Photoinduced ultrafast ring-opening reaction in trithianes in solution

Article abstract of DOI:10.1016/j.cplett.2008.09.055

The ring-opening reaction of trithianes possessing aromatic substituents has been studied using femtosecond broadband UV-Vis transient absorption spectroscopy. Photocleavage of the C{single bond}S bond occurs during the short-lived (1.5 ps) singlet excited state of trithiane. The products are formed with excess of vibrational energy and cooling of the hot molecules occurs with a 12.1 ps time constant in acetonitrile.

Full text of DOI:10.1016/j.cplett.2008.09.055

Chemical Physics Letters 465 (2008) 45–47
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Photoinduced ultrafast ring-opening reaction in trithianes in solution
b
Gotard Burdzinski a, , Bronislaw Marciniak
*
a Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, 85 Umultowska, Poznan 61-614, Poland
b Faculty of Chemistry, Adam Mickiewicz University, 6 Grunwaldzka Street, Poznan 60-780, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2008
In final form 22 September 2008
Available online 26 September 2008
The ring-opening reaction of trithianes possessing aromatic substituents has been studied using femto-
second broadband UV–Vis transient absorption spectroscopy. Photocleavage of the CAS bond occurs dur-
ing the short-lived (1.5 ps) singlet excited state of trithiane. The products are formed with excess of
vibrational energy and cooling of the hot molecules occurs with a 12.1 ps time constant in acetonitrile.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
resolves dynamics of ring-opening product formation (1.5 ps) upon
photoexcitation of trithiane and reveals its precursor – trithiane in
Polyatomic molecules possessing relatively weak carbon–sulfur
bonds may undergo a photoinduced cleavage process [1,2]. This
process likely occurs in the singlet excited electronic states. To ex-
plore the early events after photoexcitation, we apply femtosecond
UV–Vis broadband transient absorption technique in order to trace
the species involved in the photocleavage process to characterize
their spectral signature and kinetic behavior. For ultrafast studies
we have chosen 1,3,5-trithianes, because their photocleavage
mechanism has been recently investigated by stationary photo-
chemistry and laser flash photolysis studies [3–7]. Photocleavage
process leads to the formation of an intermediate I (resonance
structure between biradical and zwitterion, see Scheme 1). Reac-
tivity of I found in methanol confirm bipolar nature of I and also
indicates a heterolytic nature of the cleavage process [4]. Its forma-
tion rate has been measured to be faster than 40 ns [4]. Ultrafast
studies should clearly resolve the rise of ring-opening product I.
Recently, ring-opening of thiophene at one of the C–S bonds has
been studied theoretically to explain ultrafast decay of the photo-
excited S1 state [8]. The photoinduced ring-opening process is not
only important from a fundamental point of view, but plays a cru-
cial role in photochromic reactions [9,10] and vitamin D3 forma-
tion [11]. Its mechanism and dynamics have been extensively
studied by time-resolved spectroscopy and theoretical quantum
chemical calculations [11–26]. Typically ring-opening occurs in
solution on a picosecond time-scale, and the following time-con-
stants have been reported: <0.3 ps (cyclohexadiene [12,13],
dihydrophenanthrene [16]), 0.7 ps (spiro-oxazine [17]), 0.9 ps
(indolinospiropyran [18]), 0.95 ps (7-dehydrocholesterol [11]),
2.1 ps (indolyl-fulgimide [19]), <2–3 ps (trimethyl-thienyl maleic
anhydride [20]), 12 ps (cyclooctatriene [21]), 20–60 ps (diaryleth-
ene [22]), 150 ps (bithiophene linked 2H-chromene [23]) and
2.1–325 ps (dithienylethene derivatives [24,25]). This Letter clearly
the singlet excited state.
2. Experimental
The femtosecond transient absorption system has been de-
scribed elsewhere [27].
was synthesized and purified by the methods described elsewhere
[28,29]. The excitation wavelength was set to 266 nm, correspond-
a-TPT (a-2,4,6-triphenyl-1,3,5-trithiane)
ing to the low-energy edge of the
a-TPT absorption band (Fig. 1).
The molar absorption coefficient at 266 nm for
a-TPT in acetoni-
trile is 1800 dm3 molÀ1 cmÀ1. The sample concentration was ad-
justed to an absorbance of 0.55 in a 1 mm flow cell. The pump
pulse energy was about 10 l
J at the sample (0.65 mJ/cm2) and
the estimated number of photons absorbed per molecule in the
excitation volume is 3.4 Â 10À3. To avoid rotational-diffusion ef-
fects, the angle between polarizations of the pump beam and the
probe beam was set to the magic angle (54.7°) [30]. Transient
absorption spectra were corrected for chirp in the probe contin-
uum [31]. Kinetic analysis was performed by global fitting selected
probe wavelengths to a sum of three exponentials and a constant
offset. Convolution with a Gaussian response function was in-
cluded in the global fitting procedure. The instrument response
was approximately 300 fs (FWHM). Acetonitrile was chosen as sol-
vent to minimize contribution from photophysical side-effects
(e.g., two-photon absorption). All experiments were performed at
room temperature.
