Solid-state photodecarbonylation of diphenylcyclopropenone: A quantum chain process made possible by ultrafast energy transfer

Article abstract of DOI:10.1021/ja078301x

It has been reported that electronic excitation of diphenylcyclopropenone (DPCP) into the second excited state (S₂) results in an adiabatic ring-opening process within 200 fs to give an excited product with a lifetime of ca. 8 ps in S₂. Knowing that energy transfer in crystals may occur within 1-2 ps, we recognized that the conditions could be right for a quantum chain process with the excited-state product sensitizing a neighboring ground-state reactant to set up a domino effect. In agreement with that, we discovered that excitation or macroscopic crystals result in a remarkably efficient reaction to give diphenylacetylene as a polycrystalline powder by a process that releases a great deal of mechanical energy. By taking advantage of nanocrystalline suspensions and using dicumyl ketone as an internal actinometer and after accounting for differences in absorbance at the irradiation wavelength, we were able to establish that the quantum yield of the reaction for DPCP was Φ_DPCP = 3.30 ± 0.35, which is well above unity and consistent with a remarkable quantum chain process. Copyright

Full text of DOI:10.1021/ja078301x

Published on Web 01/10/2008
Solid-State Photodecarbonylation of Diphenylcyclopropenone: A Quantum
Chain Process Made Possible by Ultrafast Energy Transfer
Gregory Kuzmanich, Arunkumar Natarajan, Khin K. Chin, Marcel Veerman,
Christopher J. Mortko, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90024-1569
Received October 30, 2007; E-mail: mgg@chem.ucla.edu
INT*n-1 + DPCPn E-tr8 INTn-1 + DPCP*n -CO8
Robust reactivity models, progress in crystal design, and the use
of specimens in the nanometer scale have led to remarkable
DPAn + INT*n (2′)
advances in the photochemistry of crystals.1 While it is well
appreciated that the crystalline environment “allows” for certain
excited-state processes and reaction pathways, as suggested by the
topochemical postulate,2 recent views place more emphasis on the
structure of the reactant, which ultimately determines its chemical
fate.3,4 For example, on the basis of simple structural considerations,
even without predicting its crystal packing, one can predict whether
the photodecarbonylation of a crystalline ketone may occur in the
solid state!3-5 The reaction has been recently exploited in the
synthesis of structures with adjacent quaternary stereogenic centers,
including the total synthesis of (()-herbertenolide6 and optically
pure (-)- and (+)-R-cuparenone.7
INTn* f DPAn + hν or ∆ (3)
Experiments with large single crystals of DPCP monohydrate12
showed that the solid-state photoreaction occurs with remarkable
efficiency. In a strong indication of a quantum chain process,
crystals crumbled within a few minutes into a fine powder of pure
DPA (Figure 1 and Supporting Information).13 The large amount
of mechanical energy released in this process is consistent with a
very efficient reaction, a large and positive reaction volume, and
the loss of CO gas.
In order to explore the adiabaticity of the reaction, we set out to
measure its quantum yield, ΦDPCP, defined as the moles of product
formed per mole of photons absorbed. A quantum chain would
require ΦDPCP > 1.0. While it is difficult to measure the number
of photons absorbed by dry solids, we recently showed that aqueous
nanocrystalline suspensions may trap all of the photons from a light
source so that one can use a chemical actinometer.14 Having
established a ΦDCK ) 0.2 for the decarbonylation of nanocrystalline
dicumyl ketone (DCK), we decided to use it as an internal
actinometer for DPCP by measuring their relative reactivities within
the same container. Independent nanocrystalline suspensions of
DCK and DPCP prepared by the reprecipitation method15 were
mixed, exposed to low-intensity RPR-3000 (λ ) 312 ( 15 nm)
lamps, and their products quantified. We confirmed that centrifuged
and filtered nanocrystals of DPCP and DPA correspond to the same
phases as their corresponding macroscopic samples. A representative
run of the conversion versus time data in Figure 2 shows the parallel
time evolution of the two reactions and a remarkable factor of 50
between the two conversion scales.
The feasibility of the solid-state reaction relies on R-substituents
(R1-R6, Scheme 1a) that lower the bond dissociation energies of
the bonds being cleaved. The reaction starts by excitation and is
followed by sequential cleavage of the two R-bonds to form radical
pairs RP-1 and RP-2. However, as indicated in Scheme 1a, efficient
R-cleavage is necessary but not sufficient for the reaction to occur
since RP-1 can return to the starting ketone. For that reason, the
dissociation of CO from RP-1 becomes the product formation
determining step.
Exploring new avenues for the solid-state photodecarbonylation
reaction, we sought out structures that facilitate the first bond
cleavage and make the second one unavoidable. Notably, these
conditions should occur in small ring ketones where the release of
a large amount of strain energy after the initial R-cleavage makes
