Intersystem Crossing and Triplet-State Property of Anthryl- And Carbazole-[1,12]fused Perylenebisimide Derivatives with a Twisted π-Conjugation Framework

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Intersystem Crossing and Triplet-State Property of Anthryl- and

Carbazole-[1,12]fused Perylenebisimide Derivatives with a Twisted

π ‑Conjugation Framework

⊥

Zafar Mahmood, Andrey A. Sukhanov, Noreen Rehmat, Mengyu Hu, Ayhan Elmali, Yi Xiao,

Jianzhang Zhao,* Ahmet Karatay,* Bernhard Dick,* and Violeta K. Voronkova*

Cite This: J. Phys. Chem. B 2021, 125, 9317 −9332

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sı Supporting Information

ABSTRACT: Heavy atom-free triplet photosensitizers (PSs) are

particularly of interest concerning both fundamental photo-

chemistry study and practical applications. However, achieving

eﬃcient intersystem crossing (ISC) in planar heavy atom-free

aromatic organic compounds is challenging. Herein, we demon-

strate that two perylenebisimide (PBI) derivatives with anthryl and

carbazole moieties fused at the bay position, showing twisted π-

conjugation frameworks and red-shifted UV−vis absorption as

−

1

compared to the native PBI chromophore (by 75−1610 cm ),

possess eﬃcient ISC (singlet oxygen quantum yield: Φ = 85%)

Δ

and a long-lived triplet excited state (τ = 382 μs in ﬂuid solution

T

and τ = 4.28 ms in solid polymer ﬁlm). Femtosecond transient

T

absorption revealed ultrafast intramolecular charge-transfer (ICT) process in the twisted PBI derivatives (0.9 ps), and the ISC takes

.7 ns. Pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra indicate that the triplet-state wave

3

function of the twisted PBIs is mainly conﬁned on the PBI core, demonstrated by the zero-ﬁeld-splitting D parameter. Accordingly,

the twisted derivatives have higher T energy (E = 1.48−1.56 eV) as compared to the native PBI chromophore (1.20 eV), which is

1

T

1

an advantage for the application of the derivatives as triplet PSs. Theoretical computation of the Franck−Condon density of states,

based on excited-state dynamics methods, shows that the eﬃcient ISC in the twisted PBI derivatives is due to the increased spin−

−

1

orbit coupling matrix elements for the S −T and S −T states [spin−orbit coupling matrix element (SOCME): 0.11−0.44 cm .

1

2

SOCME is zero for native PBI], as well as the Herzberg−Teller vibronic coupling. For the planar benzoPBI, the moderate ISC is due

−

1

to S → T transition (SOCME: 0.03 cm . The two states share a similar energy, ca. 2.5 eV).

1

2

1

7−20

1

. INTRODUCTION

excited-state properties of PBIs,

but little eﬀort has been

made to improve their triplet excited-state generation ability.

The triplet excited state plays a crucial role in H production

Perylenebisimides (PBIs) are popular organic chromophores

with extended π-conjugated molecular structure that oﬀer

excellent thermal, photochemical, and electrochemical stabil-

ities and strong absorption of visible light.

widely used as light-absorbing or light-emitting chromophores

in organic solar cells,

state lasers, and so forth. Strong absorption of visible light and

high ﬂuorescence quantum yield make the PBIs an ideal

candidate for application in molecular assemblies,

2

21−23

24−26

through water splitting,

photocatalysis,

and triplet−triplet annihilation (TTA)

ISC is the crucial property for the triplet

photodynam-

27−29

ic therapy (PDT),

1

−3

PBIs have been

30−32

upconversion.

33

photosensitizers (PSs).

Several strategies have been

developed to enhance the ISC eﬃciency of triplet PSs, such

4

−6

7

light-emitting diodes, organic solid-

8

34−37

as the heavy atom eﬀect,

but this method suﬀers from

high cost, toxicity of heavy atoms (Ir, Ru, Pt, I, or Br atom),

and the shortened triplet-state lifetimes. Other approaches to

induce eﬃcient ISC are the n−π* ↔ π−π* transitions (El-

9

−12

two-

13

photon absorption dyes, ﬂuorescence probes, and ﬂuores-

cence bioimaging.

14−16

Moreover, the PBI core provides a

number of positions for functionalization; thus, chemical

modiﬁcation through substitution with diﬀerent moieties on

the PBI core can drastically change the photophysical

properties of PBIs. However, the lack of intersystem crossing

Received: June 8, 2021

Revised: July 23, 2021

Published: August 11, 2021

(

ISC) ability restricts the application of PBIs in photo-

chemistry. Many studies have been devoted to tune the singlet

©

2021 American Chemical Society

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a

Scheme 1. Synthesis of Twisted PBI Derivatives and the Reference Compounds

a

Key: (a) 3-Aminopentane, imidazole, reﬂuxed at 140 °C for 4 h under a N atmosphere, and yield: 95%; (b) Br , K CO , CHCl , reﬂuxed at 60 °C

2

3

for 2 h, under N , and yield: 45%; (c) 2-anthraceneboronic acid, K CO , PhCH /C H OH/H O (4:2:1, v/v), Pd(PPh ) , reﬂuxed at 90 °C for 8 h

under a N atmosphere, and yield: 75%; (d) hν, I , PhCH , stirred at rt for 4 h, and yield: 86%; (e) (9-phenyl-9H-carbazol-3-ylboronic acid), the

2

3

2

5

2

3 4

2

3

reaction conditions are similar to that of (c), yield: 73%; (f) phenyl boronic acid, the reaction conditions are similar to that of (c), yield: 65%; (g)

reaction conditions are similar to that of (d), yield: 70%; and (h) reaction conditions are similar to that of (d), yield: 66%.

3

8

39,40

Sayed rule), exciton coupling,

ers,

use of spin convert-

excited state and to unravel the ISC mechanism and triplet-

state property in these twisted compounds in detail.

4

1−43

44

and the energy level matching of the S /T states.

n n

Recently, the ISC of few twisted compounds has been studied,

Herein, we studied the ISC of PBI derivatives that have

extended π-conjugation at the bay position. Previous studies

demonstrate that the steric congestion at the bay position of

PBI can cause twisting of the PBI core and result in enhanced

and it was observed that twisting of the π-conjugation

45−49

framework of a chromophore can enhance the ISC.

PBI-based heavy atom-free triplet PSs are rarely reported.

Several strategies were used to generate a triplet excited state in

45,46

ISC.

We prepared two PBI-twisted derivatives by fusing

5

0

the PBI chromophore including triplet sensitization,

attaching heavy atoms or transition metals to the PBI

anthryl and carbazole moieties (Scheme 1) at the bay position

of PBI. The photophysical properties of these twisted

derivatives were investigated in detail by using steady-state

and time-resolved absorption and emission spectroscopic

methods. We ﬁnd that these derivatives have eﬃcient ISC

capability and a long-lived triplet excited state. Theoretical

computation of the Franck−Condon density of states of the

possible ISC pathways indicates that the eﬃcient ISC of the

twisted PBIs is due to the increased spin−orbit coupling matrix

elements (SOCMEs) and the Herzberg−Teller vibronic

coupling of S → T , induced by the twisted molecular

5

1−53

core,

replacing the oxygen atoms of the carbonyl groups

54

55

6,57

with sulfur atoms, singlet ﬁssion, charge recombination in

5

electron donor−acceptor dyads,

attaching substituents at

44

45,47

the ortho position, or based on twisting of the PBI core.

It is well known that attaching of bulky groups at the bay

position of the PBI can twist its core, but the ISC capability of

5

8,59

such system was not fully studied.

phenanthrene-fused PBI derivatives was studied. These

fused derivatives exhibit enhanced ISC, but the triplet quantum

The ISC of

4

6

1

structures, whereas the moderate ISC of the planar benzo-PBI

yield is low (Φ = 10−30%) and the triplet excited state is

T

is due to the non-negligible SOCME of S → T ISC.

