Non-radiative depletion of the excited electronic states of 9-cyanoanthracene in presence of tetrahydronaphthols.

Article abstract of DOI:10.1016/S1386-1425(02)00191-9

Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching mechanism of the lowest excited singlet (S1) and triplet (T1) states of the well known electron acceptor 9-Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol (TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex, 1 or 3(D+-H...A-), causes the formation of the neutral radical, 3(D+H-A)*, due to the complete detachment of the H-atom. It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.

Full text of DOI:10.1016/S1386-1425(02)00191-9

Spectrochimica Acta Part A 59 (2003) 525ꢃ535
/
www.elsevier.com/locate/saa
Non-radiative depletion of the excited electronic states of 9-
cyanoanthracene in presence of tetrahydronaphthols
a
T. Bhattacharya , T. Misra , M. Maiti , R.D. Saini , M. Chanda ,
a
a
b
c
c
a,
S. Lahiri , T. Ganguly ꢀ
a
Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Radiation Chemistry and Chemical Dynamics Division, Bhaba Atomic Research Centre, Mumbai 400 085, India
b
c
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
Received 21 March 2002; accepted 9 May 2002
Abstract
Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching
mechanism of the lowest excited singlet (S ) and triplet (T ) states of the well known electron acceptor 9-
Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol
TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence
1
1
(
of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar
acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching
phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN
solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-
bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-
1
or
3
ꢁ
bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex,
(D
A)ꢀ, due to the complete detachment of the H-atom.
It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.
2002 Elsevier Science B.V. All rights reserved.
Ã
/
ꢂ
3
HÁ Á ÁA ), causes the formation of the neutral radical, (Dꢁ
/
HÃ
/
#
Keywords: H-bonding; Electron transfer; Time resolved spectroscopy; Contact ion-pair; Neutral radical
1
. Introduction
Specially investigations on photoinduced electron
transfer (ET) [1ꢃ10] and excited state H-bonding
/
Photoinduced processes within various chemical
reactions and material systems are the rich field of
study for both photochemists and photophysicists.
interactions [1,11] are still the subjects of increas-
ing interest. Through photoinduced ET or excited
state H-bonding the separation of charge between
electron donor and acceptor proceeds followed by
charge recombination which might occur either by
first order or second order or both processes
depending upon the binding power of the contact
ꢀ
Corresponding author. Tel.: ꢁ
fax: ꢁ91-33-473-2805
E-mail address: sptg@mahendra.iacs.res.in (T. Ganguly).
/
91-33-473-4971/3073x266;
/
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 1 9 1 - 9

526
T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ535
/
ion-pair (CIP) initially formed. The behaviours of
the transient ion-pair state are of crucial impor-
tance for the photochemical and specially for the
photobiological reaction mechanism whose true
nature is yet to be explored. This tempted us to
study the photoinduced processes considering two
tetrahydronaphthols as electron donor molecules:
and polar environments to see the polarity effect
on the ion-pair or charge transfer (CT) species
involved. In the present paper the results are
described.
5
5
,6,7,8-tetrahydro-1-naphthol
,6,7,8-tetrahydro-2-naphthol (TH2N) (Fig. 1).
(TH1N)
and
2. Experimental
Both TH1N and TH2N possess tetralin-like ske-
letons and show wide spectrum of biological
2.1. Materials
activities [12ꢃ14]. The present study was made
/
on the photoinduced intermolecular interactions
between the above naphthols as donors and the
well-known electron acceptor 9-Cyanoanthracene
All the samples TH1N (99% pure), TH2N (98%
pure), 9CNA (97% pure), supplied by Aldrich,
were purified by vacuum sublimation. The solvents
n-heptane (NH), acetonitrile (ACN) (SRL) of
spectroscopic grade were distilled under vacuum
according to the standard procedure and tested
before use for the presence of any impurity
emission in the wavelength region studied.
(
9CNA) (Fig. 1) by steady state and time resolved
spectroscopic techniques. Transient absorption
spectral measurements by laser flash photolysis
method were made to locate the ion-pair state and
these measurements were made both in non-polar
Fig. 1. Molecular structures of TH1N, TH2N, TH2MN and 9CNA.

