Photoenolization via excited state double proton transfer induces turn on fluorescence in diformyl diaryl dipyrromethane

Article abstract of DOI:10.1039/c4cc03668a

A light triggered enolization in diformyl diaryl dipyrromethane by excited state dual proton transfer (ESDPT) induces turn on fluorescence. The role of diaryl and diformyl groups in the enolization process was confirmed by photophysical and theoretical studies.

Full text of DOI:10.1039/c4cc03668a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Photoenolization via excited state double proton
transfer induces ‘‘turn on’’ fluorescence in
diformyl diaryl dipyrromethane†
Cite this: Chem. Commun., 2014,
50, 8667
Received 14th May 2014,
Accepted 13th June 2014
K. C. Gowri Sreedevi,a Ajesh P. Thomas,b K. H. Aparna,b Renuka Pradhan,b
M. L. P. Reddy,a U. Lourderajb and A. Srinivasan*b
DOI: 10.1039/c4cc03668a
www.rsc.org/chemcomm
A light triggered enolization in diformyl diaryl dipyrromethane by excited
state dual proton transfer (ESDPT) induces ‘‘turn on’’ fluorescence. The
role of diaryl and diformyl groups in the enolization process was
confirmed by photophysical and theoretical studies.
Certain molecular systems change their chemical structures in
response to external stimuli such as chemical, electrical or light
thereby modulating their luminescence properties.1 In particular,
light stimulated responses resulting in irreversible2a or reversible2b
structural and conformational changes play a major role in the
precise analysis of biological functions.2 One such structural
transformation is the photoinduced proton transfer, involved in
several biochemical reactions in living systems.3
Recently, a computational study on pyrrole-2-carboxaldehyde
(P2C), a nonfluorescent molecule commonly used in the synthesis
of porphyrins,4 predicted the possibility of an excited state intra- or
inter-molecular proton transfer from NH to CO forming enol.5
However, such a proton transfer process has not been observed
experimentally in pyrrole derivatives to date. Herein, we report a
case of light activated fluorescent ‘‘turn on’’ caused by the photo-
induced excited state intramolecular dual proton transfer (ESDPT)
in 1,9-diformyl-5,5-diaryldipyrromethane (DAKK) resulting in a
dienol that exhibits a large Stokes shifted emission. Interestingly, it
is found that the photoenolization is favored by the presence of aryl
substituents at the meso-carbon.
Fig. 1 (a) Single crystal X-ray structure of DAKK, (b) absorption and (c) emission
changes of DAKK (2.75 Â 10À5 M) in acetonitrile upon UV irradiation along with
visible color and emission changes (inset). (d) The solid state color change of
DAKK before and after UV irradiation under visible and UV light.
The UV-Vis absorption spectrum of DAKK in acetonitrile showed
an absorption maximum at 300 nm (e = 3.3 Â 104 MÀ1 cmÀ1) which
Vilsmeier–Haack formylation of ditolyldipyrromethane6 was assigned to a p–p* transition and a very weak emission band at
using DMF and POCl3 at 0 1C for 1 h resulted in DAKK (72% around 560 nm, when excited at the absorption maximum. How-
1
yield) and was characterized by H NMR, 13C NMR, FAB mass ever, the fluorescence excitation spectrum (lem = 560 nm) showed a
spectrometry (Fig. S1–S3, ESI†) and finally confirmed by single- band between 380–500 nm with two peaks (Fig. S4, ESI†) that
crystal X-ray-diffraction analysis (Fig. 1a).
differed largely from the absorption spectrum of DAKK, predicting
the presence of a new species. Further, when a 2.75 Â 10À5
M
a School of Chemical Sciences, National Institute of Science Education and Research
(NISER), Bhubaneswar-751005, Orissa, India. E-mail: srini@niser.ac.in
b Chemical Science and Technology Division, National Institute for Interdisciplinary
Science and Technology (NIIST-CSIR), Trivandrum-695019, Kerala, India
† Electronic supplementary information (ESI) available: Spectral details, coordi-
nates of minimum energy structures, and details of photoirradiation studies of
DAKK. CCDC 923773. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc03668a
solution of DAKK in acetonitrile was exposed to UV irradiation for
definite time intervals with a band-pass filter l = 300 Æ 20 nm, the
absorption band at 300 nm decreased gradually with the appear-
ance of a new broad band with maximum intensity at 390 nm.