1

2

short-lived (3.7−19.6 μs). Triply bay-fused di-PBIs were also

47

studied for enhanced ISC due to distorted structure. A high

2. EXPERIMENTAL SECTION

singlet oxygen quantum yield (Φ ) of 59−67% and a triplet

Δ

lifetime up to 35−56 μs were observed for these derivatives,

but the relationship of molecular structure and ISC eﬃciency

and the triplet-state property were not studied in detail. For

instance, the spatial localization of the triplet-state wave

function was not studied. It is worthy to develop simple PBI

derivatives with eﬃcient ISC capability and long-lived triplet

2.1. General Methods. All chemicals used in the synthesis

are of analytical grade and used as received. The solvents were

dried before using for synthesis of the compounds. UV−vis

absorption spectra were collected on a UV-2550 UV−vis

spectrophotometer (Shimadzu Ltd., Japan). Fluorescence

emission spectra were obtained on an RF-5301 PC

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spectroﬂuorometer (Shimadzu Ltd., Japan). Fluorescence

lifetimes were measured by an OB920 luminescence lifetime

spectrometer (TCSPC detection mode. Edinburgh Instru-

ments Ltd, U.K.) using an EPL 510 nm picosecond pulsed

laser, and the instrument response function (IRF) is 100 ps;

the repetition rates of the EPL picosecond laser for diﬀerent

lifetime ranges depend on the lifetime ranges to be measured:

2.6. Femtosecond Transient Absorption Spectrosco-

py. The femtosecond transient absorption spectroscopy

instrument consists of a Ti:sapphire laser ampliﬁer−optical

parametric ampliﬁer system with a 1 kHz repetition rate and 50

fs pulse duration (Spectra-Physics, Spitﬁre Pro XP, TOPAS)

and a commercial pump−probe experimental device (Spectra-

Physics, Helios); the signal is monitored by a white light

continuum probe. The instrument resolution is 120 fs. The

pump beam spot size was measured to be 400 μm by using a

proﬁlometer and the knife edge method. According to the spot

size and energy measurements (spot diameter = 400 μm and

pump laser energy = 1 μJ), the pump beam ﬂuence is

2

0 MHz for 50 ns, 10 MHz for 100 ns, and 5 MHz for 200 ns.

.2. Synthesis of PBI-1. Compound 2 (70 mg, 0.1 mmol)

2

and I (20 mg, 0.079 mmol) were dissolved in toluene (100

2

mL). The solution was photo-irradiated by white light (xenon

lamp) for 3 h at room temperature. The reaction progress was

monitored by TLC. After the completion of the reaction, the

reaction mixture was concentrated under reduced pressure to

obtain the crude product, which was further puriﬁed by

column chromatography (silica gel, CH Cl ). PBI-1 was

−

4

−2

calculated as ∼8.0 × 10 J cm on the sample in the pump

probe experiment. The excitation wavelength was selected

according to the absorption spectrum of the compounds. The

angle between the probe and pump beam polarization

directions was set up perpendicularly in the measurement.

The data were subdivided and ﬁtted using the Glotaran-

2

1

obtained as a dark red solid (60 mg, yield: 86%). H NMR

(

CDCl , 400 MHz): δ 10.09 (s, 1H), 9.76 (s, 1H), 9.10 (s,

3

60

Application 1.5.1 and Surface Xplorer, and all data were

1

8

2

4

H), 9.04−9.02 (m, 2H), 8.92−8.89 (m, 2H), 8.56 (d, 1H, J =

.0 Hz), 8.47 (s, 1H), 8.12 (d, 1H, J = 8.0 Hz), 8.09−8.04 (m,

H), 7.65−7.55 (m, 2H), 5.24−5.13 (m, 2H), 2.43−2.27 (m,

chirp-corrected before global ﬁtting.

2

.7. Time-Resolved Electron Paramagnetic Reso-

nance Spectroscopy. The time-resolved continuous-wave

TR CW) EPR measurements were performed on an X-band

1

3

H), 2.14−2.03 (m, 4H), 1.05 (t, 12H, J = 8.0 Hz). C NMR

100 MHz, CDCl ): δ132.8, 132.7, 131.9, 131.8, 130.9, 130.4,

(

3

EPR Elexsys E-580 spectrometer (Bruker) at 80 K. The

samples were dissolved in toluene/MeTHF (3:1, v/v). The

oxygen was removed by a few freeze−pump−thaw circles. The

samples were excited by the second harmonic of a Nd:YAG

pulsed laser pumped optical parameter oscillator (Quantel

Brilliant, λ = 500 nm, pulse length = 5 ns; E/pulse = ca. 10

mJ). The spectra were simulated using the EasySpin package

1

5

29.9, 128.9, 128.5, 127.6, 127.1, 126.9, 126.7, 126.6, 126.5,

26.4, 123.8, 123.6, 123.4, 123.3, 122.7, 122.5, 120.1, 58.0,

7.9, 25.1, 25.1, 11.6, 11.6. TOF MALDI−HRMS

−

(

C H N O ), calcd: m/z = 704.2675; found, m/z =

4

8

36

2

4

7

04.2693.

.3. Synthesis of PBI-2. PBI-2 was synthesized using

2

compound 3 (75 mg, 0.1 mmol) as the starting material,

following a similar method used for the synthesis of PBI-1.

61

based on Matlab.

2

.8. Photoreduction Experiment. A xenon lamp (20

1

PBI-2 was obtained as a dark red solid (54 mg, yield: 70%). H

2

mW/cm , 35 W) was used to perform the photoreduction

experiment. Triethanolamine (TEOA) was used as a sacriﬁcial

electron donor. An Agilent 8453 UV−vis spectrophotometer

was employed to record the absorption spectra. The sample

NMR (CDCl , 500 MHz): δ 10.66 (s, 1H), 9.86 (s, 1H),

3

8

1

7

5

1

.99−8.92 (m, 3H), 8.95 (d, 1H, J = 10.0 Hz), 8.75 (d, 2H, J =

0.0 Hz), 8.01 (d, 1H, J = 10.0 Hz), 7.82−7.77 (m, 4H),

.71−7.67 (m, 1H), 7.61−7.58 (m, 2H), 7.46−7.42 (m, 1H),

.22−5.12 (m, 2H), 2.42−2.26 (m, 4H), 2.14−2.02 (m, 4H),

solution (compound + TEOA) was deaerated with N for 10

2

min prior to photoirradiation.

1

3

.06 (t, 6H, J = 10.1 Hz), 1.00 (t, 6H, J = 10.0 Hz). C NMR

100 MHz, CDCl ): δ141.3, 141.3, 136.8, 133.2, 132.8, 130.3,

2

.9. Theoretical Computations. All calculations were

62,63

(

3

carried out with the ORCA programs.

All electronic

1

28.8, 128.6, 128.2, 127.2, 127.1, 126.7, 126.6, 126.4, 124.6,

24.1, 123.9, 123.7, 122.6, 122.4, 122.3, 121.7, 120.8, 116.9,

structure calculations used DFT with the B3LYP functional

and the SVP atomic basis set. The geometry of the ground-

13.7, 110.9, 57.9, 57.7, 25.1, 25.1, 11.6. TOF MALDI−

state S was optimized with the RHF-type calculation (the

0

−

HRMS (C H N O ), calcd: m/z = 769.2941; found, m/z =

standard for singlet ground states with DFT). The T states

5

2

39

3

4

1

7

69.2962.

were optimized with the unrestricted open-shell DFT method.

For S and T states, the TD-DFT method was used. For each

2

.4. Synthesis of PBI-3. PBI-3 was synthesized using

1

2

compound 4 (60 mg, 0.1 mmol) as the starting material,

following a similar method used for the synthesis of PBI-1.

PBI-3 was obtained as an orange solid (40 mg, yield: 66%). H

of the four states S , S , T , and T , the Hessian matrices were

0 1 1 2

calculated, all eigenvalues are positive. The next step used TD-

DFT at all four optimized geometries in order to get the

relative level ordering in each geometry. This also allowed us

1

NMR (CDCl , 400 MHz): δ 9.46 (s, 2H), 8.78−8.76 (m, 6H),

3

7

2

.97 (s, 2H), 5.21−5.15 (m, 2H), 2.44−2.35 (m, 4H), 2.18−

to arrange all energies within the same scale, with the S state

0

.12 (m, 4H), 1.12−1.09 (m, 12H). TOF MALDI−HRMS

at the optimized S geometry as the global zero.

0

−

(

[C H N O ] ), calcd: m/z = 604.2362; found, m/z =

4

0

32

2

4

6

04.2384.

3. RESULTS AND DISCUSSION

2

.5. Nanosecond Transient Absorption Spectroscopy.

3

.1. Molecular Design and Structure Conﬁrmation.

The nanosecond transient absorption spectra were recorded on

an LP980 laser ﬂash photolysis spectrometer (Edinburg

Instruments, U.K.) equipped with a Tektronix TDS 3012B

oscilloscope. The samples were deaerated with N for 15 min

prior to measurements and excited with a nanosecond pulsed

laser (Opolette, the wavelength is tunable in the range of 210−

400 nm. OPOTEK, USA). The typical laser power is ca. 5 mJ

per pulse. The data was processed by L900 software.

PBI is highly ﬂuorescent, and it is feasible to functionalize at

the bay position and ortho position.

studies have shown that by modiﬁcation at the bay position,

core-twisted PBI derivatives were obtained, leading to

drastically diﬀerent photophysical properties.

the ISC ability of such system mostly remained unclear.

