Studies on unimolecular and bimolecular photoprocesses of a newly synthesized selenium compound, 7-chloro-2-phenyl-9H-[1]-benzopyrano[3,2-b]-selenophene-9-one (SeP)

Article abstract of DOI:10.1016/S1386-1425(00)00487-X

Using steady state/time resolved spectroscopic and electrochemical techniques the spectroscopic and photophysical studies were made on a novel synthesized selenophene compound SeP in nonpolar methylcyclohexane (MCH), polar aprotic acetonitrile (ACN) and p

Full text of DOI:10.1016/S1386-1425(00)00487-X

Spectrochimica Acta Part A 57 (2001) 1427–1441
www.elsevier.nl/locate/saa
Studies on unimolecular and bimolecular photoprocesses of
a newly synthesized selenium compound,
7-chloro-2-phenyl-9H-[1]-benzopyrano[3,2-b]-
selenophene-9-one (SeP)
A.K. De a, S.K. De b, A.K. Mallik b, T. Ganguly a,*
a Department of Spectroscopy, Indian Association for the Culti6ation of Science, Jada6pur, Calcutta 700 032, India
b Department of Chemistry, Jada6pur Uni6ersity, Jada6pur, Calcutta 700 032, India
Received 13 October 2000; accepted 24 October 2000
Abstract
Using steady state/time resolved spectroscopic and electrochemical techniques the spectroscopic and photophysical
studies were made on a novel synthesized selenophene compound SeP in nonpolar methylcyclohexane (MCH), polar
aprotic acetonitrile (ACN) and polar protic ethanol (EtOH) solvents at the ambient temperature as well as at 77 K.
Both from the studies on unimolecular and bimolecular photoprocesses this selenophene compound was found to
1
1
1
1
1
possess several electronic levels, Bb, La, Lb (all are of pp* nature and Lb is hidden within La band envelop like
the characteristics of most of the acenes) and 1(nOp*) state arising due to carbonyl oxygen atom. In polar ACN
environment this nOp* state disappears because it moves within the envelop of intense 1La band due to large
destabilization. Large overlapping of different band systems within the 1La band of SeP was confirmed from the
observed depolarization effect. The lack of phosphorescence of SeP both in MCH and EtOH rigid glassy matrix at
77 K has been inferred due to large vibronic interactions between closely lying triplets of the corresponding 1nOp* and
1Lb states. From the bimolecular investigations, it reveals that SeP acts as a good electron donor in presence of the
well known electron acceptor 9 cyanoanthracene (9CNA). Transient absorption spectra measured by laser flash
photolysis technique demonstrate the formation of ion-pair when the acceptor is excited. From the analysis of the
fluorescence quenching data it seemingly indicates that the major contribution in the diminution of the fluorescence
intensity of the acceptor 9CNA in presence of SeP is not only due to the photoinduced electron transfer (ET) but also
originates from static type (instantaneous) quenching processes along with external heavy atom effect. The possibility
of occurrence of photoinduced ET reaction in Marcus inverted region is hinted. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Selenophene; Time resolved spectroscopy; Photoinduced electron transfer; Static quenching; Dynamic quenching
* Corresponding author. Tel.: +91-033-4734971/+91-33-4733073 (ext. 266); fax: +91-33-473 2805.
E-mail address: sptg@mahendra.iacs.res.in (T. Ganguly).
1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(00)00487-X

