Chromophore Orientation-Dependent Photophysical Properties of Pyrene-Naphthalimide Compact Electron Donor-Acceptor Dyads: Electron Transfer and Intersystem Crossing

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Chromophore Orientation-Dependent Photophysical Properties of

Pyrene−Naphthalimide Compact Electron Donor−Acceptor Dyads:

Electron Transfer and Intersystem Crossing

Muhammad Imran, Andrey A. Sukhanov, Partha Maity, Ayhan Elmali, Jianzhang Zhao,*

Ahmet Karatay,* Omar F. Mohammed,* and Violeta K. Voronkova*

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sı Supporting Information

ABSTRACT: In order to study the eﬀect of mutual orientation of the

chromophores in compact electron donor−acceptor dyads on the spin-orbit

charge transfer intersystem crossing (SOCT-ISC), we prepared naphthalimide

(NI)−pyrene (Py) compact electron donor−acceptor dyads, in which pyrene

acts as an electron donor and NI is an electron acceptor. The connection of the

two units is at the 4-C and 3-C positions of the NI unit and the 1-position of the

pyrene moiety for dyads NI-Py-1 and NI-Py-2, respectively. A charge transfer

absorption band was observed for both dyads in the UV−vis absorption spectra.

Upon nanosecond pulsed laser excitation, long-lived triplet states (lifetime is 220

μs) were observed and the triplet state was conﬁned to the pyrene moiety. The

ISC eﬃciency is moderate to high in nonpolar to polar solvents (singlet oxygen

quantum yield: Φ = 14−52%). Ultrafast charge separation (ca. 0.81 ps) and charge recombination-induced ISC (∼3.0 ns) were

Δ

observed by femtosecond transient absorption spectroscopy. Time-resolved electron paramagnetic resonance spectroscopy conﬁrms

the SOCT-ISC mechanism; interestingly, the observed electron spin polarization pattern of the triplet state is chromophore

orientation-dependent; and the population rates of the triplet sublevels of NI-Py-1 (P :P :P = 0.2:0.8:0) are drastically diﬀerent

x

y

z

from those of NI-Py-2 (P :P :P = 0:0:1).

x

y

z

1

. INTRODUCTION

disadvantages, for instance, the complicated synthesis or non-

eﬃcient ISC. Therefore, a simple method is highly desired in

the design of heavy atom-free triplet PSs.

To populate the triplet excited state of a chromophore is

crucial in photochemistry, such as energy transfer and electron

transfer processes and has attracted much attention in recent

years. Strong absorption of light, high intersystem crossing

One promising approach to populate the triplet state

without any heavy atom is the CR-induced ISC in electron

2

1,24−26

donor−acceptor systems.

Upon photoexcitation, charge

(

ISC) ability, and long-lived triplet excited states are the

separation (CS) occurs in electron donor−acceptor dyads and

forms the charge transfer (CT) state, which can produce the

triplet state ( LE, LE: locally excited) via the CR process or

desired properties of eﬃcient triplet photosensitizers (PSs).

Triplet PSs are important for various applications such as

photovoltaics, luminescence bioimaging, artiﬁcial photosyn-

thesis, photodynamic therapy,

lation photon up-conversion.

organic compounds is populated by ISC. As the ISC is an

electron spin forbidden process, diﬀerent methods have been

used to enhance the ISC ability in aromatic compounds. The

most straightforward strategy is the utilization of heavy atoms

1

,2

3

4

,5

6−9

and triplet-triplet annihi-

The triplet excited state of

may decay to the ground state without ISC. Usually, the

separation of the electron donor and acceptor in the

conventional dyads is large, the electronic exchange energy is

1

0−13

−

1

extremely small (J < 0.01 cm ), and the purpose for

3

preparation of those dyads is not to obtain a LE state, the

3

long-lived CT state is usually populated by a radical-pair ISC

(

Br, I, Pt, Ir, etc.) based on the heavy atom eﬀect-enhanced

4,5,21

(

RP-ISC) mechanism.

However, the large separation of

14−16

spin-orbit intersystem crossing (SO-ISC) mechanism.

the electron donor and acceptor makes the synthesis of the

However, the presence of these atoms can shorten the triplet

state lifetime and increase the cost of the material. Some other

approaches have also been used to enhance the ISC in heavy-

atom free organic compounds, for instance, the S → T (n ≥

Received: April 20, 2021

Revised: July 25, 2021

n

m

11,14

1

; m ≥ 1) states energy matching,

n−π*↔ π−π* transition

1

7

18

(

EI-Sayed rule), exciton coupling, electron spin con-

19,20

vertor

ISC,

and charge recombination (CR)-induced

etc. However, all these methods exhibit some

2

1−23

©

XXXX American Chemical Society

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J. Phys. Chem. B XXXX, XXX, XXX−XXX

A

The Journal of Physical Chemistry B

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a

Scheme 1. Synthesis of the Compounds

a

Key: (a) n-butylamine, EtOH, reﬂux at 78 °C for 5 h, under N , yield: 62%; (b) pyrene-1-boronic acid, Pd(PPh ) Cl , THF/H O, K CO , at 80

2

3

2

3

°

C for 24 h, under N , yield: 46%; (c) n-butylamine, EtOH, reﬂux at 80 °C for 6 h, under N , yield: 60%; (d) similar to step (b), yield: 30%; (e)

2 2

phenyl boronic acid, K CO , Pd(PPh ) , EtoH/tol/H O (2:2:1 v/v/v), stirred at 70 °C for 9 h, under N , yield: 56%; (f) and (g) similar to step

2

3

4

2

(

e), yields: 50 and 42%, respectively.

3

0,36,38,39

compounds diﬃcult. Moreover, the RP-ISC is slow (a few ns),

electron donors/acceptors.

Moreover, the eﬀect of the

1

3

which makes the process CT → CT → LE non-eﬃcient. On

the other hand, shortening the linker will increase the

electronic coupling between the electron donor and acceptor,

which leads to large electron exchange energy, and the RP-ISC

is inhibited. Therefore, electron donor−acceptor dyads with

more simple molecular structures are desired to attain eﬃcient

ISC.

mutual orientation of electron donor and acceptor chromo-

phores on the SOCT-ISC eﬃciency and the electron spin

selectivity was rarely studied.

Recently, we demonstrated that the perylene−naphthalimide

compact dyad showed excellent ISC eﬃciency (Φ = 80%)

Δ

and the ISC mechanism was conﬁrmed by TREPR spectros-

40

copy. We also found that the dyads with perylene as an

36

electron acceptor also show eﬃcient SOCT-ISC. Previously,

electron donor−acceptor dyads with bodipy as an electron

acceptor and pyrene as an electron donor were reported to

Recently, CR-induced ISC was reported for the compact

donor−acceptor dyads, in which electron donor and acceptor

units are connected by a short linker and the two units adopt

orthogonal geometry, and the mechanism is called spin-orbit

show moderate SOCT-ISC (Φ = 25% in EtOH; Φ = 1% in

Δ

41−43

24,27−30

HEX).

Inspired by these approaches, herein we designed

charge transfer intersystem crossing (SOCT-ISC).

In

new compact donor−acceptor dyads by using a pyrene (Py)

moiety as an electron donor while naphthalimide (NI) acts as

an electron acceptor, intending to tune the excited state

properties, especially the electron spin selectivity of the ISC

this mechanism, the CR is accompanied by a molecular orbital

angular momentum change, which compensates the electron

spin angular momentum change of ISC. As a result, the CR-

induced ISC is enhanced in these compact orthogonal electron

donor−acceptor dyads. Therefore, it is of particular interest for

the preparation of heavy atom-free organic triplet photo-

sensitizers with simpliﬁed molecular structures compared with

(

Scheme 1). The conformation of the dyads is controlled by

the hindered rotation around the linker (governed by steric

hindrance) with diﬀerent substitutional positions (3-C or 4-C

positions of the NI moiety). The rotation around the linker

can inﬂuence the electronic coupling or the kinetics of CS/CR

and the ISC eﬃciency. We studied the photophysical

properties of the dyads by using the steady-state and

femtosecond/nanosecond time-resolved transient absorption

spectroscopic methods as well as TREPR spectroscopy.

22,27,31−34

other heavy atom-free triplet photosensitizers.

Until now, the reports related to SOCT-ISC are restricted to

2

7,35

24

the chromophores of anthracene,

bodipy, naphthalene-

2

8

36

diimide (NDI), and perylene. Concerning the pyrene

chromophore, it is rarely used for the preparation of dyads

showing SOCT-ISC. However, to fully understand the SOCT-

ISC eﬃciency as well as the electron spin selectivity of the ISC

of these dyads, more examples are required. For instance, the

spin dynamics of some systems are studied by time-resolved

electron paramagnetic resonance (TREPR) spectrosco-

2

. EXPERIMENTAL SECTION

2

.1. General Procedures. All the chemicals and organic

solvents used in the synthesis are analytically pure and were

used as received. The solvents used in the anhydrous reactions

were dried using molecular sieves. The UV/visible absorption

spectra were taken on a UV-2550 dual-beam ultraviolet−visible

spectrophotometer (Shimadzu Ltd., Japan). The ﬂuorescence

emission spectra were recorded on an RF-5301 PC

spectroﬂuorometer (Shimadzu Ltd., Japan). Fluorescence

lifetimes were measured on an OB920 luminescence lifetime

3

5,37,38

py.

ESP) pattern of the TREPR spectra of the triplet state

accessed by SOCT-ISC is always diﬀerent than the conven-

It was proposed that the electron spin polarization

(

27,31,35

tional SO-ISC mechanism.

This is true but only for few

cases: we found that the ESP pattern of the triplet TREPR

spectra can be the same for both the mechanisms (SOCT-ISC

and SO-ISC) and it is also dependent on the structures of

B

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spectrometer (with TCSPC detection mode, Edinburgh

Instruments Ltd., U.K.); the instrument response function

the quartz plate, with a similar method to that mentioned

above. All the ﬁlms were prepared before 24−36 h of

measurements.