Inspired by previous reports, we are interested to study the ISC

of twisted PBI derivatives with an aryl moiety fused at the bay

44,64−66

Recently, a few

2

46,47

However,

67

2

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1

Figure 1. Partial 2D H− H COSY spectra of PBI-2 recorded in CDCl at 20 °C along with two possible molecular structures of PBI-2. The

3

6

8

expected structure was proposed previously for an analogue with an alkyl substituent at the N position of the carbazole moiety.

Table 1. Photophysical Parameters of the Compounds

a

b

c

d

e

g

^λabs

ε

λ_e_m

Φ_F

τ_F/(ns)

τ_T(μs)

Φ_Δ

h

i

PBI-1

PBI-2

PBI-3

2

492/558

514

0.52/0.10

0.30

638

636

514

579

712

576

532

0.09

0.05

0.90

0.01

8.1/9.1

3.7/3.5

343

382

212

0.55/0.45 /0.42

h

i

0.85/0.60 /0.57

h

i

j

503

0.67

5.1/4.3

0.01/0.001 /0.15

h

i

361/528

485/530

531

0.13/0.33

0.24/0.20

0.37

6.1

8.8

131.7

110.1

0.01/0.57 /0.001

h

f

l

3

4

PBI

h

i

j

75

0.86

5.9/5.5

4.8/4.5

48.5

129.4

0.06/0.23 /0.24

h

f

k

512

0.90

a

In dichloromethane (1.0 × 10⁻⁵M) and in nanometer. Molar absorption coeﬃcient (10 M cm ). Fluorescence quantum yield with Bodipy

b

5

−1

c

d

e

as a standard (Φ = 72% in tetrahydrofuran). Fluorescence lifetimes, λ_e_x= 510 nm (EPL picosecond pulsed laser). Triplet state lifetime.

F

f

g

Obtained by TTET with PtOEP as the triplet energy donor, and the data was analyzed with global ﬁtting. Singlet oxygen quantum yield measured

h

i

j

k

l

with 2,6-diiodobodipy as a standard (Φ = 0.87 in acetonitrile). In n-hexane. In toluene. In acetonitrile. Not determined. Not observed.

Δ

region. For this purpose, we synthesized two PBI derivatives by

fusing organic moieties of diﬀerent electron-donating ability

the 2-position), the protons (H and H ) at the 1- and 4-

A B

postion of carbazole will exhibit singlet peaks. The assignment

(

carbazole and anthracene) at the bay position of PBI (Scheme

). Moreover, this modiﬁcation will cause steric congestion at

of doublet peaks to the protons was further veriﬁed by 2D

H− H COSY NMR, in which the interaction between these

1

the bay region and leads to twisting of the core. The synthesis

of these compounds was reported previously, but their

two doublets was conﬁrmed. However, this interaction is

impossible for the previously assigned structure, at least for the

isolated product (no 2D NMR spectral study was presented in

67,68

photophysical properties were not studied in detail.

68

Previously, the carbazole-fused PBI derivatives were

reported for optoelectronic study, and it was concluded that

the direct cyclization of carbazole aﬃxed at the bay position of

PBI leads to planar geometry (carbazole fuse from 2-position

the previous study). The absorption and emission data of

PBI-2 (Table 1) are identical to the previously reported

68

data; thus, we believe that at least in our case, we isolated a

diﬀerent product, and the structure is presented in Figure 1.

Moreover, the low ﬂuorescence quantum yield and good

solubility of PBI-2 compared to the phenyl-fused PBI (PBI-3)

planar derivative also support the twisted structure of PBI-2, as

68

with the PBI core). However, Lu et al. demonstrated that the

-position of carbazole is more reactive toward the reductive

cyclization reaction as compared to the 2-position of the

4

6

9

carbazole. To conﬁrm the molecular structure of the

carbazole-fused PBI (PBI-2), we investigated the 1D

it is established that twisting of the PBI core improves the

1

46,70

H

solubility.

Moreover, previously it was reported that

NMR and also performed the 2D COSY NMR spectra of the

isolated product (Figures 1 and S12 ).

The H NMR spectrum of PBI-2 showed two characteristic

photocyclization of anthracene at the bay position of PBI

67

(PBI-1) leads to a zigzag (twisted) geometry, while the

1

71

phenyl-fused PBI (PBI-3) has a planar structure. On the

doublet peaks of protons of the carbazole unit (H and H in

basis of previous reports on the core-twisted PBI derivatives,

we suppose that the extended π-conjugation at the core region

of PBI and the twisted geometry will induce ISC in these

A

B

Figure 1), which conﬁrmed that the carbazole fused at the 4-

position instead of the 2-position as stated in a previous report,

and it leads to twisted geometry (the planar structure was

46,47

derivatives.

6

8

observed for the previously proposed structure). The two

doublet peaks for the protons of the carbazole unit are only

possible in the structure with carbazole fused at the 4-position,

while in the planar structure (in which the carbazole fused at

In order to study the molecular conformation, the ground-

state geometries of the fused PBI derivatives were optimized

with the DFT method. For both PBI-1 and PBI-2, twisted

geometries were found (Figure 2a,b), while reference PBI-3

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Figure 2. Optimized geometries and the twist angles at the bay regions of PBI derivatives (a) PBI-1, (b) PBI-2, and (c) PBI-3. Calculated by DFT

B3LYP/6-31G (d)] using Gaussian 09.

[

Figure 3. UV−vis absorption spectra of twisted PBI derivatives along with reference compounds. (a) PBI-1, PBI-2, 2, and 3. (b) PBI-3 and PBI. c

1.0 × 10⁻M in dichloromethane. (c) Fluorescence emission spectra of PBI derivatives in dichloromethane, λ = 465 nm (A = 0.10). Optically

ex

5

=

−

matched solutions were used (all the solutions have the same absorbance at the excitation wavelength c ≈ 1.0 × 10 M, A = 0.1), 20 °C.

Figure 4. Fluorescence emission spectra of PBI-fused derivatives in solvents of diﬀerent polarity; (a) PBI-1, (b) PBI-2, and (c) PBI-3, λ_e_x= 465

nm (A = 0.10). Optically matched solutions were used (all the solution of the compounds have the same absorbance at the excitation wavelength. c

−

5

≈

1.0 × 10 M, A = 0.1), 20 °C.

has a planar structure (Figure 2c). Due to the steric hindrance

of hydrogen atoms at the bay region, the π-conjugation plane

of PBI-1 is twisted with an angle of 32°, while PBI-2 shows

twisting of 34°. Previously, phenanthrene-fused PBI derivatives

with a highly twisted geometry (torsion angle of 40.7−44.3°)

the precursor compound 2 is slightly red-shifted, and the

absorption peaks are not well resolved.

For PBI-2, a broad absorption band centered at 513 nm

without any ﬁne structure was observed (Figure 3a), which is

diﬀerent from its precursor (compound 3). The absorption

spectrum of 3 consists of two poorly resolved broad bands. In

the case of a previously studied phenanthracene-fused PBI

derivative, the absorption proﬁle possesses a structure similar

4

6

were reported, but the triplet yield was quite low.

.2. UV−Vis Absorption and Fluorescence Spectra.

3

The UV−vis absorption spectra and the ﬂuorescence emission

spectra of the compounds were studied (Figures 3 and 4). The

absorption spectrum of PBI-1 contains two sharp peaks

centered at 460 and 490 nm (Figure 3a), which is the

46

to that of the PBI chromophore but slightly red-shifted. The

reported diPBI-fused derivatives also showed structured and

47

red-shifted absorption due to a large π-conjugation system.

67

characteristic PBI vibronic progression. A shoulder band at

60 nm was also observed, which is very sensitive to solvent

polarity (Figure S13 ). However, the absorption spectrum of

However, for PBI-2, the strong electron-donating ability of the

fused carbazole moiety perturb the molecular orbitals of the

PBI chromophore, which results in a totally diﬀerent

5

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absorption proﬁle. The reference compound PBI-3 shows a

similar absorption proﬁle as that of the unsubstituted PBI

3.3. Nanosecond Transient Absorption Spectroscopy:

Triplet-State Properties. In a previous report of the bay-

fused PBI derivatives, the triplet state of the twisted derivatives

was not studied, such as using nanosecond transient absorption

(Figure 3b). The absorption of compounds was also studied in

other solvents (Figure S13). It is worth to note that PBI-1 and

PBI-2 show good solubility in organic solvents due to their

twisted nature as compared to rigid and planar PBI-3.

The ﬂuorescence emission spectra of the compounds were

67,68

(ns-TA) spectra.