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A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1. Introduction
F-4500 Fluorescence spectrophotometer respec-
tively. The degree of polarization (P) values were
measured with UV-VIS polarizer accessories pur-
chased from Oriel Instruments, USA by using the
formula:
As the origin of life itself must have seen photo-
physical and photochemical acts, it appears that
photophysical studies on the systems containing
Se should be useful since it possesses a significant
role in protecting our lives [1]. In the present
investigation a novel synthesized biologically ac-
tive selenium compound, 7-chloro-2-phenyl-9H-
[1]benzopyrano[3,2-b]-selenophene-9-ones (SeP),
was chosen, to study both of its unimolecular and
bimolecular photophysical properties by steady
state and time resolved spectroscopic techniques.
In bimolecular photoprocesses special attention
was given to reveal the mechanism of the photoin-
duced electron transfer (ET) reaction, if any as
studies on photoinduced ET reactions using sele-
nium compound as one of the reacting partners
are rather scarce.
P=[IEE−(IBE/IBB)IEB]/{IEE+(IBE/IBB)IEB].
Where IEE and IEB are the intensities of parallel
and perpendicular polarized emission with verti-
cally polarized excitation and IBB and IBE are the
intensities of horizontally and vertically polarized
emission when excited with horizontally polarized
light. IBE/IBB defines the instrumental correction
factor G (polarization characteristic of the photo-
metric system). This correction is done for any
change in the sensitivity of the emission channel
for the vertically and horizontally polarized
components.
The fluorescence lifetimes of the samples were
measured by using a time correlated single photon
counting (TCSPC) fluorimeter model 199 Edin-
burgh Instruments U.K. having conventional L
format arrangement. The excitation source con-
sists of an all metal coaxial N2 flash lamp with an
instrumental response function of about 1.2 ns
FWHM at 30 kHz repetition rate. The fluores-
cence decays were analyzed by using deconvolu-
tion fit over the entire decay including the rising
edge. The goodness of the fit has been assessed
with the help of some statistical parameters, e.g.
x2 and Durbin–Watson (DW) values. All the
solutions used for the room temperature measure-
ments were deoxygenated by purging nitrogen gas
stream for about 30 min. Electrochemical mea-
surements were done by using PAR model 370-4
electrochemistry system incorporating a model
174A polarographic analyzer, model 175 universal
programmer model RE0074X-Y recorder. Three
electrode systems including SCE were used in the
measurements. Tetraethylammonium perchlorate
(TEAP) in ACN was used as supporting elec-
trolyte. The value of fluorescence quantum yield
(ƒ0f ) of donor SeP in ACN fluid solution was
determined at room temperature (296 K) relative
to that of carbazole in the same solvent (ƒ0F=
0.4090.03) [2]. Transient absorption spectra at
the different delay times between the exciting and
analyzing pulses were measured with the help of
The investigations were carried out both at the
ambient temperature and at 77 K using steady
state/time resolved spectroscopic and electrochem-
ical techniques in nonpolar MCH as well as in
polar ACN and EtOH media. The results ob-
tained from the studies are described below.
2. Experimental
2.1. Materials and their purifications
The samples 9CNA supplied by Aldrich and
SeP, which is newly synthesized (for method of
synthesis and characterization, vide infra) were
purified by vacuum sublimation. The solvents ace-
tonitrile (ACN), methylcyclohexane (MCH) and
ethanol (EtOH) supplied by E Marck of spectro-
scopic grade were distilled under reduced pressure
and tested for absence of any emission in the
wavelength region studied.
2.2. Experimental apparatus
The steady state electronic absorption and
emission spectra of moderate to dilute (10−3
–
10−6 M) solutions of the samples at ambient
temperature were recorded by Shimadzu UV-VIS
spectrophotometer of model 2101PC and Hitachi

A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1429
nanosecond flash photolysis setup (Applied Pho-
2.3. Analytical and spectral data of 3a and 3b
tophysics) comprises of Nd:YAG laser (DCR-11,
Spectra Physics). The sample was excited by 355
nm laser light (third harmonic) (FWHMꢀ8 ns).
The pulsed Xe lamp of 250 W was used as probe.
The photomultiplier (IP 28) output was fed into a
storage oscilloscope (TDS-540 Tektronix, 500
MHz, 1 Giga sampling, GS, s−1) and the storage
data were transferred to a computer through a
GPIB/IEEE interface.
3a: m.p. 183–184°C, Anal.: Found: C, 62.69;
H, 3.05%; Calcd. For C17H10O2Se: C, 62.78; H,
1
3.08%. IR (Nujol, w, cm−1): 1660 (CO). H NMR
(300 MHz, CDCl3): l7.41–7.57 (5H, m, Ar-H),
7.62–7.73 (4H, m, Ar-H) and 8.32 (1H, dd, J=8
and 1.4 Hz, H-8). 13C NMR (75 MHz, CDCl3):
l116.61 (C-3), 117.78 (C-5), 121.72 (C-8a), 124.70
(C-7), 125.95 (C-8), 126.42 (C-3%,5%), 129.24 (C-
2%,6%), 129.90 (C-4%), 133.58 (C-6), 134.81 (C-1%),
156.14 (C-4a), 157.58 (C-2), 160.53 (C-3a), 173.14
(C-9a) and 185.90 (C-9). EIMS: m/z (rel. int.)
326(100) and 324(58.1) (M+), 298(25.9) and
296(13.1) (M+-CO), 218(50.1, M+-CO-Se).
2.2.1. Synthesis and characterization of SeP
Among the methods reported for conversion of
2%-hydroxy-ꢀ-cinnamylidene-acetophenones (1 in
Fig. 1) to 2-styrylchromones (2 in Fig. 1), the
selenium dioxide oxidation appears to be the old-
est [3]. This reaction was originally well-known
for conversion of 2%-hydroxychalcones to flavones
[4]. In connection with some other problem, we
required 2 and therefore carried out selenium
dioxide oxidation of 1. Thus, when 1a (1 mmol;
Fig. 1) was refluxed with selenium dioxide (1.2
mmol) in isoamyl alcohol, the starting material
disappeared completly within 15 h. Chromatogra-
phy of the crude product mixture over silica gel
afforded 2a (yield 52%; Fig. 1) along with an
unusual product characterized as 2-phenyl-9H-
[1]benzopyrano[3,2-b]selenophene-9-one (3a; 31%;
Fig. 1) from its special features. Similarly, 3b was
obtained (35%) from 1b. The compounds 3a and
3b belong to a class of heterocycles not known
previously. The present photophysical investiga-
tion was done with the compound 3b (SeP).
3b: m.p. 219–220°C. Anal: Found: C, 56.48; H,
2.61%; Calcd for C17H9O2SeCl: C, 56.77; H,
1
2.52%. IR (Nujol, w, cm−1): 1660 (CO). H NMR
(300 MHz, CDCl3): l7.45–7.50 (4H, m, Ar-H),
7.62–7.66 (4H, m, Ar-H) and 8.28 (1H, d, J=2
Hz, H-8).
According to the revised mechanism of sele-
nium dioxide oxidation of ketones [5], their a-po-
sition becomes bonded to selenium directly, but
not through oxygen [6]. Thus, the plausible mech-
anism delineated in Scheme 1 may be suggested
the formation of 3. That 3 was not formed by
interaction of 2 with either selenium or selenium
dioxide under the reaction condition employed
was ascertained from negative results obtained
from separate experiments involving 2 and these
two reagents.
Fig. 1. Synthesis of SeP.