(

IRF) is 100 ps. For all the compounds, ﬂuorescence lifetimes

were measured with a 340 nm EPL picosecond pulsed laser

pulse width: 800 ps, maximum average power: 40 μW). An

2.5. Electrochemical and Spectroelectrochemical

Studies. The electrochemical measurements were carried

out with a CHI610D electrochemical workstation (CHI

instruments, Inc., Shanghai, China). In nitrogen-purged

saturated DCM solution containing 0.10 M tetrabutylammo-

nium hexaﬂuorophosphate (Bu N[PF ]) as a supporting

(

Optistate DN cryostat (Oxford Instruments, U.K.) was used

for low-temperature ﬂuorescence spectra measurements.

2

.2. Synthesis of NI-Py-1. Under a N₂atmosphere,

pyrene-1-boronic acid (139.4 mg, 0.56 mmol) and 4-bromo-

,8-naphthalic anhydride (143.9 mg, 0.43 mmol) were mixed

in THF/H O (10 mL/1 mL). K CO (596.6 mg, 4.3 mmol)

4

6

1

electrolyte, the counter electrode was a platinum electrode, a

glassy carbon electrode was the working electrode, and the Ag/

2

3

was added, and the reaction mixture was bubbled with N for

AgNO (0.1 M in ACN) couple was the reference electrode. A

ferrocenium/ferrocene (Fc /Fc) redox couple was used as an

2

3

+

2

0 min. The catalyst Pd(PPh ) Cl (10 mg, 0.013 mmol) was

3

2

added, and the mixture was stirred at 80 °C for 24 h. After

cooling down, the product was extracted with DCM (3 × 20

mL), washed with water, and dried over anhydrous MgSO₄.

The solvent was removed under reduced pressure, and the

crude product was puriﬁed by column chromatography (silica

internal reference.

Spectroelectrochemistry measurements were performed

using a 0.1 cm path length quartz electrochemical cell. The

cell was equipped with a platinum gauze as the working

electrode, a platinum wire as the counter electrode, and Ag/

AgNO as the reference electrode. Bu N[PF ] was used as the

gel, DCM/PE = 1:3, v/v) to give a light yellowish solid (90

3

4

6

1

mg, yield: 46%). Mp: 194−196 °C. H NMR (CDCl , 400

supporting electrolyte. The potential was regulated with the

CHI610D electrochemical workstation (CHI instruments, Inc.,

Shanghai, China), and the spectra were recorded using an

Agilent 8453E UV−vis spectrophotometer (Agilent Technol-

ogies Inc., U.S.A.). Samples were deaerated with nitrogen for 5

min before measurement, and the nitrogen atmosphere was

maintained during the measurements.

3

MHz): δ 8.82 (d, 1H, J = 8.0 Hz), 8.67 (d, 1H, J = 8.0 Hz),

8

.36 (d, 1H, J = 8.0 Hz), 8.31 (d, 1H, J = 8.0 Hz), 8.22 (d, 3H,

J = 8.0 Hz), 8.10−8.01 (m, 2H), 7.98−7.92 (m, 2H), 7.81 (d,

1

H, J = 8.0 Hz), 7.60−7.54 (m, 2H), 4.30 (t, 2H, J = 8.0 Hz),

.87−1.79 (m, 2H), 1.59−1.49 (m, 2H), 1.06 (t, 3H, J = 8.0

1

3

Hz). C NMR (CDCl , 125 MHz): δ 164.4, 164.2, 145.9,

3

1

33.4, 133.0, 131.5, 131.4, 131.3, 130.8, 130.7, 129.6, 129.4,

28.5, 128.2, 127.9, 127.3, 127.0, 126.4, 125.8, 125.5, 124.8,

2.6. Femtosecond Transient Absorption Spectrosco-

py. The femtosecond experiments were performed using a

Ti:sapphire laser system with ∼100 fs pulse duration and 1

kHz repetition rate (Spectra Physics, Spitﬁre Pro XP) and a

commercial ultrafast transient absorption spectrometer (Ultra-

fast System, Helios). The excitation wavelength was

determined from the steady-state UV−vis absorption spectra

and generated in an optical parametric ampliﬁer (TOPAS,

Light Conversion, U.S.A.). The magic angle between the probe

and the pump beam polarization direction was used. The

surface Xplorer and Glotaran software were used for processing

24.7, 124.6, 124.5, 123.0, 122.4, 40.3, 30.3, 20.5, 13.9. TOF

+

MS EI: Calcd ([C H N] ), m/z = 453.1729; found, m/z =

3

8

23

4

53.1718.

.3. Synthesis of NI-Py-2. Compound NI-Py-2 was

2

synthesized with a method similar to that of NI-Py-1. The

product was obtained as yellow crystals (60 mg, yield: 30%).

Mp: 132−134 °C. H NMR (CDCl , 400 MHz): δ 8.94 (s,

1

3

1

8

H), 8.70 (d, 1H, J = 8.0 Hz), 8.46 (s, 1H), 8.33 (d, 2H, J =

.0 Hz), 8.29 (d, 1H, J = 8.0 Hz), 8.24 (d, 1H, J = 8.0 Hz),

.18 (s, 2H), 8.14−8.06 (m, 4H), 7.86 (d, 1H, J = 8.0 Hz),

.27 (t, 2H, J = 8.0 Hz), 1.84−1.76 (m, 2H), 1.56−1.49 (m,

44

the experimental data after chirp correction.

4

2.7. Nanosecond Transient Absorption Spectroscopy.

The nanosecond transient absorption spectra were studied on

a LP980 laser ﬂash-photolysis spectrometer (Edinburgh

Instruments Ltd., U.K.). The samples were purged with N₂

for 15 min before measurement and excited with a nanosecond

pulsed laser (Opolette 355II + UV nanosecond pulsed laser,

the wavelength is tunable in a range of 200−2200 nm.

OPOTEK, U.S.A.). Typical laser power is 5 mJ per pulse. The

signal was digitized with a Tektronix TDS 3012B oscilloscope,

and the data was analyzed with L900 software.

13

2

H), 1.03 (t, 3H, J = 8.0 Hz). C NMR (CDCl , 125 MHz): δ

3

1

64.2, 140.3, 135.4, 134.9, 133.9, 133.7, 131.9, 131.5, 131.3,

31.1, 130.9, 128.6, 128.3, 128.0, 127.7, 127.4, 127.3, 126.3,

25.6, 125.2, 125.0, 124.8, 124.4, 122.9, 122.8, 40.3, 30.3, 20.4,

+

1

3.9. TOF MS EI: Calcd ([C H N] ), m/z = 453.1729;

36

27

found, m/z = 453.1720.

2

.4. Polymer Film Preparation Method. The commer-

cially available polyurethane precursors Clear Flex 50 (part A

contains 4,4′-methylenedicyclohexyl diisocyanate and part B

contains phenylmercury neodecanoate) and polymethyl

methacrylate (PMMA), which are transparent at room

temperature, were used to prepare the doped thin ﬁlms of

compounds. For Clear Flex 50 ﬁlm preparation, part A and

part B were mixed by weight (1:2 ratio) and stirred to a clear

sticky mixture. To 150 μL of Clear Flex 50 and 20 μL of DCM

2.8. Time-Resolved Electron Paramagnetic Reso-

nance Spectroscopy. TREPR measurements were carried

out at 80 K. The samples were excited at 355 nm (10 mJ pulse,

7 ns, 10 Hz) with a 400−900 ns time window. The

measurements were done in frozen solution (ca. 10⁻M)

and loaded in a quartz tube (1.6 mm o.d. × 1.1 mm i.d). A

Bruker Elexsys E580 EPR spectrometer with a cylindrical

resonator (EN 5107D2) at X-band (9.8 GHz) frequencies was

used. The TR CW EPR spectra were obtained by the

summation of the data in diﬀerent time windows after the

laser pulse. The EPR spectra were simulated using the

EasySpin package implemented in the MATLAB programming

3

−

3

was added the respective compound (20 μL, c = 2 × 10 M in

DCM) and mixed adequately. The mixed solution (100 μL)

was added dropwise onto the quartz plate (diameter is 17 mm,

thickness is 1 mm), allowing the solvent to evaporate and the

resulting ﬁlm to dry for 24 h. For PMMA ﬁlm preparation,

PMMA (50 mg) was dissolved in THF (2 mL). Then, the

respective compound (NI-Py-1 or NI-Py-2, etc. 20 μL, c = 2 ×

45

language.

−

3

1

0

M) was added into PMMA solution (45 μL) and mixed

2.9. DFT Calculations. The geometries of the compounds

well. A total of 60 μL of miscible liquids was taken to cast on

were optimized using density functional theory with the

C

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B3LYP functional and 6-31G(d) basis set. There are no

imaginary frequencies for all the optimized structures. The

Interestingly, for NI-Py-2, a broader LE absorption band

was observed, which is diﬀerent from NI-Py-1. For both the

energy gaps between the S state and the triplet excited states

dyads at the red-end of the absorption spectra, a well-separated

0

1

of the compounds were approximated based on the optimal

ground state geometry. All these calculations were performed

CT absorption band (S → CT) in a range of 370−450 nm

0

was observed (Figure 1a). This indicates that in both dyads,

there is strong electronic interaction between the NI and the

Py moieties at the ground state. For NI-3Ph and NI-4Ph, as a

comparison with NI and NI-4Br, the absorption bands are also

red-shifted to 338 and 347 nm, respectively (Figure 1b). This

shows that the electronic coupling exists for NI-3Ph and NI-

4Ph but weaker than that between NI and pyrene moieties in

NI-Py-1 and NI-Py-2 dyads. Moreover, the weak absorption of

the NI-Ph compounds may be attributed to the small transition

dipole moments. From the charge-transfer absorption band in

NI-Py-1 and NI-Py-2 dyads, the electronic coupling matrix

element (V ) between the S and Frank Condon S state was

46

with Gaussian 09W software.

