The first structural and spectroscopic characterisation of a ring-opened form of a 2H-naphtho[1,2-b]pyran: A novel photomerocyanine

Article abstract of DOI:10.1039/c4cc03435j

Heating 4-methoxy-1-naphthol with a 1,1-diarylprop-2-yn-1-ol gave the 2,2-diaryl-6-methoxy-2H-naphtho[1,2-b]pyran together with the novel merocyanine, (E)-2-[3′,3′-bis(aryl)allylidene]-4-methoxynaphthalen-1(2H)-one. Brief UV-irradiation of the pyran favoured the formation of the (Z)-merocyanine with longer irradiation and/or acidic conditions favouring the (E)-isomer. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc03435j

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The first structural and spectroscopic
characterisation of a ring-opened form
of a 2H-naphtho[1,2-b]pyran: a novel
photomerocyanine†
Cite this: Chem. Commun., 2014,
50, 7900
Received 7th May 2014,
Accepted 4th June 2014
Stuart Aiken,a Kathryn Booth,a Christopher D. Gabbutt,a B. Mark Heron,*a
DOI: 10.1039/c4cc03435j
Craig R. Rice,a Azzam Charaf-Eddinb and Denis Jacqueminbc
www.rsc.org/chemcomm
Heating 4-methoxy-1-naphthol with a 1,1-diarylprop-2-yn-1-ol gave the 2H-naptho[1,2-b]pyrans 2 (Fig. 1) have been studied and differences
2,2-diaryl-6-methoxy-2H-naphtho[1,2-b]pyran together with the novel in their performance characteristics contrasted.3 Multi-nuclear NMR
merocyanine, (E)-2-[30,30-bis(aryl)allylidene]-4-methoxynaphthalen- spectroscopy has been demonstrated to be a useful tool for probing
1(2H)-one. Brief UV-irradiation of the pyran favoured the formation of photochromism4 and the structure of the transient merocyanine
the (Z)-merocyanine with longer irradiation and/or acidic conditions dyes resulting from the photochemically-induced electrocyclic
favouring the (E)-isomer.
ring-opening of 3,3-bis(4-fluorophenyl)-3H-naphtho[2,1-b]pyran
have been identified as the (Z)- and (E)-1-[3,3-di(4-fluorophenyl)-
Naphthopyrans (benzochromenes) and fused carbocyclic and allylidene]naphthalen-2(1H)-ones 3 and 4, respectively;5 the
heterocyclic derivatives have attracted significant academic interest colourless allenyl naphthol 5 (Ar = 4-FC6H4–) was also identified
as a consequence of their photochromism which has been widely in a subsequent study.6
exploited in commercial photochromic ophthalmic sun-lenses.1,2
Given the commercial interest in 2H-naphtho[1,2-b]pyran derived
The photochromic properties of 3H-naphtho[2,1-b]pyrans 1 and systems2 it is somewhat remarkable that there have been, to the best
of our knowledge, no examples of the characterisation of the
proposed merocyanine dyes (6 and 7) resulting from the
photochemical ring-opening of the 2H-naphtho[1,2-b]pyran
system 2. Furthermore, there does not appear to have been
any reports of the differing absorption properties of the iso-
meric merocyanine dyes. In this communication we report our
findings concerning the isolation, characterisation, structure and
interconversion of photomerocyanines derived from 6-methoxy-
2,2-bis(4-methoxyphenyl)-2H-naphtho[1,2-b]pyran.
Heating a mixture of 4-methoxy-1-naphthol, propynol 8a, PPTS
and trimethyl orthoformate in 1,2-dichloroethane under reflux7 gave
a multicomponent reaction mixture, which upon elution from silica
with 20% EtOAc in hexane, gave 9 (37%), a 2,2-diaryl analogue of the
Fig. 1 Isomeric naphthopyrans and their photomerocyanine dyes.
Brazilian hardwood (Paratecoma alba) extractive lapachenole,8
and the more polar, intensely coloured merocyanine 10 (12%)
(Scheme 1). We have previously noted the formation of permanent
merocyanine dyes, 4-(3,3-diarylylallylidene)naphthalen-1(4H)-ones,
a Department of Chemical Sciences, School of Applied Science,
University of Huddersfield, Queensgate, Huddersfield, HD1 3DH, UK.
