Precise Control of the Electronic Coupling Magnitude between the

Article Cite This: J. Phys. Chem. C 2018, 122, 3756−3772

pubs.acs.org/JPCC

Precise Control of the Electronic Coupling Magnitude between the Electron Donor and Acceptor in Perylenebisimide Derivatives via Conformation Restriction and Its Effect on Photophysical Properties Yingjie Zhao,† Xiaoxin Li,‡ Zhijia Wang,† Wenbo Yang,† Kepeng Chen,† Jianzhang Zhao,*,† and Gagik G. Gurzadyan*,‡ †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, P. R. China ‡ Institute of Artificial Photosynthesis, State Key Laboratory of Fine Chemicals, Dalian 116024, P. R. China S Supporting Information *

ABSTRACT: Perylenebisimide (PBI)−phenothiazine (PTZ) and PBI−diphenylamine (DPA) dyads were prepared, with the PTZ or DPA as the electron donor and the 6-subsituted PBI unit as the electron acceptor. The dyads were designed in such a way that electronic coupling (magnitude is the matrix elements, VDA and VDA*) between the electron donor and acceptor is controlled by conformation restriction. The effects of the electronic coupling on UV−Vis absorption and emission were studied. More significant charge-transfer (CT) absorption and CT fluorescence emission bands were observed for dyads with smaller dihedral angles between the electron donor and acceptor, thus stronger electronic coupling (VDA = 3290 cm−1 and VDA* = 4317 cm−1) was observed than those with larger dihedral angle, showing weaker coupling effect (VDA = 1210 cm−1 and VDA* = 2770 cm−1). Triplet state production was observed only for PBI−DPA but not for PBI−PTZ dyads. With an intermolecular triplet photosensitizing method, the triplet state of PBI−PTZ dyads was observed. The femtosecond transient absorption spectra confirmed the ultrafast charge separation (CS, 0.3 ps ∼ 0.6 ps) and slow charge recombination (CR, 130 ps ∼ 240 ps) process. These results indicate that the CR does not produce any triplet state in the PBI−PTZ dyads, for instance by the expected spin−orbital charge transfer intersystem crossing (SOCT-ISC). We propose that the lack of triplet state production in PBI−PTZ dyads is due to the large separation of the 1CT and the 3CT/3PBI states, and the orthogonal geometry and the CR are not exclusively sufficient criteria for SOCT-ISC. Our results on controlling the absorption, emission, and ISC by tuning the VDA magnitudes between the donor and acceptor will be useful for molecular design of compact electron donor/acceptor dyads.

1. INTRODUCTION Precise control of the excited state property of chromophores is crucial for understanding of the photophysical properties and development of novel organic optical materials, as well as for the applications of the chromophores,1,2 such as in fluorescent molecular probes,3−8 photovoltaics,9 and charge separation studies.10−14 Concerning these studies, one of the most investigated chromophores is perylenebisimide (PBI), which was very often used as an electron acceptor.15−19 In these multicomponent molecules, a photoinduced electron transfer (PET) process resulted, and the fluorescence was quenched. On the other hand, the absorption band position can be tuned by attaching an electron-donating group to the bay position or the 2-position of the PBI chromophore.18,20−23 In these molecular systems, the electronic coupling matrix elements (VDA) between the PBI moiety and the electron-donating unit are either very small16,17 or very large.18,20−23 In the former case, the two units show no electronic interaction at the ground © 2018 American Chemical Society

state, and in the latter case the two units literally merged to one π-conjugation system. On the other hand, PBI−electron donor dyads with moderate electronic coupling between the electron donor and the acceptor were not investigated. Electronic coupling (quantified by the electronic coupling matrix elements, VDA) exerts a significant effect on the photophysical properties of the chromophores,2,11,24−27 such as the optical absorption/emission (e.g., observation of the CT bands),28 charge transfer (CT), and intersystem crossing (ISC).29 Previously the electronic coupling between the arylamine electron donor and the anthracene unit was tuned by variation of the π-conjugation extent, via conformation restriction.30 It was found the CT bands of absorption and emission are highly dependent on the electronic coupling Received: December 1, 2017 Revised: January 11, 2018 Published: January 29, 2018 3756

DOI: 10.1021/acs.jpcc.7b11872 J. Phys. Chem. C 2018, 122, 3756−3772

Article

The Journal of Physical Chemistry C Scheme 1. Synthesis of PBI Derivativesa

Key: (a) 3-Aminopentane, imidazole, refluxed at 160 °C for 8 h, under N2, yield 90%. (b) Br2, CHCl3, refluxed at 60 °C for 9 h and stirred at room temperature for 36 h under N2, yield 61%. (c) K2CO3, Pd(PPh3)4, DMF/H2O, stirred at 80 °C for 3 h, yield 52%. (d) Phenothiazine, tri-tertbutylphosphine, sodium tert-butoxide, Pd(OAc)2, dry toluene, stirred at 120 °C for 4 h, under N2, yield 47%. (e) Diphenylamine, similar to step d, yield 24%. a

DPA is the electron donor, and PBI is the electron acceptor for these three dyads. These three dyads were not reported previously. Moreover, the molecular structural profile of this kind of PBI-based compact electron donor/acceptor dyad, i.e., with direct connection of the electron donor at the bay position of PBI, was not reported previously. The rationale for the molecular design is to control the rotation restriction about the C−N/C−C linker between the electron donor and acceptor, and as such the electronic coupling between the two moieties can be varied. As a result the photophysical properties of the dyads can be systematically tuned.30 The difference between our dyads and the previous reported dyads is that the PTZ and PBI moieties are directly connected through a single C−C or C−N bond, thus the electronic coupling may be stronger than the previously reported dyads, as well as the electron exchange energy (J, J ∝ VDA2).16,32−34 With our molecular structure design method, we successfully tuned the absorption and emission properties of the dyads, by variation of the rotation restriction of the electron donor/acceptors. We found that the magnitude of the CT absorption/emission bands is dependent on the electronic coupling (VDA) between the units. Molecules with decoupled units do not show the CT bands in both the

strength between the electron donor and anthracene moiety. However, to the best of our knowledge, a similar investigation has not been reported for dyads based on the PBI chromophore.31 PBI−PTZ dyads with linker at the amide position of PBI were reported,16,32,33 and two mechanisms of the electron transfer, coherent superexchange, and incoherent electron hopping were studied by controlling the distance between the phenothiazine and PBI moieties. It was found that with increasing of the bridge length between the PTZ and PBI the mechanism of the electron transfer can switch from coherent superexchange to incoherent hopping.16,32,33 Moreover, in these dyads, the separation of the PBI and the PTZ moieties is large, and thus the coupling of the two units is small. ISC was observed, and the mechanism is the radical pair ISC (RPISC).16,32,33 However, the ISC in compact PBI−PTZ dyads was not studied. Herein we prepared two compact PBI−PTZ dyads, with C− N bond and C−C bond as the linker between the two moieties, in which the orientation of the PTZ moiety is different (Scheme 1). The PBI−DPA dyad as a reference was also prepared (DPA stands for diphenylamino moiety). The PTZ or 3757


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The Journal of Physical Chemistry C

