Photoinduced substitution reaction in halothiophenes. Quantum-mechanical and photochemical studies on nitro- and cyano-derivatives

Article abstract of DOI:10.1039/b101993g

2-Iodo-5-nitrothiophene, 2,-bromo-5-cyanothiophene, 2-iodo-5-cyanothiophene, and 4-iodo nitrobenzene were studied by steady state and pulsed methods and by semi-empirical quantum-mechanical computations. The quantum yields for the direct and sensitized photoarylation of these haloromatics following the photocleavage of the carbon-halogen bond were measured in benzene. The transient species originated by direct and sensitized laser excitation were examined by nanosecond laser flash photolysis in benzene and methylcyclohexane. The transients were characterized in terms of absorption spectra and decay lifetimes, and the triplet states were also characterized by phosphorescence measurements in rigid matrices at low temperature. The quenching rate constants of the triplet states and the quantum yields for singlet oxygen production were determined in methylcyclohexane. Findings obtained with sensitizers of various triplet energy showed that an upper triplet state is involved in the photocleaveage of the carbon-halogen bond.

Full text of DOI:10.1039/b101993g

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Photoinduced substitution reaction in halothiophenes.
Quantum-mechanical and photochemical studies on
nitro- and cyano-derivatives
L. Latterini,a F. Elisei,a G. G. Aloisi*a and M. DÏAuriab
a Dipartimento di Chimica, Universita di Perugia, I-06123 Perugia, Italy
b Dipartimento di Chimica, Universita della Basilicata, I-85100 Potenza, Italy
Received 2nd March 2001, Accepted 11th May 2001
First published as an Advance Article on the web 18th June 2001
2-Iodo-5-nitrothiophene, 2-iodo-5-cyanothiophene, 2-bromo-5-cyanothiophene and 4-iodo-nitrobenzene were
studied by steady state and pulsed techniques and by semi-empirical quantum-mechanical calculations. The
quantum yields for the direct and sensitized photoarylation of these haloaromatics following the
photocleavage of the carbonÈhalogen bond were measured in benzene. The transient species (triplet states and
free radicals) originated by direct and sensitized laser excitation were investigated by nanosecond laser Ñash
photolysis in benzene and methylcyclohexane. The transients were characterized in terms of absorption spectra
and decay lifetimes. The triplet states were also characterized by phosphorescence measurements in rigid
matrices at low temperature. The quenching rate constants of the triplet states and the quantum yields for
singlet oxygen production were determined in methylcyclohexane. The results obtained with sensitizers of
di†erent triplet energy indicate that an upper triplet state is involved in the photocleavage of the
carbonÈhalogen bond.
photoreaction the main route is probably again the triplet,
although a singlet pathway cannot be excluded.
Introduction
To obtain a more general picture of the behaviour of this
class of compounds we extended the photophysical study to
halothiophene derivatives containing withdrawing groups
The photocleavage of the carbonÈhalogen bond (CÈX) in ali-
phatic and aromatic halides,1h10 haloketones11h13 and halo-
thiophenes14h18 has been widely studied over the last decades.
In the aryl halides, where the electronic excitation produces a
p,p* excited state, an intersystem crossing (ISC) to a disso-
ciative triplet was proposed.3,4,9 Direct evidence for the role
of the triplet state in the photodissociation of iodonaphtha-
lene came from experiments with triplet energy sensitizers.1
Interestingly, for the sensitized dissociation quantum yield a
marked temperature e†ect was observed2 as well as a depen-
dence on the triplet energy of the sensitizers.1 Some
experiments3,4,9 also gave evidence that the dissociative state
is an upper triplet. A clear demonstration of the involvement
of an upper triplet in the CÈBr bond cleavage was reported by
Scaiano et al. for bromonaphthalene and bromophenanthrene
by using two lasersÈtwo colors techniques. They found a nice
correlation between the photobleaching of the bromo-arene
triplets and the increase in the absorption of the benzeneÈBr~
complex.10
A photochemical and photophysical study on the photo-
cleavage of the CÈX bond in haloderivatives of thiophencarb-
aldehyde and in 2-acetyl-5-iodothiophene, with formation of
the phenylthiophene derivatives in direct and sensitized condi-
tions, was reported by our laboratory.18 The direct and sensi-
tized photoarylation, following the photocleavage of the
corresponding halothiophenes was studied under steady state
irradiation. Moreover the triplet states and free radicals
formed by excitation were investigated by laser Ñash photoly-
sis to determine their spectral and kinetic properties together
with their formation quantum yields. The results indicated
that, for these compounds also, the sensitized photo-
substitution takes place in a dissociative upper triplet state
mainly localized on the carbonÈhalogen bond. For the direct
(NO and CN) and moreover we undertook semiempirical
2
quantum-mechanical calculations to give information on their
excited singlet states.
