Effect of substituents on the fluorescence quenching of a few (anthracen-9-yl)methanamines

Article abstract of DOI:10.1016/j.jphotochem.2020.112552

This article deals with the photophysical properties of a few (anthracen-9-yl)methanamines. Analysis of absorption and the fluorescence emission spectra of these compounds indicated intramolecular photoinduced electron transfer leading to quenching of excited states, the efficiency of which depends on the geometrical constraints and electronic factors pertaining to the molecule. The decay kinetics studied using Time Correlated Single Photon Counting showed a bi-exponential decay indicating the involvement of two transient species in the excited state of these molecules. Also, we build up a relation between the rate of the photochemical reaction, free energy change and quenching of excited states.

Full text of DOI:10.1016/j.jphotochem.2020.112552

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Effect of substituents on the fluorescence quenching of a few
(anthracen-9-yl)methanamines
Rani Mathew (Visualization) (Investigation), Rekha R. Mallia
(Investigation) (Writing - original draft) (Data curation) (Formal
analysis), Suja Haridas (Project administration), Jomon P. Jacob
(Conceptualization) (Methodology) (Supervision)
PII:
S1010-6030(20)30351-8
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112552
JPC 112552
Reference:
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Journal of Photochemistry & Photobiology, A: Chemistry
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20 March 2020
1 April 2020
Please cite this article as: Mathew R, Mallia RR, Haridas S, Jacob JP, Effect of substituents
on the fluorescence quenching of a few (anthracen-9-yl)methanamines, Journal of
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doi: https://doi.org/10.1016/j.jphotochem.2020.112552
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Effect of substituents on the fluorescence quenching of a few (anthracen-9-
yl)methanamines
Rani Mathew1, Rekha R. Mallia1, Suja Haridas*, Jomon P. Jacob*
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi-682
022, Kerala, India.
1These two authors contributed equally
Highlights


Synthesized and studied the photophysical properties of a few (anthracen-9-yl)methanamines
Studied the substituents effects in the nitrogen atom on the intramolecular photoinduced
electron transfer and thereby quenching the excited state.

Also developed a relation between the quenching of fluorescence and the electron density
around the donor nitrogen atom of (anthracen-9-yl)methanamines.
Abstract
This article deals with the photophysical properties of a few (anthracen-9-yl)methanamines.
Analysis of absorption and the fluorescence emission spectra of these compounds indicated
intramolecular photoinduced electron transfer leading to quenching of excited states, the
efficiency of which depends on the geometrical constraints and electronic factors pertaining
to the molecule. The decay kinetics studied using Time Correlated Single Photon Counting
showed a bi-exponential decay indicating the involvement of two transient species in the
excited state of these molecules. Also, we build up a relation between the rate of the
photochemical reaction, free energy change and quenching of excited states.
Keywords: (anthracen-9-yl)methanamines; 9-methylanthracene; fluorescence quenching;
photoinduced electron transfer; exciplex.
Introduction
Nowadays photoresponsive supramolecular systems are of great interest due to their
application in light-directed ionic switches, photoreversible cation traps, nanoscale devices
for cation detection, light energy devices, etc.1,2 Investigations on photoinduced electron
transfer (PET)3,4 in ‛donor-acceptor’ molecules were mainly focused on studying such
photoresponsive systems.5-7 Nanosecond and picosecond laser-flash photolysis techniques
have been used by different groups to elucidate the various intermediate stages involved in
the photoinduced reactions of such systems.8 These ‛donor-acceptor’ molecules are also used
as pH indicators9,10 and as sensors in biological11 as well as in chemical12 fields.
Electron-transfer reactions of amines and anthracenes are important in several
technological applications such as imaging, photopolymerisation13 and fading of textile
dyes.14-18 More recently, 2,6-divinylpyridine-appended anthracene derivatives flanked by two
alkyl chains at the 9,10-position are shown to recognize picric acid selectively down to parts
per billion level in both aqueous and non-aqueous media. Fluorescence emission of these
compounds in solution is significantly quenched by adding trace amounts of picric acid due
to the combined effects of light-induced intramolecular charge transfer (ICT) as well as
electron transfer from the electron-rich anthracene to the electron-deficient picric acid.19
1