20.2, 13.9, 11.3. TOF MALDI−HRMS: Calcd ([C50H45N3O4S]−), m/z = 783.3131; found, m/z = 783.3134. 2.4. Compound PBI-3. Under N2 atmosphere, compound 1 (100 mg, 0.16 mmol), diphenylamine (33.2 mg, 0.19 mmol), Pd(OAc)2 (7.4 mg, 0.03 mmol), and sodium tert-butoxide (126.1 mg, 1.30 mmol) were mixed in dry toluene (5 mL). Then tri-tert-butyl phosphine (13.3 mg, 0.06 mmol) was added. The reaction mixture was stirred at 120 °C under N2 for 4 h. After completion of the reaction, the reaction mixture was cooled to RT, and water (3 mL) and CH2Cl2 (30 mL) were added. The organic layer was washed with brine solution (3 × 50 mL) and dried over anhydrous Na2SO4. Then the solvent was removed under reduced pressure, and the residue was purified with column chromatography (silica gel, CH2Cl2) to give a dark green solid (27.7 mg, yield: 24%). Mp: 189−191 °C. 1H NMR (CDCl3, 500 MHz): δ 9.19 (d, 1H, J = 10.0 Hz), 8.67−8.57 (m, 4H), 8.51 (s, 1H), 8.42 (d, 1H, J = 10.0 Hz), 7.24 (t, 4H, J = 10.0 Hz), 7.13 (d, 4H, J = 10.0 Hz), 7.04 (t, 2H, J = 10.0 Hz), 5.07−4.97 (m, 2H), 2.29−2.15 (m, 4H), 1.96− 1.85 (m, 4H), 0.94−0.88 (m, 12H). 13C NMR (CDCl3, 126 MHz): δ 145.4, 145.3, 134.5, 134.4, 132.7, 129.8, 129.3, 129.2, 129.0, 127.3, 126.6, 125.1, 124.2, 123.5, 122.5, 122.3, 57.8, 57.5, 25.1, 25.0, 11.4. TOF MALDI−HRMS: Calcd ([C46H39N3O4]−), m/z = 697.2941; found, m/z = 697.2916. 2.5. Nanosecond Transient Absorption Spectroscopy. The nanosecond transient absorption spectra were studied on an LP980 laser flash photolysis spectrometer (Edinburgh Instruments, UK), and the signal was digitized with a Tektronix TDS 3012B oscilloscope. The samples were purged with N2 15 min before measurement and excited with a nanosecond pulsed laser (Opolette 355II+UV nanosecond pulsed laser, the wavelength is tunable in the range of 200−2200 nm, OPOTEK, USA). 2.6. Femtosecond Transient Absorption Spectroscopy. The femtosecond transient absorption spectra were measured by a homemade pump−probe setup in combination with a mode-locked Ti-sapphire amplified laser system (Spitfire Ace, Spectra-Physics). Briefly, the system was amplified with wavelength 800 nm of duration 35 fs, repeltion rate 1 kHz, and average power 4 W. The output beam was then split into two parts, and 90% of this output beam was converted into UV− VIS−IR in the range of 240−2400 nm radiation using an optical parametric oscillator (Topas, Light Conversion) used as pump beam. The left 10% output beam was used to generate a white light continuum (WLC) in a 3 mm thickness rotated CaF2 plate and used as a probe beam. The weak beam was passed through a variable delay line (up to 6 ns), and its absorbance change in the presence and in the absence of the pump beam is measured. The magic angle between pump and probe beam was set at 54.7° in order to avoid rotational depolarization effects. The entire setup was controlled by a PC with the help of LabView software (National Instruments). All measurements were performed at room temperature under aerated conditions. Global target analyses were carrired out using Glotaran.35 2.7. DFT Calculations. The geometries of the compounds were optimized using density functional theory (DFT) with B3LYP functional and 6-31G(d) basis set. There are no imaginary frequencies for all optimized structures. The excitation energy and the energy gaps between the S0 state and the triplet excited states of the compounds were approximated based on the optimal ground-state geometry. All these calculations were performed with Gaussian 09W.36

absorption and emission spectra. The NIR emission of the CT excited state is highly dependent on the solvent polarity. CS, CR, and the formation of the triplet state upon photoexcitation were studied with various steady state and time-resolved absorption and emission spectroscopies. DFT computations on the molecular geometry, potential energy curves, molecular orbitals, and the electronic excited states were also carried out. We also find that the ISC of the dyads is different from the previously reported PBI−PTZ dyads.16,32,33

2. EXPERIMENTAL SECTION 2.1. General Method. All the chemicals used in synthesis are analytically pure and were used as received. Solvents were dried and distilled prior to use. Time-resolved emission spectra and fluorescence lifetimes were measured on an OB920 luminescence lifetime spectrometer (Edinburgh Instruments, UK). An Optistat DN cryostat (Oxford Instruments, UK) was used for low-temperature absorption/fluorescence spectra measurements. 2.2. Compound PBI-1. Under N2 atmosphere, compound 1 (100 mg, 0.16 mmol), phenothiazine (39.2 mg, 0.19 mmol), Pd(OAc)2 (7.4 mg, 0.03 mmol), and sodium tert-butoxide (126.1 mg, 1.30 mmol) were mixed in dry toluene (5 mL). Then tri-tert-butyl phosphine (13.3 mg, 0.06 mmol) was added. The reaction mixture was stirred at 120 °C under N2 for 4 h. After completion of the reaction, the reaction mixture was cooled to RT, and water (3 mL) and CH2Cl2 (30 mL) were added. The organic layer was washed with brine solution (3 × 50 mL) and dried over anhydrous Na2SO4. Then the solvent was removed under reduced pressure, and the residue was purified with column chromatography (silica gel, CH2Cl2) to give a dark red solid (55.5 mg, yield: 46%). Mp: > 250 °C. 1H NMR (CDCl3, 400 MHz): δ 10.00 (d, 1H, J = 8.0 Hz), 8.80− 8.75 (m, 2H), 8.70 (d, 3H, J = 4.0 Hz), 8.61 (d, 1H, J = 8.0 Hz), 7.13 (d, 2H, J = 8.0 Hz), 6.89 (t, 2H, J = 8.0 Hz), 6.80 (t, 2H, J = 8.0 Hz), 6.25 (d, 2H, J = 8.0 Hz), 5.11−4.98 (m, 2H), 2.32−2.18 (m, 4H), 1.99−1.86 (m, 4H), 0.97 (t, 6H, J = 8.0 Hz), 0.89 (t, 6H, J = 8.0 Hz). 13C NMR (CDCl3, 126 MHz): δ 140.9, 138.4, 135.4, 134.4, 133.8, 132.7, 129.3, 129.1, 129.0, 127.4, 127.3, 126.9, 123.9, 123.6, 123.2, 121.2, 115.8, 57.9, 57.7, 25.0, 24.9, 11.4, 11.3. TOF MALDI−HRMS: Calcd ([C46H37N3O4S]−), m/z = 727.2505; found, m/z = 727.2484. 2.3. Compound PBI-2. Under a N2 atmosphere, the mixture of compound 1 (100 mg, 0.16 mmol), 10-butyl-3(4,4,5-trimethyl-1,3,2-dioxaborolan-2-yl)-10H-phenothiazine (61 mg, 0.16 mmol), K2CO3 (68.0 mg, 0.49 mmol), DMF (5 mL), and water (0.5 mL) was mixed together. Then Pd(PPh3)4 (9.0 mg, 0.007 mmol) was added. The reaction mixture was stirred at 80 °C under N2 for 3 h. After the reaction was finished, the mixture was cooled to RT. Then CH2Cl2 (30 mL) was added, and the organic layer was washed with brine (3 × 50 mL) and dried over Na2SO4. Then the solvent was removed under reduced pressure, and the residue was purified with column chromatography (silica gel, CH2Cl2) to give a dark violet solid (64.7 mg, yield: 51%). Mp: 131−133 °C. 1H NMR (CDCl3, 500 MHz): δ 8.72−8.59 (m, 4H), 8.53 (s, 1H), 8.20− 8.13 (m, 2H), 7.32 (s, 1H), 7.23−7.13 (m, 3H), 6.98−6.90 (m, 3H), 5.09−5.00 (m, 2H), 3.92 (s, 2H), 2.31−2.17 (m, 4H), 1.97−1.83 (m, 6H), 1.53−1.49 (m, 2H), 1.02 (t, 3H, J = 5.0 Hz), 0.93−0.89 (m, 12H). 13C NMR (CDCl3, 126 MHz): δ 145.8, 144.6, 140.8, 136.4, 134.9, 134.8, 134.5, 132.2, 129.7, 129.3, 128.5, 128.2, 127.6, 127.5, 127.2, 127.0, 124.1, 123.5, 122.9, 122.6, 116.6, 115.7, 57.7, 57.6, 47.4, 29.7, 29.0, 25.0, 3758


