chem. 1964, 15, 144-196. (23) Marcus, R. A.; Sutin, N. Electron Transfers in Chemistry and Biology. Biochim. Biophys. Acta 1985, 811, 265-322. (24) Marcus, R. A. Electron Transfer Reactions in Chemistry: Theory and Experiment (Nobel Lecture). Angew. Chem. Int. Ed. Eng. 1993, 32, 1111-1121. (25) van Haare, J. A.; Havinga, E. E.; van Dongen, J. L.; Janssen, R. A.; Cornil, J.; Brédas, J. L. Redox States of Long Oligothiophenes: Two Polarons on a Single Chain. Chem. Eur. J. 1998, 4, 1509-1522. (26) Heeger, A. J. Semiconducting and Metallic Polymers: The Fourth Generation of Polymeric Materials. J. Phys. Chem. B 2001, 105, 8475-8491. (27) Park, B.; Yang, L.; Johansson, E. M. J.; Vlachopoulos, N.; Chams, A.; Perruchot, C.; Jouini, M.; Boschloo, G.; Hagfeldt, A. Neutral, Polaron, and Bipolaron States in PEDOT

ACS Paragon Plus Environment

22

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Prepared by Photoelectrochemical Polymerization and the Effect on Charge Generation Mechanism in the Solid-State Dye-Sensitized Solar Cell. J. Phys. Chem. C 2013, 117, 2248422491. (28) Kuang, L.; Olson, T. L.; Lin, S.; Flores, M.; Jiang, Y.; Zheng, W.; Williams, J. C.; Allen, J. P.; Liang, H. Interface for Light-Driven Electron Transfer by Photosynthetic Complexes Across Block Copolymer Membranes. J. Phys. Chem. Lett. 2014, 5, 787-791. (29) Gao, W.; Liu, T.; Hao, M.; Wu, K.; Zhang, C.; Sun, Y.; Yang, C. Dithieno[3,2-b:2',3'd]pyridin-5(4H)-one Based D-A Type Copolymers with Wide Bandgaps of Up to 2.05 eV to Achieve Solar Cell Efficiencies of Up to 7.33%. Chem. Sci. 2016, 7, 6167-6175.

ACS Paragon Plus Environment

23

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 24

TOC Graphic

ACS Paragon Plus Environment

24