Two-photon induced photodecarbonylation reaction of cyclopropenones

Article abstract of DOI:10.1039/b513248g

Irradiation of cyclopropenones (1a-c) with 800 nm pulses of ultrafast laser results in a photodecarbonylation reaction via nonresonant two-photon absorption of light. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513248g

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Two-photon induced photodecarbonylation reaction of
cyclopropenones{
Nurtay K. Urdabayev, Andrei Poloukhtine and Vladimir V. Popik*
Received (in Cambridge, UK) 19th September 2005, Accepted 16th November 2005
First published as an Advance Article on the web 12th December 2005
DOI: 10.1039/b513248g
as well as the photo-Wolf rearrangement10 and the uncaging of
Irradiation of cyclopropenones (1a–c) with 800 nm pulses of
ultrafast laser results in a photodecarbonylation reaction via
nonresonant two-photon absorption of light.
bioactive molecules11 using two-photon excitation.
This communication describes for the first time the two-photon
induced decarbonylation of cyclopropenones 1a–c, as well as the
direct determination of two-photon absorption cross-sections of
these substrates.
UV photolysis of cyclopropenones often results in efficient
decarbonylation and the formation of corresponding acetylenes
in high yield (Scheme 1).1 In the development of novel
photonucleases, which is underway in our group, we employ the
cyclopropenone photodecarbonylation for the in situ generation of
reactive cyclic enediyne compounds.2
Single-photon photochemistry of 1a–c. The UV spectra of diaryl-
substituted cyclopropenones 1a–c in methanol show the longest
wavelength band between 300 and 400 nm depending on
substitution (1a lmax 5 341 nm, loge 5 4.64; 1b lmax 5 364 nm,
loge 5 4.19; 1c lmax 5 390 nm, loge 5 4.24, Fig. 1). We have not
observed any absorption bands with lmax . 400 nm even at very
high substrate concentrations (up to 0.01 M).
Decarbonylation of cyclopropenones is usually induced by UV
irradiation. These conditions, however, are not compatible with
many biomedical applications, which require use of light in a so-
called ‘‘phototherapeutic window’’, a region of relative tissue
transparency between 650 and 950 nm. One of the approaches
allowing for the alleviation of this problem is to employ
nonresonant two-photon excitation (2PE). At high light fluxes
chromophores might simultaneously absorb two red/NIR photons
producing excited states the same as or similar to ones accessible
by excitation with UV light of twice the frequency.3
Commercialization of ultrafast lasers has resulted in the develop-
ment of numerous successful applications of the nonresonant two-
photon excitation in fluorescent spectroscopy,4 microscopy,5 and
photonics.6 While many very efficient two-photon fluorophores
have been reported,7 the field of two-photon photochemistry8
remains relatively unexplored. There are only a few examples of
two-photon induced cycloaddition and cycloreversion reactions,9
While thermal decomposition of cyclopropenones 1a–c requires
temperatures in excess of 150 uC, the 350 nm irradiation of these
compounds in methanol results in efficient decarbonylation and
the quantitative formation of the corresponding acetylene (2a–c,
Table 1, Scheme 1).
Two-photon induced decarbonylation of cyclopropenones (1a–c).
The irradiation of 1a–c in methanol with 800 nm ultrashort (ca.
95 fs) pulses from a Ti:sapphire laser results in the same process as
the UV photolysis, i.e., the loss of carbon monoxide and the
formation of the corresponding acetylenes 2a–c. The progress of
this photoreaction was monitored by the formation of 2a–c using
HPLC (Fig. 2, Table S1).12
Scheme 1
Center for Photochemical Sciences, Bowling Green State University,
Bowling Green, Ohio, USA 43403. E-mail: vpopik@bgnet.bgsu.edu;
Fax: +1 (419) 372-9809; Tel: +1 (419) 372-2470
{ Electronic supplementary information (ESI) available: Derivation of eqn
(1) and experimental details for 2PE and SPA photolyses. See DOI:
10.1039/b513248g
Fig. 1 UV spectra of ca.