In the present paper a theoretical description of the singlet
excited states of 2-iodo-5-nitrothiophene (INT), 2-iodo-5-
cyanothiophene (ICNT), 2-bromo-5-cyanothiophene (BCNT)
and 4-iodo-nitrobenzene (INB), in terms of nature, energy and
transition probability, is presented. For these compounds,
photophysical and photochemical studies were performed by
steady state and laser Ñash photolysis techniques. In particu-
lar, the quantum yields for the direct and sensitized photoary-
lation and the spectral and kinetic properties of the triplet
states and of non-triplet transients are reported together with
the quantum yields of triplet formation (from O (1* ) sensiti-
2
g
zation experiments). Moreover, for the four compounds, phos-
phorescence spectra and lifetimes were measured in
methylcyclohexane/3-methylpentane (MCH/3MP) at 77 K.
Experimental
p-Iodonitrobenzene was a commercial product (Aldrich). INT,
ICNT, BCNT were the same as used in previous studies.16,17
Absorption spectra were recorded by
a Perkin-Elmer
(Lambda 5) double-beam spectrophotometer, while emission
was tested by a Spex Fluorolog-2 spectroÑuorometer.
The steady-state irradiation of the halothiophene deriv-
atives under direct and sensitized conditions was performed
by a high-pressure mercury lamp coupled with interference
Ðlters or by the excitation system of the Spex Fluorolog-2
DOI: 10.1039/b101993g
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This journal is ( The Owner Societies 2001
2765

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Table 1 Transitions of halo-aromatics calculated by INDO/1-CI
spectroÑuorometer using ferrioxalate as an actinometer. The
irradiation for the direct photoreaction was performed on
deaerated solutions and in total absorption of light. The
photosensitized reaction was quantiÐed by irradiating deaer-
ated solutions of the donors acetophenone (A) and phen-
anthrene (Ph) at 330 nm, benzophenone (B) at 340 nm and
chrysene (Ch) at 345 nm; the photosensitizers were Baker pro-
ducts (Photosensitizers and Quencher Kit).
(singly excited, 15 occupied and 15 virtual MOs)
Compound
Transition
j
/nm
f
Nature
max
S ] S
806
544
355
331
299
258
0.0000
0.0005
0.011
0.0024
0.060
0.35
p,p*
p,p*
p,r*
p,p*
p,p*
p,p*
0
1
2
3
4
5
6
S ] S
0
S ] S
0
S ] S
0
S ] S
The photoproducts, p-phenylnitrobenzene and phenylderi-
vatives of nitrothiophene and cyanothiophene were identiÐed
by spectrophotometry, gas chromatography-mass spectrom-
etry (GC-MS) and high-performance liquid chromatography
(HPLC) comparing their characteristics with those of pure
samples. The quantum yields (estimated error ^10% ) for the
direct or sensitized photoarylation (/ and / , respect-
0
S ] S
0
S ] S
382
374
330
283
278
0.011
0.0000
0.012
0.31
p,r*
p,p*
p,p*
p,p*
p,r*
0
1
2
3
4
5
S ] S
0
S ] S
0
S ] S
0
S ] S
0.054
0
PA
SPA
ively) were determined spectrophotometrically (halothiophene
concentration ca. 10~4 M). To avoid an inner Ðlter e†ect, the
conversion percentage was held below 10% in all the experi-
S ] S
395
288
274
251
247
0.010
0.40
0.012
0.0002
0.0027
p,r*
p,p*
p,r*
p,r*
p,r*
0
1
2
3
4
5
S ] S
0
S ] S
0
S ] S
ments. The /
values were determined by taking into
0
SPA
S ] S
account the efficiency of the energy transfer process as calcu-
lated by the quenching of the donor lifetime (quenching
efÐciency \ 1 [ q/q , where q and q are the lifetimes of the
donor in the absence and in the presence of the acceptor,
respectively) and the amount of the light transmitted in each
experiment.