1430
A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
Scheme 1. Plausible mechanism of formation of SeP.
3. Results and discussion
arising due to transition involved within the n-
electron on the oxygen atom of carbonyl group
(the subscript ‘O’ for the oxygen atom) and the
antibonding p-orbital of the benzene ring. No
such long wavelength band system was found in
highly polar ACN solvent. Possibly due to large
destabilization in this high dielectric medium this
nOp* band being largely blue shifted is hidden
3.1. Electronic absorption spectra of SeP in
nonpolar MCH and highly polar ACN at the
ambient temperature
3.1.1. Assignment of the different band systems
obser6ed in different en6ironments
1
well-within the envelop of La band system.
Fig. 2a reproduces the electronic absorption
spectra of SeP in both MCH and ACN solvents.
On comparing these spectra with those of the
analogous molecule thiophenanthrene [7], which
is known to possess very similar electronic spectra
as anthracene, it seems logical to assign the p-elec-
1
From Fig. 2a it is also apparent that the La
band of SeP suffers a red shift with increase of
polarity of the surrounding medium, i.e. from
nonpolar MCH to highly polar ACN solvent.
This indicates the charge transfer character of this
band which tends to stabilize in ACN.
1
tronic absorption bands of SeP at 330 nm to La
and band systems within 225–275 nm region (two
1
3.2. Unimolecular photoprocesses of SeP in
different sol6ents at the ambient temperature
peaks around 230 and 260 nm) as Bb. For this
1
molecule no Lb band is apparent but it is possi-
ble, as it is the characteristics of most of the
acenes, that this band might be submerged under
the intense band of La. Another important obser-
3.2.1. Fluorescence of SeP in different sol6ents at
the ambient temperature
1
vation is that a very weak but genuine long
wavelength band of broad nature was noticed for
this synthesized selenium compound SeP only in
nonpolar MCH environment at the wavelength
region between 400 to 500 nm. The inset of Fig.
2a shows the magnified spectrum near 450 nm
region showing the peak position at around 487
nm of this long wavelength absorption band. This
band might be designated as singlet nOp* state
Using the excitation wavelength at 330 nm,
which is the peak position of the La absorption
1
band, a very weak fluorescence emission band of
low quantum yield (ƒf=1.32×10−3) was ob-
served for SeP in ACN surrounding medium (Fig.
2b). However, in nonpolar medium MCH, SeP
becomes totally nonemissive.
From the electronic absorption spectra of SeP
1
(Fig. 2a) it is apparent that as in MCH (nOp*)

A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1431
SeP is nonfluorescent in MCH (Fig. 2b). On the
other hand in polar solvent ACN it is possible
that the change in the order of the energy levels
might occur within 1Lb(pp*) and 1(nOp*) states
state is the lowest excited state, the fluorescence
emission should occur from this state. But possi-
bly due to the space forbidden character of the
1
transition involving (nOp*) and the ground state,
Fig. 2. (a) The electronic absorption spectra of SeP (ꢀ10−5 mol dm−3) in MCH and ACN media (inset shows the magnified
version of the longer wavelength band at 487 nm region in MCH) at the ambient temperature. (b) Fluorescence emission spectra
of SeP (uexꢀ334 nm; ꢀ10−5 mol dm−3) in MCH (1) and in ACN solvent (2) at the ambient temperature.