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3. RESULTS AND DISCUSSION 3.1. Design and Synthesis of the Compounds. PTZ is a well-known electron donor.10,33,37−40 Previously PBI−PTZ dyads were studied, but the linker between the electron donor and acceptor is at the amide position of the PBI moiety, and the distance between the electron donor and acceptor is large.16,17 There is no ground state electronic coupling between the PTZ and PBI moiety, although photoinduced electron transfer (or CS) was observed. In order to attain stronger electronic coupling between the electron donor/acceptor and the resulting variation of the absorption, emission, and ISC properties, we prepared the N−C and the C−C single bond linked dyads PBI-1 and PBI-2 (Scheme 1), and we expect large VDA and J values for these new compact dyads. In these dyads, the length of the linker between the electron donor and acceptor is minimized, and the orientation of the PTZ chromophore toward the PBI unit is different for the two dyads. We assume the PBI-2 is with more coplanar geometry than PBI-1. Moreover, the potential energy surfaces of these two compounds with rotation of the linker, i.e., the C−N bond or the C−C bond, is expected to be different. We anticipate the two units in PBI-1 to take the orthogonal geometry with the minimum energy, whereas for PBI-2 the rotation about the linker will encounter less steric hindrance. The two units are more likely completely decoupled in PBI-1 due to the orthogonal geometry. For PBI-1, we expect the spin−orbit charge-transfer ISC (SOCT-ISC).34 We also designed PBI-3, with the diphenylamine group as the electron-donating moiety,41 and the alignment of two phenyl groups will be more flexible. As such, we anticipate a fully coupled system between the two units, i.e., the π-conjugation between the diphenylamine moiety and the PBI unit may be more significant than that in PBI-1 and PBI-2. The different coupling between the electron donor (PTZ) and electron acceptor (PBI) moiety in these dyads will affect the photophysical properties of the dyads. PBI-1 was prepared with Pd(0)-catalyzed Buchward− Hartwig coupling reaction, and the yield is 47%. Pd(0)catalyzed Suzuki coupling was used for preparation of PBI-2, and the yield is 52%. Buchward−Hartwig coupling reaction was used for preparation of PBI-3.41 All the products were verified with 1H NMR, 13C NMR, and HR MS spectra. 3.2. UV−Visible Absorption and Fluorescence Emission Spectra. The UV−Vis absorption of the compounds was studied (Figure 1). The main absorption bands of all the compounds are in the range of 400−550 nm, which are the structured absorption bands of the PBI moiety. Due to the geometry restriction (orthogonal geometry, dihedral angle is

80°; see later section), the PTZ and the PBI units in PBI-1 are completely electronically decoupled at the ground state. As a result the CT transition (S0 → 1CT*) is a forbidden transition, and no CT absorption band was observed. The absorption of PBI-1 in the visible range (S0 → 1LE*; LE stands for locally excited) is highly identical to that of PBI-5, the unsubstituted PBI chromophore.30 For PBI-2, however, the dihedral angle between the PTZ and the PBI moiety is 54° (see later section), thus the electronic coupling between PTZ and PBI moieties is stronger than that in PBI-1,30 and indeed the molecular orbital analysis shows that the HOMO and HOMO−1 are delocalized over the two chromophores; as such, the S0 → 1CT* is an optically allowed transition for PBI-2. As a result, a weak, but distinct, broad absorption band in the range of 520−700 nm was observed for PBI-2. The broad feature of this CT absorption band is probably due to the geometry distribution at the ground state, supported by the shallow potential energy curves (see later section). This trend, i.e., the effect of the magnitude of electronic coupling on the absorption profile, is further confirmed by PBI3, in which the diphenyl moiety does not restrict the geometry of the amino group against PBI unit. Thus, significant πconjugation between the amino moiety and the PBI results, and the CT absorption band becomes distinct, i.e., baseline separated from the LE absorption band of the PBI moiety (Figure 1a). The molar absorption coefficient of the CT absorption band of PBI-3 (ε = 1.25 × 105 M−1 cm−1 at 618 nm) is ca. 3-fold of PBI-2 at 579 nm (ε = 3.70 × 104 M−1 cm−1, Table 1).30 To quantify the probability of the S0 → 1CT* Table 1. Absorption Properties and Electronic Coupling Matrix Element of Compounds in n-Hexane λabsa (εb) PBI-1 PBI-2 PBI-3 PBI-4 PBI-5

506 506 491 520 513

(6.44) (3.22)g/579 (0.37)h (3.10)g/618 (1.25)h (3.84) (8.66)

Mabsc

VDAd

VDAe

VDA*f

− 2.2 3.6 −i −i

− 0.15 0.41 −i −i

0.01 0.04 0.19 −i −i

−i 0.34 0.54 −i −i

i

i

c = 1.0 × 10−5 M in nm. bMolar absorption coefficient. In 104 M−1 cm−1. cThe transition dipole moment of S0 → 1CT transition, in Debay (D). dThe electronic coupling matrix element between CT state and ground state calculated by 1CT absorption in eV. eThe electronic coupling matrix element between CT state and ground state calculated by fluorescence in eV. fThe electronic coupling matrix element between 1CT state and 1LE state, in eV. gAbsorption of S0 → 1 LE band. hAbsorption of S0 → 1CT. iNot applicable. a

transition in every compound, the transition dipole moments (Mabs) of S0 → 1CT* transition were calculated using eq 130 |Mabs|2 =

3 ln 10 hc ̃ 8π 3NA nνabs

̃ ν̃ ∫band ε(ν)d

(1)

where ε(ν̃) is the molar absorption coefficient in the wavenumber scale, in M−1 cm−1; ν̃abs is the maximum of the S0 → 1CT transition band, in cm−1; and NA [mol−1], h [erg s], c [cm s−1], and n are the Avogadro constant, the Plank constant, the speed of light, and the refractive index of the solvent, respectively. For PBI-2 and PBI-3, these maxima and area of S0 → 1CT band are evaluated by Gaussian fitting. The obtained transition dipole moments are listed in Table 1. It is clearly

Figure 1. UV−Vis absorption spectra of the compounds. (a) PBI-1, PBI-2, and PBI-3. (b) PBI-4 and PBI-5. c = 1.0 × 10−5 M in n-hexane, 20 °C. 3759


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The Journal of Physical Chemistry C shown that a large transition moment was found for PBI-3 (3.6 D), and a much smaller value was observed for PBI-2 (2.2 D). From the CT absorption band in these three PBI derivatives, the electronic coupling matrix element (VDA) between the CT state and ground state was also calculated by the Hush eq 242 ⎛ 2.06 × 10−2 ⎞ CT −CT −CT 1/2 VDA (cm−1) = ⎜ ⎟(εmax νmax Δν1/2 ) R ⎝ ⎠

(2)

where R is the separation between the center of the electron donor and acceptor, in Å; εCT max is the molar absorption coefficient at the maximum of the CT absorption band, in M−1 cm−1; ν−CT max is the absorption maximum of the CT absorption band in wavenumber scale, in cm−1; and Δν−CT 1/2 is the full width of the band at the half-maximum, in cm−1. Note that RPBI‑2 = 7.57 Å and RPBI‑3 = 5.01 Å are used to estimated VDA, which are determined by DFT optimization of the geometry. The obtained VDA values (Table 1) indicate that the electronic coupling of the donor and PBI is obviously larger in PBI-3 (0.41 eV) than in PBI-2 (0.15 eV), which is in agreement with the obtained UV−Vis absorption results and also the results of fluorescence spectra mentioned below. The wave function of the 1CT state of PBI derivatives can be expressed as eq 3:43 1

ΨCT ≈ c1Φ1CT + c 2 Φ1LE

with c12 + c 2 2 = 1

Figure 2. Fluorescence emission spectra of PBI-1, PBI-2, PBI-3, PBI4, and PBI-5 in n-hexane (λex = 442 nm, A = 0.065). Optically matched solutions were used. c = ca. 1.0 × 10−5 M (slight variation of the concentration is necessary to obtain optically matched solutions) at 20 °C.

efficient even in such a nonpolar solvent, which is unusual for PET systems.38,46,47 No phosphorescence was observed for the compounds even at 77 K. For PBI-1, there is no CT transition band in the UV−Vis absorption spectrum. Interestingly, a weak emission band at 660 nm was observed in n-hexane (Figure 3a); i.e., the 1CT → S0 is no longer a forbidden transition. A more significant CT transition band was observed for PBI-2 in the UV−Vis absorption spectrum, and thus the fluorescence of PBI-2 was compared with that of PBI-1 and PBI-3 (Figure 3). For PBI-2, the locally excited (LE) state emission band at 524 nm was observed, as well as a broad structureless emission band at 780 nm. This band is attributed to the charge transfer state emission, which is in agreement with the larger electronic coupling between the PTZ and the PBI moieties.18 Interestingly, PBI-3 gives an emission band at 662 nm. This band is significantly red-shifted as compared to the emission of the PBI moiety at 512 nm, and the emission band is structureless. Thus, this band is attributed to the singlet excited state with significant CT feature. Actually this S1 state is different from the pristine PBI S1 state, due to the different πconjugation system. This observation is in agreement with the CT band in the UV−Vis absorption of PBI-3. For PBI-3, the CT emission centered at 662 nm is much more significant than the LE emission band at 512 nm, indicating the electronic coupling between the DPA and the PBI moiety is more significant than PBI-2. It should be pointed out that for all three dyads the LE emission bands centered at 520 nm were observed. The transition dipole moments of the CT fluorescence in n-hexane were calculated as 0.10, 0.72, and 2.03 D for PBI-1, PBI-2, and PBI-3, respectively (Table 2), using the following eq 630