0
S ] S
430
284
267
265
256
0.0058
0.39
0.032
0.0061
0.0006
p,r*
p,p*
p,r*
p,r*
p,r*
0
1
2
3
4
5
S ] S
o
o
0
S ] S
0
S ] S
0
S ] S
0
Phosphorescence spectra and lifetimes were measured in
methylcyclohexane/3methylpentane (MCH/3MP, 9 : 1, v/v) at
77 K with a Spex Fluorolog-2 spectroÑuorometer equipped
with a 1934D phosphorimeter.
For INB, the lowest excited singlet states S and S are p,p*
1
2
in nature, while S is p,r*. However, for the three thiophene-
3
derivatives (INT, ICNT and BCNT) the lowest state S is p,r*
and S is p,p*. On the basis of these results, the reactivity of
INT, ICNT and BCNT can involve the S state while for IBN
it does not. When the halo-aromatic contains a carbonyl
Two excitation wavelengths, j \ 347 nm from the second
1
exc
harmonic of a ruby laser (J.K. Lasers) and j \ 355 nm from
exc
2
the third harmonic of a Nd : YAG laser (Continuum), were
1
used in the nanosecond laser Ñash photolysis experiments
(laser energy \5 mJ and pulse width O20 ns). Other experi-
mental details are given elsewhere.19 Laser Ñash photolysis
measurements were carried out in a Ñow-through cell to avoid
sample degradation.
group, as for the molecules investigated previously,18 the S
1
state becomes n,p* in nature, with the orbital n localized on
the carbonyl group and with low probability of cleavage.
Fig. 1 shows the frontier molecular orbitals involved in the
descriptions of the lowest excited states of BrCNT; in particu-
Singlet oxygen quantum yields (/ , estimated error ^15%)
*
lar, the conÐguration HOMO ] LUMO mainly describes S
were determined under pulsed conditions by recording the
1
(p,r*) and the HOMO-1 ] LUMO ] 1 describes S (p,p*) of
phosphorescence of O (1* ) with a germanium diode detector
2
2
g
INT, ICNT and BCNT.
in air-equilibrated solutions.20
The maxima of the experimental electronic absorption
spectra of INT, ICNT, BCNT and INB in benzene are report-
ed in Table 2 together with the molar absorption coefficients,
e. The e values, of the order of 104 M~1 cm~1 and comparable
with those of the other halothiophenes,18 give evidence that
the absorption band recorded at lowest energies is due to
The lowest excited singlet state energies, electronic struc-
tures and transition probabilities of halothiophenes (HTP)
were investigated by semi-empirical computational methods.
The geometry of the substrates was optimized at an ab initio
(HartreeÈFock) level with Gaussian 9821 using the set of basis
3-21G. The transition energies and oscillator strengths were
calculated by the INDO/1-CI semiempirical model22 (ZINDO
program of Cerius2 3.8 package). The conÐguration inter-
action (CI) calculations included the singly excited conÐgu-
rations built from the 15 highest occupied (HOMO) and 15
lowest virtual (LUMO) molecular orbitals.
Results
Absorption spectra and quantum-mechanical calculations
To gain a better understanding of the nature of the lowest
states in the singlet manifold, quantum-mechanical calcu-
lations were performed. The electronic states of nitrobenzene
(NB) have been extensively investigated by theoretical
methods,23 but little work has been done on halonitroben-
zenes and on nitro- and halonitro-derivatives of thiophene.
For NB in the planar form,23 the lowest excited singlet is n,
p*.
Table 1 summarizes the absorption maxima, oscillator
strengths and the nature of the lowest singletÈsinglet tran-
sitions of halo-aromatics obtained by the quantum-
mechanical calculations.