1432
A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
As ACN does not form glass at 77 K, the low
temperature spectra were recorded using another
polar solvent ethanol (EtOH) as rigid glassy ma-
trix. Unfortunately in this matrix SeP does not
emit phosphorescence. Thus the internal heavy
atom effect on phosphorescence spectra of SeP
was not clear. However in MCH rigid glassy
matrix at 77 K SeP exhibits only fluorescence but
no phosphorescence. Thus the low temperature
event in this matrix differs from that observed at
the ambient temperature where no detectable
fluorescence was apparent. It seems as excitation
1
was made in La band (ꢀ330 nm), nonradiative
relaxation required to come down to the lowest
1
state (nOp*), may not occur within the lifetime of
1
the La state at 77 K. This phenomenon is not
rare at such low temperature as relaxation time
increases due to lowering down of temperature. It
seems logical to presume that fluorescence emis-
1
sion might mainly originate from La level. How-
ever, to corroborate this proposition about the
nature of the fluorescence emission band observed
in MCH matrix at 77 K the steady state polariza-
tion measurements were done on the fluorescence
excitation spectra of SeP which looks identical
with its electronic absorption spectra. To record
the excitation spectra the monitoring fluorescence
emission wavelength was chosen at 400 nm posi-
tion. Fig. 4 reproduces the polarized fluorescence
excitation spectra of SeP in MCH rigid glassy
matrix within the region of 320–380 nm which
Fig. 3. Possible radiative and nonradiative deactivation scheme
of photoexcited SeP in different environments.
causing the former level as the lowest emitting
state. In such a situation the fluorescence emission
could be expected which was actually observed in
the present investigation. Very weak fluorescence
intensity in ACN might be due to internal heavy
atom effect as SeP contains selenium. The possi-
ble radiative and nonradiative transitions within
the different electronic levels of SeP in both MCH
and ACN solvents at the ambient temperature
and in MCH rigid glassy matrix at 77 K (discus-
sion on low temperature results are made below)
are shown in the Fig. 3.
In order to check whether internal heavy atom
effect is really responsible for weak fluorescence
intensity of SeP in ACN, an attempt was made to
measure low temperature (77 K) spectra to ob-
serve the phosphorescence of SeP, if any. It is
known that heavy atom substituents tend to re-
duce the fluorescence quantum yield ƒf in favor of
1
covers the envelop of La band system.
It has been proposed above that the nature of
330 nm band observed in the electronic absorp-
1
tion spectra of SeP is mainly of character La and
1Lb band is hidden, as it was observed in cases of
1
most acenes, within this La envelop.
Fig. 4 shows depolarization effect both at the
1
red edge and at the blue edge of the La absorp-
tion band. Nearly at the middle of the band
(around 345–355 nm region) positive values of
degree of polarization, P, of moderate magnitudes
were observed. As it is apparent from Fig. 2a at
the blue edge of the 330 nm band there is a
1
1
mixture of two states Bb and La. So this might
cause depolarization. Moreover it is logical to
presume that 1Lb band might reside at the red
edge of the 1La band (generally 1Lb possesses
phosphorescence emission ƒ causing ƒp/ƒf\1.
p

A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1433
1
lower energetic level than La). Thus at this edge
also there might be a mixture of the two states 1Lb
and 1La and this might cause depolarization in
probability from the lowest triplet state and
simultaneously enhance the nonradiative transi-
tions from this level.
1
this region. In the middle of the La band, which
As an alternative mechanism it could be sug-
gested that in SeP due to presence of heavy atom
Se nonradiative transition may be promoted
largely through internal conversion (IC) process
(1La to S0) rather than intersystem crossing. In
favor of this argument it can be stated that in case
of xanthene dyes it was observed [9] that with
increase of the size of the halosubstituents (from
chlorine to bromine to iodine) the phosphores-
cence lifetime decreases simultaneously with the
quantum yield of the fluorescence and thus phos-
1
1
seems to be free from overlapping of Bb or Lb
band system, positive degree of polarization value
of moderate magnitude was observed. Thus it
seems the majority contribution for fluorescence
emission originates from 1La state because had the
emission been originated from Lb state, the value
of P in the middle of the spectra, which seems to
be entirely within La domain, should show nega-
tive value. However low positive value of P (ꢀ
1
1
1
0.09) in the middle region of the La absorption
band indicates that even in this region mixing, of
course of lesser extent, might exist between the
phorescence
proportionally.
Next, we will discuss the possible role of SeP in
bimolecular quenching reactions.
efficiency
is
not
increased
1
1
1La state and the neighboring state Lb (or Bb).
3.2.2. Why no phosphorescence was obser6ed both
in MCH and EtOH rigid glassy matrices at 77 K
Two possible mechanisms have been proposed
for the lack of phosphorescence of SeP in both
MCH and EtOH rigid glassy matrix at 77 K. It is
possible, as it was found in cases of some hetero-
cyclic molecular systems [8], that the triplet levels
corresponding to closely lying 1Lb(pp*) and
1(nOp*) states are very close to each other as
depicted in the Fig. 3. As these two triplet states
are of opposite nature, np* and pp*, large vi-
bronic interactions might occur within themselves
and this might reduce the radiative transition
3.3. Bimolecular photoprocesses
Before going to study the bimolecular photo-
processes in which one of the reacting species is
SeP, it seems important to measure its half-wave
potential by electrochemical measurements to ex-
amine its electron donating or accepting ability.
3.3.1. Electrochemical measurements
From Table 1 it is seen that in ACN solvent
SeP has a low positive value (0.7 eV) for half-
Fig. 4. Polarized fluorescence excitation spectra of SeP in MCH rigid glassy matrix (uemꢀ400 nm).