UV irradiation was accompanied by a color change from colorless
to bright yellow (Fig. 1b, inset) and exhibited a clear isosbestic point
at 316 nm.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 8667--8669 | 8667

View Article Online
Communication
ChemComm
The appearance of a new red-shifted absorption peak (390 nm) in
the excitation spectrum supports the formation of a new species.
The fluorescence intensity at 560 nm enhanced continuously with
bright green emission (Fig. 1c, inset) and a fourfold increase in the
fluorescence quantum yield (ff = 0.02) with respect to fluorescein.
Moreover, the new species formed was found to be solid state
emissive (Fig. 1d), which can potentially be applicable in the
fabrication of luminophore based devices, where the solid state
emission is desirable.7 It should be pointed that DAKK with other
external stimuli such as heat and pH variation did not show any
change in the absorption/emission spectra. The reversibility of the
processes was also checked with visible light irradiation, heat/dark
and was found to be irreversible.
Fig. 2 Changes in the absorption maxima of DAKK as a function of UV
irradiation time. Inset: emission color change of DAKK after irradiation in
different solvents.
Two possible mechanisms were considered to explain the
observed photophysical changes upon irradiation: (i) keto–enol
tautomerization by proton transfer from the pyrrolic NH to carbonyl
CO group to generate enol and (ii) cis–trans isomerization of the CO
group with respect to NH,8 where cis refers to the CO group syn to
species in each solvent at the photostationary state was calculated
from the decrease in optical density at 300 nm, which showed higher
conversion in polar solvents.
In order to analyse the role of the formyl group in the photo-
induced keto–enol tautomerism, further investigation was done with
other dipyrromethane derivatives such as MFK, which have only one
formyl group and DT, without any formyl groups (Scheme 1). The
UV-Vis absorption spectra recorded for MFK (2.75 Â 10À5 M) in
acetonitrile exhibited a peak at 297 nm which decreased gradually
with a concomitant increase at 370 nm upon UV irradiation with an
isosbestic point at 317 nm (Fig. S9, ESI†). However, the photo-
stationary state was achieved only after 11 min compared to 35 s in
DAKK and the irradiated solution of MFK remained colorless with a
weak emission at 439 nm. In contrast, DT showed no significant
change in the absorption spectra upon irradiation, further confirming
the importance of diformyl substitution and thus the formation of
enolic species by a proton transfer process. Also, the significance of
aryl substitution at the meso-carbon in the photoenolization was
studied with MAKK, which has only one aryl group at the meso-
carbon and DMKK, where aryl groups are replaced with methyl groups
(Scheme 1). There was no appreciable change observed in the
absorption spectrum for MAKK even after 10 min of irradiation
(Fig. S10, ESI†). In the case of DMKK, a continuous decrease in
absorbance was observed without any color change.
1
NH, and trans refers to the CO group anti to NH. The H NMR
spectrum of DAKK in CD3CN before and after irradiation (Fig. S5a
and b, ESI†) showed a distinct difference. Before irradiation, the
formyl proton appeared at 9.41 ppm and the NH proton at 9.72 ppm.
1H NMR spectra after irradiation displayed several additional peaks
indicating a mixture of compounds, possibly the diketo and a newly
formed species. The IR spectrum of DAKK before irradiation showed
the CO stretching frequency at 1656 cmÀ1 and a broad peak for NH
at 3211 cmÀ1 along with a shallow peak at 3435 cmÀ1 (Fig. S6, ESI†).
However, the IR spectrum after irradiation showed a weak NH band
(3227 cmÀ1) and an intense broad band at 3436 cmÀ1 that can be
assigned to the O–H stretching band, indicating the formation of
enolic species. To confirm that irradiation results in the formation of
enol, it is important to ascertain the role of the NH proton in the
process. This was investigated by the addition of a strong base such
as FÀ ions (as its tetrabutylammonium salt) to an acetonitrile
solution of DAKK prior to UV irradiation. The abstraction of NH by
the base was confirmed by the disappearance of the NH proton peak
in the 1H NMR spectrum (Fig. S7, ESI†). Interestingly, UV irradiation
after the deprotonation did not result in the ‘‘turn on’’ emission,
indicating the role of NH proton in the formation of the new species.
The fact that the abstraction of protons by a base can inhibit the
keto–enol tautomerism and the absence of ‘‘turn on’’ emission rule
out the possibility of cis–trans isomerization as the C–C single bond
rotation does not depend on the presence of the NH proton.