Fig. 1 Frontier molecular orbitals of BCNT calculated by the
ZINDO semi-empirical program (INDO/1 model).
2766
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Table 2 Ground state absorption maxima (j ), direct (/ ) and sensitized (/ ) photoarylation quantum yields of haloaromatics in benzene
max
PA
SPA
at room temperature
/
SPA
Compound
j
/nm
e/104 M~1 cm~1
/
[j /nm]
A
B
Ph
Ch
max
PA exc
290
340
279
276
1.05
0.008[313]
0.20[313]
0.34[313]
0.23[290]
0.011
O0.002
1.0
1.4
2.4
0.22
0.40
0.28
0.16
0.20
0.31
0.07
È
È
allowed p,p* transitions. The energies and the e values are in
mol~1), benzophenone (B, E \ 288 kJ mol~1), phenanthrene
T
good agreement with the computed ones.
(Ph, E \ 259 kJ mol~1) and chrysene (Ch, E \ 238 kJ
T
T
mol~1) were used as sensitizers. The quantum yields for sensi-
tized photoarylation (/ ) of the three halothiophenes and
INB are reported in Table 2. Except for BCNT, the /
values depend on the triplet energy of the sensitizer, in partic-
Steady-state experiments
SPA
SPA
Direct and sensitized photoarylation. Direct irradiation of
INT, ICNT and BCNT in benzene under steady-state condi-
tions, caused photoinduced substitution of the halogen atom
by the phenyl group. The quantum yields for the formation of
ular, for INT the highest value of / is obtained for B
SPA
(highest E ) and the lowest one pertains to Ch (lowest E )
T
T
while for ICNT the sensitized arylation quantum yield is
halved on going from B to Ph. This behaviour, that reÑects a
di†erent population of the reactive triplet with the di†erent
sensitizers, has already been reported for the photosensitized
decomposition of the iodonaphthalene1 and for other halo-
thiophenes.18 Unfortunately, it was not possible to follow the
sensitization for ICNT and BCNT by Ch because of the lack
of triplet quenching even in the presence of a high concentra-
tion of acceptor, likely due to the close triplet energies of the
two (sensitizerÈsensitized) species.
the phenylderivatives (photoarylation, / ) are in the range
PA
0.2È0.35 (see Table 2). Iodonitrobenzene irradiated at 313 nm,
also gave the corresponding p-nitrobiphenyl (NBP). This photo-
reaction was proposed by Wolf et al.24 as a synthetic tool
even though it has a very low quantum yield (D1%, Table 2).
The photoarylation of halothiophenes occurs cleanly and
quantitatively as shown by spectrophotometric and HPLC
analyses by comparing the photoproduct characteristics with
those of pure samples of the phenylthiophene-derivatives.
Only long-term irradiation with an unÐltered Hg-lamp caused
a slow decrease in the photoproduct absorbance. Spectral
changes observed in the UV region under steady state irradia-
tion are reported in Fig. 2 for INT in benzene as an example;
the presence of an isosbestic point evidences the cleanliness of
Phosphorescence properties. INT, ICNT and BCNT do not
phosphoresce at room temperature, but their emission spectra
were recorded in a MCH/3MP matrix at 77 K. The phosphor-
escence spectra of the three HTPs were similar and presented
an unstructured band around 500 nm. In the case of INB, the
phosphorescence spectrum obtained under the same experi-
mental conditions has the main maximum at 410 nm. These
the photoreaction. The spectrum 5 of this Ðgure (j \ 357
max
nm) corresponds to the absorption of the photoadduct 2-
phenyl-5-nitrothiophene. Experiments carried out in other sol-
vents (methylcyclohexane and acetonitrile) did not show
considerable changes in the absorption of halothiophenes,
even after prolonged irradiation.
Similar behaviour was observed upon irradiation of ICNT
in benzene. The absorption of an irradiated solution of ICNT
revealed a broadening and a red-shift of the absorption band.
The presence of an isosbestic point at 280 nm indicated the
formation of one photoproduct, which is 2-phenyl-5 cyano-
thiophene.