1434
A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
Table 1
Redox potential values, values of E* and Gibbs free energies (DG0) for photoinduced electron transfer (ET) reactions within the
0,0
present donor and acceptor molecules in highly polar solvent ACN at the ambient temperature
ox
1/2
red
1/2
Donors
Acceptors
E
(D/D) (V)
E
(A−/A) (V)
E0,0* (eV)
DG0 (eV)
SeP
SeP
SeP*
9CNA
9CNA*
9CNA
0.7
0.7
0.7
−1.13
−1.13
−1.13
1.83
−1.25
−1.87
3.08
3.7
* Denotes the excited singlet state S1.
wave oxidation potential measured by cyclic
voltammetry (details are given in the experimen-
tal section). This shows SeP might behave as a
strong electron donor when it would be sensitized
with a suitable acceptor. In the present investiga-
tion we chose the well known acceptor 9CNA
whose redox potential was found to be −1.13 eV
(Table 1). The Gibbs free energy change (DG0)
associated with the electron transfer (ET) reac-
tion for the present D-A systems are computed
by using the well-known Rehm–Weller relation
(1) [10,11]
3.3.2. Transient absorption spectra
With the help of Nd:YAG laser system pulsed
laser excitation was made at 355 nm wavelength.
This was done to excite only the acceptor moiety
from the mixture of the donor SeP and the accep-
tor 9CNA in ACN solvent. This excitation pro-
duces a transient species which absorbs at 440 nm
along with a weak shoulder at around 508 nm
region (Fig. 5). Following the observations made
earlier [13,14] the band at 440 nm may be as-
signed to the triplet–triplet absorption band of
monomeric 9CNA. With increase of delay times
from 0.4 ms to 1.4 ms to 2.4 ms, 440 nm band
diminishes very fast relative to the shoulder at
508 nm. This indicates that the two different
species should be responsible for these two bands.
The earlier flash photolysis experiment on
9CNA in ACN solvent showed that free 9CNA-
anion absorbs at 525 nm [12]. Thus in the present
investigation, the transient band observed at 508
nm might possibly be due to tight contact ion-pair
(SeC+/9CNA−) from which the radical anion of
the acceptor species is accessible with the help of
the present experimental set up. The interionic
distance is much less in such type of contact ion
pair than that of solvent separated ion-pair com-
plex, due to which the free radical anionic species
absorb at much longer wavelength region than the
ions in the former complex. The geminate recom-
bination takes place more efficiently in the tight
contact ion-pair than dissociation into free radical
ions and this causes the production of the
monomeric triplet state of the excited species.
Possibly this is the reason why much larger tran-
sient triplet absorption band of monomeric 9CNA
is observed relative to its anion (Fig. 5), which
appears as a shoulder at around 508 nm position
DG0=E (D/D+)−E
(A-/A)−E*
OX
1/2
RED
1/2
0,0
e2
−
.
(1)
4ym0mSR
As the contribution of the last term (coulomb
stabilization term) is negligible in ACN solvent
(ꢀ0.06 eV) [10,12], DG0 values were computed
by ignoring this term.
From Table 1 it is evident that photoinduced
ET reactions between the donor SeP and the
electron acceptor 9CNA is highly exergonic
(DG0B0), i.e. energetically favorable when one
of the chromophores are excited. However posi-
tive value of DG0, observed from the unexcited
reactants, demonstrates the fact that the possibil-
ity of occurrences of ET reactions in the ground
state within the present reacting systems is very
slim.
For the direct evidence of the occurrences of
ET reactions the transient absorption spectra of
the acceptor 9CNA in presence of the electron
donor SeP was measured at the different delay
times between the exciting and analyzing pulses
by laser flash photolysis techniques. The results
are described below.