. RESULTS AND DISCUSSION

.1. Design Rationales of the NI−Pyrene Compact

3

Electron Donor−Acceptor Dyads. To study the SOCT-ISC

of compact electron donor−acceptor dyads, we build a

bichromophoric system in which NI is used as an electron

acceptor (Scheme 1). NI has extensively been used as a

47−49

ﬂuorophore due to its robust photophysical properties.

On the other hand, the pyrene moiety is used as an electron

4

1,42

donor.

donor−acceptor dyads with diﬀerent substitutional positions

Scheme 1). Previously, NI-Py-1 was reported with diﬀerent

We synthesized NI-Py-1 and NI-Py-2 compact

DA

0

1

calculated by using eq 1

(

−

2

alkyl chains, and basic photophysical properties were studied,

i

y

z

j 2.06 × 10

−

1

CT

−CT

−

CT 1/2

j

z

V (cm ) = j

z(ε

ν

Δν

)

i.e., UV−vis absorption and ﬂuorescence properties, but the

DA

j

z

max max

1/2

50

R

(1)

k

{

detailed ISC properties were not studied.

For NI-Py-1, the attachment of pyrenyl is made at the 4-C

position of the NI moiety, and the geometry is expected to be

orthogonal due to the steric hindrance. While for NI-Py-2, the

connection is at the 3-C position of the NI unit, which gives a

large degree of freedom for rotation due to the smaller steric

hindrance imposed by the hydrogen atoms of pyrene and the

NI moieties. The synthesis of the dyads was carried out with

the Suzuki−Miyaura cross-coupling reactions. Py-Ph, NI-3Ph,

and NI-4Ph were prepared as reference compounds (Scheme

where R is the separation between the center of an electron

donor and acceptor, in Å; ε

coeﬃcient at the maximum of the CT absorption band, in

CT

is the molar absorption

max

−

1

−1

−CT

[M

cm ]; ν

is the absorption maximum of the CT

max

−

1

−CT

absorption band in a wavenumber scale, in cm ; and Δν

the full width of the band at the half-maximum, in cm . The

distance between the center of the donor and acceptor is

is

1

/2

−

1

determined by DFT optimization. Herein, R

= 7.31 Å

NI‑Py‑1

and R

= 7.68 Å are used to calculate the V . The CT

NI‑Py‑2

DA

1

). The yields of the compounds were satisfactory, and the

1

13

absorption bands were isolated by simulation with origin

software.

molecular structures were fully characterized by H NMR, C

NMR, HRMS spectroscopic methods.

^T^h^e^VDA values of NI-Py-1 and NI-Py-2 were calculated as

^.¹⁷^aⁿ^d⁰^.⁰⁸⁷^e^V^,^r^e^s^p^e^c^tⁱ^v^e^l^y^.^T^h^e^VDA of NI-Py-2 is smaller

3

.2. UV−Vis Absorption and Fluorescence Emission

0

Studies of Compounds. The UV−vis absorption spectra of

the compounds are studied (Figure 1 ). The characteristic

than that of NI-Py-1 because the orientation of the acceptor

moiety to the 3-C position slightly increases the separation

between the center of the electron donor and acceptor in NI-

Py-2.

The ﬂuorescence properties of the compounds were studied

Figure 2). Pristine Py and NI show characteristic emission

(

proﬁles with ﬂuorescence maxima at 372 and 378 nm,

respectively, with a signiﬁcant vibrational progression.

Similarly, ﬂuorescence for Py-Ph was observed at 380 nm;

under the same experimental conditions, however, the intensity

of the emission band is higher than that of pristine pyrene

(

Figure 2a). The ﬂuorescence quantum yields of Py and Py-Ph

are 8 and 13%, respectively (Table 1). The ﬂuorescence of NI-

Ph and NI-4Ph was observed at 414 and 420 nm,

Figure 1. UV−vis absorption spectra of the compounds (a) Py, Py-

Ph, NI-Py-1, and NI-Py-2 and (b) NI-3Ph, NI-4Ph, NI-4Br, and NI.

c = 1.0 × 10 M, in dichloromethane. 20 °C.

3

−

5

respectively. The ﬂuorescence quantum yields were deter-

mined as 31 and 51%, respectively (Table 1).

For NI-Py-1, the locally excited (LE) state emissions of Py

and NI moieties are completely quenched and a broad,

structureless emission band centered at 538 nm was observed

(Figure 2a), which is assigned to CT emission. For NI-Py-2,

the LE ﬂuorescence is also signiﬁcantly quenched while a

minor emission band at 384 nm and a more intense broad

band at 550 nm were observed (Figure 2a). The emission band

at higher energy is assigned as the residual LE state emission of

the pyrene moiety, whereas the band at lower energy is

attributed to CT emission. In both dyads, the ﬂuorescence of

the NI moiety is also quenched. The most probable reason, for

the quenched ﬂuorescence, is the photo-induced charge

absorption bands centered at 336 and 334 nm were observed

for pristine pyrene and NI moieties, respectively, with

signiﬁcant vibrational progression. A slightly red-shifted

absorption band was observed for Py-Ph with maxima at

3

45 nm, and the vibrational progression was lost. This result

indicates strong electronic coupling between the phenyl and

pyrene moieties. For NI-Py-1, the absorption is signiﬁcantly

red-shifted as compared with both pyrene and NI, showing

strongly structured absorption bands, with the most intensive

one at 343 nm (Figure 1a), which is characteristic absorption

of the pyrene moiety.

D

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Figure 2. (a) Fluorescence emission spectra of the compounds (λ = 325 nm, A = 0.26). Optically matched solutions were used in DCM, c = ca.

ex

.0 × 10⁻M. Fluorescence emission spectra of compounds in diﬀerent solvents: (b) NI-Py-1 (λ = 325 nm) and (c) NI-Py-2 (λ_e_x= 325 nm).

ex

5

1

−

Optically matched solutions were used (c ≈ 1.0 × 10 M, A = 0.26). 20 °C.

a

Table 1. Photophysical Parameters of the Compounds

b

c

d

e

k_r[10 s⁻¹]

f

5

k_n_r[10 s⁻¹]

g

6

h

compounds

λ_a_b_s[nm]

ε

λ_F[nm]

Φ_F[%]

τ_F[ns]

τ_T[μs]

i

Py

Py-Ph

NI

NI-3Ph

NI-4Ph

NI-Py-1

336

345

334

338

347

343/400

4.56

372

380

378

414

420

538

8

7.9

7.8

0.24

3.4

3.5

1.50

3.09

6.25

9.11

14.16

8.18

2.35

2.07

41.0

2.02

1.36

1.45

146.2

202

79.2

358

332

215

i

l

3.06

1.72

0.99

1.67

13

1.5

31

51

j

l

i

j

l

5.08/0.76

78, 73,

4.4

k

3

6, 20

i

j

l

NI-Py-2

347/400

4.51/0.50

550

62, 50,

6.0

2.33

1.43

220

k

1

4, 7

a

In DCM (1.0 × 10⁻⁵M). Molar absorption coeﬃcient: 10 M cm . Fluorescence maxima. Absolute luminescence quantum yield measured

b

4

−1

c

d

e

by an optical integrated sphere. Py and Py-Ph (λ = 330 nm) and NI-3Ph, NI-4Ph, NI-Py-1, and NI-Py-2 (λ = 340 nm). Fluorescence lifetime.

ex

f

g

Note that the IRF of a ﬂuorescence lifetime spectrometer is ca. 100 ps. Radiative decay rate constant k = Φ/τ. Nonradiative decay rate constant

r

h

i

j

k

l

k = (1 − Φ)/τ. Triplet state lifetime monitored by nanosecond transient absorption spectroscopy. In n-hexane. In toluene. In ACN. Intrinsic

nr

38,54

triplet excited state lifetime obtained by ﬁtting the experimental curves with a TTA-based kinetic model.

1

transfer from the pyrene unit to the NI unit to form a CT

state.

The solvent polarity-dependent ﬂuorescence of the com-

pounds was studied (Figure 2b,c). For both the dyads, the

ﬂuorescence is highly dependent on solvent polarity, which is

red-shifted, and the emission intensity decreased from

nonpolar solvents to moderate polar solvents and further

quenched in polar solvents; these are the typical CT emission

features. This trend is in line with ﬂuorescence quantum yields

of the dyads in solvents with diﬀerent polarity (i.e., for NI-Py-

1

, Φ = 78% in n-hexane, 36% in DCM, and 20% in ACN).

F

Figure 3. Normalized ﬂuorescence excitation (marked as Ex.) and

−

A

Note that the emission bands of the two dyads in HEX are

broad and structureless and they are also CT features. Thus,

we conclude that the emissive singlet states of the dyads are in

substantial CT features. The weak emission of NI-Py-1 and

NI-Py-2 is attributed to the ISC from the CT state (see the

UV−vis absorption spectra (in 1 − 10 ) of (a) NI-Py-1 (λ = 540

em

−5

nm) and (b) NI-Py-2 (λem = 545 nm). In DCM. c = 1.0 × 10 M. 20

C.

°

1

the energy of the n−π* and π−π* states may be changed, and

thus the ﬂuorescence becomes stronger in polar solvents, in

later section).

52

To study the energy and electron transfer process in the

dyads, the ﬂuorescence excitation (monitored at the CT

emission band) and UV−vis absorption spectra are compared

which the π−π* state is slightly lower than the n−π*.

Similarly, both pyrene and Py-Ph have the ISC ability upon

photoexcitation. The solvent polarity eﬀect on ﬂuorescence

was studied (Figure S17 ), and similar results were observed.

Thus, nonradiative decays are more eﬃcient in nonpolar

solvents for these compounds.