3. Results and discussion
Excitation of a-TPT with 266 nm radiation pumps the molecule
to the S1, S5 or S6 excited singlet states as indicated by time-depen-
dent density functional theory (TD-DFT) calculations [32,33] in the
gas phase which predict that
oscillator strength in the vicinity of 266 nm: at 281 nm (S0 ? S1,
a-TPT has three transitions with large
* Corresponding author. Fax: +48 61 829 5155.
E-mail address: gotardb@amu.edu.pl (G. Burdzinski).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.09.055

46
G. Burdzinski, B. Marciniak / Chemical Physics Letters 465 (2008) 45–47
R
R
*
S
C
R
R
R
R
R
R
S
C
C
S
C
S
S
C
S
C
H
H
hν
S
R
C
S
C
S
C
S
C
S
C
S
C
S
H
H
H
H
H
H
RCSCHSCH2R
R
H
R
H
R
H
R
H
R=C6H6
I
Scheme 1.
f = 0.0263), 267 nm (S0 ? S5, f = 0.0232) and 260.4 nm (S0 ? S6,
f = 0.0145). Therefore, excitation at 266 nm may result in the pop-
ulation of a set of excited singlet states (see Fig. 1).
The shorter time-constant (1.5 ps in Table 1) can be assigned to
the decay of the S1 population (pronounced in 500–680 nm probe
range) and to the rise of the ring-opening product (around
425 nm). This S1 state is formed with about 500 fs time constant
(see the inset of Fig. 2b), consistent with initial excitation of higher
excited singlet states (Sn > 1) in addition to S1. In contrast to the
500–550 nm probe range, signal at 350 nm is formed instanta-
neously and decays with a 500 fs time constant, which may
indicate the presence of Sn absorption. At 680 nm, almost instanta-
neous rise of the signal is also observed but 500 fs component is
weak and has a negative amplitude. It can be rationalized if S1
and Sn have very similar extinction absorption coefficients near
680 nm. Note that, analysis of the 500 fs time constant is only ten-
tative, since it is close to instrument response function FWHM
(300 fs).
Ultrafast photolysis (266 nm) of a-TPT in acetonitrile produces
the transient UV–Vis spectra of Fig. 2a. Initially, a broad transient
absorption band is formed in the probe range 320–680 nm. At later
times, within a 0.5–50 ps time window, the absorption bands near
350 and 650 nm decay, while a new band grows at 425 nm. At
longer delay times, from 50 ps to 1 ns, no changes are observed
in the transient absorption spectra. The band peaking at 425 nm
can be assigned to the ring-opening product I, which has been
identified and observed by ns laser flash photolysis studies [3]. A
320 nm band is also observed at long delay times. Mostly this band
corresponds to I (the same origin as the 425 nm band), but we can
not exclude
a contribution from the dithioester product,
C6H5C(@S)SCH(C6H5)SCH2C6H5.
Kinetic traces at selected probe wavelengths (350, 425, 500, 550
and 680 nm) are shown in Fig. 2b. Global fitting at representative
wavelengths were performed and the resulting time-constants
(0.5 ps, 1.5 ps and 12.1 ps) are given in Table 1.
Time delays [ps]:
0.04
0.5
1.1
2.3
The longest time constant (12.1 ps) can be attributed to vibra-
tional cooling in the ring-opening product. Indeed, the time depen-
dence of the band integral [34] lacks a 12.1 ps component (Fig. 3),
reflecting the fact that this time constant does not correspond to
population dynamics. Moreover, inspection of the amplitudes asso-
ciated with the 12.1 ps component (Table 1) reveals the rise of the
transient absorption signal in the band center (around 425 nm)
and the decay in the blue (around 350 nm) and red (500–
680 nm) wings. Thus, it reflects the narrowing of the transient
absorption band due to vibrational cooling of the nascent I species
[35–38]. Both the band narrowing and 12.3 ps time constant are
consistent with other reports of vibrational cooling of polyatomic
molecules in acetonitrile [39,40].