Anthracene bisimide derivatives having two triphenylamine groups as donor units at the 9,10-
positions to form a novel π-conjugated donor-acceptor system which exhibit intramolecular
electronic interactions between the donor and acceptor components.20 Furthermore,
anthracene-o-carborane dyad shows solid-state emission due to intramolecular charge transfer
in the crystalline state.21 Electron donating capacity of amine functionality has been
extensively used for designing new materials such as fluoroionophores, organic conductors,
electroluminescent materials, photovoltaics, and materials with non-linear optical activity.22-
27
It has been reported that inter28-30 and intramolecular31-35 electron and energy transfer,
exciplex formation30 etc. take place between arenes and tertiary amines. Formation of
intramolecular arene-amine exciplex and their photophysical and photochemical behaviour
has been of longstanding interest.29,35-40 In this context, we have synthesized several
(anthracen-9-yl)methanamines 1a-h41 where anthracene component acts as the excited state
electron acceptor and amine component as the donor.42 Methylene spacer is intentionally
introduced between donor and acceptor components to minimize ground state interactions.
Our systems are constructed with subtle structural variation around the donor atom. These
systems would be ideal for studying dependence of geometric requirements of ring size, steric
and electronic factors on exciplex formation and efficiency of electron transfer in these
systems. For such systems, information on the relationship between molecular geometry and
PET efficiency is of paramount importance since design of several sensors relying on
intramolecular amine PET should satisfy the caveat that greatest signal range results from
sensors with the lowest initial fluorescence. We examined the excited states of (anthracen-9-
yl)methanamines 1a-h via time-correlated single photon counting technique (TCSPC) and
derived the life times of the transient species involved in the excited states. This article
describes preliminary studies on the ground state and excited state interactions of these
(anthracen-9-yl)methanamines keeping 9-methylanthracene (2) as reference.
Results and Discussion
(Anthracen-9-yl)methanamines dyads 1a-h synthesised by us have inbuilt amine-
arene unit41 with tertiary amine components possessing varying steric and electronic makeup
(Chart 1). Absorption and fluorescence emission spectra (ex = 365 nm) of 1a-h were
recorded in methanol (10-5 M) at room temperature. Time-resolved studies were conducted
using TCSPC technique using methanol as solvent.
Chart 1. Synthesised (anthracen-9-yl)methanamines 1a-h for studying the effects of
substituents on fluorescence quenching.
2

Absorption spectra of amine-appended anthracenes 1a-h were almost identical with
that of 9-methylanthracene (2) indicating negligible ground state interaction between
anthracene and amine components (Figure 1) as stipulated by us. Excitation spectra of 1a-h
closely resembled the absorption spectra indicating that emission is taking place from the
locally excited state (Figure 2).
15
3500
2
1a
1b
1c
1d
1e
1f
1g
1h
2
1a
1b
1c
1d
1e
1f
12
9
3000
2500
 2000
1g
1h
6
1500
3
1000
500
0
0
340
360
380
400
350
400
Wave length (nm)
Wave Length (nm)
Figure 2. Excitation spectra of 1a-h and 2
Figure 1. Absorption spectra of 1a-h and 2 at
at room temperature in 10-5 M methanol.
room temperature in 10-5 M methanol.
Fluorescence emission spectra of 1a-h showed a slight red shift (1-7 nm) in
comparison to that for 2 owing to minor perturbation due to substituent effects (Figure 3). For
1a-h considerable quenching of anthracene fluorescence (28-99%) was observed (Table 1).
Compounds 1f and 1g showed high efficiency in quenching (99 and 98% respectively). This
was followed by 1e (94%), 1a (90%) and the cyclic amines 1b-d (90, 88, 82% respectively).
Among the series, 1h, as expected, showed least efficiency (28%) in quenching of anthracene
fluorescence.
PET
Fluorescence
Quenching, Quantum Yield,
Compound
emnm
%
f
0.049
0.067
0.071
0.112
0.015
0.007
0.008
0.357
416
418
417
417
417
411
415
413
90
90
88
82
94
99
98
28
1a
1b
1c
1d
1e
1f
1g
1h
Table 1. Percentage PET Quenching and Relative Quantum Yield for (anthracen-9-
yl)methanamines 1a-h compared to 9-methylanthracene (2, f = 0.400)
3