2 6 M methanol solutions of
1025
cyclopropenones 1a–c. The insert shows the spectral width of the
Ti:sapphire laser pulse.
454 | Chem. Commun., 2006, 454–456
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Table 1 Single- and two-photon photochemical properties of cyclo-
propenones 1a–c
s2PE(800)
(GM)a
e400
(M21 cm21
)
W
sR(800) (GM)a
1a 0.26 ¡ 0.04b 0.0020 ¡ 0.0005
0.0019 ¡ 0.0004c
0.0077
4.6
292
1b 0.50 ¡ 0.01b 0.00796 ¡ 0.00007 0.016
0.0078 ¡ 0.0001c
1c 0.47 ¡ 0.06b 0.20 ¡ 0.04
0.46 ¡ 0.06d 0.18 ¡ 0.04c
0.31 ¡ 0.04e 0.17 ¡ 0.01f
a
0.42
14 550
b
1
GM
5
10250 cm4
s
photon21 molecule21
.
At 350 nm.
d
Calculated from initial rates using zero-order approximation. At
c
e
f
300 nm. At 254 nm. Calculated from the consumption of 1c.
The spectral width of the Ti:sapphire laser used in this work is
represented as an insert in Fig. 1. Evidently, there is no spectral
overlap between the irradiation source and substrates 1a–c, which
is required for single-photon excitation. The conversion of the
starting material by the two-photon induced photoreaction can be
described by the equation [eqn (1)], which, in turn, has been
derived from the differential form of Beer’s law for the two-photon
absorption.13 In eqn (1) I2 is the squared light flux (photons2
cm24 s22), which is integrated for the duration of the laser pulse, n
represents the repetition rate, s is the two-photon cross-section of
the substrate (cm4 s photon21 molecule21), W2PE is the fraction of
two-photon excited molecules that undergo chemical transforma-
Fig. 2 Yields of acetylene 2a (e), 2b (,), and 2c (#) formed in the two-
photon induced decarbonylation of 1 mM methanol solutions of
cyclopropenones 1a–c. Lines shown were drawn using parameters
obtained by least-squares fitting of eqn (2). The consumption of 1c ($)
and the formation of 2c (#) are overlaid on the insert.
correspondingly with laser pulses of variable energy.14 Yields of
acetylenes 2b and 2c, which were produced under these conditions,
show quadratic dependence on laser power (Fig. 3, Table S2).12
The logarithmic representations of the yield versus pulse energy
dependences have slopes of 2.07 (1b) and 2.02 (1c), confirming
the two-photon nature of the observed photodecarbonylation
reaction.
tion, and C is the concentration of the substrate.
ð
dC
2
: :
I0 dt v C
{
~W2PE
s
(1)
dt
pulse
To convert experimentally-determined two-photon cross-
sections for the induction of the photodecarbonylation reactions,
sR, into the two-photon absorption cross-sections of cycloprope-
nones 1a–c, we need to know the fraction of two-photon excited
molecules that undergo decarbonylation, W2PE. The excited state
initially populated upon two-photon excitation might be different
from that initially populated upon single-photon excitation.
However, according to Kasha’s rule, photochemical reactions
generally occur from the lowest singlet or triplet excited states
Integration of equation [eqn (1)], the application of the 94 fs
laser pulse width,12 and the conversion to molar concentrations
result in a kinetic equation [eqn (2)]. sR is the two-photon cross-
section for the induction of the photodecarbonylation reaction and
is a product of quantum efficiency and the two-photon absorption
cross-section of the substrate, sR 5 W2PE*s.