T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ
/535
527
2
2
.1.1. Method of preparation of 5,6,7,8-tetrahydro-
-methoxy naphthalene (TH2MN, Fig. 1)
Five hundred milligrams of TH2N (0.003 mol)
nm, resolution 4 nm mmꢂ1), photomultiplier
(Hamamatsu, R-928), digital oscilloscope (Fluke-
Philips, model PM 3320A/401, 200 MHz, 250 MS
ꢂ1
was dissolved in 2.5 ml sodium hydroxide (1.75
M). The mixture was cooled in ice and to this
mixture 0.3 ml (0.003 mol) dimethyl sulphate was
added. The mixture was then warmed for 1 h at
7
was extracted with ether. Residue was sublimed
s
) data acquisition and processing system (IBM
PC-386, loaded with VLKS software developed in
Bhabha Atomic Research Centre (BARC), Mum-
bai). The laser and analyzing light beams were
located at right-angles to each other.
0ꢃ80 8C and allowed to cool. The compound
/
after removing the solvent. A colourless liquid
2.4. Electrochemical measurements
1
410 mg) was obtained (yield: 75%). HNMR
(
(
(
CCl ) (60 MHz) d 6.3ꢃ
/
2.1 (m, 4H), 3.03ꢃ
/2.55
Electrochemical measurements to determine the
redox potentials of the reactants were made by
using the PAR model 370-4 electrochemistry
system. Three electrode systems including SCE as
standard were used in the measurements. Tetra-
ethylammonium perchlorate (TEAP) in ACN was
used as supporting electrolyte as before [16].
4
m, 4H), 3.9ꢃ
/
3.6 (S, 3H), 7.06ꢃ6.65 (m, 3H).
/
2
.2. Spectroscopic apparatus
At the ambient temperature (296 K) steady state
electronic absorption and fluorescence emission
ꢂ4
ꢂ6
ꢂ3
spectra of dilute solutions (10
ꢃ
/
10 mol dm
)
of the samples were recorded using 1 cm path-
length rectangular quartz cells by means of an
3. Results and discussion
absorption spectrophotometer (Shimadzu UVꢃ
/
VIS 2101PC) and F-4500 fluorescence spectro-
photometer (Hitachi), respectively. Fluorescence
lifetimes were measured by using a time-correlated
single photon counting (TCSPC) fluorimeter con-
structed from components purchased from Edin-
burgh Analytical Instruments (EAI), model 199
UK. The experimental details are given elsewhere
3.1. Steady state fluorescence spectra
The steady state fluorescence emission spectra
of 9CNA is thoroughly quenched both in NH and
highly polar ACN fluid solutions at the ambient
temperature with addition of the bicyclic naphthol
(TH1N or TH2N) (Fig. 2). The quenching was
observed throughout the entire band envelop of
9CNA and apparently there is no indication of
formation of exciplex or some other excited state
complexes which generally emit at longer wave-
length region.
[
15]. All the solutions prepared for room tempera-
ture measurements were deoxygenated by purging
with an argon gas stream for about 30 min.
2
.3. Laser flash photolysis
Temporal optical absorption profiles were ob-
kinetic spectro-
photometry technique, using an upgraded version
of Applied Photophysics model K-347 Laser
Kinetic Spectrometer. A moderately focused na-
3.1.1. What might be the mechanism of quenching
of the excited singlet (S ) of 9CNA in presence of
bicyclic naphthols (TH1N or TH2N)
tained by laser flash photolysis*
/
1
As the electronic absorption spectra of 9CNA
are little affected both in shape and energy
position by the addition of TH1N or TH2N both
in non polar NH and highly polar ACN environ-
ment (Fig. 3), possibility of formation of ground
state complex is very slim. Moreover, the
naphthols are transparent at around 402 nm
position which has been used as excitation wave-
length in the present investigation to excite the
fluorescence spectra of 9CNA. From the above
nosecond laser pulse of 351 nm (XeF laser, fluence
ꢂ2
ꢀ/15 mJ cm
) was used to excite the deoxyge-
nated sample solution contained in a UV grade
fused silica cuvette of 1 cm pathlength. The
transient absorption was monitored by an analyz-
ing light beam from a pulsed 250 W xenon arc
lamp with associated optics, monochromator (f/
3
.4 grating, 1200 grooves per mm, blazed at 300