E-mail: m.heron@hud.ac.uk
b
´
`
CEISAM, UMR CNRS 6230, Chimie Et Interdisciplinarite, Synthese, Analyse,
´
´
´
Modelisation, Universite de Nantes, Faculte des Sciences et des Techniques,
`
BP 92208, 2, rue de la Houssiniere, F-44322 Nantes Cedex 3, France
c Institut Universitaire de France (IUF), 103 blvd St Michel, F-75005 Paris Cedex 5,
France
† Electronic supplementary information (ESI) available: Complete experimental
procedures, characterization data and copies of NMR and mass spectra for all
new compounds, computed Cartesian coordinates, frontier molecular orbitals
and vibronic spectra. CCDC 1000252. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cc03435j
Scheme 1 Naphthopyran 9 and merocyanine dyes 10 and 11.
7900 | Chem. Commun., 2014, 50, 7900--7903
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as by-products from the reaction between 1-naphthol and 9 in d6-acetone displayed a singlet at d 3.69 for the methoxy
1,1-diarylprop-2-yn-1-ols9 and Coelho et al., have reported the groups of the equivalent 4-methoxyphenyl units at C-2 and a
formation of a related dye, 4-(3,3-diphenylallylidene)-8-hydroxy- singlet at d 3.90 for the 6-methoxy group. The signal for 3-H
naphthalen-1(4H)-one, derived from 1,8-dihydroxynaphthalene appeared at d 6.31 as a doublet (J = 9.6 Hz) coupled to 4-H (d 6.77)
1
and 1,1-diphenylprop-2-yn-1-ol.10
and that for 5-H appeared as a singlet at d 6.69. The H NMR
The 1H NMR spectra of 9 (ESI,† Fig. S8) and 10 when spectrum of 10 in d6-acetone was quite different and displayed a
recorded in CDCl3 solution after 1 h were complex. From these singlet at d 3.84, at d 3.90 and at d 4.02 assigned to the non-
spectra it was apparent that 9 underwent ring-opening to afford equivalent MeO groups. The protons of the allylidene side chain
a mixture of 9 and 10, together with a minor amount of an were unequivocally assigned by HSQC, HMBC, NOE and H–1H
1
alternate merocyanine proposed as 11 (vide infra). Whilst the COSY experiments and appear at d 7.38 (20-H, partly obscured by
1H NMR spectrum of 10 in CDCl3 solution indicated that ring- other aromatic signals) and a doublet at d 7.55 (10-H); the
0
0
closure had occurred to afford the same equilibrium mixture of magnitude of the coupling constant, J1 ,2 = 12.7 Hz, consistent
9, 10 and 11. Our initial thoughts to account for this behaviour with an s-trans conformation of the allylidene unit. The 13C NMR
were that 9 and 10 were undergoing photochromic conversion spectrum displayed the expected 2-C signal at d 82.2 for the
mediated by ambient laboratory daylight resulting in an equili- pyran 9 in d6-acetone and a low field signal at d 183 confirmed
brium mixture of the three species. However, when freshly the presence of the a,b-unsaturated carbonyl function in the
preparing the 1H NMR sample of 9 in only low levels of 13C NMR spectrum of 10.
(ambient) laboratory lighting the same instantaneous develop-
To contrast with the observed isomerisation behaviour of 9
ment of the maroon colour attributed to the ring-opened and 10 in CDCl3 solution, treatment of a d6-acetone solution of
species 10 (and 11) was noted leading us to consider that it pyran 9 with a catalytic amount of TFA resulted in a mixture of
was perhaps residual acidity in the CDCl311 which was the main 9 : 10 : 11 in a ratio of B2 : 1 : 0.09, after standing over 2 h at rt
contributor to the colour development. To explore this postu- (this composition remained essentially unchanged after 5 days)
late a sample of commercial CDCl3 was washed with aqueous (ESI,† Fig. S20 and S21). Similarly treatment of a d6-acetone
K2CO3 followed by water and dried (anhyd. K2CO3)12 and then solution of dye 10 with a catalytic amount of TFA resulted in the
used to prepare a solution of 9 under low levels of (ambient) formation of essentially the same equilibrium mixture which
laboratory lighting. The resulting solution was paler in colour confirms the acid-mediated isomerisation but with the impli-
and the 1H NMR spectrum of 9 recorded after 1 h was quite cation that the dye 10 is the thermodynamically favoured ring-
different from the sample recorded in commercial CDCl3 and opened form under the applied conditions.