4.7
0.03
20
50
0.02
0.01
350 400 450 500 550 600 650
Wavelength [nm]
350 nm
425 nm
500 nm
550 nm
0.04
0.008
0.006
0.08
0.03
0.004
10000
680 nm
f
0.002
0.000
0.02
0
2
4
0.04
0.01
0.00
5000
0
10
20
30
40
50
Time [ps]
0
0.00
400
250
300
350
Fig. 2. (a) Transient absorption spectra recorded from 0.5 to 50 ps after photoex-
citation of -TPT in acetonitrile at 266 nm. (b) Time-dependence of the transient
absorption signal at selected 350, 425, 500, 550 and 680 nm probe wavelengths
A = A1 exp(Àt/
with best fit curves (solid lines). The model function was 1) + A2
exp(Àt/s2) + A3 exp(Àt/s3) + A4, where Ai are amplitudes and si are time constants
a
Wavelength [nm]
Fig. 1. Steady-state UV–Vis absorption spectrum of
left axis). The vertical bars indicate the positions and oscillator strength f of the
electronic transitions calculated by TD-DFT at the B3LYP/6-111+G(d,p) level (right
axis). The optimized structure of
shown in the inset.
a
-TPT in acetonitrile (solid line,
D
s
(global parameters). The inset presents early times of signal at 550 nm probe
wavelength (circles) which rises slower (500 fs), than instantaneously formed long-
lived population convoluted the instrument response function (dashed line).
a-TPT at the B3LYP/6-31G(d) level of theory is

G. Burdzinski, B. Marciniak / Chemical Physics Letters 465 (2008) 45–47
47
Table 1
Scheme 2 summarizes all photoinduced processes in
a
-TPT. The
Global fit parameters for the kinetics in Fig. 2b
ring-opening product I is formed with a 1.5 ps time constant
(kmax = 425 nm). To the best of authors’ knowledge, this Letter pre-
sents the first ultrafast study of ring-opening process caused by C–
S bond cleavage.
k
s1
A1
s2
(ps)
A2
s3
(ps)
A3
A4
(nm) (ps)
350
425
500
550
680
0.50 0.20
0.012 1.5 0.30
À0.003
0.002 12.1 3.0
À0.020
0.006 0.015
À0.010 0.042
0.002 0.009
0.002 0.004
À0.009
0.006
0.007
0.006
Acknowledgments
À0.007
À0.001
0
0.001
Femtosecond UV–Vis spectra were recorded at University in
Lille 1 [27]. Thanks to Julien Rehault for the help with measure-
ments. We thank Dr. Jin Wang for performing theoretical calcula-
tions using the GAUSSIAN 03.
0.012
0.010
0.008
0.006
0.004
0.002
0.000
References
[1] I.W.J. Still, Organic sulfur chemistry, in: F. Bernardi, I.G. Csizmadia, A. Mangini
(Eds.), Theoretical and Experimental Advances, Elsevier, Amsterdam, 1985, p.
596.
[2] S.T Reid, in: D. Bryce-Smith (Ed.), Photochemistry, Burlington House, London,
1982–1988.
[3] E. Janeba-Bartoszewicz, G.L. Hug, E. Andrzejewska, B. Marciniak, J. Photochem.
Photobiol., A: Chem. 177 (2006) 17.
[4] G.L. Hug, E. Janeba-Bartoszewicz, P. Filipiak, H. Kozubek, B. Marciniak, Pol. J.
Chem. 82 (2008) 883.
[5] K. Asmus, G. Hug, K. Bobrowski, Q. Mulazzani, B. Marciniak, J. Phys. Chem. A
110 (2006) 9292.
[6] E. Janeba-Bartoszewicz, G. Hug, H. Kozubek, W. Urjasz, B. Marciniak, J.
Photochem. Photobiol., A: Chem. 140 (2001) 133.
[7] E. Andrzejewska, G. Hug, M. Andrzejewski, B. Marciniak, Macromolecule 32
(1999) 2173.
[8] S. Salzmann, M. Kleinschmidt, J. Tatchen, R. Weinkauf, C.M. Marian, Phys.
Chem. Chem. Phys. 10 (2008) 380.
0
5
10
15
20
25
Time [ps]
[9] N. Tamai, H. Miyasaka, Chem. Rev. 100 (2000) 1875.
[10] D. Geppert, L. Seyfarth, R. De Vivie-Riedle, Appl. Phys. B 79 (2004) 987.
[11] N.A. Anderson, J.J. Shiang, R.J. Sension, J. Phys. Chem. A 103 (1999) 10730.
[12] S. Lochbrunner, W. Fuss, W.E. Schmid, K.L. Kompa, J. Phys. Chem. A 102 (1998)
9334.
Fig. 3. Band integral (circles) calculated accordingly with formula proposed by
Kovalenko et al. [34]. The solid line presents a two exponential function with set
time constants (0.5 and 1.5 ps).
[13] S.H. Pullen, N.A. Anderson, L.A. Walker, R.J. Sension, J. Chem. Phys. 108 (1998)
556.