528
T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ535
/
Fig. 2. Fluorescence emission spectra of 9CNA (lex
ꢄ
/
402 nm) (Cꢄ
/
2.09ꢅ
10ꢂ3 mol dmꢂ3) in ACN fluid solution at 296 K in
/
ꢂ3
ꢂ4
10ꢂ4; (3) 1.32ꢅ
ꢂ3
ꢂ3
presence of TH1N. Concentration of TH1N (mol dm ) in (0) 0; (1) 4.4ꢅ
/
10 ; (2) 8.79ꢅ
10 . Inset: Fluorescence emission spectra of 9CNA (lex 402 nm) (Cꢄ
solution at 296 K in presence of TH1N. Concentration of TH1N (mol dm ) in (0) 0; (1) 3.88ꢅ
/
/
10 ; (4) 1.76ꢅ
/
10 ; (5)
mol dm ) in NH fluid
10 ; (2) 7.77ꢅ
10ꢂ4; (3) 1.16ꢅ
ꢂ3
ꢂ3
ꢂ3
ꢂ3
2
.20ꢅ
/
10 ; (6) 2.64ꢅ
/
ꢄ/
/
2.3ꢅ10
/
/
ꢂ3
ꢂ4
/
/
ꢂ3
ꢂ3
ꢂ3
10
; (4) 1.55ꢅ
/
10 ; (5) 1.94ꢅ
/
10
.
observations it indicates that both static quench-
ing, inner filter effect and non radiative energy
transfer process, as only 9CNA whose lowest
region the quenching should be due to rapid
outer-sphere ET and the bimolecular fluorescence
quenching rate, k , is close to the diffusion-
q
1
0
3
ꢂ1
ꢂ1
excited singlet state (S ) lies lower than the
1
controlled limit (ꢀ
ACN). Thus our next attempt was to determine
kq using SternꢃVolmer (SV) relationship [6,20].
/10
dm mol
s
in
corresponding level of TH1N (or TH2N) was
excited, have no significant role in quenching
mechanism of 9CNA fluorescence emission.
From the redox potential values measured by the
Cyclic Voltammetry (Table 1) it reveals that there
is a possibility of occurrence of photoinduced ET
/
The possibility of the occurrence of hydrogen-
bonding interaction between excited 9CNA and
ground state naphthols, studied in the present
investigation, could not be ruled out. Nevertheless,
to substantiate this possibility the fluorescence of
9CNA in ACN was measured in presence of
reaction between excited (S ) 9CNA and ground
1
state naphthol (TH1N or TH2N), the former being
electron acceptor and the latter one electron
donor. The values of the Gibbs free energies or
driving energies (DG8), computed from the well
5,6,7,8
(TH2MN), the method of synthesis of which has
tetrahydro-2-methoxy
naphthalene
been provided in Section 2. As this compound has
OH
known Rehmꢃ
the systems 9CNAꢀꢁ
are found to be around ꢀ
/
Weller relationship [2,17,18], for
TH1N and 9CNAꢀꢁTH2N
0.65 eV (Table 1). If
been prepared by replacing H-atom of the Ã
/
/
/
group of TH2N with the ÃCH group, the
/
3
/
ꢂ
/
possibility of H-bonding of this molecule with
9CNA should be slim but the ET reactions with
the quenching observed be due to only photo-
induced ET process, then the values of DG8
indicate that the occurrence of ET reactions
should be in intermediate region [19]. In this
9CNA may be stronger as due to the addition of Ã
/
CH , the electron donating capability of the donor
part will be enhanced. But interestingly no quench-
3

