Laser flash photolysis study of triphenylimidazole

Article abstract of DOI:10.1016/S1386-1425(99)00192-4

Laser flash photolysis of the photocyclization of triphenylimidazole (TPI) in ethyl alcohol at 308 nm. indicates that the dihydrophenanthroimidazole (DHPI) intermediate is produced rapidly, has a lifetime of 0.25 ms, and returns predominantly back to triphenylimidazole. Analysis of the decay channels for this intermediate indicates two rate constants: (1) k₁ = 3.3×10³ s^-1, associated with reversion back to triphenylimidazole and (2) k₂ = 0.67×10² s^-1 which is associated with the conversion of the dihydrophenanthroimidazole to the photoproduct, 2-phenyl-9,10-phenanthroimidazole. The photoproduct is readily observed as an increasing component in the biexponential fluorescence decay data. Fluorescence lifetimes for triphenylimidazole and 2-phenyl-9,10-phenanthroimidazole (PPI) in ethyl alcohol were determined to be 1.76 and 8.21 ns, respectively, with no additional components in the fluorescence decay as the photochemistry proceeds. An additional transient absorption observed in the 450 nm. region, with a lifetime of 0.7 μs, decaying faster than the dihydrophenanthroimidazole intermediate, is assigned to the triplet state of triphenylimidazole.

Full text of DOI:10.1016/S1386-1425(99)00192-4

Spectrochimica Acta Part A 56 (2000) 901–904
www.elsevier.nl/locate/saa
Laser flash photolysis study of triphenylimidazole
A.C. Testa *
Department of Chemistry, St. John’s Uni6ersity, 8000 Utopia Parkway, Jamaica, New York, NY, 11439, USA
Received 22 June 1999; accepted 21 July 1999
Abstract
Laser flash photolysis of the photocyclization of triphenylimidazole (TPI) in ethyl alcohol at 308 nm. indicates that
the dihydrophenanthroimidazole (DHPI) intermediate is produced rapidly, has a lifetime of 0.25 ms, and returns
predominantly back to triphenylimidazole. Analysis of the decay channels for this intermediate indicates two rate
constants: (1) k1=3.3×103 s−1, associated with reversion back to triphenylimidazole and (2) k2=0.67×102 s−1
,
which is associated with the conversion of the dihydrophenanthroimidazole to the photoproduct, 2-phenyl-9,10-
phenanthroimidazole. The photoproduct is readily observed as an increasing component in the biexponential
fluorescence decay data. Fluorescence lifetimes for triphenylimidazole and 2-phenyl-9,10-phenanthroimidazole (PPI)
in ethyl alcohol were determined to be 1.76 and 8.21 ns, respectively, with no additional components in the
fluorescence decay as the photochemistry proceeds. An additional transient absorption observed in the 450 nm.
region, with a lifetime of 0.7 ms, decaying faster than the dihydrophenanthroimidazole intermediate, is assigned to the
triplet state of triphenylimidazole. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Laser flash photolysis; Photocyclization; Triphenylimidazole; Fluorescence; Transient absorption
1. Introduction
drophenanthroimidazole intermediate (DHPI) is
formed, absorbing at 350 and 550 nm, and has a
relatively slow decay channel with a lifetime of 1.5
ms [2]. Among some of the unresolved issues
regarding the photochemistry of this molecule are:
(i) the geometrical changes of the ortho-phenyls at
the 4 and 5 positions from their initial 40° torsion
angle, relative to the imidazole plane, to the dihy-
drophenanthroimidazole intermediate, ending
with a planar phenanthrene group as 2-phenyl-
9,10-phenanthroimidazole (PPI); (ii) additional
decay channels of this intermediate; (iii) the role
of the triplet state in the relaxation of TPI, and
(iv) the singlet lifetimes of TPI, DHPI, and PPI,
We have previously characterized the photocy-
clization of triphenylimidazole (TPI) as a uni-
molecular singlet state reaction with
quantum yield. In vacuum degassed ethyl alcohol
its fluorescence yield is 0.19 and photochemical
quantum yield is 2.2×10−3 [1,2]. The behavior of
this system is related to the photocyclization of
stilbene derivatives [3]. Using conventional ms
flash photolysis we observed that the dihy-
a low
* Tel.: +1-718-9906161; fax: +1-718-9901876.
E-mail address: testaa@stjohns.edu (A.C. Testa)
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(99)00192-4