The spectrophotometric analysis of the BCNT in benzene
solutions after irradiation showed the appearance of a new
absorption band (j \ 300 nm), whose intensity increased
max
with the irradiation time. The chromatographic analysis con-
Ðrmed the formation of only one product, 2-phenyl-5 cyano-
thiophene.
In the case of INB, the spectra recorded at increasing irra-
diation time presented a rise in the intensity and a slight red-
shift of the UV absorption band. These changes were due to
the formation of p-nitro-biphenyl (NBP), as was indicated by
the chromatographic analysis using a pure sample of NBP.
This result was further conÐrmed by comparing the absorp-
tion spectra of the photoproduct and of the commercially
available p-nitro-biphenyl (j \ 305 nm).
Fig. 2 Ground-state absorption spectra of INT in benzene (1) and
max
The sensitization of the photoreaction by triplet energy (E )
after 30 (2) 60 (3), 120 (4) and 180 min (5) of irradiation time (j
\
T
exc
donors was also investigated. Acetophenone (A, E \ 309 kJ
313 nm).
T
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2767

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spectra allowed the energy of the emitting triplet (measured at
the maximum emission wavelength) in the range 220 and 250
kJ mol~1 to be obtained. For all the samples, the phosphor-
escence decays follow Ðrst-order kinetics with lifetimes in the
range 0.5È6 ms, depending on the compound. The emission
spectra, lifetimes and triplet energies are very similar to those
previously reported for other thiophene derivatives.18 The
characteristics of phosphorescence of INT, ICNT and BCNT
are reported in Table 3 together with those of INB for com-
parison. It is to be noted that the triplet energy of INB is
about 50 kJ mol~1 higher than those of the other three com-
pounds.
Laser Ñash photolysis experiments
Direct laser excitation of INT, ICNT and BCNT produces, in
general, triplet states and radical species absorbing in the 370È
700 nm range. In particular, upon excitation of BCNT in
methylcyclohexane a transient signal was detected at the end
of the laser pulse whose spectrum presented the main
maximum at 430 nm (Fig. 3a) and which decayed with Ðrst-
order kinetics having a lifetime of 5.9 ls. The absorption
signal recorded at longer wavelengths (450È650 nm) decayed
with mixed (Ðrst- and second-) order kinetics. These obser-
vations indicated the presence of di†erent transient species. In
particular, the band at 430 nm was assigned to the T ] T
Fig. 3 Time-resolved absorption spectra of (a) BCNT in MCH
recorded at 0.16 (L), 1.9 (|), 4.0 (È) and 7.5 ()) ls; (b) ICNT in
MCH recorded at 0.07 (L), 0.12 (|), 0.45 (È) and 1.5 ()) ls; (c) INT
in MCH recorded at 0.07 (L), 0.15 (|), 0.45 (È) and 1.5 ()) ls after
1
n
transitions on the basis of the following arguments: (i) it was
produced within the laser pulse by a monophotonic process;
(ii) it decayed by Ðrst-order kinetics; (iii) it was quenched by
triplet energy acceptors; (iv) it was quenched by oxygen with a
the laser pulse (j \ 355 nm).
exc
rate constant (k ) close to the di†usion limit; (v) its location
was similar to those observed for the triplets of other halo-
and the related kinetics is second-order. For possible inter-
ference on the absorption spectra of the halogen atoms and
their complexes with MCH see ref. 18. The direct excitation of
INT and ICNT, in MCH also produces the triplet state
(absorption 440 nm for both compounds) and thienyl radicals
whose contribution to the absorption (450È650 nm) is more
evident than for BCNT (Fig. 3b and c). In the region where
the triplet absorbs (j O 440 nm) the signal decayed with Ðrst-
order kinetics, while in the range where the radicals also
adsorb it is of mixed-order. For the INT and ICNT, the triplet
lifetimes (Table 4) were 0.3 and 0.8 ls, respectively; they are
considerably shorter than those obtained for the Br-derivative,
probably due to the heavy atom e†ect. In the case of INB in
MCH no transient species were detected under direct or sensi-
tized excitation, suggesting that the observation is limited by
the time resolution of our set-up. In fact, for nitrobenzene a
triplet lifetime on the picosecond timescale was measured.25
In benzene the triplet absorption maxima are again around
440 nm. However, only in the case of BrCNT in benzene did
the signal of the triplet absorption decay with Ðrst-order
kinetics and a triplet lifetime of 10 ls. In the cases of ICNT
and INT the transient spectra are dominated by the non-
triplet species (Fig. 4b and c) since the signals decayed with a
mixed-order kinetics all over the spectral region investigated.