80
60
40
20
0
1a
1b
1c
1d
1e
1f
1g
1h
2
400
450
500
550
Wave Length (nm)
Figure 3. Fluorescence emission spectra (ex = 365 nm) of 1a-h and 2 at room temperature in
10-5 M methanol
Close examination of the structural features of 1a-h reveals the diversity in their
structure and hence, reactivity. We grouped selected anthracene into four types based on the
nature of N-substituents viz aliphatic (1a,e), alicyclic (1b-d,g), aromatic (1h) and benzylic
(1f). In order to gain a better insight on the effect of structure and geometry on fluorescence
quenching of (anthracen-9-yl)methanamines 1a-h, we optimized the energy minimized
structures using Gaussian (Chart 2). Results from structure optimization revealed the
geometry of 1a-h are mostly similar with no indication for steric crowding around donor-
acceptor components, ruling out role for steric factors on fluorescence quenching and hence,
suggesting a dominant role for electronic factors on fluorescence quenching. Size of N-
substituents, however, may have some control on efficiency of electron transfer since B-strain
induced by lone pair-bond pair repulsion in tertiary amines is partially relieved by electron
transfer.43
1a
1b
1c
4

1f
1e
1d
1h
1g
Chart 2. Energy minimized structures of (anthracen-9-yl)methanamines 1a-h using
Gaussian.
In the detailed examination of electronic effects, it is interesting to note that 1e having a
stronger basic amine component,44 shows higher quenching capability when compared to 1a.
In the case of cyclic tertiary amine appended anthracenemethanamines 1b-d, show
fluorescence quenching in the order 90, 88, 82% respectively. This is due to subtle changes in
basicity of tertiary amine components. The basicity of cyclic amines follow the order
pyrrolidine44 > piperidine44 > azepane.45 Consequently, fluorescence quenching in
anthracenemethanamines 1b-d follows the order 1b > 1c > 1d. Effect of electron availability
on nitrogen is further demonstrated by the observation in benzimidazole appended anthracene
1h where the lone pair on nitrogen is part of an aromatic ring system showed the least
magnitude of fluorescence quenching of anthracene fluorophore via PET among 1a-h. With
decreasing availability of electrons on nitrogen, quenching efficiency also decreases. In other
words, quenching efficiency depends on the electron on nitrogen atom, evidenced by the
magnitude of free energy change estimated for these molecules (vide infra).
Compound 1g showed high efficiency (98%) in PET quenching. Exceptional behaviour
of 1g from its relatives in the series 1a-h is attributable to the electron transfer facilitated by
electronic factors: amine radical cation generated in this case is stabilized by transannular
electron shuttling between ring oxygen and nitrogen atoms present in the morpholine
appendage. The highest fluorescence quenching efficiency shown by 1f (99%) among the
series is expected to be due to the presence of a benzyl moiety on the nitrogen. Presence of an
additional arene unit in the molecule increases the possibilities of excited state interactions
like intramolecular arene-arene and amine-arene along with usual anthracene-amine
interaction. This increases the magnitude of PET there by resulting in more effective
5