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A.C. Testa / Spectrochimica Acta Part A 56 (2000) 901–904
since their is a changing fluorescence spectrum as
the photocyclization proceeeds. This singlet state
photochemical process is summarized as follows:
ethyl alcohol pumped through a closed circulating
system. A tungsten–halogen lamp was used as the
analyzing source, with an optical path of 4 cm,
and the detection system comprised a cooled
gated intensified diode array. The transient spec-
tra were measured in two segments, covering the
wavelength range 320–440 nm and 400–600 nm.
Fluorescence lifetime measurements utilized 1
cm cells in a right angle configuration, and a
photon counting detection system. A frequency
doubled Rhodamine pumped Nd:YAG laser hav-
ing a pulse width of 5 ps served as the excitation
source. These solutions were purged with argon
before measuring the fluorescence lifetimes. UV
absorption spectra were recorded with a Perkin-
Elmer Lambda 2 spectrophotometer and steady
state fluorescence spectra were recorded with a
Perkin-Elmer
Model
LS-50
In an earlier study we suggested that the ineffi-
ciency of this process is due to reversion of DHPI
intermediate back to starting TPI. Since we were
only able to see the DHPI relaxation channel of
1.5 ms in ms flash photolysis, we have initiated a
laser flash photolysis study of TPI, complemented
with fluorescence lifetime measurements, to ad-
dress some of the above issues. An attempt to
characterize the geometrical changes of this pho-
tocyclization by time resolved resonance Raman
was made difficult by the large fluorescence con-
tribution from the DHPI produced in this system
during laser excitation.
spectrofluorophotometer.
3. Results
Laser flash photolysis of a 5.8×10−5 M solu-
tion of TPI in ethyl alcohol, employing a circula-
tor pump to refresh the solution under
investigation, gives rise to transient absorption
spectra which are shown in Fig. 1 for the region
320–440 and in Fig. 2 for the region 400–600 nm.
The excited singlet state is not observed due to its
short lifetime. The spectra clearly indicate absorp-
tion bands at 350 and 550 nm, which are intense
and cover the temporal range from 1 ms to 5 ms
and whose bands agree with the DHPI intermedi-
ate that we previously observed in conventional
flash photolysis [2]. Noteworthy is that the optical
density of the transient at 350 nm did not show
any signs of decay during the temporal range 200
ns to 1.0 ms. In fact the transient absorption
spectra are superimposable for the region 320–
410 nm at delay times of 0.2, 0.5 and 1.0 ms i.e. an
equilibrium population of DHPI is established,
which decays by two dominant routes. Since the
decay of DHPI does not occur until after 1 ms, its
population is the same as it was at 200 ns, i.e. the
quantum yield of DHPI formation is small. This
behavior, together with our earlier report of the
slow 1.5 ms relaxation channel for DHPI [2] is
2. Experimental
2.1. Materials
TPI was obtained from Aldrich Chemical and
recrystallized from ethyl alcohol to yield a color-
less powder. Spectrograde solvents were used as
received.
2.2. Apparatus
Laser flash photolysis at 308 nm (XeCl) em-
ployed a 20 ns pulsewidth operating at 10 Hz,
with a 200 ml solution of 5.8×10−5 M TPI in

A.C. Testa / Spectrochimica Acta Part A 56 (2000) 901–904
903
conventional ms flash photolysis for the conver-
sion of DHPI“TPI. These two kinetic processes
lead to a 0.25 ms lifetime for the DHPI intermedi-
ate. We have previously shown that the photocy-
clization proceeds via the excited singlet, and
addition of cis-piperylene does not influence the
quantum yield.
There is clear evidence of a second, shorter
lived transient absorption at 400–450 nm with a
half-life of 0.7 ms, which is most evident in spec-
trum (1) of Fig. 2. The short fluorescence lifetime
of the starting TPI and the photoproduct, PPI
(see below) preclude this transient being a singlet
state, and we have assigned this transient to the
triplet state of TPI. In view of the small photo-
chemical yield, and €F+€T:1, i.e. the dominant
relaxation mode appears to be intersystem cross-
ing. Within 1 ms essentially all the triplets have
decayed, while DHPI is longer lived, as is seen in
Figs. 1 and 2. The decreasing triplet population of
TPI at 450 nm is evident in the temporal range
0.2–1.0 ms, whereas the population of DHPI at
350 nm is constant during the same period.
Fig. 1. Time resolved transient absorption spectra for the laser
flash photolysis of 5.8×10−5 M triphenylimidazole in ethyl
alcohol covering the range 320–440 nm (308 nm exc.). Al-
though the spectra shown cover the submicrosecond regime, it
is noteworthy that the transient absorption spectra in the
region 320–410 nm. are superimposable at the smaller delay
times of 0.2, 0.5 and 1.0 ms.
Experiments employing two pulsed lasers, i.e.
308 nm, to excite the singlet state of TPI and 370
nm to probe transients showed increased fluores-
cence, which is due to absorption and fluorescence
of DHPI, and PPI, since TPI does not absorb the
latter wavelength. Transient DHPI fluorescence is
seen at delay times of 175 ns and 1 ms, confirming
that its formation occurs in the nanosecond time
scale; however, it does not compete favorably
with the formation of the triplet state of TPI.
Additional information regarding the mecha-
nism of this photocyclization was obtained by
fluorescence lifetime measurements. Irradiation of
an argon purged solution of 2.4×10−6 M TPI in
ethyl alcohol at 308 nm indicates a dual fluores-
cence, whose contribution changes as the starting
TPI is converted cleanly to PPI. A summary of
the fluorescence lifetime data is presented in Table
1, where it is seen that the lifetimes for TPI and
PPI are constant at 1.76 and 8.21 ns, as the
photolysis proceeds. The major change occurring
is reflected in columns 5 and 6, which show that a
decreasing TPI concentration is replaced with an
increase in PPI. These data are indicative of a
clean photochemical event from TPI to PPI, in
Fig. 2. Time resolved transient absorption spectra for the laser
flash photolysis of 5.8×10−5 M triphenylimidazole in ethyl
alcohol covering the range 400–600 nm (308 nm exc.).
consistent with the following two decay channels
for DHPI: (i) a faster decay corresponding to
DHPI“TPI, and (ii) a slower decay of DHPI“
PPI. Analysis of the shorter lived component in
the transient spectra at 350 nm region indicates a
relaxation time of 0.3 ms, which is significantly
faster than the 1.5 ms decay channel observed in