(Figure 3). The excitation spectra in a range of 370−450 nm

are normalized with the UV−vis absorption, and the slightly

lower magnitude in the LE absorption region shows that other

nonradiative decay channel exists upon photoexcitation into

the LE absorption bands, such as ISC of NI or the pyrenyl

moiety.

The phosphorescence emission spectra of dyads at 77 K in

THF were studied (Figure S18). Upon addition of 20%

iodoethane, the dyads showed a weak phosphorescence peak in

the range from 510 to 650 nm. The phosphorescence probably

arises from the pyrene moiety. The phosphorescence lifetime

of NI-Py-2 is determined as 6.5 μs in the frozen THF matrix,

which is slightly shorter than the previously reported

It is known that NI has ISC ability due to the π−π* →

51

n−π* transition (S → T energy level matching). For the

1

4

reference compounds, NI-3Ph and NI-4Ph, the ﬂuorescence is

intensiﬁed with increasing the solvent polarity (Figure S17);

this implies that in solvents with diﬀerent polarity, the order of

17

phosphorescence lifetime of the pyrene moiety (∼9.4 μs).

E

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Figure 4. Fluorescence lifetime decay traces of the dyads: (a) NI-Py-1 at 480 nm (in toluene), at 538 nm (in DCM), and at 582 nm (in ACN), (b)

NI-Py-2 at 483 nm (in toluene), at 546 nm (in DCM), and at 592 nm (in ACN), (c) NI-Py-2 at the 384 nm LE emission band in diﬀerent

−

5

solvents. The solvents used in the measurements are indicated. Excited with a picosecond pulsed laser (340 nm). c = 1.0 × 10 M. 20 °C.

Figure 5. Temperature-dependent ﬂuorescence spectra of (a) NI-Py-1 and (b) NI-Py-2 in deaerated DCM. (c) Corresponding decay curves of NI-

−

5

Py-2 at the 384 nm LE band (inset: ﬂuorescence decay trace of the CT emission band at 550 nm). c = 1.0 × 10 M. λ_e_x= 340 nm.

Similar results were observed for the NI-Py-1 dyad. Moreover,

in a previous study, the phosphorescence spectrum of the NI-

Py analogue dyad was observed almost in the same spectral

appear at 550 nm (in DCM) and 590 nm (in ACN). The

ﬂuorescence lifetimes of pyrene, Py-Ph , pristine NI, and NI-Ph

derivatives are also measured (Figure S19).

5

0

region.

To further study the properties of CT emission features of

the dyads, the temperature-dependent ﬂuorescence spectra

were studied in DCM (Figure 5). For both dyads, the

ﬂuorescence intensity was decreased and red-shifted at lower

The ﬂuorescence lifetimes of the dyads were studied by

using a time-correlated single-photon counting (TCSPC)

detection technique (Figure 4). The luminescence lifetime of

NI-Py-1 monitored at the CT emission band in toluene is 3.8

ns (Figure 4a), and interestingly, the ﬂuorescence lifetimes are

5

3

temperature, which is anomalous. For NI-Py-2, with

decreasing the temperature from 297 to 197 K, the CT band

ﬂuorescence intensity is signiﬁcantly reduced, while the LE

band at 384 nm is almost the same (Figure 5b). Similarly, for

NI-Py-1, the CT emission band is reduced as temperature

decreases (Figure 5a) but with less extent as compared with

NI-Py-2. These results are anomalous but can be rationalized

4

.4 and 6.0 ns in DCM and ACN solvents, respectively (with

mono-exponential features). These extended ﬂuorescence

lifetimes in polar solvents are reasonable because the CT →

1

S₀transition is actually a forbidden transition (weakly

allowed), and thus the ﬂuorescence lifetime can become

longer.

by the twisted intramolecular charge transfer emissive S

state. With decreased temperature, the thermal energy also

1

Meanwhile, in the case of NI-Py-2 (Figure 4b), the

luminescence lifetimes slightly decreased with increasing the

solvent polarity, and the mono-exponential decay was observed

with lifetimes of 8.1, 6.0, and 5.0 ns in toluene, DCM, and

ACN, respectively. For NI-Py-2, the ﬂuorescence lifetime at

the LE emission band is also measured (Figure 4c), and an

extended ﬂuorescence lifetime was observed (τ_L_E= 10.9 ns, in

DCM) as compared with CT emission (τ_C_T= 6.0 ns, in

DCM). For NI-Py-2, the ﬂuorescence lifetime is slightly

52

decreased and the mutual orientation of the two moieties

adopted more orthogonal conformation (energy minimized).

Therefore, the intensity of the CT emission band is reduced

1

because the radiative transition of CT → S is forbidden. As

0

the geometry approaches orthogonal, the CT emission band

will be red-shifted due to the lower energy of the CT state

52

showing orthogonal geometry.

The eﬀect of temperature on ﬂ uorescence lifetimes was also

studied (Figure 5c and Figure S20). For NI-Py-2, at the LE

emission band with decreasing temperature, the ﬂuorescence

lifetime is slightly increased with a biexponential decay feature,

shortened in polar solvents, with the trend being opposite to

1

that of NI-Py-1. As the CT → S transition is weakly allowed,

0

thus the CT ﬂuorescence lifetime can become longer than the

LE emission. However, the longer emission wavelength (which

1

for instance, at 297 K (τ = 0.19 ns, 53% and τ = 13.8 ns,

can reduce the CT/S energy gap) makes the nonradiative

1

2

0

4

7%) to (τ = 0.68 ns, 11% and τ = 16.2 ns, 89%) at 197 K.

transition more signiﬁcant, which may shorten the ﬂuorescence

1

2

1

Interestingly, for the CT emission band, the ﬂuorescence

lifetime is also prolonged but with a mono-exponential decay

feature (6.4 ns, at 297 K) with lowering the temperature (9.1

ns, at 197 K). Similar results were observed for NI-Py-1 when

lifetime for the CT state. That could be the reason for

shortening of the ﬂuorescence lifetimes of NI-Py-2. These

results are in agreement with the red-shifting of CT emission

bands of the dyads. For instance, NI-Py-1 shows CT bands at

5

38 and 577 nm in DCM and ACN, respectively. Meanwhile,

the temperature decreased from 297 K (τ = 4.4 ns) to 157 K

F

in NI-Py-2, the CT emission bands are more red-shifted and

(τ = 9.0 ns) (Figure S20).

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The singlet oxygen quantum yields (Φ ) of the compounds

Δ

were studied (Table 2). For reference compounds NI-3Ph, NI-

Table 2. Singlet Oxygen Quantum Yields (Φ ) of the

Δ

a

Compounds in Diﬀerent Solvents

compounds HEX (31.0) TOL (33.9) DCM (40.7) ACN (45.6)

Py

30

49

33

13

65

72

15

25

17

26

34

46

40

15

38

36

40

36

14

20

26

38

45

34

30

25

53

64

32

36

35

38

32

46

22

21

54

64

52

47

Py-Ph

Py-Br

NI

NI-3Ph

NI-4Ph

NI-3Br

NI-4Br

NI-Py-1

NI-Py-2

b

c

a

Singlet oxygen quantum yield (Φ ) with anthracene as the standard

Δ

(Φ

= 0.70 in MeOH) in diﬀerent solvents. The values in the

Δ

−

1

parentheses are the E (30) values of the solvents, in kcal mol .

Figure 6. Cyclic voltammogram of (a) reference Py-Ph and NI-3Ph

and (b) dyads NI-Py-1 and NI-Py-2. Ferrocene (Fc) was used as an

internal reference (set as 0 V in the cyclic voltammograms). Scan

T

b

40

c

55

Literature values. Literature values.

−

3

rates: 50 mV/s. c = 1.0 × 10 M. UV−vis absorption spectral

changes of NI-Py-1 measured with the spectroelectrochemical

method: (c) upon oxidation under 1.17 V and (d) upon reduction

under −1.70 V. All the experiments were carried out in deaerated

DCM containing Bu N[PF ] (ca. 0.1 M) as supporting electrolytes

4

Ph, and Py-Ph, the singlet oxygen quantum yield is higher in

nonpolar solvents (Φ = 65, 72, and 49% in n-hexane,

Δ

respectively). However, the Φ values are decreased with

Δ

increasing the solvent polarity, while Py-Ph shows a relatively

solvent polarity-independent singlet oxygen production. In

polar solvents, for instance, in ACN, Py and NI show moderate

4

6

and Ag/AgNO as the reference electrode. 20 °C.

3

Table 3. Electrochemical Redox Potentials of the

Φ values of 35 and 46%, respectively. For NI-Py-1 and NI-

Δ

a

Compounds

Py-2 dyads, the Φ is dependent on solvent polarity, and in

Δ

nonpolar solvents, the Φ is lower (14−20%). Meanwhile, in

compounds

E_O_X(V)

E_R_E_D(V)

−1.84

Δ

medium polarity solvents such as DCM, the Φ values are 32

Δ

NI-Py-1

NI-Py-2

Py

Py-Ph

NI-3Ph

NI-4Ph

+0.78

+0.75

and 36% for NI-Py-1 and NI-Py-2, respectively.

−1.80

Interestingly, the Φ is increased further in solvents with

b

Δ

higher polarity; for instance, in ACN, Φ values of 52 and 47%

were observed for NI-Py-1 and NI-Py-2, respectively (Table

b

Δ

+0.77

b

−1.82

−1.84

2

). This observation is diﬀerent from the previously reported

b

dyads, where most of the compounds showed negligible Φ in

both nonpolar and polar solvents. Interestingly, the current

dyads (NI-Py-1 and NI-Py-2) showed the highest Φ in polar

solvents. We attribute this solvent polarity-dependent O

sensitizing behavior of the dyads to the diﬀerent energy level

matching of CT and triplet states in diﬀerent solvents. This

a

Δ

Cyclic voltammetry in N -saturated DCM containing 0.10 M

2

Bu NPF . The counter electrode is a Pt electrode, the working

4

6

+

Δ

electrode is a glassy carbon electrode, ferrocene (Fc/Fc ) is the

1

2

internal reference, and Ag/AgNO couple is the reference electrode.

