Interaction between a pyridyl and a naphthyl/pyrenyl moiety in covalently linked systems

Article abstract of DOI:10.1016/S0009-2614(01)01366-5

Photophysical properties of two bichromophoric systems comprising a pyridyl moiety and a naphthyl/pyrenyl moiety covalently linked through a dimethylene bridge have been investigated. The spectral behavior of the systems suggests the formation of an intramolecular complex between the two terminal moieties of the molecules in the ground state. Interestingly, no intermolecular complexation could be observed with a 1:1 mixture of the constituting components. This contrasting behavior has been interpreted taking into consideration the difference in the entropy change associated with the complexation process. Based on the effect of solvent polarity on the absorption and emission behavior of the complex and taking into consideration the molecular geometry, it is concluded that the interaction between the aromatic hydrocarbon and pyridyl moieties is primarily driven through an overlap of the π-clouds.

Full text of DOI:10.1016/S0009-2614(01)01366-5

4 January 2002
Chemical Physics Letters 351 62002) 61±70
www.elsevier.com/locate/cplett
Interaction between a pyridyl and a naphthyl/pyrenyl
moiety in covalently linked systems
a
N.B. Sankaran , Amitava Das , Anunay Samanta
b
a,
*
a
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
b
Received 5April 2001; in ®nal form 23 August 2001
Abstract
Photophysical properties of two bichromophoric systems comprising a pyridyl moiety and a naphthyl/pyrenyl
moiety covalently linked through a dimethylene bridge have been investigated. The spectral behavior of the systems
suggests the formation of an intramolecular complex between the two terminal moieties of the molecules in the ground
state. Interestingly, no intermolecular complexation could be observed with a 1:1 mixture of the constituting compo-
nents. This contrasting behavior has been interpreted taking into consideration the di€erence in the entropy change
associated with the complexation process. Based on the e€ect of solvent polarity on the absorption and emission be-
havior of the complex and taking into consideration the molecular geometry, it is concluded that the interaction be-
tween the aromatic hydrocarbon and pyridyl moieties is primarily driven through an overlap of the p-clouds. Ó 2002
Elsevier Science B.V. All rights reserved.
1. Introduction
ponents. The components are chosen such that an
interaction between the two components results in
quenching of the ¯uorescence of the system.
However, in the presence of the guests, the inter-
action gets cut o€ 6or substantially reduced) lead-
ing to a recovery of the original ¯uorescence.
We have been examining the ¯uorescence be-
havior of various structurally simple multi-com-
ponent systems with a view to exploring the
potential of these systems in sensing the metal
ions, and the transition metal ions in particular [7±
12]. Taking into consideration the fact that pyri-
dine is an excellent coordinating ligand for the
metal ions, we thought that systems involving
pyridine as the guest-binding site could be ideal for
metal ion sensing applications. This is primarily
There is a great deal of current interest in
molecular systems capable of performing light-in-
duced logic operations [1±6]. Several multi-com-
ponent systems have been designed and developed
in recent years, which function as molecular pho-
tonic devices. One of the most commonly em-
ployed strategies for the design of photonic devices
is to develop systems consisting of a photon in-
teraction site 6¯uorophore) and a guest-binding
site. A spacer unit usually links these two com-
* Corresponding author. Fax +91-40-301-2460.
E-mail address: assc@uohyd.ernet.in 6A. Samanta).
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614601)01366-5