T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ
/535
529
ing was apparent in the fluorescence spectra of
CNA with the addition of the methoxy-substi-
further support our propositions in favour of the
occurrence of H-bonding interaction which acts as
a prime process in quenching reactions. Had the
ET be the main process in quenching, the reaction
should have been diffusion-controlled in both NH
and ACN as observed earlier by Mataga et al. [21].
9
tuted naphthol. From the above findings it
appears that intermolecular H-bonding is primar-
ily responsible for the observed quenching and
photoinduced ET, if at all present, might have
insignificant role in quenching mechanism. From
the observed linear plots (Fig. 4(a) and (b)) using
simple SV relation it indicates that some dynamic
processes are involved. It is to be pointed out in
this connection that since the fluorescence quench-
ing occurs in the region of TH1N (or TH2N)
concentrations where the 9CNA absorption spec-
trum is not at all affected, the simple SV equation
was used for the analysis of the quenching reac-
tions. As shown in Table 2, in the quenching of
Thus within the present donorꢃacceptor systems,
/
H-bonding process seems to play the major role in
quenching mechanism. Lower k values in ACN
q
indicate that the excited state hydrogen bonding
interaction between 9CNA and TH1N (or TH2N)
in polar solvent ACN becomes much weaker than
in non polar NH environment due to solvation of
both 9CNA and naphthol molecules by strongly
polar ACN which hinders hydrogen-bonding in-
teraction. We already showed that the fluorescence
of 9CNA is not affected by TH2MN in both NH
and ACN. This demonstrates clearly that the
hydrogen bonding interaction is crucial for the
occurrence of quenching phenomena. Moreover,
the stronger hydrogen-bonding ability in the
9CNA fluorescence emission by naphthol the
reaction is diffusion controlled only in non polar
NH environment and k values for the same Dꢃ
/
q
Aꢀ (ꢀdenotes excited singlet state) pair decrease
significantly in ACN. The above observations
Fig. 3. Steady state electronic absorption spectra of TH2N (Cꢄ
/
6.69ꢅ
/
10ꢂ5 mol dmꢂ3, curve 1), 9CNA (Cꢄ
/
4ꢅ
/
10ꢂ5 mol dmꢂ3
,
1 cm. Inset: Steady state electronic absorption
curve 2) and mixture (curve 3) of TH2N and 9CNA in ACN fluid solution at 296 K. lꢄ
/
ꢂ4
ꢂ3
ꢂ5
ꢂ3
spectra of TH2N (Cꢄ
/
1.26ꢅ
/
10 mol dm , curve 1), 9CNA (Cꢄ
/1.28ꢅ10 mol dm , curve 2) and mixture (curve 3) of TH2N
/
and 9CNA in NH fluid solution at 296 K. lꢄ
/1 cm.

530
T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ535
/
Table 1
Values of redox potentials of the donors and the acceptor 9CNA and free energy changes, DG
8
ET, associated with photoinduced ET
processes for the present Dꢃ
/
A systems in ACN fluid solution at the ambient temperature
a
O1 / X2 (D/Dꢁ) (V)
RED
1/2 (Aꢂ/A) (V)
a
System
E
E
0
E ,0 (eV)
DGET (eV)
8
THN1Nꢁ
THN1Nꢁ
THN2Nꢁ
THN2Nꢁ
/
9ACN
9CNAꢀ
9ACN
9CNAꢀ
ꢁ
/
1.31
1.31
1.30
1.30
ꢂ
/
1.13
1.13
1.13
1.13
ꢃ
3.08
/
ꢁ
/
2.44
0.64
2.43
0.65
/
ꢁ
/
ꢂ
/
ꢂ
/
/
ꢁ
/
ꢂ
/
ꢃ/
3.08
ꢁ
/
/
ꢁ/
ꢂ/
ꢂ/
ꢀ
, Denotes the first excited singlet state (S
1
).
singlet (0,0) transition energy of the excited chromophore.
a
E
ꢀ
,0, is the singletꢃ
/
0
excited state than in the ground state is confirmed
from the observations of significant changes in the
fluorescence spectra of the acceptor 9CNA caused
by the hydrogen bonding at much smaller donor
as an electron acceptor. In ACN, the transient
absorption of 9CNA only exhibits a band at
around 430 nm whose decay time is about ꢀ2
/
ms. This band has been assigned [22] to the band of
the cationic species of 9CNA. No prominent pure
(
TH1N or TH2N) concentrations than those
necessary for the changes of the absorption spectra
of this species.
tripletꢃtriplet transition band of 9CNA is appar-
/
To observe directly the CT state resulted from
the excited state hydrogen bonding interactions
and to examine the polarity effect of the environ-
ment on the CT state, detailed investigations on
the transient absorption spectra measured by laser
flash photolysis technique seems to be of crucial
importance. The results obtained from time re-
solved studies have been discussed in the follow-
ing.
3
.2. Transient absorption measurements
The transient absorption spectra of both
9
CNAꢃ
/
TH1N and 9CNAꢃTH2N systems in
/
highly polar ACN fluid solution are shown in the
Fig. 5(a) and (b)). These transient spectra were
measured by exciting with l ꢀ351 nm of XeF
laser. This excitation would help to excite only
CNA (the donor THN moiety remains undis-
turbed) to its S . If the CT interaction between the
(
/
ex
9
1
ꢁ
ꢂ
hydrogen bonded pair (DÃ
/
H Á Á ÁA ) in the
excited state be responsible for the fluorescence
quenching, one should observe the transient CT
state. By using picosecond laser flash photolysis
earlier authors [21] found the transients for
cationic and anionic species of CT state formed
through H-bonding interaction for some other
Fig. 4. (a) Sternꢃ
cence emission intensity measurements in case of singlet (S
excitation of 9CNA in the presence of TH1N in ACN fluid
solution at 296 K. (b) SternꢃVolmer (SV) plots from fluores-
cence lifetime measurements (time-resolved) in case of singlet
) excitation of 9CNA in the presence of TH1N in ACN fluid
solution at 296 K.
/
Volmer (SV) plots from steady state fluores-
1
)
/
organic donorꢃ
the present investigation we have chosen 9CNA
/
acceptor molecular systems. In
(S
1