(3)

The relatively small contribution of ground state configuration to 1ΨCT was neglected in the above expression.43 The values of c1 and c2 = (1 − c12)1/2 can be estimated from the change of the S0 → 1LE absorption band intensity between the PBI derivative with unsubstituted PBI using eq 443 c12 ≈ ε(ν)̃ LE /ε(ν)̃ ref

(4)

where ε(ṽ)LE and ε(ṽ)ref denote the maximum molar absorption coefficients of the S0 → 1LE transition of the PBI derivative and unsubstituted PBI, in M−1 cm−1. Here ε(ṽ)ref is 8.66 × 104 M−1 cm−1 in n-hexane (Table 1). Consequently, on the basis of the absorption band analysis, the electronic coupling matrix element between the 1CT and 1 LE states (VDA*) can be estimated by eq 543 ⎛ E A − hcv CT ̃ ⎞ abs ⎟ VDA*(eV) ≈ (c1/c 2)⎜ LE −12 ⎝ 1.6 × 10 ⎠

EALE

(5)

1

where is the LE state energy, and the value is taken as 2.41 eV ± 0.05 eV for PBI derivatives; h [erg s] and c [cm s−1] are the Plank constant and the speed of light. ṽCT abs is the maximum of the S0 → 1CT transition band, in cm−1. The obtained VDA* values (Table 1) are significantly larger than VDA for PBI-2 and PBI-3, which indicates that the electronic coupling between the 1LE and 1CT states is remarkably stronger than that between the ground and 1CT states. It is well-known that the rate of nonadiabatic electron transfer is proportional to the square of the electronic coupling matrix element (VDA or VDA*).44,45 Therefore, the rate of the CS process (1LE → 1CT) is faster than the CR process (1CT→ S0) which is discussed in the later section. The fluorescence emission spectra of the compounds were studied (Figure 2). The unsubstituted PBI gives the most intense fluorescence at 519 nm (ΦF = 100%, Table 2). Under the same conditions (optical solutions were used); however, the fluorescence of PBI-1 was quenched. Note that the solvent is hexane, and this result indicates that the electron transfer is

|M fl|2 =

3hk r 64π 4(nνfl̃ )3

(6)

−1

where ν̃fl [cm ] is the fluorescence maximum in the wavenumber scale and kr [s−1] is the radiative rate constant. The obtained Mfl listed in Table 2 is admirably identical with the results of fluorescence spectra. The visual effect of the emission of the compounds is shown in Figure 3e and 3f. PBI-3 gives strong red emission, whereas PBI-1 and PBI-2 are nonfluorescent under the same conditions. It is also clear that the fluorescence emission is highly dependent on the solvent polarity. Moreover, these results clearly demonstrated that the UV−Vis absorption and 3760


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The Journal of Physical Chemistry C Table 2. Fluorescence Properties of 1LE → S0 Bands and 1CT → S0 Bands of Compounds in n-Hexane λFa g

ΦFb h

518 /659 524g/765h 515g/662h 556g 519g

PBI-1 PBI-2 PBI-3 PBI-4 PBI-5

g

τFc h

0.4 /0.03 1.2g/0.1h 1.1g/15.9h 68.7g 100g

g

krd h

3.5 /9.0 4.4g/1.1h 4.0g/13.7 5.3g 4.4g

g

knre h

1.1 /0.03 2.7g/0.94h 2.7g/11.6h 178.9g 229.4g

g

Mflf h

28.2 /11.1 22.4g/94.2h 24.5g/6.1h 8.2g 0g

0.10 0.72 2.03 −i −i

In nm. bFluorescence quantum yields, in percent. Bodipy (BDP, ΦF = 72% in THF) was used as a standard for PBI-4 and PBI-5 (λex = 470 nm); Ru(bpy)3[PF6]2 (ΦP = 6.2% in deaerated MeCN) was used as standard for PBI-1, PBI-2, and PBI-3 (λex = 440 nm). cFluorescence lifetimes (λex = 510 nm), in ns, c = 1.0 × 10−5 M. dRadiative rate constant kr = Φ/τ, in 106 s−1. eNonradiative rate constant knr = (1−Φ)/τ, in 107 s−1. fThe transition dipole moment of 1CT→ S0 transition in n-hexane, in Debye. gThe lifetime of the localized emission (1LE → S0). hThe lifetime of the CT emission (1CT→ S0). iNot applicable. a

Figure 3. Fluorescence emission spectra of the compounds in different solvents. (a) PBI-1 (λex = 480 nm, A = 0.176); (b) PBI-2 (λex = 455 nm, A = 0.104); (c) PBI-3 (λex = 445 nm, A = 0.143); and (d) PBI-1, PBI-2, and PBI-3 in n-hexane (λex = 440 nm, A = 0.111). Optically matched solutions were used. Photographs of the emission of (e) PBI-1, PBI-2, PBI-3, and PBI-5 in n-hexane and (f) PBI-3 in different solvents, excited by a 254 nm UV lamp at 20 °C.

fluorescence emission properties of the electron donor/ acceptor dyads can be precisely controlled by variation of the electronic coupling between the electron donor and acceptor. The electronic coupling can be controlled by rotation restriction or geometry constraint. From the CT fluorescence band in these three PBI derivatives, the electronic coupling matrix element (VDA) between the ground state and CT state can be estimated by the eq 744,45 VDA

⎡ 1.4 × 105k ⎤1/2 r ⎥ (cm ) = ⎢ 3 2 ⎣ n R c νmax ⎦

ground state and CT state,44 but the values of the same compound obtained by different equations are slightly different. It may indicate that VDA calculated by CT absorption mainly shows the coupling between the ground state and the 1CT* (Franck−Condon state), while that calculated by CT fluorescence manifests the coupling between 1CT* (after relaxation) and the ground state. The time-resolved emission spectra of PBI derivatives were also studied (Figure 4). For PBI-1, the CT emission decays slower than the LE emission. For PBI-2, the CT emission decays much faster than the LE emission. For PBI-3 in nhexane, the CT emission intensity is much higher than the LE emission at 515 nm, and the CT emission decay is obviously slower than LE emission which is confirmed by lifetime measurements (Figure 4f). The fluorescence lifetimes of the compounds were studied (Figure 4d, e, f). For PBI-1, the decay of the fluorescence at 520 nm (3.5 ns LE emission) and 659 nm (9.0 ns CT emission) is drastically different (Figure 4d). For PBI-2, the fluorescence lifetime of the CT band (1.1 ns) is shorter than the LE emission (4.4 ns). However, for PBI-3 in nhexane, the fluorescence lifetime of the CT emission is three times longer (13.7 ns) than the LE emission (4.0 ns). However,

−1

(7)

where kr is the radiative rate constant, in s−1; n is the solvent refractive index; Rc is the separation between the center of electron donor and acceptor, in Å; and vmax indicates the position of the CT fluorescence maximum. The obtained VDA values (Table 1) are in the similar trend with the values estimated by eq 2 using CT absorption; i.e., the order of the electronic coupling extent for three PBI derivatives is PBI-3 > PBI-2 > PBI-1. The VDA values calculated by these two equations indicate the electronic coupling between the 3761


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Figure 4. Time-resolved fluorescence emission spectra of (a) PBI-1; (b) PBI-2; and (c) PBI-3, λex = 445 nm. (d), (e), and (f) are the decay curves of the selected wavelengths for (a), (b), and (c), respectively, λex = 510 nm. c = 1.0 × 10−5 M in n-hexane at 20 °C.