62
N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
why we have synthesized and studied the photo-
physical behavior of two multi-component sys-
tems, 1-61-naphthyl)-2-64-pyridyl)ethane 6NPY)
and 1-61-pyrenyl)-2-64-pyridyl)ethane 6PPY) 6vide
Scheme 1), in which the ¯uorophore 6a naphthyl or
a pyrenyl moiety) is connected to a pyridyl moiety
through a ¯exible dimethylene spacer unit.
Even though a good amount of literature is
available dealing with the interaction of the ali-
phatic and aromatic amines with the aromatic
hydrocarbons such as naphthalene, anthracene,
pyrene, etc. [13±19], very little information could
be obtained from the literature that deals with the
interaction of these hydrocarbons with pyridine.
This is presumably due to the fact that pyridine
does not form an exciplex readily with the aro-
matic hydrocarbons such as naphthalene or py-
rene, whereas the aliphatic amines such as
diethylamine, triethylamine, etc., and aromatic
amines such as N; N-dimethylaniline do. This be-
havior can be rationalized taking into consider-
ation the fact that an sp3 hybridized nitrogen atom
is more available for complex formation compared
to sp2 hybridized nitrogen atom of pyridine. The
only information that we could obtain from the
literature on systems involving both pyridine and
naphthalene or pyrene is that when the naphthyl
and pyridyl moieties are connected through two
Scheme 1.
an intramolecular complex in the ground state.
The lack of an intermolecular complexation be-
tween the interacting partners under the same
condition has been ascribed to a smaller change in
entropy associated with the complexation process.
2. Experimental section
2.1. Materials
2.1.1. Synthesis
NPY. To a solution of 4-picoline 60.93 g, 10
mmol) in ice cold tetrahydrofuran 6THF) 625ml)
under argon, a solution of LiNPri2 610 mmol,
freshly prepared by mixing 6.64 ml of 1.6 M n-
butyllithium in hexane and 1.33 ml of dry iso-
propylamine at room temperature under argon)
was added dropwise in over 30 min. The deep
purple solution was allowed to equilibrate at ꢁ5
°C for 1 h, to which 1-naphthyl carbaldehyde 61.56
g, 10 mmol) dissolved in 15ml THF was added
dropwise keeping the reaction temperature at ꢁ20
°C. Then the mixture was allowed to warm to
room temperature and stirred overnight. After the
reaction was quenched by ice, the reaction solution
was evaporated to dryness in vacuo. An oily mass
of the crude intermediate alcohol was obtained.
About 50 ml of water was added to it and the
desired diol was extracted in a CH2Cl2 layer. It
was evaporated to dryness under vacuo and the
pale yellow solid obtained was used without puri-
®cation for subsequent dehydration reaction. The
ꢀCH2†
linkages in an ‰m; nŠphane type of
m or n
structure, a new broad ¯uorescence could be ob-
served at low temperature [16]. This ¯uorescence
was assigned to an `exciplex' formed between the
two chromophores. In a recent paper, Hirsch et al.
[17] reported the photophysical behavior of some
systems involving a pyridyl moiety 6or N-meth-
ylpyridinium moiety) and a 9-anthryl moiety cov-
alently linked through di€erent spacers. Even
though intramolecular charge separation between
the anthryl and the N-methylpyridinium moieties
could be observed in some of these systems, there
was no evidence of the terminal pyridyl and
anthryl moieties interacting with each other when
the two were connected through a methylene r
bridge.
In this Letter, it is shown that the terminal
moieties 6naphthyl/pyrenyl and pyridyl) of the
multi-component systems shown in Scheme 1 in-
teract through-space leading to the formation of