ox
thiophene derivatives previously investigated.18 On the basis
of the kinetic analysis of the signals (mixed Ðrst- and second-
order), the absorption in the range 450È650 nm was attributed
to the overlap between the triplet and the thienyl radical
formed by carbonÈhalogen cleavage. In fact, as in methyl-
cyclohexane the quantum yield of the photoreaction is negligi-
ble, the main decay pathway of radicals is the recombination
Table 3 Phosphorescence properties of haloaromatics in methylcy-
clohexane-3-methyl-pentane (9 : 1) at 77 K
Compound
j
/nm
q /ms
E /kJ mol~1
max
P
T
410(sh), 460
525(sh), 560
473(sh), 520
496(sh), 535
0.6
293
226
250
242
1.2
0.53
6.0
For the mixed ÐrstÈsecond-order kinetics t
values in the
1@2
range 1È6 ls were measured at 500 nm. Attempts were made
Table 4 Spectral and kinetic properties of the lowest triplet state produced upon direct excitation of haloaromatics in methylcyclohexane at
room temperature together with the singlet oxygen and triplet (lower limit) quantum yields
Compound
j
/nm
q /ls
k /109 M~1 s~1
/
/min
max
T
q
*
T
440, 540
440, 540
430, 530
0.3
0.8
5.9
1.3
1.5
0.9
0.35
0.66
0.75
0.70
0.83
0.80
2768
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to distinguish kinetically between the triplet and thienyl tran-
sients but we failed because of the similar lifetimes of the two
transients. Furthermore, transient spectra features in benzene
are complicated by the presence of additional transients: the
halogen atomÈbenzene complexes (X~Èbenzene, where X~
stands for I~ or Br~). In particular, the halogen atoms produc-
ed by photolysis form well-known complexes with solvent
molecules which absorb around 500 nm (I~Èbenzene, e \ 2450
M~1 cm~1)18,26 and around 550 nm (Br~Èbenzene, e \ 2300
M~1 cm~1),18 depending on the halogen atom. Therefore, in
the same range of the radical absorption there is the absorp-
tion of the complexes and the tail of the triplet. Moreover, the
absorption contribution of thienyl radicals seems higher than
that of the X~-complexes whose maxima are hardly recognized
(in contrast to that observed for similar compounds).18 Only
for BCNT (Fig. 4a) does a secondary maximum due, at least
in part, to Br~Èbenzene complex appear at 550 nm. Unfor-
tunately, under these experimental conditions it was impossi-
ble to evaluate the absorption of X~Èbenzene complexes and
their concentrations and hence a determination of the forma-
tion quantum yields was prevented by the overlap of the tran-
sient absorptions.
HTPs, and the fraction which produced the radicals through
an upper triplet state.
Since it was important to know the triplet yields, we used
an indirect method to determine them taking advantage of the
ability of the triplets to sensitize singlet oxygen, O (1* ). In
2
g
fact, the halothiophen triplets were efficiently quenched by
molecular oxygen with bimolecular rate constants (k B 1
ox
] 109 M~1 s~1) close to the di†usional limit. The triplet
quenching was accompanied by the appearance of a lumines-
cence in the IR region detected by a Ge-photodiode. This
emission was assigned to the O (1* ) phosphorescence for the
2
g
following reasons: (i) it was produced with the instrumental
time resolution; (ii) it decayed by Ðrst-order kinetics with life-
times very close to those already reported for O (1* );29 (iii) it
2
g
was not observed in the absence of the substrate or by bub-
bling with argon. The emission signal of an air-equilibrated
solution extrapolated at ““zero timeÏÏ (I ) was proportional to
*
the concentration of O (1* ), which depends on the quantum
2
g
yield of the O (1* ) production, / , and on the laser energy,
I . Therefore, the slopes of the I vs. I plots, after calibration
of the experimental set-up by use of phenalenone as standard
2
g
*
L
*
L
(/ \ 0.97 ^ 0.03),30 gave / of the thiophene derivatives
*
*
The energy transfer experiments were performed in benzene
to measure the efficiency of the energy transfer process to be
(Table 4). In air-equilibrated solutions, the quenching effi-
ciency of HTP triplets, deÐned as the ratio (qAr [ qair)/qAr \
used in the determination of /
phenanthrene (Ph) and chrysene (Ch) as energy donors. The
using benzophenone (B),
P where qAr and qair correspond to the triplet lifetimes in
SPA
Q
Ar-saturated and air-equilibrated solutions, respectively, were
triplet state of B was quenched with rate constant around 1010
di†erent for the various HTP and therefore the / values were
T
calculated taking into account the di†erent P values.