quenching. The decay kinetics of 1f shows multi-exponential type decay that indicates the
presence of more than two transient species during the excitation.
In continuation, we have established inverse relation between change in free energy
(GET) and percentage of fluorescence quenching. The change in free energy for
photoinduced electron transfer reaction was evaluated on the basis of Rehm-Weller
equation.46-48 (eq. 1)
GET = Eox – Ered – E0,0 + e2/4d
(1)
where, E0,0 is the singlet excitation energy in eV, Eox is the oxidation potential of the
donor and Ered is the reduction potential of the acceptor,  is the dielectric constant of the
solvent and d is the distance between donor and acceptor. Reduction potential of 9-
methylanthracene (-1.97 eV)49, and singlet state energy of 9-methylanthracene (3.21 eV) were
used for calculations. It is important to note that these calculations are made by considering
the amine donor and anthracene acceptor as separate moieties.
The change in free energy value for the electron transfer from the tertiary amine moiety
to the singlet excited state of the anthracene chromophore is found to be < 0 for 1a-h, which
predicts a facile quenching of the anthracene fluorescence by the tertiary amine component in
1a-h through intramolecular electron transfer mechanism (Table 2). The values are in
agreement with the quenching characteristic shown by these compounds. GET is highest
among the series for 1h (-0.08 eV), which shows the lowest quenching characteristics. On the
other hand, for 1e, f and g, GET is low (-0.64, -0.69, -0.66 eV respectively) which shows
high quenching in the series.
GET (eV) GET (kcal mol-1)
Compound ED/D+
0.80
0.81
0.83
0.89
0.60
0.55
0.58
1.16
-0.44
-0.43
-0.41
-0.35
-0.64
-0.69
-0.66
-0.08
-10.09
-9.87
1a
1b
1c
1d
1e
1f
-9.42
-8.04
-14.71
-15.86
-15.17
-1.83
1g
1h
Table 2. Calculated free energy GET for anthracene/amine electron transfer; EA/A- = -1.97
eV (vs. SCE); E0/0 = 3.21 eV; ED/D+ (oxidation potential of the donor) values are taken from
literature.50,51
Emission spectrum of 2 clearly reveals a vibronically resolved emission band and no
broad emission. Similarly compounds 1a-h, also have a vibronically resolved emission band
but there was no exciplex emission observed in the fluorescence emission spectra in solvents
like methanol and acetonitrile. Absence of exciplex emission indicates that these solvents are
inefficient to stabilise the possible exciplex species of 1a-h and electron transfer may be
taking place directly to form a radical cation-radical anion pair resulting in quenching of
locally excited state of anthracene in these molecules. There are reports of exciplex emission
6

in the analogous N,N-dimethyl(phenanthren-9-yl)methanamines52 in highly polar solvent
mixtures like DMSO/DMF and H2O. The decay kinetics of 1a-h was studied using time-
correlated single photon counting technique (TCSPC) in methanol at room temperature
(Figure 4). Compounds 1a-h exhibited bi-exponential decay, whereas uni-exponential decay
was observed in the case of 2 (Table 3). There was an enhancement of anthracene
fluorescence lifetime 2 in 1a-h compared to 2. The lifetime 1 of 1a-h are smaller than 2 as
expected indicating that 1 corresponds to short-lived transient species formed in the excited
state.
1a
1e
2
1b
1f
1c
1g
1d
1h
104
103
102
101
100
0
10
20 30
time (ns)
40
50
60
Figure 4. Decay curves of 1a-h and 2 in 10-5 M methanol
In the photo-excited state, we anticipate that electron transfer takes place from the
amine component to anthracene giving rise to a short-lived intramolecular charge transfer
(CT) complex 3. Radiationless energy decay will give rise to a radical cation-radical anion
pair 4 followed by charge recombination to generate the parent (anthracen-9-yl)methanamine
in the excited state (Scheme 1). Emission from CT complex 3 has not been detected in the
emission spectrum of 1a-h in methanol or acetonitrile. We propose that 3, the transient short-
lived (1) species remains undetected by the spectrometer.
Compound
% Composition
96.16
95.57
96.52
97.82
86.94
*
% Composition
2 ns
1 ns
0.87 ± 0.09
1.61 ± 0.06
2.31 ± 0.21
0.52 ± 0.00
1.64 ± 0.03
*
0.92 ± 0.25
2.80 0.74
-
7.83 ± 0.02
7.89 ± 0.01
8.16 ± 0.02
7.43 ± 0.01
6.37 ± 0.01
8.80 ± 0.04
6.69 ± 0.01
5.92 ± 0.01
4.07 ± 0.00
3.84
4.43
3.48
2.18
13.06
*
13.35
0.07
-
1a
1b
1c
1d
1e
1f
1g
1h
2
86.65
99.93
100
*multiexponential decay
Table 3. Excited state lifetime of 1a-h; 