displayed signals attributed to a B10 : 1 mixture of compounds
The structure of 10 (Fig. 3) was unequivocally determined by
9 : 11 (see Fig. 2 and ESI,† Fig. S10), with no evidence of 10; X-ray diffraction from a single crystal grown by slow evapora-
a feature which implies that acidity was important for the tion from EtOAc/hexane solution. Bond length alternation of
isomerisation of 11 to 10.
the (E)-pentadienone unit was apparent with C3–C4 (1.486 (3) Å)
The 1H NMR spectra of freshly prepared d6-acetone solutions and C12–C13 (1.436 (3) Å) proffering single bond like character
(without any protection from (ambient) laboratory daylight) of 9 with C3–C12 (1.362 (3) Å) and C13–C14 (1.355 (3) Å) possessing
and 10 were next examined (ESI,† Fig. S4 and S13). Whilst the more double bond character; such data is comparable with
d6-acetone solution of 10 was deep maroon, as expected, the that noted for 4-(3,3-diarylylallylidene)naphthalen-1(4H)-ones.9
solution of the pyran 9 was much less intensely coloured relative In addition to the expected propeller-like arrangement of the
to that in commercial CDCl3 solution. The 1H NMR spectrum of geminal 4-methoxyphenyl rings, which arise as a consequence
of the minimisation of ortho-H-atom interactions, there is a
slight deviation of the allylidene unit from the plane of the
naphthalenone (torsion angle 161.091 for the allylidene unit
C3–C12–C13–C14); it is possible that this deviation is a con-
sequence of the crystal packing in which the naphthalenone
rings are arranged in an anti-planar array with a naphthalenone
Fig. 2 1H NMR spectrum of 9 in commercial CDCl3 (lower) and base
washed CDCl3 (upper) with expansion d 5.7 - 8.7.
Fig. 3 Crystallographic structure of 10 (thermal ellipsoids shown at 50%
probability) and crystal packing.
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Fig. 4 Absorption spectra (acetone) of naphthopyran 9 and dye 10.
Fig. 6 1H NMR spectrum of naphthopyran 9 in d6-acetone after 30 s UV
irradiation.
ring centroid spacing of 3.619 Å, as the consequence of a
potential dipole–dipole interaction.13
The photochromic response of 9 and the absorption spectrum
of 10 were next examined in acetone solution. Dye 10 exhibits lmax
at 496 nm and has a molar extinction coefficient (em) of 2.64 ꢀ
104 molꢁ1 dm3 cmꢁ1 the latter is comparable to em of merocyanine
dyes derived from 2-tetralone14 but lower than those derived from
1-naphthol.9 The absorption spectrum of 9 before UV irradiation
confirms the weakly coloured solution with an absorbance of
o0.05 a.u. at 507 nm, whereas upon UV irradiation for 30 s a
deep maroon solution was observed with an absorption maximum
at 507 nm (Fig. 4). Progressively longer irradiation times resulted
in a small hypsochromic shift of lmax but with little if any increase
in intensity after ca. 220 s of UV irradiation. The decay of the
intensity of the absorption at 507 nm was measured as a function
of time and the half-life (t1/2) was calculated to be 42.3 min at 20 1C
(Table S1, ESI†).
Of some significance was the fact that the absorption maximum
of the coloured ring-opened form of pyran 9, resulting from 30 s of
UV irradiation, was bathochromically shifted by 11 nm relative to
that obtained for dye 10. It was thought that the different absorp-
tion maxima may be due to the photochemical ring-opening of the
pyran 9 resulting in the alternate geometrical (Z)-isomer 11 of the
isolated dye 10 [(E)-isomer] (Fig. 5).
slightly smaller bond length alternation, hinting at a marginally
more delocalized structure. For both dyes, TD-DFT predicts a
first dipole-allowed transition that can be mainly ascribed to a
HOMO–LUMO contribution (see ESI† for representation). For
10, TD-DFT predicts a first excited-state at 541 nm, with an
oscillator strength, f, of 0.86.15 For 11, theory predicts a small
bathochromic (+11 nm or ꢁ0.05 eV, lmax = 552 nm) hypo-
chromic (ꢁ0.05, f = 0.81) shift of the spectra, which seems to fit
the experimental trends. To ascertain this result, we have also
performed vibrationally resolved calculations of the optical
spectra for each dye to reach theoretical estimates of em and
they also provided a weaker absorption for 11 than for 10.