N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
63
2.2. Methods
oily mass was dissolved in a minimum volume of
dry pyridine 630 ml) and a solution of POCl3 61.9
g, 12 mmol; dissolved in 10 ml of pyridine) was
added dropwise under argon at ꢁ5 °C with stir-
ring. After the addition, the reaction mixture was
allowed to come to the room temperature and
stirred for 3 h. Then the reaction was quenched
with ice and the volume was reduced under vacuo.
pH of the solution was adjusted to 3 and the un-
desired organic material was extracted in CH2Cl2.
Solution pH was then adjusted to ꢁ6.5and the
desired product was extracted in CH2Cl2 and dried
in vacuo. Crude solid was puri®ed by column
chromatography using silica column and CH2
Cl2±CH3OH mixture 697±3%, v/v) as eluent. Sec-
ond fraction was collected. This 6850 mg) was re-
duced by 10% Pd/C catalyst at 40 °C under 300 psi
H2 pressure for 4 h using 25ml acetonitrile 6AN)
as solvent. Desired hydrogenated product was
further puri®ed by column chromatography using
silica column and CHCl3±CH3OH 698±2%, v/v)
mixture as eluent. Major fraction was collected.
The absorption spectra were recorded on a
Jasco model 7800 spectrophotometer. Fluores-
cence spectra were recorded on a Jasco FP-777
spectro¯uorimeter. The ¯uorescence decay curves
were recorded using an IBH model 5000 single-
photon-counting spectro¯uorimeter. The instru-
ment was operated with a thyratron-gated ¯ash
lamp ®lled with hydrogen at a pressure of 0.5atm.
The lamp was operated at a frequency of 40 kHz,
and the pulse width of the lamp under the oper-
ating condition was ꢁ1.2 ns. The lifetimes were
estimated from the measured ¯uorescence decay
curves and the lamp pro®le using a nonlinear least-
squares iterative ®tting procedure [21]. The good-
ness of the ®t was evaluated from the v2 values and
the plot of the residuals. In view of the long ¯uo-
rescence lifetime of pyrene and naphthalene [14]
and its sensitivity to dissolved oxygen, the lifetimes
were obtained from N2 bubbled solutions of the
samples. The steady-state and time-resolved mea-
surements have been carried out with suciently
dilute solutions of the samples 6the concentration
was maintained between 1 and 4 Â 10À5 M) such
that the contribution due to the intermolecular
interaction is eliminated. The excitation wave-
lengths were chosen based on whether we want to
excite the naphthyl/pyrenyl moiety or the complex.
While the naphthyl moiety was excited at 290 nm
for steady-state measurements, it was excited at
280 nm in time-resolved studies because of a larger
intensity of the excitation lamp at this wavelength.
The ¯uorescence quantum yield of the complex
was estimated using 4-aminophthalimide ꢀ/f of 0.1
in methanol) as the reference compound [22].
‡
Yield: 600 mg 669%); E-I MS: m/z 233 6M , 100%);
1H NMR 6200 MHz, CD2Cl2†: d 8.44 62H, d), 7.0±
7.9 67H, m), 6.99 62H, d), 3.26 62H, dd), 2.94 62H,
dd).
PPY. Synthetic procedure is identical to that
for NPY except that pyrene-1-aldehyde was used
instead of 1-naphthyl carbaldehyde. The reduction
was done at 50 °C under 410 psi H2 pressure.
‡
Yield: 600 mg 675%); E-I MS: m/z 307 6M , 100%);
1H NMR 6200 MHz, CD2Cl2): d 8.47562H, d),
7.945±8.237 67H, m), 7.71 62H, d), 7.08 62H, d), 3.6
62H, dd), 3.11 62H, dd).
2.1.2. Puri®cation of the materials
Pyridine and 4-methylpyridine 6Merck) were
puri®ed by standard procedures [20]. 1-Methyl-
naphthalene and pyrene 6Aldrich) were recrystal-
lized from ethanol prior to the experiments. Unless
otherwise mentioned, the spectroscopic experi-
ments were performed in THF and AN. These
solvents were procured locally and rigorously pu-
ri®ed by following standard procedures [20]. Prior
to the actual experiments it was made sure that the
solvents were free from impurities and no ab-
sorption or ¯uorescence could be observed from
the solvent in the spectral region of interest.
3. Results
3.1. Absorption
The UV±Vis absorption spectra of NPY 6Fig. 1)
consist of a structured absorption between 250 and
300 nm along with an additional broad band 6300±
370 nm) with the maximum around ꢁ328 nm in
THF. While the structured absorption is clearly
due to the naphthyl and pyridyl moieties, the

64
N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
observation suggests that charge transfer character
of the complex is rather negligible.
Interestingly, in the case of PPY, no intramo-
lecular complex formation between the terminal
groups is apparent from the absorption spectra of
the system in THF or AN 6Fig. 1b). Moreover, a
1:1 mixture of pyrene and 4-methylpyridine does
not give rise to any new absorption that can be
attributed to the formation of a complex.
3.2. Emission
Both NPY and PPY exhibit multiple emission
bands, the relative intensities of which depend on
the excitation wavelength. As seen from Fig. 2a,
the emission spectrum of NPY consists of a short
wavelength emission due to the naphthyl moiety
and another long wavelength broad band
ꢀkmax ꢁ 400 nm), which presumably arises from
the intramolecular complex. That an intermolec-
ular interaction between two molecules of NPY is
not responsible for the appearance of the second
emission band is evident from the fact that the
relative intensity of the two emission bands is not
dependent on the concentration of the systems.
When excited at 290 nm, the spectrum of NPY is
dominated by emission due to the naphthyl moi-
ety. However, when excited at 310 nm, the emis-
sion spectrum consists of a small contribution due
to the naphthalene moiety; the major component
of the emission is due to the complex. The emis-
sion is entirely due to the complex when excited at
330 nm or higher wavelength. With the change of
the solvent from THF to AN 6or an even more
polar solvent, ethanol), neither the intensity dis-
tribution of the two bands nor the band positions
change signi®cantly. A small red shift of about 6±7
nm could be observed. While the ¯uorescence ex-
citation spectrum 6Fig. 3a) obtained on monitor-
ing the 345nm emission resembles the absorption
spectrum of naphthalene, when monitored at 410
nm, the excitation maximum corresponds very
closely to that due to the complex 6as indicated in
the absorption spectra).
Fig. 1. Absorption spectra of NPY 6a) and PPY 6b) in THF
6solid line) and AN 6dashed line).
longer wavelength band cannot be attributed to
either of them. Obviously, this band arises due to
an intramolecular interaction between the two
moieties in the ground state. Interestingly, no such
band could be observed from a 1:1 mixture of 1-
methylnaphthalene and 4-methylpyridine in THF.
Even though the structureless feature of the
broad band and its location suggest it to be due to
a charge transfer transition, the band is found to
be rather insensitive to the polarity of the medium.
In fact, contrary to an expected red shift for a
typical charge transfer band, 1±2 nm blue shift of
this band could be observed on changing the sol-
vent from THF to a more polar solvent, AN. This
The ¯uorescence behavior of PPY has been il-
lustrated in Fig. 2b. When excited at 330 nm, the
emission spectrum is typically that expected from
the pyrene moiety. It is interesting to note that