the return to the ground state unfavorable. To test this, we choose
diphenylcyclopropenone (DPCP, Scheme 1b),8 which has a well-
documented solution photochemistry9,10 and has been shown to
dissociate adiabatically along the S2 surface to form CO and
diphenylacetylene (DPA). While there is some debate on whether
de-excitation occurs in the final product9 or along the reaction
surface,10 we reasoned that ultrafast energy transfer in crystals
would favor an otherwise very unlikely quantum chain
process.11 Ideally, excitation to (S2) DPCP* would lead to the
irreversible formation of an excited intermediate or product INT*,
which would transfer energy (E-tr) to a neighboring DPCP, form
another INT*, and so on, until the nth intermediate finally
deactivates (eqs 1-3).
The amounts of diphenyl acetylene DPA (NDPA) and dicumene
(NDC) were measured at various time intervals (Figure 2) in triplicate
experiments for various loading ratios using suspensions with
(CTAB and SDS) and without surfactants. With >50 independent
Scheme 1
DPCP1
9
hν8 INT1* -CO8 DPA1*
(1)
INT1* + DPCP2 E-tr8 INT1 + DPCP*2 -CO8 DPA1 + INT*2
(2)
9
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J. AM. CHEM. SOC. 2008, 130, 1140-1141
10.1021/ja078301x CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
are consistent with every photon initiating the ring-opening reaction
of no more than 4-8 molecules (i.e., 3.30 molecules per photon in
this case). It should be noted that the reaction is exothermic by
∼10 kcal/mol, and that each mole of 300 nm photons injects ∼75
kcal/mol into the sample. A large fraction of this energy may lead
to the vaporization of H2O, which requires the cleavage of several
hydrogen bonds, and to the recrystallization of the product. To test
this hypothesis, we investigated the photochemistry of solvent-free
crystals grown from dry benzene.18 While the reaction was also
fast at ambient temperature, the samples melted and recrystallized
in situ.13 It was also of interest to see whether suspended
nanocrystals of the hydrate have a mechanical response that is
similar to that of macroscopic specimens.19 Indeed, using dynamic
light scattering, we found that the average crystal size changed from
1000 nm before reaction to 220 nm at 100% conversion.
Figure 1. Two micrograph frames illustrating the conversion of crystalline
DPCP to powder DPA. For a series of frames illustrating the entire
transformation into a fine powder within 15 min.12
In conclusion, the release of ring strain upon R-cleavage is a
viable strategy to engineer decarbonylation reactions in crystals.
With quantum yield values of ΦDPCP ) 3.30 ( 0.35, our results
indicate that DPCP undergoes an efficient quantum chain reaction
in the crystalline state. Given that the product is a highly emissive
chromophore, this remarkable reaction may be used to design
strategies for signal amplification.
Figure 2. Representative plot of conversion versus time for a suspension
with equimolar DPCP and DCK in H2O/CTAB. The 50-fold difference in
reactivity is due to a ca. 3.4 times greater absorbance by DPCP, and a 16-
fold difference in quantum yield (ΦDCK ) 0.2 and ΦDPCP ) 3.30 ( 0.35).
Acknowledgment. Financial support by the National Science
Foundation (Grants DMR0605688 and CHE0551938) is gratefully
acknowledged.
Supporting Information Available: Photochemical procedures,
reaction data, and fluorescence spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
References
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(16) By comparing suspended crystals with an optically dense solution of
DPCP, we confirmed that irradiation to S1 (λ g 335 nm) gives similar
quantum yields (ΦDPCP ≈ 1.0), as expected for a reaction from S1.
(17) Powel, R. C; Soos, Z. G. J. Lumin. 1975, 11, 1.
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Figure 3. Fluorescence excitation (heavy line) and emission (dotted line)
of (top) a nanocrystalline suspension of DPA with submicellar CTAB/H2O
and (bottom) a micellar solution of DPA in CTAB/H2O.
experiments, taking into account the different absorbances (A) of
DPCP and DCK, and using the formula
ΦDPCP ) (ADPCP/ADCK)(NDPA/NDC)ΦDCK
we obtained ΦDPCP ) 3.30 ( 0.35.16 Incidental evidence that no
reaction takes place in solution or in micelles was obtained by
fluorescence analysis. While DPCP is not emissive, the fluorescence
emission of DPA formed in solid suspensions was nearly identical
to that obtained from a bulk solid, which is readily distinguishable
from that obtained from experiments carried out in solution or in
micelles (Figure 3).
A quantum yield greater than 1.0 is consistent with an adiabatic
barrierless reaction involving a species capable of transferring
energy to the ground-state reactant before deactivation. While there
is some disagreement on the series of events leading to the ground
state,9,10 femtosecond excitation of DPCP to S2 in solution leads to
ring opening within ca. 200 fs to give an excited-state species with
a lifetime of ca. 8 ps before relaxing to S1. Given that singlet
excitons in crystals have hopping times of ca. 1-2 ps,17 our results
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