3

b

Not observed.

1

strong solvent polarity dependency of the Φ value is an

Δ

indication of the SOCT-ISC mechanism.

negligible interaction between the NI and Py units in the dyads

3

.3. Electrochemical Study. The electrochemical proper-

ties of the compounds were studied by cyclic voltammetry

Figure 6 and Table 3). Pyrene and Py-Ph show a reversible

at ground states (S ). Thus, for both the dyads upon

photoexcitation, the pyrene unit will act as an electron donor

0

(

and the NI unit as an electron acceptor. Compared with the

+

40

oxidation wave at +0.75 and +0.77 V (vs Fc/Fc ), respectively,

and no reduction wave was observed (Figure 6a and Table 3).

For NI-3Ph and NI-4Ph, a quasi-reversible reduction wave was

previously reported perylene-NI dyad, the reduction

potential of the NI moiety is similar (−1.83 V). The oxidation

potential of the pyrene moiety of NI-Py-1 and NI-Py-2 is

+0.78 V, as compared with the perylene-NI dyad (+0.55 V),

indicating that the pyrene moiety is a weaker electron donor

compared to the perylene moiety.

Furthermore, spectroelectrochemistry was also measured to

study pyrene radical cation and NI radical anion absorption in

NI-Py dyads. By applying a positive potential (1.17 eV, vs Ag/

+

observed at −1.82 and −1.84 V (vs Fc/Fc ), respectively, and

no oxidation wave was observed (Figure 6a). For NI-Py-1, a

reversible oxidation wave at +0.78 V and a quasi-reversible

reduction wave at −1.84 V were observed (Figure 6b).

Similarly, for NI-Py-2, a reversible oxidation wave at +0.78 V

and a quasi-reversible reduction wave at −1.80 V were

observed (Figure 6b), which are assigned to the oxidation of

the pyrene moiety and the reduction of the NI moiety,

respectively. The similar oxidation potentials of the dyads NI-

Py-1 and NI-Py-2 (ca. +0.78 V) and Py-Ph (+0.75 V) and the

similar reduction potentials of the dyads (ca. −1.8 V, vs Fc/

AgNO ) to NI-Py-1, we observed the generation of absorption

3

bands centered at 355, 492, 600, and 656 nm (Figure 6c). It is

almost similar to that observed for Py-Ph under similar

experimental conditions (Figure S21a). Thus, these bands are

assigned to the pyrenyl radical cation absorption. When a

+

Fc ) and NI-3Ph and NI-4Ph (ca. −1.8 V, vs Fc/Fc ) indicate

negative potential (−1.70 eV, vs Ag/AgNO ) was applied,

3

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Table 4. Driving Forces of Charge Separation (ΔG⁰_C_S) and the Energy Levels the of Charge Separated State (E_C_S_S) of the

Dyads in Diﬀerent Solvents

0

ΔG (eV)

E_C_S_S(eV)

CS

compounds

HEX

TOL

DCM

ACN

HEX

TOL

DCM

ACN

a

NI-Py-1

NI-Py-2

−0.01

−0.33

−0.34

−1.23

−1.26

−1.47

−1.50

3.61

3.62

3.29

3.30

2.40

2.37

2.16

2.12

b

a

b

E₀₀= 3.63 eV. E = 3.63 eV. E₀₀is the approximated energy level based on the crossing point of UV−vis absorption and ﬂuorescence emission

after being normalized at the absorption/emission bands of the pyrene moiety.

00

absorption bands at 428, 502, 698, and 780 nm were observed

(Figure 6d) for NI-Py-1 . This result is in agreement with that

observed for NI-4Ph (Figure S21b), and therefore, we can

assign these bands to NI radical anion absorption. Similar

results were observed for the NI-Py-2 dyad (Figure S21c,d).

All these results are in agreement with the previous

5

6,57

observations,

and the data are helpful for study of the

charge separated state in femtosecond transient absorption

spectra (see Section 3.5).

The Weller equations (see eqs S2 and S3 in the Supporting

Information) were used to determine the charge separation

state (CSS) and the driving force of intramolecular charge

separation (Gibbs free energy changes of the photo-induced

electron transfer process, ΔG ), and the results are

CS

summarized in Table 4; further details are given in the

Supporting Information . The ΔG values indicated that the

CS

charge separation is thermodynamically allowed even in

nonpolar solvents for both the dyads, for instance, in Hex

(

ΔG = −0.01 eV) and in toluene (ΔG_C_S= −0.33 eV) for NI-

CS

Py-1. The ΔG values are more negative in polar solvents, and

CS

Figure 7. Nanosecond transient absorption spectra of (a) NI-Py-1

and (b) the decay trace at 460 nm. (c) Transient absorption spectra

a similar trend of ΔG values is observed for NI-Py-2. These

CS

results are in agreement with the experimental observations.

of NI-Py-2 and (d) the decay trace at 470 nm. λ = 355 nm. c = 2.0 ×

ex

−

5

3

.4. Nanosecond Transient Absorption (ns-TA) Spec-

10 M in deaerated DCM. 20 °C. Note: the triplet lifetimes are

intrinsic triplet state lifetimes obtained by ﬁtting the decay traces with

troscopy: Triplet Excited States Properties of the

Compounds. Nanosecond transient absorption (ns-TA)

spectra were measured to study the triplet state production

of the compounds (Figure 7). For NI-Py-1 upon nanosecond

pulsed laser excitation, a negative peak centered at 340 nm was

observed, which is in agreement with the UV−vis absorption

spectra, and thus it is assigned as the ground state bleach

GSB) band. Moreover, a broad excited state absorption

ESA) band in a range of 400−840 nm was observed (Figure

a). The apparent triplet state lifetime of NI-Py-1 is 105.8 μs

38,54

a TTA-based kinetic model.

For NI-Py-2, a diﬀerent transient proﬁle was observed upon

laser excitation as compared with NI-Py-1, except the GSB

band centered at 345 nm (Figure 7c). A broad ESA band was

observed in a range of 400−600 nm, although this ESA band is

diﬀerent than NI-Py-1, but it is similar to the ESA band of Py-

Ph (Figure 8e). In DCM, the intrinsic triplet state lifetime of

NI-Py-2 was determined as 220 μs (Figure 7d), which is much

longer than the apparent triplet lifetime, τ = 84.8 μs. For these

dyads, we propose that the triplet state is most probably

(

7

(

−

5

c = 2.0 × 10 M, in deaerated DCM). Moreover, the

transient species lifetime of NI-Py-1 is signiﬁcantly reduced

under aerated conditions (0.68 μs), which conﬁrms the

formation of triplet species. The intrinsic triplet state lifetime

of the transient species of NI-Py-1 was determined as 215 μs

by monitoring the decay trace at 460 nm (Figure 7b) and by

ﬁtting the decay traces with a kinetic model with the triplet−

triplet annihilation quenching eﬀect considered (complete

details of TTA-based triplet lifetime ﬁtting are given in the

3

localized on the pyrene moiety because the pyrene state is

3

47,58

lower (2.10 eV) than the NI state (2.21 eV).

demonstrated by the presence of the signiﬁcant GSB band

note that the NI moiety in the dyads gives much weaker

This is also

(

absorption than the pyrene moiety) (Figure 1). This

assignment is further supported by the TREPR spectral

study (see the later section). The ns-TA spectra of NI-Py

dyads were also studied in HEX and ACN (Figures S22 and

S23 ). The pyrene-localized triplet state was observed with

almost similar transient features to those observed in the DCM

solvent. However, the triplet state lifetimes are short in HEX (τ

= 29.5 μs) and long in ACN (τ = 300.4 μs).

30,38,54

Supporting Information).

It should be noted that for the compounds showing high

ISC yields and long triplet state lifetimes, the reported lifetimes

are shorter than the intrinsic triplet state lifetime. The TTA

self-quenching eﬀect can signiﬁcantly reduce the lifetime, and

therefore it is important to get the real triplet state lifetimes.

The triplet state in NI-Py-1 is mainly localized on the pyrene

moiety (T → T , transition of the Py moiety), supported by

For reference compounds NI-3Ph and NI-4Ph, the ISC is

eﬃcient in nonpolar solvents and it varies with solvent polarity.

It is known that for the unsubstituted NI moiety, the ISC is

1

n

5

1

TREPR and triplet spin density distribution (see the next

sections).

due to the S → T , a π−π* → n−π* transition. The n−π*

1

4

state energy is more sensitive to the solvent polarity than the

H

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−

5

Figure 8. Nanosecond transient absorption spectra of (a) NI-3Ph, in deaerated toluene (c = 2.0 × 10 M), and (b) the decay trace at 475 nm. (c)

−

5

Nanosecond transient absorption spectra of NI-4Ph, in deaerated toluene (c = 2.0 × 10 M), and (d) the decay trace at 540 nm. (e) Nanosecond

−

5

transient absorption spectra of Py-Ph, in deaerated n-hexane (c = 1.0 × 10 M), and (f) the decay trace at 450 nm, λ = 355 nm. 20 °C. τ is the

ex

int

38,54

intrinsic triplet state lifetimes obtained by ﬁ tting of the data with a TTA-based kinetic model.

Figure 9. Femtosecond transient absorption spectra of (a) NI-Py-1 at diﬀerent delay time, (b) respective evolution-associated diﬀerence spectra

EADS) obtained after global ﬁtting, and (c) decay traces at selected wavelengths. In DCM. λ_e_x= 350 nm. 20 °C.