N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
65
Fig. 2. Fluorescence spectra of NPY and PPY in THF 6solid line) and AN 6dashed line) as a function of the excitation wavelength. 6a)
NPY, spectra labeled i, ii and iii represent those obtained on excitation at 290, 310 and 330 nm, respectively. 6b) PPY, spectra labeled i
and ii represent those obtained on excitation at 330 and 390 nm, respectively.
even though no intramolecular complex 6between
the pyrenyl and pyridyl moieties) could be ob-
served for this system in absorption, a long
wavelength ¯uorescence band 6with a maximum
ꢁ450 nm) is clearly observable when the system is
excited at ꢁ390 nm. The ¯uorescence excitation
spectrum corresponding to the 450 nm emission
shows an absorption maximum at ꢁ375nm 6Fig.
3b). This observation suggests that the new emis-
sion at 450 nm arises due to the excitation of a
ground state species that absorbs at around 375
nm. Interestingly, at this wavelength, PPY virtu-
ally shows no absorption. The di€erence in the
absorption and ¯uorescence excitation behavior of
PPY can only be rationalized by assuming that the
extent of formation of the complex formed be-
tween the pyrenyl and pyridyl moieties is too small
or the extinction coecient of the complex is too
low at around 375nm. Since the complex is highly
¯uorescent it is easier to detect this species through
¯uorescence compared to absorption. The solvent
polarity dependence of the emission behavior of
PPY is found to be very similar to that observed
with NPY.
The ¯uorescence decay behavior of NPY and
PPY has been studied monitoring both the emis-
sion bands. The decay parameters have been col-
lected in Table 1 and a representative decay curve
along with the best ®t has been shown in Fig. 4.
When monitored at the short wavelength region,
where the naphthyl or the pyrenyl moiety emits,
the ¯uorescence decay consists of a long- and a
short-lived component. The relative weightage of
the short to the long component is roughly 1:2 in
the case of NPY. In the case of PPY, the short-
lived component is the predominant one in THF,
though the reverse is true in AN. While the long
wavelength emission is also ®tted to a biexponen-
tial decay behavior to obtain the best v2 and better
residuals, the contribution of the second compo-

66
N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
Fig. 3. Fluorescence excitation spectra of NPY and PPY in THF 6solid line) and AN 6dashed line) monitored at di€erent emission
wavelengths. 6a) NPY, spectra labeled i and ii correspond to the monitoring wavelengths of 345and 410 nm, respectively. 6b) PPY,
spectra labeled i and ii correspond to the monitoring wavelengths of 400 and 460 nm, respectively.
nent is found to be too little compared to the
dominant one. The predominant component has a
lifetime of 2.1±2.7 ns in the case of PPY and 0.2 ns
in the case of NPY.
Quite obviously, the spacer dimethylene group
plays a crucial rule in dictating the nature as well
as the magnitude of the interaction between the
two moieties in linked system.
Studies on intramolecular and intermolecular
excimer or exciplex have shown that the entropy
change associated with its formation depends
strongly on whether the complexing partners are
linked or not [23]. While the entropy change as-
sociated with the formation of an intermolecular
complex is typically À80 J KÀ1 molÀ1, the same in
the case of an intramolecular complex varies be-
tween )20 and 30 J KÀ1 molÀ1 [23]. This large
di€erence in the magnitude of the entropy change
can account for a greater stability of an intramo-
lecular complex compared to an intermolecular
one, as the free energy change associated with the
4. Discussion
As mentioned earlier, addition of 4-methyl-
pyridine to a solution of 1-methylnaphthalene or
pyrene results in no change in the absorption or
emission spectrum 6in the region of interest)
though a quenching of the ¯uorescence of the
hydrocarbon could be observed. One may wonder
why 1-methylnaphthalene and pyrene do not form
an intermolecular complex with 4-methylpyridine
even though in covalently linked systems they do.