1
2 is the local anthracene fluorescence lifetime and
is the life time of a transient species.
7

With respect to 2, lifetime of the locally excited state of anthracene chromophore (2) is
higher for 1a-h. The decay kinetics shows that the major component present in the excited
state is the longer-lived species corresponding to the local emission from anthracene. Steady
state fluorescence emission spectra of 1a-h show efficient quenching of anthracene
fluorescence by the amine component. These observations suggest that the decay of short-
lived exciplex/charge transfer species takes place via back electron transfer giving rise to the
local excited state of anthracene (1*). Apparently, the minor emitting component with shorter
lifetime which is detected only in the TCSPC is expected to be a transient species 3* formed
through charge transfer and is expected to be in rapid equilibrium with 1* as shown in
Scheme 1.53 Radiationless energy loss from 3* gives the intramolecular radical cation-radical
anion pair 4 that by back electron transfer reverts to 1. We attribute fast equilibrium between
1* and 3* for enhanced lifetime for the locally excited state. Anomalous behaviour of 1g is
consistent with this argument; transannular electron shuttling in 3g* effectively drives the
equilibrium between 1g* and 3g* in favour of 3g.*
Scheme 1. Proposed mechanism of PET quenching for (anthracen-9-yl)methanamines 1a-h.
We propose that the locally excited state of anthracene (1*) is populated during the
process and the enhancement of lifetime is attributable to the time delay in electron transfer-
back electron transfer process. The highest 2 (8.8 ns) in the series is for 1f possessing a N-
benzyl substituent, which is understandable on the basis of additional excited state species
available due to the presence of benzyl group, which are in equilibrium with the local excited
state of the molecule via reversible electron/charge transfer.
The enhancement in lifetime of anthracene fluorescence can also be due to intersystem
crossing to form the triplet excited state of the species. The anthracene triplets formed may
again undergo a non-emissive electron transfer process or regeneration of singlet-excited
state via triplet-triplet annihilation (P-type) or thermal repopulation (N-type). However, there
is no evidence so far for the plausible occurrence of a triplet pathway in PET quenching in
similar systems.5,42
Fluorescence quantum yields for 1a-h in methanol solvent were calculated using
equation 2,
0
ODa If f
f
=
f0
ODa If0 0
(2)
8