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A.C. Testa / Spectrochimica Acta Part A 56 (2000) 901–904
Table 1
either of the two decay channels of DHPI. The
fluorescence lifetime data support this interpreta-
tion since the values of 1.76 and 8.21 ns, assigned
to TPI and PPI, respectively, remain constant,
and vary only in their contributions, as the photo-
cyclization proceeds.
Fluorescence lifetime data for triphenylimidazole in ethyl
alcohola
b
b
A1
~
(ns)
A2
~
(ns)
2
f1
f2
1
1.
2.
3.
4.
5.
6.
14 453
14 114c
13 955
1.77
1.74
1.75
220 8.03
506 8.21
604 7.96
615 8.18
660 8.33
700 8.54
0.936
0.855
0.835
0.829
0.816
0.801
0.064
0.145
0.165
0.171
0.184
0.199
In view of the TPI fluorescence lifetime of 1.76
ns and its fluorescence quantum yield of 0.19 in
ethyl alcohol, it is apparent that the rate constant
for the initial photocyclization step from the TPI
singlet state to DHPI is 54×108 s−1. This result
may not be surprising since AM1 calculations
with MOPAC [5] for the optimized geometry of
the ground state of TPI predicts the phenyl
groups at positions 4 and 5 to have a torsional
angle of 38° relative to the imidazole plane. The
dominant radiationless decay of the excited singlet
state is intersystem crossing to the triplet state in
the nanosecond regime, which together with the
fluorescence yield of TPI, leads to the small pho-
tochemical yield.
13 916d 1.76
13 930
13 643
1.75
1.76
a Conc.=2.4×10−6 M; 308 nm excitation; 380 nm fluores-
cence. IF(t)=A1e−1 t/~+A2e−2 t/~
.
b f1=A1~1/(A1~1+A2~2) and f2=A2~2/(A1~1+A2~2) [4].
c Measured at 360 nm.
d Measured at 410 nm.
which both molecules fluoresce. A similar behav-
ior was observed in acetonitrile, where the corre-
sponding lifetimes are 1.80 and 9.17 ns The
fluorescence of the DHPI is not seen in these
experiments, since it does not absorb the 308 nm
excitation, i.e. only TPI and PPI contribute to the
fluorescence. In contrast, the dual laser experi-
ment using 308 nm to produce singlets and 370
nm to probe transients led to a significant increase
in fluorescence, due to the conversion of TPI to
DHPI, followed by absorption and emission of
the latter.
In summary, the flash photoexcitation of TPI
shows that the DHPI intermediate is present at
100 ns, has a lifetime of 0.25 ms, and only begins
to decay after 1 ms. The disappearance of the
triplet state is much faster than that of DHPI.
There are two major relaxation routes for DHPI:
(1) a major decay channel of DHPI back to TPI
with a k1=3.3×103 s−1 and (2) a slower decay
process with k2=0.7×102
s
−1, which we re-
ported earlier, that accounts for the low quantum
yield conversion of DHPI to the photoproduct,
PPI.
4. Discussion
.
The results presented are consistent with a uni-
molecular photochemical process in which the
DHPI intermediate is present at 100 ns, has two
decay channels, with only one fifth of their popu-
lation proceeding to the photoproduct, PPI. From
the two relaxation modes of DHPI we have deter-
mined its lifetime to 0.25 ms. In contrast, the
triplet state, absorbing in the 450 nm region, is the
dominant relaxation mode of the TPI excited
singlets. The triplet is evident in the first 100 ns,
and its lifetime of 0.7 ms is much shorter than
References
[1] J. Hennessy, A.C. Testa, J. Phys. Chem. 76 (1972) 3362.
[2] P. Takach, A.C. Testa, J. Photochem. Photobiol. 37 (1987)
347.
[3] W.H. Laarhoven, Pure Appl. Chem. 56 (1984) 1225.
[4] J.R. Lakowicz, Principles of Fluorescence Spectroscopy,
Plenum Press, New York, 1983, pp. 56–59.
[5] Semi-empirical quantum mechanical program, V. 6.0, from
Serena Software, Bloomington IN, 1993.