A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1435
(in inset of Fig. 5 magnified spectrum of the
3.4. Fluorescence quenching phenomena of the
9CNA− anion is shown). The observations of
transient absorption bands of ion-pair were in
accord to our expectation as the large negative
values of DG0 for the SeP–9CNA* system was
observed (Table 1) from the electrochemical
measurements.
acceptor 9CNA in presence of the electron donor
SeP in ACN fluid solution at the ambient
temperature
As photoinduced ET reactions are generally
facilitated in highly polar medium ACN, investi-
gations on quenching reactions were made in this
solvent.
To keep in conformity with the transient mea-
surements by flash photolysis techniques, the ac-
ceptor 9CNA was excited to its lowest excited
singlet state (ꢀ377 nm) in presence of the unex-
cited donor SeP. From Fig. 6a it is apparent that
at 377 nm position, where the acceptor 9CNA
As photoinduced ET reactions are nonradiative
transitions, one should expect the quenching phe-
nomena in the steady state fluorescence emission
spectrum of the acceptor 9CNA in presence of the
electron donor SeP in ACN solvent.
The results obtained from the fluorescence
quenching studies at the ambient temperature
have been presented below.
Fig. 5. Transient absorption spectra of 9CNA (excitation wavelength ꢀ355 nm; third harmonic of Nd:YAG laser system), obtained
by laser flash photolysis techniques, in ACN at the ambient temperature in presence of the donor SeP at the different delay times.
The delay times are (1) 0.4 ms; (2) 1.4 ms; (3) 2.4 ms (inset shows the enlarged version of 500–600 nm wavelength range to
demonstrate 9CNA− anionic band).

1436
A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
Fig. 6. (a) Electronic absorption spectra of the donor SeP (curve 1), acceptor 9 CNA (curve 2) and the mixture of the donor and
acceptor (curve 3) in ACN solvent at the ambient temperature. (b) Fluorescence emission spectra of 9CNA fluorescer (uexc=402
nm) in ACN solvent of concentration (2.78×10−6 mol dm−3) at the ambient temperature in presence of the quencher (donor) SeP
whose concentration (mol dm−3) in (1) 0; (2) 2.00×10−5; (3) 5.00×10−5
.
component is present concurrently with the dy-
namic process ET in quenching mechanism,
Stern–Volmer (SV) plot of fluorescence intensity
variation of the acceptor ( f0/f ) as a function of
quencher (donor SeP) concentrations was drawn
(D-A* system) [ f0 and f represent the relative
integrated fluorescence emission intensities of the
acceptor without and with the quencher (donor)
concentration respectively]. As depicted in Fig. 7,
SV plot shows clear positive deviation of quench-
ing data from linearity and is found to be curved
upwards. This phenomenon was observed in
many cases of quenching studies where static
quenching was present. However, unlike the situa-
tion generally observed in pure static quenching
phenomena, the fluorescence lifetime of 9CNA
was found to be affected significantly on gradual
addition of the donor molecules SeP in ACN
solvent (Table 2). This modifies our proposition
made from SV plot and suggests that the quench-
ing may not be pure static quenching in nature
and the positive deviation from the simple SV
relation might be introduced by the transient
component of dynamic quenching. In favor of
such proposition it could be mentioned that f0/f
absorbs, the donor is completely transparent.
Thus excitation at 377 nm avoids the competitive
absorption by the donor as well as its filtering
effect on the light emitted by the acceptor. From
the same figure it is also found that the electronic
absorption spectrum of the mixture of the donor
and the acceptor in ACN is just the superposition
of the individual absorption spectra of the inter-
acting partners SeP and 9CNA. This observation
excludes the possibility of the formation of
ground state complex whose presence could be
responsible for the observed fluorescence quench-
ing of the acceptor 9CNA. Moreover, as the
acceptor possesses lower lying electronic energy
levels than those of the donor SeP, the energy
transfer possibility, which is the another quench-
ing reaction, is forbidden in the present case as
the acceptor moiety was excited.
Thus the observed weak fluorescence quenching
(Fig. 6b) might be due to photoinduced ET reac-
tions whose existence within the present reacting
systems has already been confirmed from the tran-
sient absorption measurements as discussed
above. However, some sort of static quenching
might also be operative. To test whether static

Table 2
Room temperature data on fluorescence lifetime (~) of the acceptor 9CNA in presence of different quencher concentrations, values of KSV, kq, R, DG0 and u for
9CNA a–SeP system in ACN solvent
/
V (dm3 mol−1
)
kq (dm3 mol−1 s−1
)
R (A)
DG0 (eV)
−1.25
–
u (eV)
0.85
–
,
Conc. of 9CNA
(mol dm−3
Conc. of SeP
Solvent
ACN
~ (90.4)
(ns)
KSV
)
(mol dm−3
)
(dm3 mol−1
)
(r/A)
,
7.87×10−6
0
11.5
10.2
9.1
11.6
11.6
11.7
0.07
13
(17)
6.4×106
10
6.95×10−7
2.78×10−6
0
7.87×10−6
EtOH
–
–
–
–
1.11×10−5
1.2×10−4
7
(
a Denotes the first excited singlet state. u(=uI+us) was computed by using the procedure described elsewhere [10].
1
144