Thus, in order to conﬁrm the triplet

excited-state population in the twisted PBI derivatives, the ns-

TA spectra of the compounds were measured (Figure 5 ).

studied (Figure 3c). PBI-3 is highly ﬂuorescent (Φ = 0.90),

F

and it shows the characteristic emission proﬁle similar to that

of the unsubstituted PBI, except the 10−20 nm of blue

7

1

shifting. For PBI-1 and PBI-2, a red-shifted and broad

structureless emission band centered at ca. 635 nm was

6

7,68

observed.

The ﬂuorescence of PBI-1 and PBI-2 is greatly

quenched as compared to the reference compounds. The

ﬂuorescence quantum yields of PBI-1 and PBI-2 in dichloro-

methane are 0.09 and 0.05, respectively, which are much lower

than the unsubstituted PBI (Φ = 0.86).

F

These two compounds showed red-shifting and quenching

of emission with increasing solvent polarity (Figure 4a,b). The

−

1

large Stokes shifts (3700−5500 cm ) and the weak emission

in polar solvents of the twisted PBI derivatives indicate the

intramolecular charge-transfer (ICT) character of the emissive

state. For PBI-3 and PBI, however, no clear trend in emission

was observed with increasing solvent polarity (Figures 4c and

S14 ). Previously, the ﬂuorescence property of the twisted

phenanthrene-fused PBI derivative was studied and no such

46

ICT feature was observed. Those derivatives exhibited a

vibronically resolved structured emission proﬁle, and a small

Stokes shift of ca. 330 cm⁻was observed (in which one

phenanthrene unit was fused at the bay position of PBI). The

small Stokes shift and structured emission indicate that

emission takes place from the LE (locally excited) state in

1

Figure 5. Nanosecond transient absorption spectra of the twisted PBI

derivatives (a) PBI-1 and (b) PBI-2 upon nanosecond pulsed laser

excitation (λex = 500 nm). (c,d) Respective decay curves monitored at

−

5

46

5

35 nm in deaerated dichloromethane, c = 1.0 × 10 M at 20 °C.

these derivatives.

The τ values in (c,d) are the intrinsic triplet-state lifetimes of the

The ﬂuorescence emission decay kinetics of the compounds

was studied (Figure S15). For PBI-1 and PBI-2, drastically

diﬀerent ﬂuorescence lifetimes were observed in dichloro-

methane. The ﬂuorescence of PBI-1 decays with the mono-

exponential feature with a lifetime of 8.1 ns, while the

ﬂuorescence of PBI-2 decays much faster, showing a shorter

lifetime of 3.7 ns, which is slightly shorter than the

unsubstituted PBI (τ_F= 4.8 ns). The diﬀerence in the

ﬂuorescence lifetimes of PBI-1 and PBI-2 may be due to the

diﬀerent ICT nature of the excited singlet states. The

ﬂuorescence of PBI-3 also decays mono-exponentially, and

the ﬂuorescence lifetime of 5.1 ns is comparable to that of the

unsubstituted PBI (Figure S17).

compounds.

Upon photoexcitation, both twisted derivatives exhibited a

nearly identical absorption proﬁle, only a slight change in the

ground-state bleach (GSB) was observed due to the diﬀerence

in their ground-state absorption (Figure 5a,b). For PBI-1, two

GSB absorption bands centered at 380 and 485 nm,

respectively, were observed; one excited-state absorption

(

ESA) band centered at 540 nm was observed, which is the

characteristic T → T exited-state absorption of the PBI

moiety.

1

n

44,53,72

It should be noted that the ESA band overlaps

with the GSB band. For PBI-2, a GSB band in the range of

3

00−450 nm was observed, as well as an ESA band in the

The singlet oxygen quantum yields (Φ ) of the twisted PBI

Δ

range of 450 −700 nm. Similar results were observed in other

solvents (Figures S19 and S20 ). The overlap of the ESA and

GSB bands in the current twisted derivatives is diﬀerent from

derivatives were studied (Table 1), as a preliminary estimate of

the triplet-state production of the compounds. Previously, the

singlet excited-state properties of the current twisted

derivatives were studied, but the triplet excited-state properties

57

the unsubstituted PBI chromophore, or less twisted PBI

7

2

moiety, because the molecular orbital and T₁→ T_n

transitions are diﬀerent for the current twisted PBI compounds

and the unsubstituted PBI compounds (see the theoretical

computation section).

67,68

remained unraveled.

The twisted PBI-1 and PBI-2 show

very high Φ values of 0.55 and 0.85, respectively, but a much

Δ

lower Φ value (0.01 in dichloromethane) was observed for

Δ

PBI-3, which has a planar geometry. Since the PBI moiety is

devoid of ISC ability, the enhanced ISC in these derivatives is

due to the twisting of the core of the molecules Moreover,

these derivatives show eﬃcient singlet oxygen generation

ability in solvents of diﬀerent polarity. Previously, phenan-

threne-fused PBI derivatives with twisted geometry have been

studied, but the ISC eﬃciency in those derivatives is low

For PBI-1 and PBI-2, apparent triplet lifetimes of 115.8 and

78.5 μs were observed, respectively, by monitoring the decay

traces at 535 nm and using the imbedded software of the ns-

TA spectrometer (L900 software). However, this apparent

lifetime should be signiﬁcantly shorter than the actual triplet

lifetime of the compounds due to the TTA quenching eﬀect, as

a result of the strong absorption, eﬃcient ISC, and the long

triplet lifetime of PBI-1 and PBI-2. For this reason, the

4

6

(

<0.3).

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Figure 6. (a) Femtosecond transient absorption spectra of PBI-1 at diﬀerent delay times, (b) corresponding evolution associated diﬀerence spectra

EADS), and (c) decay traces at selected wavelengths. (d) Femtosecond transient absorption spectra of PBI-2, (e) corresponding EADS, and (f)

decay traces at selected wavelengths. λ_e_x= 500 nm, in dichloromethane, 20 °C.

(

intrinsic triplet lifetime of the compounds was determined by

ﬁtting the decay traces at two diﬀerent concentrations with a

special kinetic model, which was developed by some of us

air atmosphere). The minor quenching of the triplet state

under aerated atmosphere is attributed to the poor O₂

diﬀusion in the polymer matrix), while PBI-2 in polymer

ﬁlm showed τ = 4.74 ms in a N atmosphere (4.52 ms under

73,74

previously, to eliminate the self-quenching TTA eﬀect.

PBI-1, a long-lived triplet excited state was observed (Figure

c), with an intrinsic triplet lifetime of 343 μs (vs. apparent τ_T

115.8 μs). For PBI-2, an intrinsic triplet lifetime of 382 μs

was observed (Figure 5d, vs. apparent triplet lifetime of τ =

For

T

2

aerated condition). These results indicate that in solution, the

triplet lifetime is not only quenched by TTA but also other

relaxation channels such as vibration relaxation, which is

inhibited in the solid matrix so that the triplet lifetime is

5

=

T

77,78

7

8.5 μs) by ﬁtting of the decay traces with the TTA kinetic

prolonged.

This is beneﬁcial for some applications of the

model (in dichloromethane). The triplet decay feature of the

twisted derivative was also studied in polar solvent, and it was

observed that the triplet decay becomes slightly faster in polar

solvent; for instance, the apparent triplet lifetime of PBI-1 in

acetonitrile is 83.2 μs (Figure S20).

triplet PSs such as PDT; in some cases, the PDT agents were

doped in the solid matrix.

49

3.4. Femtosecond Transient Absorption Spectrosco-

py. To gain further insights into the excited-state dynamics in

the twisted PBI derivatives, the femtosecond transient

absorption (fs-TA) spectra of the compounds were measured

(Figures 6 and S26 ). The fs-TA spectral studies were

performed in two diﬀerent solvents (toluene and dichloro-

methane). Upon photoexcitation of PBI-1 at 490 nm in

dichloromethane, intense GSB centered at 492 nm appeared

promptly, as well as a broad ESA band in the range of 575−

The triplet-state lifetimes observed for PBI-1 and PBI-2 are

much longer than those of the previously reported twisted PBI

derivatives. For instance, in mono- and bis-phenanthrene-fused

PBI derivatives, the triplet-state lifetimes were determined as

46

3

.7 and 19.6 μs. For the di(perylenebisimide)s showing a

twisted π-conjugation framework, the triplet-state lifetimes

47

44,76,79

were observed as 35−56 μs. For the Pt(II) bisacetylide

800 nm, assigned to S → S transition (Figure 6a).