Figure 5. Normalized fluorescence excitation spectra and UV−Vis absorption spectra. (a) PBI-1 (λem = 680 nm, c = 1.0 × 10−5 M); (b) PBI-2 (λem = 765 nm, c = 2.0 × 10−6 M); and (c) PBI-3 (λem = 700 nm, c = 1.0 × 10−6 M). In n-hexane at 20 °C.

in toluene, the fluorescence lifetimes of the CT emission band (3.9 ns) and the LE emission band (4.0 ns) are almost the same (Supporting Information, Table S1). The 1CT → S0 transition is a weakly allowed transition, and thus the fluorescence lifetime can become longer than the LE emission. However, the longer emission wavelength (the small 1CT/S0 energy gap) makes the nonradiative transition more significant, which may shorten fluorescence lifetime for the 1CT state. We also measured the fluorescence lifetimes of the compounds in deaerated solutions, but no significant change was observed as compared to that in aerated solution (Supporting Information, Table S1), indicating that the fluorescence emission was not perturbed by any long-lived triplet excited states. The fluorescence excitation spectra for three derivatives were compared with the UV−Vis absorption spectra (Figure 5). For PBI-2 and PBI-3, the normalized excitation spectra are basically in agreement with the absorption spectra. The slightly weaker bands at the LE range indicate the 1CT state can be produced from 1PBI* (local excited state) almost quantitatively. In other words, the quantum yield of 1LE* → 1CT* is not unity, which is reasonable since 1LE* → S0 fluorescence emission (LE emission) was observed, and thus direct 1LE* → S0 decay competes with the charge separation process. Interesting results were observed for PBI-1 (Figure 5a). A distinct band in the 550−650 nm range was observed in the

excitation spectrum, but such a band is missing in the absorption spectrum. This result indicates that the excitation in the range 550−650 nm is more efficient to produce the CT excited state than that with excitation at the LE transition band.48 We assume that an energy barrier exists for the 1LE* (FC) → 1CT* (emissive) relaxation (the yield of this process is probably low), but such a barrier does not exist for the 1CT* (FC) → 1CT* (emissive state) relaxation. The temperature-dependent UV−Vis absorption spectra of compounds were studied (Figure 6a−c). For PBI-1, as the temperature decreases, the absorption in the range 400−550 nm (localized on the PBI moiety) increases. For PBI-2, the absorption in the range 400−550 nm increases with temperature decreasing from 300 to 210 K but decreases with the temperature decreasing from 210 to 90 K. The absorption of the shoulder peak centered at 630 nm becomes more significant with temperature decreasing. For PBI-3, as the temperature decreases, the absorption in the range 400−520 nm (localized on the PBI moiety) increases, and the absorption of the CT band (550−750 nm) increases and red shifts. These features can be rationalized by the potential energy curves of the compounds, and the geometries of PBI-2 and PBI-3 become more coplanar, which makes the S0 → 1CT*(FC) transition more likely. It is unlikely the hydrogen bonding plays a role in the changes.49 3762


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Figure 6. Temperature-dependent UV−Vis absorption spectra of (a) PBI-1, (b) PBI-2, and (c) PBI-3. In MeOH:EtOH (1:4, v/v) mixture solvent, c = 2.0 × 10−5 M. Temperature-dependent fluorescence emission spectra of (d) PBI-1 (c = 2.0 × 10−5 M), (e) PBI-2 (c = 2.0 × 10−5 M), and (f) PBI-3 (c = 1.0 × 10−5 M) in n-hexane at different temperatures. λex = 445 nm.

Figure 7. Cyclic voltammograms of the compounds. (a) PBI-1, (b) PBI-2, and (c) PBI-3. Ferrocene (Fc) was used as internal reference (E1/2 = +0.64 V (Fc+/Fc)). Conditions: in deaerated CH2Cl2 containing 0.10 M Bu4NPF6 as supporting electrode, Ag/AgNO3 as reference electrode. Scan rates: 50 mV/s. c = 1.0 × 10−3 M at 20 °C.

The temperature-dependent fluorescence emission spectra of compounds were also studied (Figure 6d−f). For PBI-1 and PBI-2, both the LE and CT emission are red-shifted, and the intensity decreased with decreasing temperature. For PBI-3, the CT emission band intensity decreased and red-shifts with decreasing temperature, while the LE emission peak shows no obvious change. It should be pointed out for a fluorophore that normally a blue-shifted and intensified fluorescence should be observed at lower temperature due to the weaker solvation effect and the inhibited nonradiative decay of the excited state at lower temperature. However, the change of the CT emission of the PBI derivatives is opposite to the above general trend. The reason may be the emission band of 600−700 nm is from the twisted intramolecular charge transfer (TICT) state,50,51 especially at low temperature (see the potential energy curve studies, where less thermal energy is available at low temperature), in which the two units linked by a C−C (or C−N) single bond in the molecule can rotate during the geometry relaxation. For the TICT state, the energy level decreases with the dihedral angle of the two units approaching 90°. At the orthogonal geometry, the two units are decoupled

to a large extent, and the CT emission will decrease; however, the emission will be red-shifted due to the stabilized TICT state. The red-shifted emission of the CT band is in agreement with this postulation. An energy barrier for the 1LE* (Franck− Condon state) → 1CT* (emissive) may be also responsible for the temperature dependence of the CT emission. 3.3. Redox Properties: Cyclic Voltammogram of the Compounds. In order to study the Gibbs free energy changes of electron transfer as well as the energy level of the chargeseparated state (CSS) in PBI derivatives, the electrochemical properties of PBI-1, PBI-2, and PBI-3 were studied with cyclic voltammograms (CV, Figure 7).46,52,53 The reference compounds PTZ, N-butylphenothiazine, and triphenylamine were also studied, and all three compounds show only one reversible oxidation wave and no reduction wave (Supporting Information, Figure S18 and Table 3), which indicates that both the PTZ and DPA moieties in the PBI derivatives serve as an electron donor. For reference compound PBI-5, two reversible reduction waves at −1.11 V and −1.32 V were observed (vs Ag/AgNO3 electrode). There is no oxidation wave within the potential range used in the electrochemical 3763


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of the geometry; RCC (PBI-1) = 5.30 Å; RCC (PBI-2) = 7.69 Å; RCC (PBI-3) = 4.83 Å; RD is the radius of the electron donor; RA is the radius of the electron acceptor; εREF is the static dielectric constant of the solvent used for the electrochemical studies; and ε0 is permittivity of free space. The solvents used in the calculation of free energy of the electron transfer is nhexane (εS = 1.88), CH2Cl2 (εS = 8.93), and acetonitrile (εS = 37.5). The Gibbs free energy changes of the CS and CR processes were studied based on the electrochemical data and the spectral measurements. The results show that the CS is thermodynamically allowed even in a nonpolar solvent such as hexane. This result is in agreement with the experimental observations. 3.4. Nanosecond Transient Absorption Spectroscopy: The Triplet Excited State Property of the Compounds. In order to investigate the triplet excited state formation of the compounds in PBI derivatives, the nanosecond transient absorption spectra of the compounds were studied (Figure 8). For PBI-3 in n-hexane, two ground state bleaching (GSB) bands in the range of 400−500 nm and 580−700 nm were observed, upon 532 nm pulsed laser photoexcitation. The first bleaching band is weak because it overlaps with the excited state absorption (ESA) of the triplet state of PBI. The ESA band centered at 515 nm is the T1 → Tn absorption of the PBI triplet excited state.52,54 The decay trace monitored at 516 nm gives the 3PBI* lifetime of 13.7 μs. In aerated solution, the lifetime was greatly reduced (138 ns, Supporting Information, Figure S19). The ns TA spectra in acetonitrile were also recorded, but no signal was observed. The transient absorptions of PBI-1 and PBI-2 were also measured; however, no triplet excited state was observed. There are two possible reasons for failure to observe triplet states for PBI-1 and PBI-2: i.e., (i) either the triplet state quantum yields (i.e., ISC efficiency) are extremely low or (ii) the triplet state lifetimes are too short (the instrument response function, IRF, of our ns TA spectrometer is ca. 10 ns). In order to elucidate the exact reason for the lack of triplet state formation in PBI-1 and PBI-2 upon photoexcitation, the intermolecular triplet photosensitizing method was used, with triplet photosensitizer as a triplet energy donor, and then by intermolecular triplet−triplet energy transfer (TTET), the T1 states of PBI-1 and PBI-2 should be observed if (i) is true. Conversely, no triplet state for PBI-1 and PBI-2 should be observed with our ns TA spectrometer if (ii) is true. 2,6-Diiodobodipy was used as the triplet energy donor and the PBI derivative as the acceptor to study the TTET (Figure

Table 3. Electrochemical Redox Potentials of the Compoundsa E1/2(Ox) (V) PBI-1 PBI-2 PBI-3 PBI-5 phenothiazine N-butylphenothiazine triphenylamine