N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
67
Table 1
Fluorescence decay parameters of NPY and PPY in THF and AN
System
Solvent
kex 6nm)
kem 6nm)
s1 6ns)
ꢀB1†
s2 6ns)
ꢀB2†
v2
NPY
THF
280
330
3258.1 1.1
420
5
60.06)
0.2
1.05
60.15)
2.3
60.008)
1.18
61.2)
AN
280
330
3253.9
420
33.9
1.15
60.14)
11.8
60.08)
0.2
61.4)
1.28
60.003)
PPY
THF
AN
3153753.1
3754652.7
192
46
1.26
1.28
60.5)
60.06)
60.12)
60.002)
3153756.5 122
3754652.1
1.00
60.04)
60.16)
60.30)
48
1.04
60.0004)
Fig. 4. Fluorescence decay behavior of NPY in THF at 325nm. The excitation wavelength was 280 nm. The ®gure also shows the
residuals and the best biexponential ®t 6solid line) to the decay curve with the ®tting parameters as follows: s1 ˆ 8:1 ns, s2 ˆ 51:1 ns,
A1 ˆ 0:06 and A2 ˆ 0:15.
formation of a complex, which determines the
stability of a complex, is equal to DH À TDS. A
simple calculation suggests that at room temper-
ature 625 °C) it may not be possible to detect in-
termolecular complexes when the DH values are
smaller than À24 kJ molÀ1. Hence, an unfavor-
able entropy change is responsible for the lack of
formation of an intermolecular complex between

68
N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
4-methylpyridine and 1-methylnaphthalene or py-
rene.
Interestingly, since the broad absorption band
due to the complex hardly shows any dependence
on the solvent polarity; a charge transfer nature of
the complex is untenable. The e€ect of the medium
rather suggests an intramolecular complex of p±p
character 6with negligible charge transfer character
of absorption and ¯uorescence), similar to what is
commonly encountered in the formation of dimers
and excimers of aromatic hydrocarbons [15].
Molecular modeling of the present systems shows
that the two p-systems cannot be fully parallel to
give rise to a sandwich geometry that is necessary
for the maximum p-overlap. Both molecular
modeling and the literature suggest that at least 3-
methylene bridges are essential for a perfectly
sandwich geometry [24]. In our systems, where the
terminal groups are linked through two methylene
bridges, the two p-systems make a small angle.
However, the distance between the two almost
parallel rings is small enough to give rise to a
signi®cant overlap of the two p-clouds. Since the
redox properties of the terminal moieties are not
very di€erent, the complexes behave more like a
hetero-dimer with a negligible charge transfer
character.
It is to be noted that in other linked systems,
involving naphthalene and pyridine, such as in
‰2; 2Šnaphthalinopyridinophane [16], in which the
pyridine ring system is connected at 2,6 positions
with the 1,4 positions of the naphthalene ring
through two dimethylene linkages, the crystal
structure data show that the two aromatic ring
systems are almost planar 6the interplanar angle is
reported to be 14.3°). This system does exhibit an
absorption tail extending to a longer wavelength
6similar to what we have observed in the case of
NPY) and a long wavelength emission. The ¯uo-
rescence was attributed to an exciplex formed as a
result of the p; p overlap of the two ring systems.
While our systems are much more ¯exible com-
pared to the ‰2; 2Šphanes, these results nevertheless
show that it is indeed possible for a naphthyl 6or
pyrenyl) moiety to interact with a pyridyl moiety,
when forced in a constrained geometry, giving rise
to new absorption and emission features.
The ¯uorescence excitation spectra of the pre-
sent systems show that the new emission band is
certainly not due to an exciplex, as reported pre-
viously. The UV±Vis absorption behavior of NPY
clearly shows that the complex is formed in the
ground state. Even though a ground state inter-
action between the pyridyl and pyrenyl moieties is
not evident from the absorption features of PPY,
the ¯uorescence excitation spectrum correspond-
ing to the new emission of PPY con®rms that the
¯uorescence indeed occurs from the excitation of a
weak intramolecular complex formed in the
ground state.
The redox properties of the molecular compo-
nents have been compiled 6Table 2) with a view to
understanding whether a charge/electron transfer
is thermodynamically allowed between the pyridyl
and naphthyl/pyrenyl moieties. The oxidation po-
tentials of pyrene, naphthalene and pyridine are
1.16, 1.54 and 2.09 V, respectively, and the re-
duction potentials are À2:10, À2:63 and À2:62 V,
respectively. Since the electron accepting tenden-
cies of pyridine and naphthalene are very similar
Table 2
Redox potentialsa and the ®rst excited singlet state energies of the molecular components along with free energy changes expected for a
hypothetical PIET in NPY and PPY
Component
Eox
6V)
Ered
6V)
Es
DG
6kcal molÀ1
)
6kcal molÀ1
)
Pyridine
Naphthalene
Pyrene
2.09b
1.54
1.16
)2.62
)2.63
)2.10
92. 0c
77. 0c
)8. 52d
)2. 29d
a These values are taken from [29]; unless otherwise mentioned, SCE is the reference electrode.
b Measured values 6vs. Ag/AgCl electrode) have been corrected for SCE by adding 0.27 V.
c From [30].
d These values refer to the multi-component systems involving naphthalene or pyrene.