T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ
/535
531
Table 2
Fluorescence ouenching data for the present Dꢃ
absence of the donor (quencher), KSV is the Sternꢃ
/
A systems at the ambient temperature, t
0
is the fluorescence lifetime of the acceptor in
/
Volmer constant, k is the dynamic fluorescence quenching rate constant
q
SV (dm molꢂ1)
3
k
3
(dm mol
ꢂ1
sꢂ1) (9
10%)
/
System
t
0
(ns) (9
/
0.4)
k
q
a
b
b
b
b
9a
9b
9b
THN1Nꢁ
THN2Nꢁ9CNAꢀꢁ
THN1Nꢁ9CNAꢀꢁ
THN2Nꢁ9CNAꢀꢁ
/
9CNAꢀꢁ
/
ACN
ACN
NH
17.2
17.2
11.8
11.8
123.88 , 108.04
7ꢅ
9ꢅ
1.8ꢅ
2.0ꢅ
/
10 , 6ꢅ
/
10
a
9a
/
/
152.88 , 138.26
/
10 , 8ꢅ
/
10
a
10a
10b
10b
/
/
211.40 , 202.27
/
10 , 1.7ꢅ
10 , 1.8ꢅ
/
10
a
233.39 , 213.82
10a
/
/NH
/
/
10
ꢀ
1
, Denotes the electronic excited singlet state S .
a
These values are computed by using the simple SV relation: f
fluorescence emission intensity measurements).
0
/fꢄ
/
1ꢁ
/
K
SV[Q]ꢄ [Q] (obtained from steady state
/
1ꢁ
/
k
q
t
0
b
These values are computed by using the simple SV relation: t
0
/tꢄ1ꢁ
/
/
K
SV[Q]ꢄ1ꢁ/k t [Q] (obtained from time resolved
/
q 0
measurements).
ent whose lifetime is known to be of about 560 ms
In the transient absorption spectra of the
[
23]. Manring et al. [24] reported that the triplet
mixture of 9CNA and a naphthol in ACN fluid
solution measured at the different delay times
between the exciting and analyzing pulses the
band at 640 nm could be assigned to the band of
the cationic species of the naphthols following the
observation made by Shizuka et al. [25] for similar
bicyclic molecules. According to the earlier reports
state of this molecule could be artificially devel-
oped by promoting the intersystem crossing (ISC)
process by a heavy atom containing molecule.
They used thioanisole. In the present investigation
it was observed that in presence of a naphthol
(
TH1N or TH2N), the transient absorption of
CNA at 430 nm region shows biexponential
decay. The decay analysis shows the two lifetimes,
t1ꢀ2 ms (f ꢀ0.01) and t ꢀ30 ms (f ꢀ0.99) (f1
[
22,24] the 570 nm absorption band should be the
9
ꢂ
band of the 9CNA radical anion (9CNA ).
Another band at 548 nm might be due to the
formation of 9CNA anion of the CIP. Photoreac-
tion studies made by earlier authors [26] using
organic donor and acceptor molecules in polar
solvents showed that excited CT reactions give
both solvent-separated and CIPs and anions of the
CIP reside in the shorter wavelength (higher
energy) region. From the decay analysis at around
/
/
/
/
1
2
2
and f2 are fractional contributions). Thus this
observation clearly demonstrates the formation
of another species apart from cations (ꢀ2 ms). In
/
high probability this species should be due to
quenched triplet of 9CNA. It is inferred that the
CT state produced in the S1 state enhances
intersystem crossing rate which results in the
formation of the triplet state of 9CNA. As from
the decay analysis it is observed that the triplet
5
80 nm it was observed that for both the pairs
CNAꢀꢃTH1N and 9CNAꢀꢃTH2N 2nd order
9
/
/
fitting shows the best fit for the decay (Fig. 6(a)
and (b)). On the other hand, the decay at 548 nm
corresponds very well to the first order fitting (Fig.
lifetime (ꢀ30 ms) is much less than the lifetime of
/
the unperturbed triplet of 9CNA, it indicates that
the lowest triplet (T ) of 9CNA is also involved,
1
7
(a) and (b)). This corroborates our presumption
after being formed, in hydrogen bonding interac-
tion with the ground state of TH1N (or TH2N)
and this results in reduction of the triplet lifetime
of the acceptor 9CNA. The formation of the CT
about the formation of CIP which absorbs at 548
nm region because the back CT (charge recombi-
nation) within the CIP s obeys the first order decay
kinetics. Second order fitting demonstrates the
absorbance at 580 nm should be due to solvent-
separated (or dissociated) anions which by second
order diffusion mechanism would combine with
donor cations in a relatively slower process. Thus
3
ꢁ
ꢂ
species, (D Ã
/
HÁ Á ÁA )ꢀ, in the excited triplet
state has been confirmed from the observations of
the products containing the cationic and anionic
species of the reactants (vide infra).