+0.45 +0.32 +0.77 −b +0.18 +0.28 +0.55

E1/2(Red) (V) −1.06, −1.12, −1.13, −1.11, −b −b −b

−1.31 −1.33 −1.29 −1.32

a

Cyclic voltammetry in N2-saturated CH2Cl2 containing a 0.10 M Bu4NPF6. Counter electrode is Pt electrode, and working electrode is glassy carbon electrode, with Ag/AgNO3 couple as the reference electrode. bNot observed.

measurement (Supporting Information, Figure S18), which indicates that the PBI moiety is an electron acceptor rather than an electron donor. For PBI-1, there are two reversible reductive waves observed at −1.06 V and −1.32 V which are identical with the PBIs and a reversible oxidation wave at +0.45 V which can be attributed to the phenothiazine moiety. Similar results are also observed for PBI-2 and PBI-3, and the redox potentials of the compounds are collected in Table 3. The intramolecular electron transfer process of compounds was studied by calculating the free energy changes using the Weller eq (eqs 8−10).46,47 ΔGcs0 = e[EOX − E RED] − E00 + ΔGS

ΔGS = −

(8)

e2 e2 ⎛ 1 1 ⎞⎛ 1 1⎞ − + − ⎟ ⎜ ⎟⎜ εS ⎠ 4πεsε0R CC 8πε0 ⎝ RD RA ⎠⎝ εREF (9)

ΔGCR = −(ΔGCS + E00)

(10)

where ΔGS is the static Coulombic energy, which is described by eq 9. e = electronic charge; EOX = half-wave potential for one-electron oxidation of the electron-donor unit; ERED = halfwave potential for one-electron reduction of the electronacceptor unit; E00 = energy level approximated with the crossing point of UV−Vis absorption and fluorescence emission after normalization at the singlet excited state; εS = static dielectric constant of the solvent; RCC = center-to-center separation distance between the electron donor (PTZ or DPA) and electron acceptor (PBI), determined by DFT optimization

Figure 8. Nanosecond transient absorption spectra of PBI-3. (a) Transient absorption spectra with different delay times. (b) Decay trace at 516 nm. λex = 532 nm. c = 3.0 × 10−5 M in deaerated n-hexane at 20 °C. 3764


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Figure 9. Intermolecular triplet energy transfer with 2,6-diiodobodipy as the triplet energy donor and PBI-1 as the triplet energy acceptor, studied with ns TA spectra. (a) The spectra of the mixture of 2,6-diiodobodipy with PBI-1 and (b) selected transient absorption spectra at a few specific delay times of the mixtures. (c) The decay traces of the mixture. c [2,6-diiodobodipy] = 5.0 × 10−6 M, c [acceptors] = 2.0 × 10−5 M. (d) Stern− Volmer plot for lifetime quenching of the 2,6-diiodobodipy triplet state with increasing concentration of PBI derivatives. c [photosensitizers] = 1.0 × 10−5 M in deaerated n-hexane. λex = 532 nm, 20 °C.

9).55 The triplet excited state of the 2,6-diiodobodipy was obviously quenched in the presence of PBI derivatives (Figure 9), and the featured ESA of the triplet state was PBI developed, which means the effective intermolecular triplet energy transfer occurs. For PBI-1 and PBI-2, a new ESA band at 490 nm appears, which is the ESA of the triplet excited state of PBI.56 Furthermore, the decay trace at 490 nm is biphasic (Figure 9c), which includes the formation/accumulation of the 3PBI* state and then the decay of this triplet state. For PBI-3, the absorption of the T1 state of PBI red-shifts to 510 nm because the bleaching band at around 490 nm in nhexane overlaps with the ESA absorption band, and the transient profile is very close to that accessed with PBI-3 alone (Figure 8), validating our TTET approach to access the triplet states of the dyads. The TTET processes of PBI-4 and PBI-5 were also measured, in which the triplet excited state of the 2,6diiodobodipy was obviously quenched and a strong ESA band of the 3PBI* range from 400 to 500 nm was observed (Supporting Information, Figure S21). The kinetics of the intermolecular TTET, the accumulation of the T1 state of PBI, and the decay of the T1 states were monitored at specific wavelengths (Figure 9c). The T1 state lifetime of diiodobodipy was greatly reduced in the presence of PBI derivatives, indicating TTET. Based on eq 11,51 the intermolecular triplet energy transfer efficiency was calculated as 95%, 92%, and 93% for PBI-1, PBI-2, and PBI-3, respectively. ΦET = 1 − τ2/τ1

On the other hand, the formation of the triplet state by TTET is clearly indicated by the growth of the transient at 490 nm for PBI-1 and PBI-2 and 515 nm for PBI-3, for which the kinetics is very close to the decay of the T1 state of diiodobodipy. The decay time constants of the transient at 490 nm (for PBI-1 and PBI-2) and 515 nm (for PBI-3) are 29.6, 33.7, and 52.6 μs, respectively. These results confirm that the lack of the T1 state of PBI-1 and PBI-2 in n-hexane is due to the lack of ISC; i.e., the CR process does not lead to formation of the T1 state. It should be pointed out that the production of the triplet state in PBI derivatives cannot be produced by intermolecular charge recombination because the intermolecular electron transfer is highly endoenergetic in n-hexane. On the basis of the electrochemical and spectroscopic data, the Gibbs free energy changes (ΔGCS) of the process for PBI-1, PBI-2, and PBI-3 are +1.23 eV, +1.17 eV, and +1.31 eV, respectively, calculated by the Weller equation (with the lowest triplet state energy level of diiodobodipy as the E00 values). The quenching of the triplet excited state of diiodobodipy by PBI derivatives was studied by the Stern−Volmer equation (eq 12, Figure 9d): τ0

τ = 1 + KSV[Quencher]

(12)

Stern−Volmer quenching constants of PBI-1, PBI-2, PBI-3, PBI-4, and PBI-5 were calculated as KSV = 7.47 × 105 M−1, 4.31 × 105 M−1, 3.85 × 105 M−1, 2.88 × 105 M−1, and 4.42 × 105 M−1, respectively. The quenching efficiency f Q was calculated to be 64%, 44%, 51%, 28%, and 50%, respectively (see Supporting Information). The KSV value of the compounds

(11) 3765


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Table 4. Driving Forces of Charge Recombination (ΔGCR) and Charge Separation (ΔGCS) and the Energy of the ChargeSeparated State (CSS) for PBI Derivatives in Different Solventsd ΔGCS (eV) PBI-1a PBI-2b PBI-3c

ΔGCR (eV)

ECSS (eV)

hexane

DCM

MeCN

hexane

DCM

MeCN

hexane

DCM

MeCN

−0.60 −0.19 −0.39

−1.17 −0.21 −0.72

−1.35 −1.37 −1.01

−1.87 −2.24 −2.14

−1.25 −1.26 −1.75

−1.08 −1.03 −1.46

1.87 2.24 2.14

1.25 1.26 1.75

1.08 1.03 1.46

E00 = 2.43 eV. bE00 = 2.40 eV. cE00 = 2.47 eV. dE00 is the energy level approximated with the crossing point of UV−Vis absorption and fluorescence emission after normalization at the singlet excited state.

a

Figure 10. (a) Femtosecond transient absorption spectra of PBI-1 in n-hexane. (b) Corresponding species-associated difference spectra (SADS) obtained from target analysis. (c) Kinetic traces at selected wavelengths. (d) fs TA spectra of PBI-1 in acetonitrile. (e) Corresponding SADS obtained from target analysis. (f) Kinetic traces at selected wavelengths with their fitting lines. Conditions: λex = 475 nm, 20 °C.