1

n

complex with PBI as the ligand (ISC is enhanced by the heavy

Previously, the fs-TA spectra studies on twisted phenanthrene-

fused PBI and triply linked di-PBIs derivatives also showed the

singlet excited-state absorption in a similar range (700−900

atom eﬀect), a short-lived triplet state (τ = 0.37 μs) was

T

7

5

observed. Ir(III) and Pt(II) complexes containing PBI

ligands also exhibited a short-lived PBI-localized triplet state

τ_T= 22.3 μs and 0.25 μs, respectively).

46,47

nm).

Within 1 ps, the ESA band slightly blue-shifted,

5

1,53

(

1,6,7,12-

which is attributed to the vibrational relaxation/ICT process.

Remarkably, within few picoseconds, the singlet ESA band

decreased in intensity and a new band centered at 538 nm rises

gradually, which is assigned to T → T transition. The triplet

TetrabromoPBI gives a short triplet-state lifetime (τ = 1.04

T

4

μs). Previously, it was reported that some PBI derivatives

with aryl substituents attached at the ortho position of the PBI

core exhibit eﬃcient ISC, but the triplet lifetime (30.6 μs) is

1

n

absorption band (538 nm) continues to grow on a nanosecond

timescale similar to that as previously observed in phenan-

76

also much shorter.

46

The triplet excited state of the twisted derivatives was also

studied in a solid polymer ﬁlm (Figures S21 −S23. Doped in

Clear Flux 50, polyurethrene precursors, a ﬂexible transparent

rubber). The transient absorption proﬁle of the twisted

derivatives in a thin ﬁlm is similar to that observed in a ﬂuid

solution. Interestingly, for PBI-1, a much longer triplet lifetime

threne-fused PBI derivatives, in which ISC takes 1 ns.

To obtain the exact rate constants of the excited state

dynamics, global ﬁtting and target analysis of transient

absorption data were performed to obtain the evolution-

associated diﬀerence spectra (EADS). For PBI-1, four

components were required for the satisfactory ﬁtting of the

data. The initial spectral component represents the population

of 5.18 ms in a N atmosphere was observed (4.74 ms under

2

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of Frank−Condon (FC) singlet excited state as is evident by

the GSB and a broad positive excited-state absorption (Figure

6

b). The second spectral component (red line) appearing with

a time of 0.5 ps represents the relaxation from the FC state to

form the ICT state. In the following evolution, occurring in

1

38 ps (blue line), the ICT* partially relaxed. We suppose

that this might be due to some solvation relaxation. Finally, the

triplet state populates as is indicated by the ﬁnal spectral

evolution which appears with 8.2 ns and persists for a long

time. The spectral feature of the ﬁnal component resembles

well the ns-TA spectral proﬁle of the compound. Thus, for

PBI-1, the ISC rate or the triplet rise time is 8.2 ns, nearly

1

Figure 7. Representative TREPR spectra of the frozen solutions (T =

identical to the ICT* decay time (τ_F= 8.1 ns). The

8

0−130 K) of (a) PBI-1 and (b) PBI-2 in toluene/MTHF (3:1, v/v)

brominated twisted PBI derivative and doubly phenanthrene-

fused PBI derivatives were previously reported to have ultrafast

obtained upon excitation at 500 nm with pulsed laser at energies of 10

−5

mJ per pulse. c = 3.0 × 10 M. The spectra in red are the simulations.

46,80

ISC (0.38 or 0.25 ps).

In contrast, singly phenanthrene-

^f^u^s^e^d^P^B^I⁽^τISC = 1 ns) and triply linked DIPBIs derivatives

(e, a, e, a, e, a) was observed in twisted hexaphyrin

derivatives.

46,47

85,86

(

^τISC = 970 ps) showed ISC at a relatively slower timescale.

Thus, the ISC rate signiﬁcantly varied (from few picoseconds

to nanoseconds) in the diﬀerent twisted derivatives having

diﬀerent extents of the π-conjugated framework.

The zero-ﬁeld splitting (ZFS) parameters of PBI-1 are |D| =

4

|

7.1 mT and |E| = 1.9 mT, while for PBI-2, |D| = 44.6 mT and

E| = 2.2 mT were obtained by the simulation of TREPR

spectra (Table 2). The D value is similar to the planar PBI

For PBI-2 in dichloromethane upon excitation at 510 nm,

the transient absorption spectra exhibited a negative

absorption band at 524 nm, which can be attributed to GSB

Table 2. ZFS Parameters (D and E) and Relative Population

Rates (P_x_,_y_,_z) of the Zero-Field Spin States of the Twisted

(

Figure 6d), and it is consistent with UV−vis absorption

a

(Figure 3a). An ESA band in the range of 600−800 nm

PBI Derivatives and Reference Compound

46,47

corresponding to S → S transition was also observed.

1

n

b

D (mT)

E (mT)

^Px

0.4

0.3

0.2

0.43

^Py

0.6

0

^Pz

ΔP*

The global analysis shows that the vibrational relaxation or the

ICT takes place at 0.9 ps (Figure 6e), while the triplet state in

PBI-2 also populates at a relatively longer time scale (τ_I_S_C= 3.7

ns), similar to that observed previously in a twisted thiophene-

c

PBI-1

PBI-2

47.1

− 47.1

44.6

−1.9

1.9

−2.2

2.2

0

0.7

0

0.33

0.43

0.75

d

c

0.8

0

81

d

fused diPBI derivative (τ_I_S_C= 4.7 ns).

− 44.6

0.57

a

To further verify the ISC mechanism, the decay traces at

singlet- and triplet-state ESA bands were compared, and it was

observed that within the singlet-state decay, the triplet state

populates gradually (Figure 6c,f). fs-TA spectral measurements

of twisted PBI derivatives were also performed in toluene

Obtained from simulation parameters of the triplet-state TREPR

spectra of the indicated molecule in a toluene/MTHF (3:1, v/v)

b

c

solvent matrix at 80 K. ΔP* = |P

0

− P

|/|P

− P |. By assuming D >

x

y

z

y

d

. By assuming D < 0.

(

Figure S26). The excited-state dynamics of the compounds

moiety (|D| = ca. 46.5 mT), previously observed with the PBI-

shows a slight change in toluene. The ISC takes 7.9 and 2.5 ns

for PBI-1 and PBI-2, respectively.

Cz donor/acceptor dyads exhibiting the SOCT-ISC mecha-

5

7

nism of triplet population. However, the E value is

signiﬁcantly smaller than those of planar PBI triplet PSs (|E|

= ca. 4.2 mT). On the basis of ZFS parameters, we conclude

that the triplet wave function in twisted derivatives is

predominantly localized on the PBI core, the distribution of

the spin density on the fused anthracene and carbazole

moieties should not be signiﬁcant; otherwise, a much smaller D

parameter should be observed (this is supported by the spin

density surface analysis of the triplet state, see later section). In

other words, the relatively smaller E value compared to D in

twisted PBI derivatives suggest that the unpaired electrons are

not delocalized in the twisted framework, as previously

3

.5. TREPR Spectra: Electron Spin Selectivity of the

ISC and the Triplet-State Wave Function Conﬁnement

in the Twisted π-Conjugation Framework. To further

investigate the electron spin selectivity of the ISC induced by

the twisted π-conjugation system and the spatial conﬁnement

of the triplet-state wave function of the PBI derivatives, time-

resolved electron paramagnetic resonance (TREPR) spectral

study was carried out (Figures 7 and S27). This information

can hardly be obtained with ordinary optical spectroscopic

methods. TREPR spectroscopy is a useful tool to investigate

the ISC mechanism, the spatial localization of the triplet

excited state, and the electron spin selectivity of the ISC based

on the electron spin polarization (ESP) phase pattern and ZFS

parameters (D and E values) observed with TREPR spectra,

85

observed in the twisted hexapyrin derivative.

The relative population of the triplet sublevel was

determined by assuming the two conditions, that is, D > 0

and D < 0 (Table 2). It is worth to mention that diﬀerent

conventions for assigning the D/E sign result in diﬀerent spin

level referencing. Predicting the exact sign of the D/E value

from EPR experiment is challenging. For some compounds

with large D values, sometimes the sign was determined, from

the temperature dependence of the EPR spectra, based on the

change of the ratio of the intensities of the weak-ﬁeld and

strong-ﬁeld signals. However, in the case of compounds with a

small D value, the diﬀerence is small. By comparison of the

7

4,82−84

respectively.

The TREPR spectra of PBI-1 and PBI-2 observed in frozen

toluene:MeTHF (3:1, v/v) at 0.25 μs are presented in Figure

7

a,b, respectively. For both PBI-1 and PBI-2, the TREPR

spectra of the triplet state show the ESP phase pattern of (e, a,

e, a, e, a), which is diﬀerent from the ESP of the PBI triplet

56

state (e, e, e, a, a, a) accessed by the normal SOC eﬀect.

Interestingly, the ESP of the triplet state is very close to that

generated by charge recombination.