Solvent dependent analyses showed that the absorption and
emission spectra of DAKK after photoirradiation exhibited a positive
Density functional theoretical (DFT) calculations were performed
to gain a better insight into the keto–enol tautomerization process,
particularly to understand the role of diaryl substitution at the meso-
carbon of DAKK in the enolization process by using the B3LYP
functional and 6-31+G* basis set.10 Although the absorption and
solvatochromism (Fig. S8 and Table S1, ESI†). The large red shift in
max
em
the l suggested the excited state to be highly polar in nature than
the ground state, confirming the involvement of a polar enolic
species in the process further eliminating the possibility of cis–trans
isomerization. Photophysical changes in DAKK due to irradiation in a
series of solvents of varying polarity (cyclohexane to DMSO) were
monitored by the changes in the absorption maximum of the enol
peak as a function of irradiation time (Fig. 2). The keto–enol
tautomerism was found to follow the first-order kinetics9 and the
rate constant (k) values at 298 K increased with solvent polarity
(Table S2, ESI†). The percentage of conversion from the diketo Scheme 1 Keto and enol forms of dipyrromethanes considered in this study.
8668 | Chem. Commun., 2014, 50, 8667--8669
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Table 1 Relative energy, absorption and emission properties of KK and EE
tautomers of DH, DM and DA computed at the B3LYP/6-31+g* level
of theory
DMEE and DAEE are 507, 521 and 527 nm, respectively, and
can be compared to the experimental emission maximum of
560 nm for DAEE (Table 1). Clearly, the DFT calculations
reiterate that the appearance of the new red shifted peak is
due to the formation of the enolic tautomer.
Molecule
Tautomer
DH
KK
DM
KK
DA
KK
EE
EE
EE
The relative energies (Table 1) of the EE minima in the S1 state
revealed that DHEE and DMEE are B29 kcal molÀ1 higher than that
of the KK form, whereas only B20 kcal molÀ1 for DAEE. The
additional stabilization of B10 kcal molÀ1 seen in DAEE clearly
demonstrates that the KK to EE tautomerization of DA in the S1
state is energetically more favorable compared to that of DH and
DM systems. Although the tautomerization barrier in the S1 state
could not be calculated, it can be expected that such stabilization of
the transition state connecting the KK and EE forms may be seen
for DA. The above results explain the appearance of a new red
shifted absorption due to enol formation and the importance of
aryl substitution to enable keto–enol tautomerization in the excited
state, a phenomenon not seen for other derivatives. The complete
photophysical processes during irradiation of DH and DA systems
Relative energy (kcal molÀ1
)
S0
S1
0.0
0.0
34.0
28.5
0.0
0.0
34.0
27.6
0.0
0.0
35.9
20.0
Vertical excitation wavelength (nm)
S1
S2
S3
316
316
293
318
309
306
316
316
295
319
310
306
318
318
302
345
330
327
a
labs
—
—
o250
—
300
390
Emission wavelength (nm)
lem
lem
386
—
507
—
385
—
521
—
386
—
327
560
b
a
Experimental absorption maximum. b Experimental emission maximum.
emission wavelengths may not match with the experiments owing to are given in Fig. S14 (ESI†) for comparison.
the gas-phase nature of the calculations, qualitative and important
To summarize, we have demonstrated the photoenolization
insight into the experimental observations can be deduced from of DA, via ESDPT. To the best of our knowledge, this is the first
the DFT results. The ground and excited state minimum energy report on light triggered enolization of a dipyrromethane
structures of DAEE showed distinct differences in the orientation of derivative with ‘‘turn on’’ fluorescence and emissive even in
the aryl and pyrrole rings (Fig. S12, ESI†). The relative energy the solid state. DFT calculations confirm the importance of the
calculations showed that in the ground state, the DAEE, DMEE and diaryl group at the meso-carbon that stabilizes the dienol form
DHEE are B34.0 kcal molÀ1 higher in energy than the KK forms in the excited state compared to monoaryl or dimethyl groups.
irrespective of the meso-carbon substitution (Table 1).
The computed excitation energies and the molecular orbitals shifted emission of the molecule as sensors will be explored.
contributing to the excitations are given in Table 1 and Fig. S13 KCGS, APT and RP thank CSIR, New Delhi, for fellowship.