Q
M~1 s~1 (k \ 2.7 ] 1010 M~1 s~1),27 while the quenching
diff
rate constant for Ch was ca. 109 M~1 s~1 for INT and negli-
Unfortunately, due to the lack of Ñuorescence it was not
gible for the other two HTPs. The quenching of the donor
possible to measure the singlet lifetimes, however in the pres-
ence of heavy atoms they should be short so that it can be
assumed that O (1* ) is produced only by energy transfer
triplet was accompanied by the appearance of the T ] T
absorption band (440 nm) together with the thienyl band. The
1
n
2
g
determination of the triplet quantum yields (/ ) and molar
from the lowest triplet state T . Assuming furthermore that
T
1
absorption coefficients with the usual energy transfer method
the fraction of quenched triplet producing O (1* ) is unitary
2
g
(based on the triplet lifetimes and absorption changes
measurements)28 was not possible due to the overlap of the
absorptions of the various transients in both solvents and to
the reactivity of an upper triplet state (see below). Indeed, in
the sensitization experiments it was not possible to distinguish
(S \ 1) we determined the lower limits of the / values which
*
T
are reported in Table 4. In general, they are high, as expected
for molecules containing heavy atoms such as sulfur and halo-
gens.
between the fraction of the donor, which populated T of the
Discussion
1
The dissociation of CÈBr and CÈI bonds in halobenzene needs
energy of 313 ^ 5.0 and 286 ^ 5.0 kJ mol~1, respectively.31
Since it is likely that the energy required for the various halo-
thiophenes is comparable, the dissociation of CÈX bonds of
these compounds can occur in the lowest excited singlet state.
In fact the maximum energy of absorption band at longer
wavelengths is around 340 kJ mol~1 for INT and higher for
the others. However, the energy of the lowest triplet state of
INT, ICNT and BCNT obtained from phosphorescence mea-
surements was 220È250 kJ mol~1 (and 300 kJ mol~1 for INB)
and hence should not be sufficient to dissociate the CÈX bond
as was observed for other HTPÏs.18 On the other hand it was
stressed that p,p* triplet9 and p,p* singlet4 in haloaromatics
are unable to give dissociation because the antibonding p
density is placed between carbon atoms. Moreover the Ñash
photolysis experiments of the present work show that the trip-
lets and the radical species were both produced within the
laser pulse and this excludes the detected triplet (the lowest,
T ) from being the precursor of the radicals. The decrease in
1
the sensitized photoarylation, paralleling the decrease in the
sensitizer energy (Table 1), also gives evidence that the lowest
triplet is not reactive (or only slightly reactive). In fact, even
though the triplets of benzophenone (E \ 308 kJ mol~1),
T
phenanthrene (E \ 259 kJ mol~1) and chrysene (E \ 238 kJ
T
T
mol~1) were efficiently quenched by INT and those of the two
Fig. 4 Time-resolved absorption spectra of (a) BCNT in benzene
recorded at 0.2 (L), 1.4 (|), 3.9 (È) and 10.5 ()) ls; (b) ICNT in
benzene recorded at 0.1 (L), 0.34 (|), 2.4 (È) and 7.9 ()) ls; (c) INT
in benzene recorded at 0.1 (L), 0.34 (|), 0.76 (È) and 5.0 ()) ls after
former sensitizers by ICNT and BCNT (k D 1010 M~1 s~1),
q
the /
values of Table 2 show a marked decrease, except for
SPA
BCNT, on decreasing the energy of the sensitizer (three times
for INT). The ICNT and BCNT do not quench the triplet of
the laser pulse (j \ 347 nm).