56,57

A similar ESP pattern

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result with theoretical computations, the sign of the D value of

the current twisted PBI derivative seems probably negative.

However, for consistency and to comply with assignment used

for the recently reported PBI series, we assumed the sign of D

The electron spin density surfaces of the T states of the

1

twisted PBI derivatives were also studied (Figure 8). For PBI-

1, the spin density of the triplet state is mainly conﬁned on the

PBI moiety, and only minor spin leakage to the anthryl moiety

was observed. Similar results were observed for PBI-2. For

PBI-3, the spin density of the triplet state is also mainly

localized on the PBI unit, slightly leaked to the fused phenyl

ring. These results are in agreement with the ZFS D parameter

of the twisted PBI derivatives observed in the TREPR spectral

>

0 and converted the P , P , and P of the current twisted PBI

x y z

derivatives.

Assuming D > 0, the relative population rates of the three

triplet sublevels for PBI-1 is P /P /P = 0.4:0.6:0, which is

x

y

z

similar to that of the previously reported PBI-Ph -PTZ dyad

1

57

(

P /P /P = 0.35:1:0), both showed overpopulation of the T

study, which are similar to the native PBI chromophore.

x

y

z

y

56

state (P > P , P ). Note that previously the ZFS D and E

In addition to the electronic ground state S , the geometries

y

x

z

0

parameters of the triplet state of PBI derivatives with phenoxy

substituents at the bay position were assigned as positive and

negative, respectively. Another twisted derivative (PBI-2) also

of the excited states S , T , and T were optimized. Figure 9

1

2

shows the energy level diagrams at each of these optimized

geometries (indicated at the top of each column). All energies

are given relative to the energy of the optimized ground state

showed overpopulation of the T state with relative population

y

rates of P /P /P = 0.2:0.8:0. However, the ESP pattern of

S .

0

x

y

z

twisted PBI derivatives was similar to that of the previously

reported PBI-Cz dyads, but the relative population rates of the

triplet sublevels are diﬀerent. The PBI-Cz dyads showed

Analysis of the TDDFT wave functions shows that these

excited states originate from excitations from the two highest

occupied orbitals (HOMO, HOMO − 1) to the two lowest

virtual orbitals (LUMO, LUMO + 1). All of these orbitals are

part of the π-electron system, except for HOMO − 1 of PBI,

which is a linear combination of the oxygen lone pairs.

According to these calculations, PBI and PBI-3 have a

57

overpopulation of the T state (P /P /P = 1:1.08:0). This is

x

y

z

a consequence of the diﬀerent choice of the sign of E

parameters.

3

.6. Theoretical Computations. In order to rationalize

−

1

the photophysical properties, the frontier molecular orbitals of

PBI-fused derivatives were studied. The frontier molecular

orbitals of compounds PBI-1, PBI-2, and PBI-3 are depicted

in Scheme 2. For PBI-1, HOMO is mainly localized on the

similar S excitation energy (21075 and 20800 cm ). The S

1

and T states of PBI originate from the HOMO → LUMO

1

excitation. Since this orbital pair has a large exchange integral,

−

1

the energy gap is rather large (10944 cm ). The T state is

2

above the S state at all geometries; hence, it is most likely not

involved in ISC. The situation is similar for the other planar

1

Scheme 2. Selected Frontier Molecular Orbitals of PBI-1,

PBI-2, and PBI-3 Calculated at the DFT (B3LYP/6-31G

molecule PBI-3 with a slightly smaller adiabatic S /T energy

1

a

−1

(d)) Level with Gaussian 09W in Toluene

gap (8695 cm ). In contrast to PBI, the T state is now

2

slightly below S and ISC to this state should be considered.

1

Fusion of the PBI chromophore with either anthracene in

PBI-1 or with carbazole in PBI-2 not only forces twisting of

the π-electron system but also introduces a possible electron

donor. Indeed, analysis of the wave functions reveals that the

S₁state in these compounds has a substantial CT character. As

a proof, we show the static dipole moments of all four states of

the four compounds in Table 3.

In line with this observation, the calculated S → S

0

1

−

1

excitation energy in PBI-1 (15651 cm ) and PBI-2 (16692

−

1

cm ) is substantially red-shifted with considerably lower

oscillator strength. Judging from the dipole moments, the

corresponding triplet state is the T state. As the dipole

2

moment shows, the T state corresponds to the local excitation

1

(

as in compounds PBI and PBI-3). Hence, two possible

channels exist for ISC, S (CT) → T (LE) and S (CT) →

1

T (CT). Due to the diﬀerent excitation characteristics of S

2

1

−

1

and T , the gap in PBI-2 (4836 cm ) is only half of the size

1

compared to PBI and considerably smaller in PBI-1 (2999

−

1

cm ). This smaller energy gap should result in a larger FC-

weighted density of states and hence in larger ISC rate

constants.

a

The energy levels of the orbitals are presented (in eV).

The ISC process is caused by coupling between the initial

singlet and the ﬁnal triplet state via the spin−orbit operator.

The matrix elements of this operator were calculated with the

TD-DFT method at the equilibrium geometries of T and T

and are listed in Table 4. The matrix elements are given in

Cartesian coordinates; for the orientation of the molecules, see

Figure S36 . The z-component is identical to the M = 0

component of the triplet state, the M = 1 and M = −1

components are complex conjugate linear combinations of the

x- and y-components.

fused anthryl unit while LUMO is mainly conﬁned on the PBI

moiety but also spreads slightly to the fused anthryl

chromophore. This result indicates the possibility of charge

transfer in PBI-1 upon photoexcitation. Similar results were

observed for PBI-2. For PBI-3, which has the planar geometry,

the HOMO and LUMO are not separated, thus no CT

1

2

67

behavior is expected for the S state. These results are in

1

agreement with the solvent polarity-dependent ﬂuorescence

emission wavelength of the compounds (Figure 4).

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Figure 8. Spin density distribution of the triplet state of PBI-1, PBI-2, and PBI-3 in toluene. Calculated at the B3LYP/6-31G (d) level using

Gaussian 09W.

Figure 9. Energy diagram of the compounds (a) PBI, (b) PBI-1, (c) PBI-2, and (d) PBI-3 at the optimized geometries of the four states S , S , T ,

0

1

and T (indicated at the top of every column), respectively. Singlet states are marked in black and triplet states in red. The large dot in each column

2

indicates the state that was optimized.

In PBI and PBI-3, the S state and the T state have the

the non-planar systems PBI-1 and PBI-2, non-zero coupling

1

same leading conﬁguration. Hence, the coupling S /T is

elements are found. The square modulus of the three Cartesian

1

−

1

forbidden by the high symmetry in PBI (D ) and PBI-3 (C ).

components is 0.11 cm for PBI-1, and for PBI-2, the value is

2h

2v

−

1

The planarity of the compounds PBI and PBI-3 also results in

0.44 cm . This is a clear indication that the SOCME is

very small values for the S /T coupling (El-Sayed rule). For

increased by the twisted π-conjugation system in the carbazole

1

2

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2

Table 3. Static Dipole Moments (Debye) of the States S , S ,

SO

0

1

k^H^T= ²

π

∂⟨i|H |f ⟩

⟨u|q|v⟩

1

Z

e

−E /kT

iu

T , and T of the Four Compounds

∑

1

2

ℏ

∂q

q, u, v

S₀

S₁

T₁

T₂

PBI

0.00

2.73

5.34

1.39

0.00

16.54

18.91

4.67

0.00

3.33

7.08

1.78

0.00

15.15

17.74

4.93

δ(E − E )

iu

jv

(6)

PBI-1

PBI-2

PBI-3

In the general case, interference terms occur and are

calculated in ORCA.

The ISC rates of the compounds were calculated by the so-

called excited-state dynamics (ESD) technique implemented in

ORCA. This technique considers the FC density of states as

well as the vibronic contributions (Herzberg−Teller, HT) to

the ISC rate constant. The rate constants can be studied as a

function of energy gap (Figure 10 ) and temperature, while the

or anthryl-fused PBI derivatives. These values are, however,

−

1

still smaller than 1 cm . As we will see below, the FC term

leads only to a minor contribution to the ISC rates, whereas

the main contribution is from the Herzberg−Teller term.