(

π−π* state, and thus the ISC eﬃciency of NI-3Ph and NI-4Ph

on the pyrene moiety in the dyads. Although for NI-Py-1, the

triplet state ESA is rather diﬀerent and broad in a range of

400−840 nm, as the substitution at 4-C of the NI moiety,

which makes diﬀerent electronic coupling in NI-Py-1. For

pyrene upon photoexcitation, a broad ESA band in a range of

360−550 nm was observed upon the addition of 30%

iodoethane. The lifetime of transient species was determined

as 146.2 μs (Figure S24 ).

The triplet excited state lifetimes of the compounds were

also determined in polymer ﬁlms, prepared by doping the

compounds with the polymethylmethacrylate (PMMA) matrix

and commercially available transparent polyurethane rubber

(Clear Flex 50) (Figure S26 ). Interestingly, in both the media,

the triplet state lifetimes of the compounds were extended up

to the millisecond timescale as compared to that in the

deaerated solution. NI-Py-1 exhibited triplet lifetimes of 25.4

ms (in PMMA) and 24.8 ms (in Clear Flex 50); similarly, for

NI-Py-2, triplet state lifetimes were determined as 27.5 and 24

ms, respectively (in doped PMMA and Clear Flex 50 thin ﬁlms

40

could be solvent polarity-dependent as shown previously.

Similarly, the ISC of the pyrene moiety is known for a long

59

time, and the ISC is due to the S → T energy matching.

n

m

The nanosecond transient absorption spectra of NI-3Ph,

NI-4Ph, and Py-Ph were studied (Figure 8). For NI-3Ph, an

ESA band in a range of 370−670 nm was observed, which is

60

the typical feature of the NI triplet state absorption. A minor

GSB band is present at 350 nm, and the intrinsic triplet state

lifetime was determined as 358 μs in toluene (Figure 8a,b).

Similarly, the nanosecond transient absorption spectra of NI-

4

Ph were studied, with an intrinsic triplet state lifetime of 332

μs (Figure 8c,d). The NI localized triplet ESA band in a range

of 370−670 nm was observed, similar to that we reported

previously. On the other hand, for Py-Ph, a strong GSB band

observed at 340 nm and an ESA band in a range of 370−550

nm were detected upon laser excitation (Figure 8e). The

intrinsic triplet state lifetime was determined as 202 μs (Figure

4

0

8

f), which is longer than the apparent triplet state lifetime (τ =

2 μs). It is important to notice that the observed ESA band of

under a N atmosphere). Similarly, extended triplet state

2

Py-Ph is comparable with the ESA band of NI-Py-2.

Furthermore, the strong GSB bands in dyads match well

with the GSB bands of Py-Ph, while for NI-3Ph, the GSB band

is very weak and the GSB band of NI-4Ph is also weaker. This

observation further supported that the triplet state is localized

lifetimes were also observed for reference compounds (Figure

S27 ).

3.5. Femtosecond Transient Absorption (fs-TA) Spec-

troscopy. In order to investigate charge separation, charge

recombination, and intersystem crossing rate constants in the

I

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compounds, the ultrafast transient absorption spectra of NI-Py

dyads as well as the reference compounds were studied. First,

we measured pyrene in a nonpolar solvent, which displayed a

positive ESA band at 580 nm, which can be attributed to the

centered at 680 nm is slightly blue-shifted, while the 465 nm

band starts decreasing (evolution from red to blue in Figure

9b), which could be due to vibrational cooling. The CR

process takes place at a longer time scale (3.0 ns) and produces

the long-lived pyrene localized triplet state (evolution from

blue to green in Figure 9b). The last component with an

inﬁnite lifetime is beyond the range of our fs-TA experiment

setup. The selected kinetic traces at 465 and 670 nm are

presented in Figure 9c. The transient absorption spectra of NI-

Py-1 measured in ACN show almost similar features to those

observed in DCM (Figure S30) but with slower CS (∼1.6 ps)

and CR (∼5.1 ns). Compared with the previously reported

perylene-NI dyad in DCM (CS takes 0.66 ps, CR takes 8 ns),

these dyads show almost similar CS rate constants, while the

transient absorption of the S state (Figure S28 ). This band

2

decays rapidly and a new positive band rises at 473 nm, which

can be attributed to the absorption of the pyrene S state.

1

These dynamics can be ascribed as S → S internal conversion

2

1

(

IC), which takes about 0.51 ps. No triplet state was observed

for pyrene with the fs-TA spectra, which is in agreement with a

long ﬂuorescence lifetime of 7.9 ns. For Py-Ph, two ESA bands

at 580 and 635 nm appeared after excitation within 0.23 ps (S₂

→

S_n), and these bands decay rapidly while a broad ESA rise in

a range of 465−540 nm in 22 ps (S → S IC, Figure S29). On

2

1

40

later delay times, the S state band decays, and a new ESA band

CR is slightly faster.

1

centered at 445 nm appears, which is the T → T transition of

the pyrene moiety; this band is similar to the ns-TA spectra of

Apart from some diﬀerences, NI-Py-2 has similar optical

characteristic behaviors in transient absorption spectra to those

observed for NI-Py-1 (Figure S31 ). The broad ESA band from

580 to 720 nm is relatively weak, charge separation occurred

upon photoexcitation within 0.23 ps, and the ISC rate was

found to be about 3.2 ns. Similar results were observed in ACN

with slightly slower kinetics CS of 3.9 ps and ISC of 4.5 ns.

The observation of the pyrene radical cation in both the dyads

is in good agreement with the spectroelectrochemical

measurements (Figure 6c and Figure S21c ).

1

n

this compound (Figure 8e). These results of pyrene and Py-Ph

59,61,62

are in accordance with the previously reported literature.

Upon photoexcitation of NI-Py-1 at 350 nm, the transient

absorption spectra in DCM were recorded (Figure 9). There

are a few ESA bands that appeared in ranges of 400−550 nm

and 580−720 nm (Figure 9a). The ESA band increases, and

shortly after, it decreases at longer delay times. This shows that

the ESA signal includes several excited state processes, for

instance, the singlet excited state (S → S , transition), radical

Additionally, we measured the femtosecond transient

absorption spectra of NI-Ph derivatives in toluene (Figure 10

1

n

·

+

·−

cation of pyrene (Py ), or radical anion of NI (NI )

absorptions and the triplet state (T → T , transition). All

1

n

these excited states show the ESA signals almost in the same

range from 400−550 nm, which can well overlap with each

6

1,63−65

other.

The initial absorption at 465 nm along with a

shoulder band at about 550 nm can be attributed to pyrene

singlet excited state absorption (S → S , transition). After

1

n

that, the signal of the pyrene radical cation is intensiﬁed in the

same range; meanwhile, another broad band centered at 680

·

+

nm (the absorption band of Py ) was developed, indicating

the formation of the CS state. Subsequently, with the decay of

the CS state, a broad triplet ESA band centered at 464 nm was

3

retained, corresponding to the T → T transition of the Py

1

n

Figure 10. Femtosecond transient absorption spectra of (a) NI-4Ph

at diﬀerent time delays and (b) decay trace at 514 nm. In toluene. λ_e_x

= 350 nm. 20 °C.

state, indicating the occurrence of ISC. Note that the ESA

bands at 420 and 500 nm of NI are probably overlapped with

the broad ESA bands of Py and Py. We assign the broad

band at 464 nm at later times to triplet state absorption since it

does not decay completely on the time scale of our fs-TA

measurements (ca. 5.5 ns). The photophysical process of NI-

Py-1 can be presented as S → CT → LE. It is noteworthy to

mention here that for both the dyads, we did not observe

pyrene higher singlet excited state S (as observed in the case

of pristine pyrene and Py-Ph) because of the ultrafast charge

·

−

·+

3

and Figure S32 ). NI-4Ph initially shows stimulated emission at

420 nm and a sharp ESA band centered at about 514 nm along

with a broad tail at about 625 nm, which is assigned to the

singlet excited state (S₁→ S_ntransition) (Figure 10a).

However, these ESA bands ﬁrst increase, and then after a short

time, they start decreasing and blue-shifting; the new ESA

1

3

1

2

3

55

band is assigned to the T → T transition of the NI state.

1 n

1

separation to form the CS state (via the S → CT path).

Through single wavelength ﬁtting (Figure 10b), the ISC time

constant is determined as ca. 20.1 ps and the excited state

dynamics can be summarized as *S → T → T . Similarly, for

1

To obtained the time constants of the CS and CR processes,

global analysis of the transient absorption data was performed,

and evolution-associated diﬀerence spectra (EADS) were

obtained for the description of photodynamic in the dyads

1

4

1

NI-3Ph upon 350 nm photoexcitation, an ESA band at about

465 nm appeared, which became broader at later delay times

and extended up to 700 nm; it can be assigned to the triplet

excited state absorption of the NI moiety (Figure S32). Thus,

in the nonpolar solvents, the triplet excited state of NI-Ph

derivatives populates via the enhanced ISC due to the n → π*

to π → π* transition (singlet/triplet energy level matching).

3.6. Time-Resolved Electron Paramagnetic Reso-

nance (TREPR) Spectroscopy. Previously, the EPR spectra

of pyrene were measured in a pyrene-doped ﬂuorene single

(

Figure 9b). For NI-Py-1, four time constants were necessary

for a satisfactory ﬁt of the data. The initial spectral component

represents the population of the singlet excited state in less

than 1 ps with the partial ultrafast CS process. The singlet

excited state decays within 0.81 ps due to charge separation,

the intensity of the ESA band at 465 nm is increased, and a

new positive ESA in a range of 580−720 nm rises (evolution

from black to red with a lifetime of 82 ps indicates that the

charge separation is eﬃcient in the dyad). The broad band

66

crystal. The values of zero-ﬁeld splitting (ZFS) parameters |

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D| and |E| were determined as 2398 and 539 MHz,

respectively. However, no TREPR spectrum was supplied.