In future, the application of the light triggered large Stokes-
(ESI†) respectively. It can be seen that for DAKK, the transitions We thank Dr Arun Kumar, NISER, for X-ray structure of DA.
resulted from charge redistribution in the pyrrole rings, whereas
in DAEE, there is an extension of conjugation from pyrrole to the
phenyl ring. For all the three KK structures, the S1 and S2 states
Notes and references
1 (a) Y. Sagara and T. Kato, Nat. Chem., 2009, 1, 605; (b) H. Maeda,
are due to n–p* (HOMO À 6 to LUMO, HOMO À 6 to LUMO + 1,
Y. Bando, K. Shimomura, I. Yamada, M. Naito, K. Nobusawa,
H. Tsumatori and T. Kawai, J. Am. Chem. Soc., 2011, 133, 9266.
respectievely) transitions and are degenerate, while S3 is due to
p–p* (HOMO to LUMO) transition. In the case of EE structures,
all the first three excited states are p–p* [HOMO to LUMO (S1),
HOMO À 1 to LUMO (S2), and HOMO À 1 to LUMO + 1 (S3)] in
nature and the S2 and S3 states are nearly degenerate. The
2 (a) G. Mayer and A. Heckel, Angew. Chem., Int. Ed., 2006, 45, 4900;
(b) Q. Chen, D. Zhang, G. Zhang, X. Yang, Y. Feng, Q. Fan and D. Zhu,
Adv. Funct. Mater., 2010, 20, 3244; (c) T. Kobayashi, T. Komatsu,
´
´
M. Kamiya, C. Campos, M. Gonzalez-Gaitan, T. Terai, K. Hanaoka,
T. Nagano and Y. Urano, J. Am. Chem. Soc., 2012, 134, 11153.
3 (a) D. R. Morris and L. P. Hager, J. Biol. Chem., 1966, 241, 3582;
(b) A. Taurog, Endocrinology, 1976, 98, 1031; (c) S. A. Aziz and
C. O. Knowles, Nature, 1973, 242, 417–418.
vertical excitation energies are similar (316–293 nm) for DHKK
,
DMKK, and DAKK, again showing the absence of substituent
effects in the excited states of the KK forms (Table 1). The enols,
DHEE and DMEE also showed similar absorption energies
(319–306 nm) and interestingly these are also similar to those
of the KK forms. However, DAEE showed a red shift of 43 nm
4 (a) H. Fischer and W. Gleim, Justus Liebigs Ann. Chem., 1935, 521, 157;
(b) A. Wiehe, C. Ryppa and M. O. Senge, Org. Lett., 2002, 4, 3807.
5 N. Kumar, S. Chakravorti and P. Chowdhury, J. Mol. Struct., 2008,
891, 351.
6 K. C. G. Sreedevi, A. P. Thomas, P. S. Salini, S. Ramakrishnan,
K. S. Anju, M. G. D. Holaday, M. L. P. Reddy, C. H. Suresh and
A. Srinivasan, Tetrahedron Lett., 2011, 52, 5995.
(l = 345 nm) compared to that of the p–p* (S3) state of DAKK
.
The red shift in the vertical absorption wavelength seen in
DAEE is consistent with the experimental observation and can
be attributed to the extended charge delocalization seen in the
p-orbitals of the pyrrole and the aryl rings that are involved in
the transitions in DAEE. The emission energies of the KK forms
of all the three molecules are similar (B386 nm) indicating that
there is little effect of the substituents on their emission
7 R. Deans, J. Kim, M. R. Machacek and T. M. Swager, J. Am. Chem.
Soc., 2000, 122, 8565.
8 (a) C. A. Rice, I. Dauster and M. A. Suhm, J. Chem. Phys., 2007,
126, 134313; (b) B. M. Giuliano, I. Reva and R. Fausto, J. Phys. Chem.
A, 2010, 114, 2506.
9 Y. Chiang, A. J. Kresge, Q. Meng, R. A. More O’Ferrall and Y. Zhu,
J. Am. Chem. Soc., 2001, 123, 11562.
10 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648; (b) P. J. Stephens,
F. J. Devlin, C. F. Chablowski and M. J. Frisch, J. Phys. Chem., 1994,
98, 11623; (c) R. H. Hertwig and W. Koch, Chem. Phys. Lett., 1997, 268, 345.
properties. The emission wavelengths calculated for DHEE
,
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 8667--8669 | 8669