[14] P. Celani, S. Ottani, M. Olivucci, F. Bernardi, M.A. Robb, J. Am. Chem. Soc.; 116
(1994) 10141.
[15] B. Heinz et al., J. Am. Chem. Soc. 129 (2007) 8577.
[16] S.T. Repinec, R.J. Sension, A.Z. Szarka, R.M. Hochstrasser, J. Phys. Chem. 95
(1991) 10380.
[17] N. Tamai, H. Masuhara, Chem. Phys. Lett. 191 (1992) 189.
[18] N.P. Ernsting, T. Arthen-Engeland, J. Phys. Chem. 95 (1991) 5502.
[19] S. Malkmus et al., Chem. Phys. Lett. 417 (2006) 266.
[20] H. Miyasaka, S. Araki, A. Tabata, T. Nobuto, N. Mataga, M. Irie, Chem. Phys. Lett.
230 (1994) 249.
[21] P.J. Reid, S.J. Doig, R.A. Mathies, J. Phys. Chem. 94 (1990) 8396.
[22] S. Shim, T. Joo, S. Chul Bae, K.S. Kim, E. Kim, J. Phys. Chem. A 107 (2003) 8106.
[23] B. Moine, G. Buntinx, O. Poizat, M. Frigoli, C. Moustrou, A. Samat, Mol. Cryst.
Liq. Cryst. 431 (2005) 63.
τ ~500 fs
τ = 1.5 ps
Sn
S1
ring opening
VC (τ = 12.1 ps)
τ ~ 10 μs
I
[24] J. Ern, A.T. Bens, A. Bock, H.-D. Martin, C. Kryschi, J. Lumin. 76–77 (1998) 90.
[25] J. Ern et al., Chem. Phys. 246 (1999) 115.
[26] Y.Q. Guo, A. Bhattacharya, E.R. Bernstein, J. Chem. Phys. 128 (2008) 34303.
[27] G. Buntinx, R. Naskrecki, O. Poizat, J. Phys. Chem. 100 (1996) 19380.
[28] Z.E. Hawehi, D. Hempel, J. Prakt. Chem. 21 (1963) 286.
[29] J.B. Douglas, W.R. Hydro, J. Am. Chem. Soc. 73 (1951) 3507.
[30] H.E. Lessing, A.V. Jena, Chem. Phys. Lett. 42 (1976) 213.
[31] T. Nakayama, Y. Amijima, K. Ibuki, K. Hamanoue, Rev. Sci. Instrum. 68 (1997)
4364.
S0
-TPT
Scheme 2.
[32] M. Casida, C. Jamorski, K. Casida, D. Salahub, J. Chem. Phys. 108 (1998) 4439.
[33] R. Stratmann, G. Scuseria, M. Frish, J. Chem. Phys. (1998) 8218.
[34] S.A. Kovalenko, R. Schanz, V.M. Farztdinov, H. Henning, N.P. Ernsting, Chem.
Phys. Lett. 323 (2000) 312.
[35] F. Laermer, T. Elsaesser, W. Kaiser, Chem. Phys. Lett. 156 (1989) 38.
[36] H. Miyasaka, M. Hagihiro, T. Okada, N. Mataga, Chem. Phys. Lett. 188 (1992)
259.
Involvement of the triplet state of
a-TPT in the ring-opening
process is unlikely as its presence would require extremely fast
intersystem crossing (500 fs). Ultrafast changes in the molecular
spin state occur rarely in organic compounds [41].
Time-resolved investigations of other ring-opening reactions
[11–13,18,19] agree with our findings on a-TPT: bond cleavage oc-
[37] D. Schwarzer, J. Troe, M. Votsmeier, M. Zerezke, J. Chem. Phys. 105 (1996)
3121.
curs in the S1 singlet excited state, the product is formed vibration-
ally hot and undergoes subsequent cooling by solvent molecules.
For example, recent time-resolved IR studies of indolylfulgimide
in acetonitrile revealed that its excited state lifetime is 4 ps, the
E-isomer product in ring-opening process is born with excess vibra-
tional energy and the cooling occurs on a 10–15 ps time scale [42].
[38] T. Elsaesser, W. Kaiser, Ann. Rev. Phys. Chem. 42 (1991) 83.
[39] G. Burdzinski, J.C. Hackett, J. Wang, T. Gustafson, C. Haddad, M. Platz, J. Am.
Chem. Soc. 128 (2006) 13402.
[40] S.A. Kovalenko, R. Schanz, H. Henning, N.P. Ernsting, J. Chem. Phys. 115 (2001)
3256.
[41] J. Zugazagoitia, C.X. Almora-Diaz, J. Peon, J. Phys. Chem. A 112 (2008) 358.
[42] F.O. Koller et al., J. Phys. Chem. A 112 (2008) 210.