The TREPR spectrum of pyrene in sodium dodecyl sulfate

With simulation of the TREPR spectrum of Py-Br, the

population rates of the sublevels of the T state are P :P :P =

1

x

y

z

0:0.33:0.67. The spectrum did not show any shape change

along with longer delay times. It should be pointed out that the

transient species lifetime determined with the TREPR

spectroscopy is about 5 μs, which is much shorter than the

triplet state lifetime (τ = 160 μs) of Py-Br determined with the

nanosecond transient absorption spectroscopy (Figure S25).

Spin lattice relaxation (SLR) may contribute to this

discrepancy.

(

SDS) aqueous micelle was studied, but the observed signal

was assigned to the pyrene cation, generated by two-photon

6

7

ionization. Previously, the ﬂuorescence properties of the

pyrene-pyrazole nitronyl and imino nitroxides dyads were also

studied, but the triplet state and the electron spin polarization

68

of the ISC of the compounds were not studied.

Herein, ﬁrst, we studied the TREPR spectrum of the triplet

state of Py-Br, with randomly oriented sample in frozen

solution at 80 K (Figure 11 a). An electron spin polarization

No satisfactory triplet TREPR spectrum was obtained for

native pyrene due to the low ISC yield (Φ = ∼30% in HEX).

Δ

Previously, we reported the TREPR spectrum of the triplet

57

state of NI-4Br, for which the ESP is (e, a, e, a, e, a). The

ZFS parameters were determined as |D| = 2590 MHz and |E| =

150 MHz, respectively. The population rates of the sublevels of

the T state of NI-4Br is P :P :P = 0.33:0.67:0. For NI-3Br,

1

x

y

z

we observed an ESP of (e, e, e, a, a, a) (Figure 11b) and almost

similar ZFS parameters (|D| = 2750 MHz and |E| = 120 MHz).

However, the population rates of the sublevel of the T state of

1

NI-3Br are P :P :P = 0:0:1, which are drastically diﬀerent

x

y

z

from NI-4Br. Furthermore, the |P − P |/|P − P | values of NI-

x

y

z

y

3

Br (0/1) and NI-4Br (0.50) are also drastically diﬀerent from

each other (Table 5).

This is an interesting example where the Br atoms attached

at diﬀerent positions of the chromophore may induce diﬀerent

population rates of the sublevels of the T state, i.e., the

1

electron spin selectivities of the bromo-NI analogues are

diﬀerent. It is noteworthy to mention here that the D

parameters of pyrene and NI moieties are only slightly

diﬀerent but the E parameters are drastically changed. This

diﬀerence may be useful for the assignment of triplet state

localization of the NI-Py-1 and NI-Py-2 dyads.

Figure 11. TREPR spectra of (a) Py-Br, (b) NI-3Br, (c) NI-Py-1,

and (d) NI-Py-2, recorded at 80 K in frozen toluene and 400 ns after

pulsed laser excitation at 355 nm, 10 mJ per pulse. Positive signal

amplitudes represent enhanced absorptive (A) and negative

amplitudes stand for emissive (E) polarization of the respective

transition. Red lines are computer simulation, and black curves are

experimental TREPR spectra.

The dyads NI-Py-1 and NI-Py-2 show ISC ability, and thus

the triplet state was studied with TREPR spectra (Figure

11c,d). The (a, e, a, e, a, e) ESP features of NI-Py-1 are

diﬀerent from both Py-Br and NI-3Br, and therefore the ISC

mechanism of NI-Py-1 should be diﬀerent from the SO-ISC.

The |D| and |E| parameters of NI-Py-1 were determined as

2

450 and 510 MHz, respectively. Based on the ZFS

parameters, especially the E values, we purpose that the triplet

state of NI-Py-1 is conﬁned to the pyrene moiety. This is in

agreement with the ns-TA spectral results (Figure 7a).

Interestingly, for NI-Py-2, which has diﬀerent connections to

the NI moiety, a TREPR spectrum with diﬀerent ESP (e, e, e, a,

a, a) features was observed. Moreover, the population rates of

patttern (e, e, e, a, a, a) was observed, which is typical for the

triplet state of an organic chromophore generated by the spin-

2

7,36,37

orbit intersystem crossing (SO-ISC).

The |D| and |E|

values were determined as 2350 and 510 MHz, respectively

(

Table 5). These D and E values are close to those observed

the T state are drastically diﬀerent from those of NI-Py-1

1

66

for pyrene when doped in the ﬂuorene crystal.

(

Table 5).

This geometry-dependent feature is typical for the electron

Table 5. Zero Field Splitting Parameters (D and E) and

Triplet-State Population Sublevels (P , P , and P ) Obtained

from Simulation of the Triplet-State TREPR Spectra of the

Compounds

donor/acceptor dyads showing the SOCT-ISC mecha-

27,38

x

y

z

nism.

The T state of NI-Py-2 should be also conﬁned

1

on the pyrene moiety based on the ZFS E parameter (|E| = 479

MHz). The complete simulation parameters of the TREPR

spectra are summarized Table 5.

3.7. Computational Study. The ground state geometries

of the compounds were optimized with DFT calculations

|

D|

|E|

|P − P |/

x y

a

compounds (MHz) (MHz)

P_x

P_y

P_z

|P − P |

z

y

NI-Py-1

NI-Py-2

Py-Br

2450

2300

2350

2750

2590

510

479

510

120

150

0.2

0

0.33

0.8

0

0.33

0

1

0.67

1

0

0.75

0/1

0.97

0/1

0.50

(

Figure 12). For NI-Py-1, the dihedral angle between the NI

and the perylene moieties is 113.6°, and this is almost similar

50

to the previously reported NI-Py analogue (φ = 107°).

NI-3Br

b

Meanwhile, for NI-Py-2, the torsion about the C−C linker has

more freedom, and the dihedral angle is much smaller (54.4°).

This result indicates that some extent of electronic coupling is

expected between donor and acceptor units being connected

NI-4Br

0.67

a

Unique ratio of populations, which determines the triplet ESP.

b

57

Literature values.

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The frontier molecular orbitals of the compounds were

studied (Figure S33). For both NI-Py-1 and NI-Py-2, the

highest occupied molecular orbital (HOMO) is localized on

the pyrene moiety and the lowest unoccupied molecular orbital

(

LUMO) is localized on the NI moiety, indicating that the

electron transfer is possible from the pyrene unit to NI unit.

However, a minor delocalization of the molecular orbitals was

observed, which is responsible for the CT absorption bands

(

Figure 1). The molecular orbitals of HOMO-1 and LUMO+1

are slightly delocalized, and fully conﬁned molecular orbitals

are beneﬁcial for eﬃcient ISC. The frontier molecular orbitals

of Py-Ph were also studied, and more delocalized molecular

orbitals were found as compared with the dyads.

The triplet state spin density distribution of the dyads at the

optimized triplet state was presented (Figure 14 a,d). For NI-

Figure 12. Dihedral angles (in degree) of the compounds at the

linkage C−C bonds based on optimized ground state geometries at

the B3LYP/6-31G(d) level with Gaussian 09W.

by a C−C single bond, and therefore we observed CT

absorption bands in the UV−vis absorption spectra (Figure 1).

In comparison, similar torsion angles were observed for the

respective reference compounds NI-4Ph (53.7°) and NI-3Ph

(

36.5°), which indicate larger torsion freedom, as a result of

attachment of the smaller phenyl substituents. The diﬀerent

dihedral angles between the two units may result in diﬀerent

electronic coupling, and thus CT absorption bands with

diﬀerent magnitudes were observed for the dyads.

The optimized ground state geometry is not suﬃcient to

investigate the conformational ﬂexibility of the torsion of the

electron donor/acceptor mutual orientation. Therefore, we

constructed the potential energy surface (PES) of the torsion

of the C−C linker between the NI and Py moieties (Figure

1

3 ). For NI-Py-1 , the PES is rather ﬂat in a dihedral angles

Figure 14. Isosurfaces of spin density distribution of NI-Py-1 (upper

panel) and NI-Py-2 (lower panel) at the optimized (a and d) triplet

state, (b and e) radical anion, and (c and f) radical cation geometries

(isovalues are 0.0025). Calculated at the B3LYP/6-31G(d) level using

Gaussian 09W.

Figure 13. (a) Calculated ground state (S ) potential energy surface

0

of the compounds as a function of the dihedral angle between the two

chromophores. (b) Magniﬁed view of (a) in a low energy range. The

straight line in (b) near the bottom shows the room temperature

accessible thermal energy range (0.026 eV). Calculations were

performed at the (B3LYP/6-31G(d)) level with Gaussian 09W.

Py-1, the triplet state is mainly localized on the Py moiety but

there is also a minor contribution from the NI unit. Similarly,

triplet state spin density of NI-Py-2 is localized on the Py

moiety but the leakage to the NI unit is also observed. This is

because of relative triplet state energy levels of both the

moieties: the triplet energy level of the Py moiety is 2.10 eV

range of 60−120°. This range is similar to the recently

4

0

reported perylene-NI dyad (60−120°). The geometry

accessible at room temperature is mainly in a range of 52−

1

28°.

Interestingly, for NI-Py-2, the potential energy curve is

47,58

shallow with two energy minima at 56 and 130° with the

thermally accessible dihedral angle at room temperature being

in a range of 41−74°. However, eﬃcient triplet state

production was observed for NI-Py-2. It is well known for

SOCT electron donor/acceptor systems that orthogonal

geometry is much favorable, but our results imply that other

factors may also aﬀect ISC eﬃciency, i.e., the chromophore

and the NI triplet energy level is 2.21 eV.

This result

indicates that upon photoexcitation, the triplet state will be

mainly localized on the pyrene moiety, which is in agreement

with the ns-TA and TREPR observations (Figures 7 and 11).

The electron spin density surfaces of the radical anion and

radical cation were also calculated (D states). For both the

0

dyads, the spin density of the radical anion is localized on the

NI moiety with a small leakage to the Py moiety (Figure

1

3

orientation or CT and LE state energy level matching.