0
1
100 fs
ð
B
C
A
2
0
:
C~C0 exp {s
:
: :
I dt v t
@
R
(2)
{100 fs
100 fs
ð
DC
2
:
: :
I0 dt v t
~{sR
(3)
C0
{100 fs
Least-squares fitting of the experimental data to the equation
[eqn (2)] allowed us to calculate values of sR (Fig. 2, Table 1). The
two-photon induced decarbonylation of bis(2-methoxy-1-
naphthyl)cyclopropenone (1c) was quite efficient and allowed us
to calculate sR not only from the rate of product formation but
also from the kinetic data on the consumption of 1c (insert in
Fig. 2). In addition to the use of the exponential equation [eqn (2)],
we have applied a zero-order kinetic equation [eqn (3)] to calculate
the two-photon cross-section for the induction of the decarbonyla-
tion reaction using data in the region of low conversion (Table 1).
Values produced by all three calculations are in a very good
agreement.
In order to verify that an observed reaction is, in fact, a two-
photon process, we have analyzed the expected quadratic
dependence of the rate of product formation on the light flux.
Cyclopropenones 1b and 1c were irradiated for 25 min and 8 min
Fig. 3 Yield of acetylenes 2b and 2c formed on variable power 800 nm
laser irradiation of cyclopropenones 1b and 1c.
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regardless of the excitation method and the initial exited state.15
Thus, we can assume that the quantum yield of the two-photon
initiated process is equal to its single-photon counterpart,
W2PE 5 WSPE. The latter values were determined by chemical
actinometry at 350 nm (Table 1). To check the validity of Kasha’s
rule for excitation of cyclopropenones, we also measured the
quantum yield for the decarbonylation reaction of 1c at 254 nm
and 300 nm, which showed very little variation from the 350 nm
values (Table 1). The two-photon absorption cross-sections of
cyclopropenones 1a–c at 800 nm were calculated using single-
photon quantum yields, s2PE(800) 5 sR(800)/WSPE, and are shown in
Table 1.
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is usually based on the donor–p–acceptor–p–donor motif, where
–p– is an extended conjugated p-system.16 The very small two-
photon cross-section of bis-p-anisylcyclopropenone (1a) is,
therefore, not surprising since the p-system of this molecule is
2
rather small (total area of a p-conjugated system is 305 A ).
˚
However, extension of the conjugation by the p-phenylene unit on
2
both sides of the cyclopropenone in 1b (area 5 463 A ) does not
˚
bring much gain in the two-photon cross-section. The somewhat
smaller bis(2-methoxy-1-naphthyl)cyclopropenone 1c (area 5
2
390 A ), on the other hand, shows a twenty six times larger
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two-photon absorption cross-section than 1b. A much better
correlation is observed between the two-photon absorption cross-
sections and the extinction coefficients at 400 nm (Table 1). The
fact that cyclopropenones 1a and 1b have very low absorption at
400 nm indicates that the energy of two 800 nm photons is
insufficient to achieve excitation to the lowest SPE-accessible
excited state. Apparently, there are no other lower lying states,
which can be populated by 800 nm two-photon excitation.
In summary, we have shown the feasibility of the two-photon
induced decarbonylation of cyclopropenones, which produces
quantitative yields of the corresponding acetylenes. The two-
photon absorption cross-sections of cyclopropenones 1a–c were
determined by the direct method based on the conversion of the
substrate under irradiation with ultrashort pulses. The develop-
ment of cyclopropenones with larger two-photon absorption cross-
sections, as well as time-resolved investigation of the two-photon
induced decarbonylation is underway in our group.
The authors thank the National Institutes of Health (grant
CA91856-01A1) and the National Science Foundation (grant
CHE-0449478) for the support of this project, as well as the Ohio
Laboratory for Kinetic Spectroscopy for the use of instrumenta-
tion and technical assistance. A. P. thanks the McMaster
Endowment for a research fellowship.
12 See ESI{.
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intensity of the laser pulses in the range from 125 to 835 mJ pulse21
.
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456 | Chem. Commun., 2006, 454–456
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