0
where, ODa and ODa are the absorbance of standard and unknown, respectively. If0
and If are the areas of fluorescence peaks of the standard and unknown, respectively. 0
(1.3749, n-hexane) and f (1.3284) are the solvent refractive indices of standard and
0
unknown. f is the fluorescence quantum yields of the standard (0.4, 9-methylanthracene in
n-hexane). The relative fluorescence quantum yield of 1a-g showed considerable decrease in
comparison to with 2. Among the anthracenemethanamines examined by us, 1h (0.38) shows
highest f (Table 4).
Applying steady-state approximation to fluorescence quenching, rate of quenching kex
is determined from the equation (eq. 3)
kex = [(f0/f)-1] / 
(3)
where, f0/f is the ratio of the fluorescence quantum yield of the reference to that of
1a-h and  is the lifetime of 2 (4.065 ns) in methanol. The values obtained are in accord
with the GET values (Table 4). High rate constant for 1e-g explains the ease of quenching in
the compounds whereas low rate constant in the case of 1h indicates a lower rate of
quenching.
Compound
ODa (at
365 nm)
If
kex x 109 s-1
f
GET (eV)
0.025
0.019
0.021
0.020
0.053
0.028
0.016
0.026
73.3
76.2
88.0
0.049
0.067
0.071
1.7362
1.2039
1.1405
0.6234
6.3358
12.7726
11.3056
0.0296
-0.44
-0.43
-0.41
-0.35
-0.64
-0.69
-0.66
-0.08
1a
1b
1c
1d
1e
1f
133.7 0.112
46.8
12.5
8.1
0.015
0.007
0.008
1g
1h
548.2 0.357
Table 4. f is the relative quantum yield keeping 9-methylanthracene (f0 = 0.4, in n-
hexane) as standard; ODa = optical density; If = integrated emission intensity; kex= rate
constant.
Scheme 2. Products formed by the PET reaction of (anthracen-9-yl)methanamines 1a-h.54
(Anthracen-9-yl)methanamines 1a-h undergo photochemical reaction to give a variety
of products (Scheme 2).54 We have proposed plausible mechanisms to account for the
observed transformations with 4 as a common intermediate. Interestingly, rate of
disappearance of (anthracen-9-yl)methanamines 1a-h varied from substrate to substrate. It
9

was observed that the rate of quenching, faster the reaction. This observation is easily
explainable on the differential rate of generation of 4 as a consequence of excited state
electron transfer (both forward and backward as indicated in Scheme 1). From these studies,
we can infer that the percentage of PET quenching is inversely correlated with the rate of the
reaction (Table 5).the reaction could be qualitatively correlated with percentage fluorescence
quenching of (anthracen-9-yl)methanamines: higher the rate of fluorescence
Percentage of
Fluorescence
Quenching
Time taken for the
complete conversion
of starting material,
Compound
(compared to 2) min
90
420
420
390
420
90
1a
1b
1c
1d
1e
1f
90
88
82
94
99
98
28
70
70
1g
1h
> 600
Table 5. Comparison of the percentage of PET quenching with time taken for the complete
conversion of starting materials 1a-h.
We have thus illustrated the PET behavior of (anthracen-9-yl)methanamines 1a-h
with respect to 9-methylanthracene (2) in methanol solvent. The formation and behaviour of
exciplex/charge transfer complex are found to be dependent on several factors. However, an
exciplex emission is not observed in these cases when methanol was used as solvent. Ease of
PET quenching is dependent upon the redox potentials of the donor (amine) and acceptor
(anthracene) components in these systems.
Figure 5. Schematic Picture illustrating electronic photo initiated processes
10

Conclusion
We have examined the photoinduced electron transfer quenching of (anthracen-9-
yl)methanamines by studying their photophysical properties photochemical transformations.46
From these studies we have come to the conclusion that percentage of fluorescence
quenching was directly correlated with the electron availability on the donor nitrogen atom.
(Anthracen-9-yl)methanamines showing high percentage of fluorescence quenching, have
higher reaction rate and lower value of change in free energy. From the structural
optimization we concluded that geometry has negligible effect on the percentage of
fluorescence quenching of (anthracen-9-yl)methanamines examined by us.
Experimental
General Techniques
All reactions were carried out using oven dried glasswares. All experiments were
done with distilled and dried solvents by using standard protocols. All starting materials were
purchased from either Sigma-Aldrich or Spectrochem Chemicals and were used without
further purification. Fluorescence decay measurements of the (anthracen-9-yl)methylamines
1a-h and 9-methylanthracene (2) were carried out using the time correlated single photon
counting technique (TCSPC) with micro channel plate photomultiplier tube (MCP-PMT) as
detector and picosecond laser as excitation source. Details of the experimental setup for
photopohysical studies and methods of preparation, purification and characterization of 1a-h
are available elsewhere.
Author Statement
Rani Mathew: Visualization, Investigation Rekha R. Mallia: Investigation, Writing - Original Draft,
Data Curation, Formal analysis Suja Haridas: Project administration Jomon P. Jacob:
Conceptualization, Methodology, Supervision
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
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