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Scheme 2. Jablonski Diagram Indicating the Photophysical Processes Involved in (a) NI-4Ph and (b) NI-Py-1 upon

a

Photoexcitation

a

The energy levels of the singlet excited states are derived from the spectroscopic data, and CT state energy levels in diﬀerent polarity solvents are

obtained from the CT emission band. The triplet state energy levels are estimated by the TD-DFT method. The numbers in superscripts indicate

the spin multiplicity.

1

4b,e). On the other hand, the electron density of the radical

physical properties of the dyads were studied with steady-state

and femtosecond/nanosecond time-resolved transient absorp-

tion spectroscopies, time-resolved electron paramagnetic

resonance (TREPR) spectroscopy, and density functional

theory (DFT) calculations. UV−vis absorption shows a clear

charge transfer absorption band, and ﬂuorescence is quenched

with increasing the solvent polarity for both the dyads. This

indicates that photo-induced charge transfer is eﬃcient. Charge

separation occurs upon photoexcitation and the ISC eﬃciency

cation is mainly restricted to the Py moiety (Figure 14c,f).

As the linkage in NI-Py-1 is at the 4-C position of the NI

moiety to the Py unit, thus we expect better electronic

communication between the two units. In the case of NI-Py-2,

the attachment of the NI moiety is at the 3-C position to Py,

which may exert less steric hindrance and give more freedom

of rotation. This result further implies that the Py moiety

serves as an electron donor and the NI unit is the electron

acceptor.

is solvent polarity-dependent (Φ = 14−52%), which is an

Δ

Based on the above studies, the overall photophysical

processes of compounds are summarized in Scheme 2. For NI-

indication of the SOCT-ISC mechanism. Diﬀerent from the

previous study, these dyads show moderate to high singlet

oxygen quantum yields in solvents with diﬀerent polarities

(from nonpolar to polar). On the other hand, the reference

compounds (NI-3Ph and NI-4Ph) show high singlet oxygen

production in nonpolar solvents (eﬃcient ISC due to S → T

4

Ph, the ISC occurs within 20.1 ps via S → T transition as

1 4

these states share the same energy level and then followed by

IC to the T state. For NI-Py-1, upon pulsed laser excitation,

ultrafast charge separation occurs (0.81 ps) to give the CT

1

4

state. The energy of the CT state is dependent on solvent

polarity. For instance, the CT state of NI-Py-1 is 2.80 eV in

HEX, whereas it decreased to 2.13 eV in ACN. The energy gap

energy level matching). Furthermore, upon nanosecond pulsed

laser excitation, we observed two diﬀerent excited-state

absorption (ESA) features for NI-Py-1 and NI-Py-2 dyads

with long-lived triplet states (intrinsic lifetime τ_i_n_t= ∼220 μs)

and the triplet state is localized on the pyrene moiety

(supported by TREPR data, the ZFS E parameter). The

ultrafast CS process (∼0.81 ps) and the ISC rate (3.0 ns) are

observed by femtosecond transient absorption spectroscopy.

With TREPR spectroscopy, we conﬁrmed the SOCT-ISC

mechanism, and interestingly, the observed ESP pattern is

chromophore orientation-dependent and the population rates

of the triplet sublevels of the dyad are drastically diﬀerent from

the spin-orbit coupling induced ISC (for monobrominated

pyrene and NI). Moreover, the population rates of triplet

sublevels are also diﬀerent with diﬀerent substitution positions

of the Br atom in bromo-NI analogues. The ZFS D and E

parameters clearly indicate that the triplet state of the dyads is

conﬁned on the pyrene moiety. Our results are useful for an in-

depth understanding of the photophysics of compact electron

donor−acceptor dyads and helpful for the designing of new

heavy atom-free triplet photosensitizers.

3

between the CT state and LE in diﬀerent polarity solvents can

play an important role in the solvent polarity dependency of

the SOCT-ISC eﬃciency. The subsequent CR (take ∼3.0 ns)

leads to the formation of the pyrene localized triplet state.

4

7,50,58

According to the DFT calculations and literature values,

3

the LE of the pyrene moiety should be the lowest laying

triplet in the dyads (supported by ns-TA and TREPR). The

ISC eﬃciency of these dyads is moderate in nonpolar solvents

3

and high in polar solvents. This can be due to the high LE

state energy (2.03 eV) of the pyrene moiety, which is much

1

closer to the CT energy in polar solvents. Similarly,

photophysical processes of NI-3Ph and NI-Py-2 are

summarized in Scheme S1 .

4

. CONCLUSIONS

In summary, naphthalimide (NI)−pyrene (Py) compact

electron donor−acceptor dyads (NI-Py-1 and NI-Py-2) were

prepared to study the eﬀect of mutual orientation of the

chromophores on the spin-orbit charge transfer intersystem

crossing (SOCT-ISC) eﬃciency. The synthesis is based on the

diﬀerent connections of the pyrene donor unit to the NI

acceptor. The purpose of this study is to explore the SOCT-

ISC eﬃciency of the dyads with diﬀerent mutual orientations

of the donor and acceptor and its eﬀect on the electron spin

polarization (ESP) pattern of the triplet state. The photo-

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.jpcb.1c03537.

■

*

Experimental methods, characterization data, cyclic

voltammetry, additional steady-state UV−Vis absorp-

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Atom-Free Photosensitizers for Enhanced Photodynamic Therapy

under Hypoxia. J. Am. Chem. Soc. 2019, 141, 16243 −16248.

AUTHOR INFORMATION

Corresponding Authors

Jianzhang Zhao − State Key Laboratory of Fine Chemicals,

School of Chemical Engineering, Dalian University of

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

■

(7) Majumdar, P.; Nomula, R.; Zhao, J. Activatable Triplet

Photosensitizers: Magic Bullets for Targeted Photodynamic Therapy.

J. Mater. Chem. C 2014, 2, 5982−5997.

(8) Nguyen, V.-N.; Yan, Y.; Zhao, J.; Yoon, J. Heavy-Atom-Free

0

000-0002-5405-6398 ; Email: zhaojzh@dlut.edu.cn

Photosensitizers: From Molecular Design to Applications in the

Photodynamic Therapy of Cancer. Acc. Chem. Res. 2021, 54, 207−

Ahmet Karatay − Department of Engineering Physics, Faculty

of Engineering, Ankara University, 06100 Ankara, Turkey;

Email: ahmet.karatay@eng.ankara.edu.tr

Omar F. Mohammed − Division of Physical Sciences and

Engineering, King Abdullah University of Science and

Technology (KAUST), Thuwal 23955-6900, Kingdom of

Saudi Arabia; orcid.org/0000-0001-8500-1130 ;

Email: omar.abdelsaboor@kaust.edu.sa

2

20.

(9) Li, X.; Kolemen, S.; Yoon, J.; Akkaya, E. U. Activatable

Photosensitizers: Agents for Selective Photodynamic Therapy. Adv.

Funct. Mater. 2017, 27, 1604053.

(10) Zhao, J.; Ji, S.; Guo, H. Triplet −Triplet Annihilation Based

Upconversion: From Triplet Sensitizers and Triplet Acceptors to

Upconversion Quantum Yields. RSC Adv. 2011 , 1 , 937 −950.

(11) Ye, C.; Zhou, L.; Wang, X.; Liang, Z. Photon Upconversion:

Violeta K. Voronkova − Zavoisky Physical-Technical Institute,

FRC Kazan Scienti ﬁ c Center of RAS, Kazan 420029, Russia;

Email: vio@kfti.knc.ru

From Two-Photon Absorption (TPA) to Triplet −Triplet Annihila-

tion (TTA). Phys. Chem. Chem. Phys. 2016, 18, 10818−10835.

(

12) Guo, X.; Liu, Y.; Chen, Q.; Zhao, D.; Ma, Y. New

Bichromophoric Triplet Photosensitizer Designs and Their Applica-

tion in Triplet-Triplet Annihilation Upconversion. Adv. Opt. Mater.

Authors

Muhammad Imran − 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 RAS, Kazan 420029, Russia

Partha Maity − Division of Physical Sciences and Engineering,

King Abdullah University of Science and Technology

2

(

018, 6, 1700981.

13) Yanai, N.; Kimizuka, N. New Triplet Sensitization Routes for

Photon Upconversion: Thermally Activated Delayed Fluorescence

Molecules, Inorganic Nanocrystals, and Singlet-to-Triplet Absorption.

Acc. Chem. Res. 2017, 50, 2487−2495.

(14) Zhao, J.; Wu, W.; Sun, J.; Guo, S. Triplet Photosensitizers:

From Molecular Design to Applications. Chem. Soc. Rev. 2013, 42,

5323−5351.

(

KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia;

orcid.org/0000-0002-0293-7118

(15) Stacey, O. J.; Pope, S. J. A. New Avenues in the Design and

Potential Application of Metal Complexes for Photodynamic

Therapy. RSC Adv. 2013, 3, 25550−25564.

Ayhan Elmali − Department of Engineering Physics, Faculty of

Engineering, Ankara University, 06100 Ankara, Turkey

(16) Zhao, J.; Ji, S.; Wu, W.; Wu, W.; Guo, H.; Sun, J.; Sun, H.; Liu,

Complete contact information is available at:

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Y.; Li, Q.; Huang, L. Transition Metal Complexes with Strong

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The authors declare no competing ﬁnancial interest.

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18) Broring, M.; Kruger, R.; Link, S.; Kleeberg, C.; Kohler, S.; Xie,

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

and 21911530095), the State Key Laboratory of Fine

Chemicals and Fundamental Research Funds for the Central

Universities (DUT19TD28) for ﬁnancial support. V.K.V. and

A.A.S. thanks the Zavoisky Physical-Technical Institute, FRC

Kazan Scientiﬁc Center of RAS for ﬁnancial support.

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