The rate constant for ISC from an initial electronic state |i⟩

to a ﬁnal electronic state |f⟩ can be expressed by Fermi’s golden

rule as eq 1

2

k_I_S_C= ²

π

⟨iu|H |fv⟩

1

Z

e

δ(E − Ε )

SO

−E /kT

iu

∑

iu

jv

ℏ

uv

(1)

where H^S^Ois the spin−orbit coupling operator, u and v label

the vibrational states within the two electronic states, E is the

iu

vibronic energy of each state, δ() is the Dirac function, and eq

2

shows the partition function

−

E_i_u/kT

Z = ∑ e

Figure 10. ISC rate constants calculated with the ESD method for the

S −T transitions (ﬁlled symbols) and the S −T transitions (open

(2)

iu

1

2

−

1

symbols) vs. variation of the energy gap in a range ±2000 cm from

the predicted energy gap by TD-DFT. The rate constants are shown

on a logarithmic scale.

The integral over the electronic coordinates is a function of

the nuclear coordinates and is usually expanded in powers of

the normal coordinates q (eq 3)

SO

∂

⟨i|H |f ⟩

SOCMEs and their vibronic derivatives need to be calculated

only once for each M-sublevel of the triplet state. For these

calculations, the SOCMEs from TD-DFT calculations were

used. Since the experimental S −T energy gap is not known,

SO

⟨

i|H (q)|f ⟩ = ⟨i|H |f ⟩ + ∑

q

∂

q

(3)

1

ORCA uses the geometry of the ﬁnal state as the reference.

When only the ﬁrst term is retained, we obtain the FC rate

constant

−

1

the ISC rate constants were calculated in a range ±2000 cm

from the predicted energy gap. The rate constants are shown

on a logarithmic scale; hence, a straight line corresponds to an

exponential energy gap law. Apparently, PBI and PBI-3 obey

this rule perfectly (Figure 10). The SOCMEs are zero for both

molecules, and the ISC rate is entirely due to vibronic

2

k^F^C= ²⟨i|H |f ⟩ (FCWD)

π

SO

ℏ

(4)

2

−1

coupling. At the calculated energy gap, k = 1.54 × 10 s for

where the FC-weighted density of states is given by eq 5

ISC

3

−1

PBI and 1.73 × 10 s for PBI-3. Both rates are so small that

they could not lead to signiﬁcant population of the triplet state.

This explains the negligible triplet yield of PBI, but not the

1

Z

2

−E /kT

iu

(

FCWD) =

∑

|⟨u|v⟩| e

δ(E − E )

iu

jv

(5)

uv

moderate yield of PBI-3 (Φ = 15%). On the other hand, PBI-

Δ

5

When the SOCME at the equilibrium geometry of the ﬁnal

states vanishes (e.g., due to symmetry), the vibronic coupling

term remains as eq 6

1 and PBI-2 show similar and large rates (5.59 × 10 and 1.01

× 10 s , respectively). Although these values are more than 2

orders of magnitude larger than those for PBI and PBI-3, they

6

−1

Table 4. SOCMEs (cm⁻¹units) for the State Pairs (S , T ) and (S , T ) Evaluated at the Optimized Geometry of the Triplet

1

2

a

State

S₁−T₁

S₁−T₂

Z

X

Y

Z

X

Y

PBI

0.0000

−0.0977

−0.4178

−0.0001

0.0000

0.0439

−0.0545

0.0003

0.0000

−0.0280

−0.0462

0.0000

−0.0303

0.1239

0.0015

0.0303

0.0008

0.0000

0.0004

−0.0103

0.0822

PBI-1

PBI-2

PBI-3

0.0004

−0.0260

a

All values are purely imaginary numbers.

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Figure 11. (a) UV−vis absorption spectra of PBI-1 in the presence of TEOA in deaerated ACN upon photoirradiation; (b) reversibility of the

formation of PBI⁻radical anion upon photoirradiation and re-exposure to air (oxidize the radical anion to the neutral form); (c) photographs

showing the reversible color change of the PBI-1/TEOA mixed solution upon sequential photoirradiation and re-exposure to air. c [PBI-1] = 1.0 ×

•

−

5

−4

2

10

M, c [TEOA] = 1.0 × 10 M. The white light source is an un-ﬁltered xenon lamp; the power density is 20 mW/cm . 20 °C.

can only account for a triplet yield of ca. 0.4% given that the

singlet lifetimes are 8.1 and 3.7 ns (Table 1), respectively.

Hence, we need to consider the contribution of the S −T

intensity within 80 s. On the basis of theoretical computations

and comparison with previous reports, we assigned these NIR

absorption bands to the absorption of PBI-1 radical anion

1

2

−

•

coupling (Figure 10). All three molecules have a non-zero

(PBI-1) , that is D → D transitions of the PBI-1 radical

0 n

−

1

−•

SOCME for the S −T coupling: 0.04 cm for PBI-1, 0.14

anion (Figures S32 and S33 and Table S2). The (PBI-1)

absorption is quite diﬀerent from radical anion absorption of

the native PBI chromophore.

1

2

−

1

−1

cm for PBI-2, and 0.03 cm for PBI-3. We ﬁnd the

5

7,95

following values for the adiabatic energy gap and the rate

Previously, carbazole-

−

1

6 −1

constant: ΔE = 1899 cm and k_I_S_C= 2.20 × 10 s for PBI-1,

perylenebisimide (PBI-Cz) donor/acceptor dyads were studied

−

1

5

−1

−•

ΔE = 1298 cm and k = 7.78 × 10 s for PBI-2, and ΔE

for the generation of PBI by a photochemical method. The

ISC

−

1

7

−1

−•

=

1054 cm and k_I_S_C= 3.93 × 10 s for PBI-3. These values

PBI absorption in those dyads appeared at 698, 794, and 953

would account for triplet yields in the range 2−20% for the

observed singlet lifetimes. Although these rates depend on the

energy gap, this dependence is not simply exponential, as can

be seen in Figure 10 (open symbols). Since these calculations

are based on a harmonic approximation of the vibrations, the

results contain some uncertainty. Nevertheless, they are in the

right range to explain the observed triplet yields of PBI-1, PBI-

nm, respectively, which is diﬀerent from the current twisted

57

PBI-1.

After the photoreduction, the PBI-1 solution was exposed

−

•

−

•

to air, the PBI-1 absorption bands centered at 676, 758, and

896 nm decreased, and the ground-state PBI-1 absorption

band at 487 nm was fully recovered. These results indicated

−

•

the oxidation of radical anion PBI-1 by the O in air. The

2

, and PBI-3 and the vanishing triplet yield of PBI. Thus, the

orange color of PBI-1 changes to light blue upon radical anion

formation and the original color of the compound gets restored

upon exposure of the photoirradiated solution to air (Figure

11c). The radical formation and re-oxidation to its neutral

precursor is reversible until more than ﬁve cycles without any

signiﬁcant decomposition, which indicates the excellent

S → T → T route is possible (note that it is diﬃcult to

1

2

1

observe the T state with femtosecond transient absorption

spectra).

2

3

.7. Photochemical Studies: Reversible Photoproduc-

tion of Radical Anion in the Presence of Sacriﬁcial

Electron Donor TEOA. PBIs are an excellent class of organic

dyes which are of particular interest in photocatalysis,

photoconduction, and n-type transistor fabrication due to

feasible reduction into their radical anion. Previously, the PBI

radical anion was prepared by electrochemical reduction,

or in the presence of a chemical reductant such as tetrabutyl

ammonium cyanide or dithionite (Na S O ),

−

•

reversibility of persistent PBI-1 radical anion formation

(Figures 11 b and S30). Similar results were observed for PBI-2

(Figure S28 ). It is also worthy to note that the twisted

derivatives can act as an excellent candidate for dye-sensitized

solar cells (DSSCs), as the oxidation potential of the PBI-2

87

88

1

89

90,91

+

radical anion (−1.24 V vs. Fc/Fc ) is more negative compared

92−94

+

96

and by

to the conduction band of TiO (−1.19 V vs. Fc/Fc ).

2

4

2

44,57

photochemical method.

However, formation of a radical

The photophysical processes of PBI-2 are summarized in

Scheme 3. Upon photoexcitation, its singlet excited state is

populated, and then the CT state is formed, which is supported

by the solvent polarity-dependent ﬂuorescence emission

wavelength. Then, from the ICT state, the ISC occurs based

on theoretical computation, which indicates the energy

matching of S −T states in PBI-2, which assists direct

anion of twisted PBI derivatives was not reported. Herein, we

demonstrated the reversible radical anion formation of twisted,

fused PBI derivatives by photochemical method, which is in

the presence of sacriﬁcial electron donor upon photoirradiation

and transformation to neutral compounds again upon exposure

to air (Figure 11). PBI-1 was easily reduced to its radical anion

1

2

−

•

(

PBI ) in the presence of sacriﬁcial electron donor triethanol-

transition of S state to the triplet state. However, advance

1

amine (TEOA) even by irradiating with a simple torch light

within few seconds (Figures 11, S28, and S29).