eV (PBI-1) and +1.03 eV (PBI-2) in MeCN (Table 4), which lie below the 3PBI* state energy (+1.2 eV). However, for PBI3, the CT state energy is +1.46 eV, higher than 3PBI* state energy, and the CS from the triplet state is not possible. Moreover, the Gibbs free energy changes were calculated by the Weller equation (with the lowest triplet state energy level of PBI as the E00 values). The ΔGCS’s of PBI-1, PBI-2, and PBI-3 are −0.12 eV, −0.17 eV, and +0.26 eV, respectively, which indicates that the CS process driven by 3PBI* is exoergic for PBI-1 and PBI-2 but endergonic for PBI-3 in MeCN. No CT state was observed with ns TA after CS from 3PBI* for PBI-1 and PBI-2, which indicates the lifetime of the CT state must be very short. These results are useful for the explanation of the femtosecond transient absorption of PBI derivatives discussed below. 3.5. Femtosecond Transient Absorption Spectroscopy. In order to study the excited-state dynamics of PBI derivatives, mainly the CS and CR processes as well as the possible ISC process, the femtosecond transient absorption (fs TA) spectra of the compounds were measured in n-hexane and acetonitrile following the excitation at 475 nm (Figure 10 and Supporting Information, Figures S25 and 26). In n-hexane, PBI-1 exhibits the GSB band in the range 410− 530 nm, a positive band from 520 to 700 nm, and a fastdecayed stimulated emission (SE) band (530−580 nm). The positive band of the transient spectrum is a superposition of

is on the order PBI-1 > PBI-2 > PBI-3, which indicates the order of triplet excited state energy levels of PBI in three compounds is in the opposite order because the TTET is more efficient with larger energy gap between donor and acceptor. For PBI-2, the KSV value is similar to the PBI-5, which means the 3PBI* in PBI-2 is similar to the energy level of PBI without any substitution (1.20 eV). For PBI-3, the energy level of 3 PBI* is slightly higher than 1.20 eV, whereas the energy level in PBI-1 is lower than this value. The TTET processes of the 2,6-diiodobodipy and PBI derivatives in acetonitrile were also studied under similar conditions with that in n-hexane (Supporting Information, Figures S22−24). For PBI-1 and PBI-2, the lifetime of the triplet excited state of diiodobodipy is also greatly reduced, but no 3PBI* signal was observed. These results indicate that the TTET also occurs effeciently between the diiodobodipy and PBI derivatives, and the 3PBI* is produced by TTET; however, the lifetime of the triplet excited state of PBI in PBI-1 and PBI2 is too short in acetonitrile and below the IRF of our instrument. For PBI-3, a strong signal of 3PBI* is observed which indicates that the triplet excited state lifetime of PBI in PBI-3 is much longer than PBI-1 and PBI-2. The intramolecular CS driven by the triplet excited state of PBI may be the reason for the short lifetime of 3PBI* of PBI-1 and PBI-2; i.e., the CS process quenched the 3PBI* state produced by TTET. The energy levels of CT states are +1.08 3766


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The Journal of Physical Chemistry C excited state absorption (ESA) of 1PBI* (S1 → Sn transitions) and the absorption of PBI−•, so the SE band is important to distinguish the absorption of two species; i.e., the positive band can be attributed to the absorption of PBI−• after the SE band disappears.33 The SE band centered at 550 nm disappears after 0.6 ps, but the GSB band still exists, which indicates that the GSB band thereafter is mainly due to the existence of CSS rather than 1PBI*. Meanwhile, the peak centered at 630 nm can be attributed to PBI−•, which red-shifts to 680 nm in acetonitrile. The band centered at 530 nm, which red-shifts to 540 nm in acetonitrile, is due to the absorption of the PTZ radical cation (PTZ•+).17,57 The kinetic traces (Figure 10c) also give the important information. The time scale of the fast-decay part in the kinetic curve of 513 nm matched the fast-ascending part in the kinetic curve at 630 nm, indicating the CS process from the 1PBI* state and the formation of the PBI−•−PTZ+• state. On the basis of the transient spectra feature, we used the target analysis and assumed that two states, the S1 state of PBI and the CSS containing PBI−• and PTZ+•, are involved and that the population of these two states is in the sequential order. Then the SADS of each state and the rate constant of the CS and CR are obtained (Figure 10b). The species A shows that the SE band centered at 550 nm can be attributed to the singlet excited state of PBI, and the decay time of this state is 0.6 ps, followed by charge separation (CS). The species B corresponds to the CSS, which shows a broad positive absorption band from 520 to 730 nm with a peak at 630 nm. The lifetime of the CSS is 130 ps, which can be assigned to the time constant for charge recombination (CR). The CSS returns back to the ground state by CR, and no PBI triplet state was observed for PBI-1 (in n-hexane). All these transient states disappear within 1 ns which is well below the time resolution of our ns TA instruments (IRF: 10 ns); therefore, no signal was observed in the ns TA spectroscopy study for PBI-1. In acetonitrile (Figure 10e), the species A, which shows a strong GSB band in the range 420−530 nm, a SE band (540− 580 nm), and a positive band from 580 to 750 nm were attributed to the 1PBI* state with the lifetime of 0.3 ps. This result also indicates that the CS process takes 0.3 ps. The species B shows a weaker GSB band and a positive band in the range of 520−730 nm. The peaks at 540 and 680 nm of this positive band were attributed to the PTZ+• and PBI−•, respectively. Species B is assigned as the CSS with a lifetime of 0.8 ps, which indicates the fast time scale of the CR process. During this process, both the negative band and positive band mostly decay, which also indicates the fast CR process, leading to a small residual signal (species C). Species C shows a very weak positive peak at 520 nm as well as a negative band at 505 nm, and the species decays on 7.4 ps. This weak residual signal is probably due to the vibration relaxation within the CSS.53 The CR process in acetonitrile (0.8 ps) is obviously faster than that in hexane (130 ps). The ΔGCR values are −1.87 eV in nhexane and −1.08 eV in MeCN, respectively. Together with the slower CR (130 ps) than the CS process (0.6 ps) in n-hexane, we assume the CR in hexane is in the Marcus inverted region. Similarly, for PBI-2 in n-hexane, the SE band from 540 to 580 nm disappears within 0.3 ps (Supporting Information, Figure S25). However, the GSB band at 450−520 nm still exists, and the peak at 690 nm of the broad ESA band at 530− 720 nm can be attributed to the absorption of the PBI radical anion PBI−•,21,58 which is clear evidence for the formation of

CSS. In the SADS, the spectrum of species A contains the SE band from 550 to 600 nm, which is assigned to the 1PBI*. The species B corresponds to the PBI−•−PTZ+• state. The time constant of the CS is 0.3 ps ,and the CR is 240 ps in n-hexane, which is obviously longer than that of PBI-1 due to the different ΔGCR. However, in acetonitrile, the time constant of CR for PBI-2 dramatically reduces to 0.7 ps. No triplet excited signal of PBI-2 was observed after CR in two solvents. The femtosecond transient absorption of PBI-3 is also measured (Supporting Information, Figure S26). In n-hexane and acetonitrile, the negative band range from 550 to 650 nm is much stronger than that at 400−530 nm, which is quite different from the steady-state UV−Vis absorption of PBI-3 (Figure 1). In hexane, both the negative band at 620 nm and positive band at 700 nm show a fast decay (0.3 ps) at the early time and no change within 1 ns after that, while in acetonitrile, the transient species decay to the ground state within 100 ps after a fast decay (0.3 ps). For PBI-3, the CT emission band is centered at 662 nm in n-hexane, and no CT emission in MeCN in steady-state fluorescence spectra was observed (Figure 3); therefore, the fast decay (0.3 ps) of the negative band (at 620 nm in hexane and 600 nm in MeCN) cannot be assigned as the stimulated emission of the CT state. Moreover, the absorption of PBI −• and DPA +• appears at 700 and 670 nm, respectively,58,59 and the reason for the quite different negative band in fs TA and steady UV−Vis spectrum of PBI-3 is unclear. In n-hexane, no 3PBI* signature was observed in fs TA, indicating that the triplet formation probably occurs on a more than 1 ns time scale or that the 3PBI* signal may be overlapped with another transient signal which is hard to identify. The time constants of the CS and CR for PBI derivatives in n-hexane and acetonitrile are listed in Table 5. For the two Table 5. Lifetimes of the CS and CR Processes in the PBI Derivatives n-hexane PBI-1 PBI-2

acetonitrile

τCS [ps]

τCR [ps]

0.6 0.3

130 240

a

b

τCS [ps]a

τCR [ps]b

0.3 0.3

0.8 0.7

a

CS time (reciprocal of the rate constant). bCR time obtained from target analysis (reciprocal of the rate constant).

compounds, the similar CS kinetics may be ascribed to the short distance between the donor and acceptor in the molecules. However, the CR kinetics are different. The CR of PBI-2 (240 ps) is much slower than that of PBI-1 (130 ps) in n-hexane, and the free energy for CR in these two compounds is −1.87 eV and −2.24 eV for PBI-1 and PBI-2, respectively. It indicates that the CR occurs in the Marcus inverted region; i.e., the rate constant of CR becomes smaller with larger driving force. In n-hexane, the CR process is proved to be obviously slower than the CS process. It is conformed with the electronic coupling law (Table 1) that the coupling between the 1LE and 1 CT states (VDA*) is stronger than that between 1CT and ground states (VDA). 3.6. DFT Calculation. The geometries of the molecules at the ground state were optimized (Table 6). For PBI-1, the dihedral angle between the PBI and the PTZ units is ca. 80° (note the PTZ moiety is in puckered conformation). This orthogonal geometry will make the two units electronically decoupled, which is in agreement with the UV−Vis absorption, in which no significant CT bands were observed. Only a very 3767