exc
Phys. Chem. Chem. Phys., 2001, 3, 2765È2770
2769

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Ch, probably because of a reversible energy transfer with T
and/or an inability to populate the too-high upper triplet. The
References
1
dependence of /
on the E is explained by the fact that the
1
2
F. Wilkinson, J. Phys. Chem., 1962, 66, 2549.
SPA
T
lowest triplet of halothiophenes (estimated energy 220È250 kJ
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(with the exception already mentioned), but is unable to
produce photodissociation. Only the donors, which can popu-
late the upper reactive triplet, are able to sensitize the photo-
reaction. The same behaviour was observed for halothiophene
carbaldehyde and acetyliodothiophene for which the reactive
triplet was estimated between 250 and 300 kJ mol~1.18 For
the compounds studied in the present paper the position of
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likely also that of the reactive triplets. It is to be noted, from
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for BCNT does not change. This can be an indication that in
this compound the energy of the reactive triplet is lower than
that of other halothiophenes.
Moreover, Table 2 shows that B does not sensitize the photo-
arylation of INB and hence this compound should not have
a reactive triplet at energies smaller than 300 kJ mol~1. Since
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mol~1), even though it remains very small (0.01), it is likely
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has the highest E energy in Table 3.
T
Assuming that / remains unchanged in benzene, and
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the sum of the quantum yields for the population of the lowest
T
PA
T
16 M. DÏAuria, Gazz. Chim. Ital., 1989, 119, 419.
triplet (/ ) and of the reactive state (/ ) is close to unity. This
T
PA
17 (a) M. DÏAuria, A. De Mico, F. DÏOnofrio and G. Piancatelli,
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ticularly taking into account that from semi-empirical calcu-
lations the p,r* singlets are the lowest for these HTPs (Table
1). The singlet mechanism should be less probable for 5-
bromo- and 5-iodo-2-thiophencarbaldehyde and 2-acetyl-
thiophene for which, on the basis of semi-empirical calcu-
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nature and the p,r* are the upper ones (S ). If the precursor of
dissociation is a triplet state it should be an upper one where
2
an antibonding orbital mainly localized on the CÈX bond is
half Ðlled. The photoarylation quantum yield of INT sensi-
tized by Ch (Table 1) is the lowest amongst the sensitizers, but
is higher than the values previously reported for BTCA, ITCA
and AIT.18 Since the sensitized photoreaction quantum yields,
for a given donor, should be related to the energy of the reac-
tive upper triplet of the acceptor, it is likely that for INT this
triplet is a little lower than those of the HTPs reported pre-
viously.18 However it cannot be excluded that the low /
the INTÈCh system is due to a small reactivity of the lowest
of
SPA
triplet T .
1
Perusal of Table 2 shows that / and /
values are com-
SPA
values with ben-
PA
parable. However for ICNT and BCNT /
zophenone are higher than / . This behaviour, already
observed with high energy sensitizers for thiophene derivatives
described earlier,18 can be due to a di†erence in the popu-
lation of the reactive triplet in the direct and sensitized excita-
tion due to an ISC lower than unity or to a di†erence in
reactivity of the states if, in the direct photocleavage, a singlet
mechanism is operative.
SPA
PA
26 R. E. Buhler, J. Phys. Chem., 1972, 76, 3220.
27 S. L. Murov, in Handbook of Photochemistry, Marcel Dekker,
New York, 1973.
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30 R. Schmidt, C. Tanelian, R. Dunsbach and C. Wol†, J. Photo-
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31 R. C. Weast, Handbook of Chemistry and Physics, CRC Press,
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New York, 1968.
Acknowledgements
Financial support by the Consiglio Nazionale delle Ricerche,
Universita di Perugia and Ministero della Pubblica Istruzione
(Rome) is gratefully acknowledged.
2770
Phys. Chem. Chem. Phys., 2001, 3, 2765È2770