The evolution of the UV−vis absorption changes of the

compound (PBI-1) upon photoirradiation in the presence of

typical sacriﬁcial electron donor TEOA was monitored (Figure

theoretical computations and femtosecond transient absorp-

tion studies support that the major ISC channel is SOC

between the S and T states caused by twisted geometry. The

other competing pathway which PBI-2 follows is the radiative

relaxation of singlet excited state through the ICT process.

From the ICT state, it decays back to the ground state by

emitting radiation or contribute in triplet population. A similar

photophysical process was observed in PBI-1. While in PBI-3,

which has a planar geometry, the singlet excited state directly

1

1a). Upon photoirradiation, the initial absorption bands

centered at 487 nm decreased with concomitant appearance of

new absorption bands centered at 676, 758, and 896 nm, and

these new NIR bands gradually rise and attain maximum

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Scheme 3. Jablonski Diagram Demonstrating the

Photophysical Processes Involved in PBI-2 upon

Photoexcitation

Furthermore, the ZFS parameter |D| was determined as 47.1

mT, which is similar to the native PBI chromophore showing |

D| = 46.5 mT, indicating that the spin density of the triplet

excited state of the twisted PBI derivatives are mainly

distributed on the PBI core and spreading to the fused anthryl

or the carbazole moiety is not signiﬁcant; otherwise, the D

value should be smaller. This postulation is supported by

theoretical computation on the triplet-state spin density.

Theoretical computation of the FC density of states of the

coupling S −T (n = 1,2) states, based on excited-state

a

1

n

dynamics (ESD) methods, indicates that the twisting of the π-

conjugation framework indeed increases the SOCMEs

magnitudes of S −T and S −T states (SOCME: 0.11−0.44

1

2

−

1

cm ), as well as the Herzberg−Teller vibronic coupling, in

comparison the planar native PBI compounds shows negligible

SOCME values (at least <0.01 cm ). For the planar

benzoPBI, the computation indicates that the moderate

triplet-state yield is due to S → T ISC (SOCME: 0.03

−

1

a

The energy levels of the excited state are derived from the

spectroscopic data. CT energy levels in diﬀerent solvents are derived

from the CT emission. The triplet energy levels are estimated by the

TD-DFT method. The numbers in superscripts indicates the spin

multiplicity.

1

2

−

1

−1

cm ) and the small S −T energy gap (1054 cm ). Upon

1

2

photoexcitation in the presence of sacriﬁcial electron donor,

the twisted derivatives exhibited eﬃcient and reversible

conversion into the near-infrared-absorbing PBI radical

anion, which demonstrate their potential application in

photocatalysis and photovoltaics. These results are useful for

designing of new heavy atom-free triplet PSs showing twisted

π-conjugation-induced ISC.

relaxed back to the ground state by radiative decay, which is in

accordance with its high ﬂuorescence quantum yield and low

triplet yield (Scheme S2).

It should be pointed out that the states S and T have a

1

2

similar orbital constitution (i.e., CT characteristic); hence, the

possibility of a S → T ISC must be considered. According to

1

2

the calculations, the energy gap is negative for PBI and less

ASSOCIATED CONTENT

−

1

■

than 2000 cm for the other compounds. When solvent eﬀects

for DCM are included in the energy calculations, these gaps

reduce to 1448, 497, and 534 cm⁻for PBI-1, PBI-2, and PBI-

*

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.jpcb.1c05032.

1

3

, respectively. For toluene as a solvent, the gaps become even

−1

smaller, namely, 1410, 379, and 312 cm . These values are

smaller than the typical accuracy of TD-DFT calculations. The

fact that no singlet oxygen production was observed with PBI-

1

General experimental methods; H NMR, 2D NMR, and

HRMS spectra of the compounds; theoretical compu-

tation data; and photophysical data (PDF)

3

in toluene or dichloromethane can be explained if we assume

that the S → T path is energetically not accessible in these

1

2

solvents for PBI-3.

AUTHOR INFORMATION

■

4

. CONCLUSIONS

Corresponding Authors

In summary, we studied the detailed photophysical property of

anthryl- and carbazole-fused PBI derivatives (PBI-1 and PBI-

Jianzhang Zhao − State Key Laboratory of Fine Chemicals,

School of Chemical Engineering, Dalian University of

Technology, Dalian 116024, P. R. China; orcid.org/

0000-0002-5405-6398 ; Email: zhaojzh@dlut.edu.cn

Ahmet Karatay − Department of Engineering Physics, Faculty

of Engineering, Ankara University, Ankara 06100, Turkey;

Email: Ahmet.Karatay@eng.ankara.edu.tr

2

, respectively) showing twisted π-conjugation frameworks.

Through the nuclear magnetic resonance (1D and 2D COSY

HNMR) study, we conﬁrmed the nonplanar structure of the

carbazole-fused PBI (PBI-2), which is diﬀerent from that of

the previously reported planar analogue (fusing position is

diﬀerent). Eﬃcient ISC (singlet oxygen quantum yield Φ =

Bernhard Dick − Lehrstuhl fu

̈

r Physikalische Chemie, Institut

Δ

8

5%) and long-lived triplet excited state (τ = 382 μs) were

fur Physikalische und Theoretische Chemie, Universit ä t

̈

T

observed for the twisted PBI derivatives, whereas the planar

Regensburg, Regensburg 93053, Germany; orcid.org/

0000-0002-9693-5243 ; Email: Bernhard.Dick@

chemie.uni-regensburg.de

analogue benzo-PBI (PBI-3) exhibits poor ISC ability (Φ =

Δ

1

5%). The triplet-state lifetimes are much longer than that

observed with the previously reported twisted PBI derivatives

0.1−67 μs). Femtosecond transient absorption spectra show

Violeta K. Voronkova − Zavoisky Physical-Technical Institute,

FRC Kazan Scientiﬁc Center of Russian Academy of Sciences,

Kazan 420029, Russia; Email: voi@kfti.knc.ru

(

that the ISC takes 3.7−8.2 ns in the current twisted PBI

derivatives, which is slower than the ISC of the previously

reported twisted PBI derivatives. The pulsed laser excited time-

resolved electron paramagnetic resonance (TREPR) spectros-

copy indicates that the electron spin polarization (ESP) phase

pattern of the triplet state is (e, a, e, a, e, a), which is diﬀerent

from the native PBI triplet state accessed with the normal spin

orbital coupling eﬀect, which shows the ESP phase pattern of

Authors

Zafar Mahmood − State Key Laboratory of Fine Chemicals,

School of Chemical Engineering, Dalian University of

Technology, Dalian 116024, P. R. China

Andrey A. Sukhanov − Zavoisky Physical-Technical Institute,

FRC Kazan Scientiﬁc Center of Russian Academy of Sciences,

Kazan 420029, Russia

(

e, e, e, a, a, a) in the triplet-state TREPR spectrum.

9

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The Journal of Physical Chemistry B

pubs.acs.org/JPCB

Article

Noreen Rehmat − State Key Laboratory of Fine Chemicals,

School of Chemical Engineering, Dalian University of

Technology, Dalian 116024, P. R. China

Mengyu Hu − State Key Laboratory of Fine Chemicals, School

of Chemical Engineering, Dalian University of Technology,

Dalian 116024, P. R. China

Ayhan Elmali − Department of Engineering Physics, Faculty of

Engineering, Ankara University, Ankara 06100, Turkey

Yi Xiao − State Key Laboratory of Fine Chemicals, School of

Chemical Engineering, Dalian University of Technology,

Dalian 116024, P. R. China

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Complete contact information is available at:

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(14) Lu, X.; Guo, Z.; Feng, M.; Zhu, W. Sensing Performance

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Author Contributions

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⊥

Z.M., A.A.S., and N.R. contributed equally to this work.

(

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The authors declare no competing ﬁnancial interest.

(16) Chen, X.-X.; Wu, X.; Zhang, P.; Zhang, M.; Song, B.-N.;

ACKNOWLEDGMENTS

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Huang, Y.-J.; Li, Z.; Jiang, Y.-B. Multicomponent Covalent Dye

Assembly for Tight Binding and Sensitive Sensing of l-DOPA. Chem.

Commun. 2015, 51, 13630−13633.

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J.Z. thanks the NSFC (U2001222, 21673031, 21761142005,

and 21911530095), the Fundamental Research Funds for the

Central Universities (DUT19TD28), and the State Key

Laboratory of Fine Chemicals for ﬁnancial support. A.S. and

V.V. acknowledge the ﬁnancial support from the government

assignment for FRC Kazan Scientiﬁc Center of RAS. B.D.

thanks the Dalian University of Technology for support

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Intersystem Crossing and Triplet-State Property of Anthryl- And Carbazole-[1,12]fused Perylenebisimide Derivatives with a Twisted π-Conjugation Framework

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