532
T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ535
/
in ACN medium both decay kinetics are observed
which indicates that both ion-pairs and free ions
are present at the same time.
The slope of the single exponential decay at 548
nm (Insets of Fig. 7(a) and (b)) yields the ion-pair
lifetime (t ). The yield fR of the dissociated ion
ip
radicals or solvent separated ion-pair (SSIP) is
obtained experimentally [27] by taking the ratio of
the absorbance due to dissociated ions at long
delay times (ꢁ50 ms) and the initial value esti-
/
Fig. 6. (a) Plot of ln A (A representing the absorbance of the
solvent-separated or dissociated anions in presence of TH1N in
ACN at around 580 nm) as a function of delay times. The solid
line is the 1st order fitting. Inset: 2nd order fitting for the same
2
shows the best fitting (r ꢀ/0.99). (b) Plot of lnA (A representing
the absorbance of the solvent-separated or dissociated anions in
presence of TH2N in ACN at around 580 nm) as a function of
delay times. The solid line is the 1st order fitting. Inset: 2nd
order fitting for the same shows the best fitting.
mated by extrapolating the ion absorbance to tꢄ
/0
in Fig. 8. The charge recombination (kCR) and
charge dissociation (k ) rates were computed
from the relations Eq. (1) and Eq. (2),
Fig. 5. (a) Transient absorption spectra of 9CNA (ex. Wave-
ꢂ2
length ꢀ
/
351 nm, laser fluence ꢀ
/
15 mJ cm ) in ACN (2ꢅ
/
dis
ꢂ4
ꢂ3
1
0
mol dm ) at the ambient temperature in presence of the
ꢂ2
ꢂ3
donor TH1N (conc. ꢀ
delay times: (1) 1.25 ms; (2) 2.0 ms; (3) 3.0 ms; (4) 5 ms; (5) 10 ms;
6) 20 ms. (b) Transient absorption spectra of 9CNA (ex.
/
1.2ꢅ
/
10
mol dm ) at the different
ꢂ1
tip ꢄ(kCR ꢁkdis)
f ꢄk t
(1)
(2)
(
ꢂ2
R
dis ip
Wavelength ꢀ
/351 nm, laser fluence ꢀ
/
15 mJ cm ) in ACN
ꢂ4
ꢂ3
(
1.5ꢅ
of the donor TH2N (conc. ꢀ
different delay times: (1) 0 ms; (2) 1.25 ms; (3) 2.5 ms; (4) 3.75 ms;
5) 5 ms; (6) 10 ms; (7) 15 ms; (8) 20 ms; (9) 25 ms; (10) 30 ms.
/
10 mol dm ) at the ambient temperature in presence
ꢂ3
ꢂ3
The values of t , f , k and kdis are shown in
ip R CR
/8.81ꢅ/10
mol dm ) at the
the Table 3. The yield of ion-dissociation (or
solvent-separated ions) are found to be about
(