1438
A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
value, measured by steady state measurements,
and value of ~0/~, measured by time resolved
techniques, at the different donor (quencher) con-
centrations used are not similar to each other.
From the above observations it could be in-
ferred that in overall quenching mechanism
though dynamic quenching, through photoin-
duced ET process, is operative, the static (tran-
sient type) component is so overwhelming that the
plot becomes curved upwards.
for the present donor–acceptor system in ACN
solvent by considering the relation (2) as used by
earlier authors [15–18]
2y
J2
(DG0+u )2
ꢁ
n
S
kET
=
exp −
(2)
'
4kBTuS
ꢀ4ykBTuS
J2=J20 exp[−i(R−(rD+rA)]; J0 denotes the
transfer integral at R=rD+rA; i is the attenua-
tion coefficient with R; the other symbols were
described elsewhere [18].
We computed Marcus first order ET rates kET
Fig. 7. Plot of f0/f versus [Q] of 9CNA*–SeP in ACN solvent at the ambient temperature. The nonlinear least square curve fitting
procedure was used to treat the data according to the model: f0/f=(1+V[Q])(1+KSV[Q])(exp k[Q]) (see text), the graph of residual
is plotted below.

A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1439
We tried to dissect the quenching data into its
dynamic and static (instantaneous type) compo-
nents by using the model used by Eftink and
Ghiron (Eq. (3)) [19].
f0//f=[1+V(Q)](1+KSV[Q])
(3)
where V is the transient (static type) parameter
and KSV is the collisional component. 1+V[Q] is
approximately equal to exp(V[Q]). This assump-
tions was made by most of the authors to treat
their data related to weak quenching reactions
[19] which was actually observed in the present
case.
The data were treated according to the above
model (Eq. (3)), the modified form of SV relation,
which comprises both static and dynamic part,
but fitting seems to become much improved (nor-
malization changes from 0.06 to 0.04) when an
additional term exp(k[Q]) is introduced as shown
below:
Fig. 8. Variation of first order electron transfer rate constant
,
(kET) with the distance of separation (R/A) between the
present D and A molecules, where DG0 is −1.25 eV. kET
values were obtained theoretically by using Eq. (2).
f0/f=(1+V[Q])(1+KSV[Q]) exp(k[Q])
(4)
Using the same procedure as adopted earlier
[18], the dependence of kET on separation distance
R is computed from the relation (2).
Fig. 8 represents the variation of kET with the
distance of separation (R) between the present
donor and acceptor systems. Clearly a maximum
was observed in this curve as it could be expected
for a highly exothermic reaction (DG0 for SeP–
9CNA* system in ACNꢀ −1.25 eV). The maxi-
mum value of kET was found not at the encounter
It seems the quenching observed in the present
case is somewhat complicated in nature and some
other radiationless reactions, apart from ET and
transient quenching, are also operative.
One possible reaction might be pure static
quenching which possibly might interfere with the
above two processes. Interestingly when the
quenching measurements were done in another
polar solvent EtOH, the same type of positive
deviation in SV curve was noticed. But time re-
solved studies (by measuring fluorescence life-
times) indicate that the nature of quenching here
is slightly different from that observed in ACN. In
EtOH, the fluorescence lifetime of the acceptor
remains unaffected (Table 2) in the presence of
the donor SeP. Thus the quenching might be
assigned to pure static quenching unlike the situa-
tion observed in the case of quenching reaction
between the same donor and acceptor system in
polar aprotic ACN solvent where transient
quenching (fluorescer lifetime changes in presence
of the quencher molecule) seems to be operative
as discussed above.
,
distance (ꢀ8.5 A) but at somewhat higher dis-
,
tance (ꢀ10 A, Fig. 8). It is logical to presume
,
that at the separation distance (R)ꢀ10 A, the
probability of occurrence of ET reactions should
be maximum in ACN solvent.
From the above observations it seemingly indi-
cates that in ACN environment a certain fraction
of the excited states of 9CNA acceptor is actually
quenched by ET mechanisms by the quenchers
,
(SeP) which reside at a distance of 10 A around
the fluorophore 9CNA. But most of the excited
states of the fluorophore are deactivated almost
instantaneously just after being formed as a
quencher molecule happens to be randomly posi-
tioned in the proximity (within the contact dis-
tance) of the fluorescer 9CNA at the time of
excitation.
Thus it could be surmised that the mechanisms
of bimolecular quenching processes involved
within SeP and 9CNA systems seem to be the