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Table 6. Optimized Ground State Conformations and the Dihedral Angles of Selected Atoms of PBI Derivatives Calculated by DFT (B3LYP/6-31G (d))

weak CT band was observed in the emission spectrum of PBI-1 (Figure 3a). For PBI-2, however, the dihedral angle between the PTZ and the PBI moieties is significantly reduced to 54°, and this smaller dihedral angle may induce larger electronic coupling between the two units. Accordingly significant CT absorption and fluorescence emission bands were observed for PBI-2 (Figure 1 and 3b). For PBI-3, the dihedral angle between the C−N−C plane and the PBI unit decreased further to 49°, which renders more significant coupling between the electron-donating moiety and the PBI. Thus, the CT bands in the UV−Vis absorption and fluorescence emission are most significant among the three compounds. The PBI moiety in all three dyads takes a twisted geometry, not a planar one. This is a common feature for the baysubstituted PBI derivatives,20 and it may exert a significant effect on the photophysical property of the PBI chromophore, such as fluorescence quantum yield. Clear evidence is shown in Table 2 that the fluorescence quantum yield of PBI-4 (68.7%) is much lower than PBI-5 (100%). The geometry of the dyads presented in Table 6 is the energy-minimized geometry at the ground state. However, the potential energy curves (PECs) generated by rotation about the C−N bond or the C−C bond (the linker) will be more useful for rationalization of the photophysical properties of the dyads because other conformations are accessible with the thermal energy. Thus the related potential energy curves were constructed with flexible optimization (only the dihedral angles were fixed in the optimization, Figure 11). The results show that the PEC of PBI-1 is steep, with the geometry of 100° as the energy minimum, and rotation of the PTZ moiety about the C−N bond will induce significant steric hindrance; as such the molecule will spend most of its time with the orthogonal geometry. For PBI-2, the PEC is more shallow; thus, the dyad had large rotation freedom, and the geometry is more likely changed to approach the coplanar geometry. Note that there are two minima for PBI-2 at 51° and 126°, respectively, and both are closer to coplanar geometry than that of PBI-1. This result is in agreement with the UV− Vis absorption and the fluorescence spectra of the compounds.30

Figure 11. Ground state potential energy curves of the PBI-1, PBI-2, and PBI-3, as a function of the rotational dihedral angle of the selected atoms calculated by DFT (B3LYP/6-31G (d)).

For PBI-3, the rotation energy raises sharply when the conformation deviates from the energy minimal geometry, with dihedral angle ranging from 60° to 120°, because the phenyl plane of the diphenylamine approaches to the PBI plane that leads to repulsion between the two moieties. Therefore PBI-3 at the ground state prefers to adopt the geometry with the dihedral angle of either 49° or 139°, which can enhance the electronic coupling between the PBI and diphenylamine moiety. These results are in agreement with the UV−Vis absorption and the fluorescence emission studies. Our results show that by controlling the geometry and the rotation PEC of the dyads the photophysical properties of the PBI derivatives can be fine-tuned. The frontier molecular orbitals of the compounds were studied (Figure 12). The LUMO orbitals of PBI-1, PBI-2, and PBI-3 are all completely confined on the PBI moieties. However, the HOMO orbitals are mostly localized on the PTZ and DPA moieties; therefore, the transition of HOMO → LUMO is in CT feature. For PBI-1, the HOMO orbital is confined on the PTZ moiety and shows no overlap with its LUMO orbital; therefore, the transition of HOMO → LUMO is optically forbidden. However, the HOMO−1 orbital of PBI2 is delocalized and leads to the transition being allowed, which is in agreement with the results of UV−Vis absorption spectra. Since the HOMO orbital of PBI-3 is highly delocalized on both 3768


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Figure 12. Selected frontier molecular orbitals of PBI-1, PBI-2, and PBI-3 calculated by DFT (B3LYP/6-31G (d)). The energy levels of the orbits are presented (in eV).

Scheme 2. Simplified Jablonski Diagram Illustrating the Photophysical Processes Involved in (a) PBI-1, (b) PBI-2, and (c) PBI3 Upon Direct Photoexcitation (Right Side in Each Scheme) and Intermolecular Triplet Photosensitizing (Left Side in Each Scheme)a

a

The energy levels of the excited state are derived from the spectroscopic and electrochemical study data. D stands for the electron donor (PTZ or DPA unit), and A stands for the electron acceptor (PBI unit). The number of the superscript designates the spin multiplicity. The data are from the results in n-hexane.

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the PBI and DPA moieties, the S0 → 1CT* transition is strongly allowed because of the larger transition matrix elements (0.41 eV, Table 1) and overlap of the molecular orbitals. The photophysical processes of the PBI derivatives are shown in Scheme 2. For all these three compounds, the singlet CT states are produced by CS from the 1PBI* state. On the basis of TTET results in n-hexane, the 3PBI* state rather than 3 CT* state was observed; therefore, the energy of 3CT* must lie higher than 3PBI* in nonpolar solvents. However, the 3CT* cannot be produced by ISC from 1CT* because the 1CT* → 3 CT* ISC process is spin forbidden. Hyperfine coupling interaction (HFI) induced ISC is unlikely because the energy gap (2J) between these two states must be large in such compact dyads, which is caused by the strong electronic coupling between the donor and acceptor in the molecules.44,60 Moreover, the rate of the HFI-induced ISC occurs on a few nanoseconds,32 while the fast CR process (on ps time scale (see the fs TA section)) will compete with the ISC process efficiently. Therefore, the only possible ISC channel is SOCTISC from the 1CT* state to the 3PBI* state. However, for both PBI-1 and PBI-2, no triplet states were observed following CR, in both n-hexane and MeCN, despite the orthogonal geometry of the molecules, especially in PBI-1. Thus we postulate that there is no triplet state that is close in energy to the 1CT* state.34 In the case of PBI-3, the 3PBI* signal was observed in n-hexane with the lifetime of 13.7 μs but not in MeCN. We assume that the 1CT* state is with similar energy to a Tn state. In MeCN, both the decreased 1CT energy level (thus unmatched 1CT/Tn states) and the enhanced CR may significantly reduce the ISC yield.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b11872. General experimental methods, 1H, 13C NMR, and HRMS spectra, steady state UV−Vis absorption and fluorescence emission spectra, cyclic voltammogram, nanosecond and femtosecond transient absorption spectra, and the TD/DFT calculation details of the compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jianzhang Zhao: 0000-0002-5405-6398 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC (21273028, 21421005, 21673031, 21473020 and 21761142005) and the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT15ZD224, DUT2016TB12) for financial support.

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REFERENCES

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4. CONCLUSIONS In summary, we prepared perylenebisimide (PBI)−phenothiazine (PTZ) compact dyads (PBI-1 and PBI-2) as well as PBI− diphenylamine (DPA) dyad (PBI-3), with the aim to tune the electronic coupling (matrix elements, VDA) between the electron donor (PTZ or DPA) and the electron acceptor (PBI), thus to control the photophysical properties of the compounds. Electronic coupling (VDA and VDA*) was controlled by molecular conformation restriction. We found that with less steric hindrance between the electron donor and acceptor the coupling increased (VDA ranging from 0.15 to 0.41 eV and VDA* ranging from 0.34 to 0.54 eV, respectively), and CT absorption and emission bands appeared. On the contrary, with orthogonal geometry between the electron donor and acceptor, the two units are decoupled, and no CT bands were observed. The kinetics of the charge separation (CS. 0.3−0.6 ps) and charge recombination (CR. 130−240 ps) were confirmed by femtosecond transient absorption spectroscopy. However, nanosecond transient absorption spectroscopy indicates that no ISC occurs for PBI-1 and PBI-2, although by intermolecular sensitizing, the triplet state can be observed. ISC was observed for PBI-3. These results indicate that CR does not necessarily produce the triplet state, indicating the requirements for the SOCT-ISC include at least orthogonal geometry, charge separation/recombination, and most likely also the matching of 1CT* state with a triplet state in energy level. These results are useful for in-depth understanding of the charge recombination in organic molecules, ubiquitous processes in photovoltaics, artificial photosynthesis, photocatalysis, etc. 3770


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