T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ
/535
533
Table 3
R
The values of yield, f , of the dissociated ion radicals, ion-pair lifetime, tip, rate of charge recombination (kCR) and rate of charge
dissociation (kdis). All the values are measured at the ambient temperature
System
t
ip (ms)
f
R
k
CR (sꢂ1
)
k )
dis (sꢂ1
4
4
TH1Nꢁ
/
9CNAꢀꢁ
/
ACN
16.2
17.3
0.40
0.14
3.6ꢅ
/
10
2.6ꢅ
/
10
4
10
4
10
TH2Nꢁ9CNAꢀꢁ
/
/ACN
5.0ꢅ
/
0.8ꢅ
/
1
ꢀ, Denotes the electronic singlet state S .
three fold larger in the case of TH1N relative to
the value observed for TH2N. As charge recombi-
4
10 s ) than the
ꢂ1
nation rate is faster (kCR
charge dissociation rate (kdis
ꢀ
/
5ꢅ
/
3
ꢂ1
ꢀ
/
8ꢅ10 s )in the
/
case of the latter donor, the yield of ion-dissocia-
Fig. 8. The time profile of absorbance (A) of the CIP of 9CNA
anion radical in presence of TH1N at 548 nm. The solid line
represents the first order fitting of the decay.
tion in this case becomes low (ꢀ
above results the following Scheme 1 has been
proposed for the present donorꢃacceptor interac-
/
0.14). From the
/
tions in highly polar ACN medium
In non-polar NH, the transient absorption
spectra of 9CNA in presence of both TH1N and
TH2N donors do not exhibit any band of ions
(
Fig. 9). In NH the lifetime of the pure triplet
which occurs at 433 nm region is about 1700 ms
23]. From the decay analysis at around this region
of the measured transient absorption spectra the
triplet lifetime was found to be ꢀ30 ms. This
[
/
Fig. 7. (a) Plot of 1/A (A representing the absorbance of the
CIP of 9CNA anion radical in presence of TH1N in ACN at
around 548 nm) as a function of delay times. The solid line is
the 2nd order fitting. Inset: 1st order fitting for the same shows
2
the best fitting (r ꢀ/0.999). (b) Plot of 1/A (A representing the
absorbance of the CIP of 9CNA anion radical in presence of
TH2N in ACN at around 548 nm) as a function of delay times.
The solid line is the 2nd order fitting. Inset: 1st order fitting for
the same shows the best fitting.
Scheme 1.

534
T. Bhattacharya et al. / Spectrochimica Acta Part A 59 (2003) 525ꢃ535
/
quenching phenomena [28] but such process prac-
tically seems to be inactive in the present case
where 9CNA was chosen as an acceptor.
Scheme 2.
quenching seems to be due to H-bonding in the
triplet state. As from the steady state measure-
ments it demonstrates that H-bonding in non-
polar NH is much stronger than in highly polar
ACN, stronger CT interaction may occur in the
former case. As a result of the stronger CT
interaction, the proton of the donor would be
further shifted towards the acceptor moiety which
would further lower the ionization potential of the
p-electron system of the donor and enhance the
electron affinity of the p-electron system of the
acceptor. Eventually this proton shift in CT state
causes the formation of neutral radical due to the
complete detachment of the H-atom as shown in
Scheme 2 below.
4
. Concluding remarks
In presence of both TH1N and TH2N photo-
induced ET reaction possesses insignificant role in
the mechanism of nonradiative depletion of the
excited states of the well known electron acceptor
9CNA. It is inferred that H-bonding interaction is
crucial for the quenching phenomena. The present
observations made from both steady state and
transient absorption spectral measurements in
non-polar environment demonstrate that both
TH1N and TH2N due to their excellent H-bond-
ing ability could be used as antioxidants.
Possibly this is the reason for the disappearance
of the ionic absorption bands in the transient
absorption spectra of 9CNAꢃTH1N (or TH2N)
/
Acknowledgements
systems when the polarity of the environment was
changed from highly polar ACN to non-polar NH.
Though in presence of another well known
electron acceptor 2-nitrofluorene (2NF) highly
exothermic ET reactions with tetrahydronaphthols
were found to be the key process responsible for
The authors are grateful to Dr S. Goswami of
the Department of Inorganic Chemistry for elec-
trochemical measurements. Thanks are also due to
Dr G. Chowdhury of the Department of Organic
Chemistry for helping in purification of the
samples. We wish to express our thanks to
Professor S. Basak, K. Giri of Saha Institute of
Nuclear Physics, Kolkata, India for fluorescence
lifetime measurements. T. Bhattacharya and T.
Misra acknowledge the support by a grant from
the Council of Scientific and Industrial Research
(CSIR), New Delhi, India.
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