N.B. Sankaran et al. / Chemical Physics Letters 351 .2002) 61±70
69
and naphthalene 6or pyrene) is easier to oxidize
compared to pyridine, the naphthyl 6or pyrenyl)
moiety should act as a donor in a hypothetical
photoinduced electron or charge transfer process.
A rough estimate of the free energy changes
ꢀDG† associated with the photoinduced intra-
molecular electron transfer 6PIET) process has
been made using the Rehm±Weller equation:
DG ꢀkcal molÀ1† ˆ 23:06‰EoxꢀD† À EredꢀA†Š À E00,
where EoxꢀD†; EredꢀA†, and E00 are the oxidation
potential of the donor, the reduction potential of
the acceptor and the excitation energy of the ¯u-
orophore [25,26]. The calculated DG values, shown
in Table 2 6À8:5and À2:3 kcal molÀ1, respec-
tively, for NPY and PPY), show that the PIET is
possible from the excited naphthalene 6or pyrene)
to pyridine. This perhaps explains why an addition
of pyridine to a solution of pyrene or naphthalene
leads to the quenching of the ¯uorescence of the
latter.
Even though the results unambiguously point
to a ground state interaction between the two
terminal chromophores leading to the formation
of a complex that emits at a longer wavelength, a
closer look at the time-resolved ¯uorescence data
of the systems does indicate an excited state in-
teraction too between the terminal moieties. Had
there been only a ground state interaction, the
¯uorescence lifetimes of the systems 6when moni-
tored the short wavelength emission) would have
been very similar to those of 1-methylnaphthalene
or 1-methylpyrene. However, the measured life-
times of NPY and PPY are signi®cantly lower than
67 ns [27] and 176 ns [28] reported for 1-methyl-
naphthalene and 1-methylpyrene, respectively, in-
dicating an excited state interaction also between
the two terminal moieties. The rate constants for
this quenching interaction 6presumably PIET),
estimated using these values as s0 and the mea-
sured lifetime of the short-lived component as s
and using kq ˆ 1=s À 1=s0, are between 1.1 and
2:4 Â 108 MÀ1 sÀ1 in NPY and 1.5and
3:2 Â 108 MÀ1 sÀ1 in PPY. The short-lived and the
long-lived components are presumably due to the
existence of two predominant conformers 6a
compact form and an open one) of the molecules.
The absence of any growth 6a negative pre-expo-
nential factor) in the ¯uorescence decay pro®les of
the complex suggests that no ¯uorescent complex
is formed as a result of the excited state interaction
between the terminal moieties.
5. Conclusions
It has been concluded from the photophysical
behavior of NPY and PPY that the nature of the
interaction of a pyridyl moiety with a naphthyl or
pyrenyl moiety strongly depends on how/whether
the two moieties are connected. While an inter-
molecular interaction between the two does not
lead to the formation any observable complex,
when linked through a dimethylene group, the p±p
interaction of the two terminal moieties gives rise
to the formation of hetero-dimer-type stable ¯uo-
rescent complex. This di€erence in the behavior of
the linked and unlinked systems has been ascribed
to the di€erence in the entropy change associated
with the formation of the two types of complexes.
The metal ion sensing ability of the present sys-
tems will be reported separately.
Acknowledgements
We thank the Council of Scienti®c and Indus-
trial Research 6CSIR) for the ®nancial support.
The time-resolved ¯uorimeter used for this study
has been obtained through a grant from Depart-
ment of Science and Technology, Government of
India. NBS likes to thank CSIR for a fellow-
ship.
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