1440
A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
mixture of dynamic (photoinduced ET) and static
type (both pure and transient) quenching modes,
the existences of which are however, confirmed
from both steady state and time resolved tech-
niques. Further the possibility of external heavy
atom effect on the fluorescence quenching of the
acceptor 9CNA observed with gradual addition of
the donor (quencher) SeP could not be ignored as
the latter contains heavy atom ‘Se’. Thus it can be
inferred that in addition to the dynamic and static
quenching, another quenching reaction ‘external
heavy atom effect’ should possibly be involved in
the overall quenching mechanism. Possibly due to
presence of various quenching reactions, as pro-
posed above, from the different experimental find-
ings, the quenching data could not be fitted using
the model proposed by Eftink and Ghiron which
was used for the systems where only collisional
(dynamic) and static modes were present. From
the observations made in the present investiga-
tion, a model, as shown by Eq. (4), has been
proposed where an additional parameter ‘k’ has
been included in the proposed model of Eftink
and Ghiron and the experimental quenching data
fit in this model in a much better way as shown
above. The best fit of the quenching data to Eq.
(4) is reproduced in the Fig. 7.
the fluorescer but it might happen that after the
excitation of the 9CNA ring, the quencher (SeP)
,
can diffuse the extra 8–9 A (contact distance,
,
rD+rAꢀ8.5 A) so rapidly that the quenching still
appears instantaneous. This points to the dynamic
character predicted by the transient effect model
[19]. Thus from the present investigation it may be
surmised that SeP acts as a weak quencher but it
shows diverse character in quenching mechanism
of the excited singlet state (S1) of the acceptor
9CNA. It deactivates nonradiatively the S1 level
of 9CNA as an electron donor as well as heavy
atom quencher and also through pure static and
transient quenching mechanisms.
From the present investigation it seemingly in-
dicates that dynamic quenching is only due to
presence of highly exothermic (DG0ꢀ −1.25 eV)
photoinduced ET process within the present react-
ing systems. The very weak, much smaller than
the diffusion-controlled rate kd in ACN (ꢀ1.9×
1010 dm3 mol−1
s
−1) [20,21], dynamic quenching
−1) points to the
rate (6.4×106 dm3 mol−1
s
possibility of the occurrences of ET reaction in
Marcus inverted region (mir) where outer sphere
ET producing radical ion pair might not be so
fast as to give diffusion controlled ET quenching
rate. The possibility of occurrence of ET reaction
in mir is further corroborated from the finding of
larger −DG0 value than that of u, nuclear reorga-
nization energy parameter (Table 2). However for
conclusive evidence for mir, investigations using
other well known electron acceptors with SeP are
needed. The work is under progress.
The best fit values of KSV and V are 0.07 and 13
dm3 mol−1 respectively (Table 2). Large values
(ꢀ3300) were observed in case of k which might
arise from more than one quenching process (pos-
sibly from external heavy atom effect and static
quenching).
The values of KSV and V are in accord to our
expectation. Very small value of KSV (ꢀ0.07)
indicate very weak dynamic quenching rate (ꢀ
6.4×106 dm3 mol−1
s
−1) which actually appears
4. Concluding remarks
from the quenching studies as discussed above.
From the value of V(ꢀ13 dm3 mol−1), the ra-
dius r of the active volume was computed using
the well-known relationship
The newly synthesized selenophene compound
SeP exhibits singlet (n0p*) in nonpolar MCH and
1Lb(p,p*) state in highly polar ACN as a lowest
excited level. No emission was found from the
former state, whereas a very weak fluorescence
emission from the latter level possibly due to
internal heavy atom effect was observed. The
nonemissive behavior of the former state might be
due to space forbiddenness with the ground side.
The lack of phosphorescence of SeP has been
V
N%
4
3
= yr3 (N% is the Avogadro’s number
per mmol).
,
The value of r was estimated to be 17 A. This
indicates that at the exact moment of excitation
the quencher may not be in physical contact with

A.K. De et al. / Spectrochimica Acta Part A 57 (2001) 1427–1441
1441
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inferred due to occurrence of vibronic interactions
3
3
of the closely lying triplets nOp* and pp*. From
the experimental findings and analysis of the
quenching data it is revealed that primary contri-
butions in fluorescence quenching of 9CNA in
presence of SeP are due to static/instantaneous
modes along with external heavy atom effect.
Though the presence of highly exothermic pho-
toinduced ET reactions within the present react-
ing system was confirmed from the measured
values of DG0 and transient absorption measure-
ments using laser flash photolysis techniques, its
role in overall quenching mechanism is not very
significant as evidenced from observed upward
curvature of SV plot. SeP seems to play diverse
role in quenching mechanisms. Investigations with
other electron acceptors are now underway.
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Acknowledgements
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(1983) 40.
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[17] J.R. Bolton, J.A. Schmidt, T. Ho, J. Liu, K.J. Roach,
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[20] P. Jana, R. De, T. Ganguly, J. Luminescence 59 (1994) 1.
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We like to express our heartiest thanks to Dr
Samita Basu of Saha Institute of Nuclear Physics,
Calcutta for measurements of transient absorp-
tion spectra by laser flash photolysis techniques.
S. K. De gratefully acknowledges the financial
assistance from the UGC, New Delhi.
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