To establish the presence of proposed isomer 11, a room
temperature solution of 9 in d6-acetone was subjected to UV
irradiation for ca. 30 s. Immediately upon cessation of irradia-
1
tion the H NMR spectrum of the intensely coloured solution
was acquired (Fig. 6). The 1H NMR spectrum revealed new
signals at d 3.84, 3.85 and 3.89 each accounting for a methoxy
group and a singlet at d 6.11 which is attributed to 3-H. The
aromatic region of the spectrum was more complex and a
signal of particular note was a low field doublet resonating at
d 8.65 with a coupling constant of 12.2 Hz which has been
assigned to 20-H; the partner signal for 10-H resonates at d 6.88
and is obscured by other aromatic signals. Longer irradiation
(40 min) of a solution of 9 in d6-acetone resulted in the
emergence of a further set of signals which were characteristic
of the dye 10 (ESI,† Fig. S6 and S7). The 1H NMR spectrum of a
sample of dye 10 after ca. 40 min of UV-irradiation was also
examined. Interestingly, in this instance photo-isomerization of
10 resulted in a mixture of 9, 10 and 11 indicating that UV
irradiation can affect the reverse geometrical isomerisation.
We have also used theoretical calculations to analyse the
spectra of 10 and 11 (see ESI†). By looking, at the three protons
of the conjugated path shown in Fig. 5, it is apparent that the
measured 10-to-11 isomeric effects on the chemical shifts
To obtain further insight on the differences between 10 and
11, we have used theoretical calculations (ESI† for details). For
10, the computed geometry agrees well with XRD with devia-
tions of ca. 0.01–0.02 Å. Indeed, the experimental (theoretical)
C3–C4, C3–C12, C12–C13 and C13–C14 bond lengths are 1.485
(1.480), 1.362 (1.377), 1.436 (1.421) and 1.355 (1.374) Å, respec-
tively. In 11, these bond distances become 1.477, 1.383, 1.420 and
1.377 Å, respectively, which are similar but nevertheless show a
0
are Dd(3-H) ꢁ0.71 (d6.11–6.82), Dd(1 -H) ꢁ0.67 (d6.88–7.55) and
0
Dd(2 -H) +1.27 (d8.65–7.38) and theory reproduces these trends:
0
0
Dd(3-H) ꢁ0.62 (d6.31–6.93), Dd(1 -H) ꢁ0.99 (d7.32–8.31) and Dd(2 -H)
Fig. 5 Relationship between pyran 9 and dyes 10 and 11.
7902 | Chem. Commun., 2014, 50, 7900--7903
+1.78 (d9.45–7.67), respectively, therefore confirming the experimental
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´
Grant (Marches-278845) and a recrutement sur poste strategique,
respectively. This research used resources of the GENCI-CINES/
IDRIS, of the CCIPL (Centre de Calcul Intensif des Pays de Loire)
and of a local Troy cluster.
Notes and references
1 (a) U. Weigand and H. Zinner, WO Pat., 2014/009020, 2014;
(b) J. Takenada, J. Momoda, K. Teranishi, T. Takahashi, M. Sando
and S. Izumi, US Pat., 2014/0054520, 2014; (c) W. Xiao and B. Van
Gemert, WO Pat., 2013/090220, 2013; (d) A. Kumar, R. L. Yoest, C. Li,
D. S. Jackson and H. Nguyen, US Pat., 2012/0120473, 2012; (e) S. Aiken, J.-P.
Cano, C. D. Gabbutt and B. M. Heron, WO Pat., 2008/028930, 2008.
2 (a) Y. Shimizu, S. Izumi and J. Momoda, WO Pat., 2013/042800, 2013;
(b) L. Sukhomlinova, T. Kosa, B. Taheri, T. White and T. Bunning,
US Pat., 2013/0248350, 2013; (c) D. A. Clarke, B. M. Heron, C. D.
Gabbutt, J. D. Hepworth, S. M. Partington and S. N. Corns, US Pat.,
2002/6387512, 2002.
3 (a) J. D. Hepworth and B. M. Heron, in Functional Dyes, ed. S.-H.
Kim, Elsevier, Amsterdam, 2006, p. 85; (b) B. Van Gemert, in Organic
Photochromic and Thermochromic Compounds, Volume 1, Main Photo-
chromic Families, ed. J. C. Crano and R. J. Guglielmetti, Plenum
Press, New York, 1998, p. 111.
4 S. Delbaere and G. Vermeersch, J. Photochem. Photobiol., C, 2008, 9, 61.
5 S. Delbaere, B. Luccioni-Houze, C. Bochu, Y. Teral, M. Campredon
and G. Vermeersch, J. Chem. Soc., Perkin Trans. 2, 1998, 1153.
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7 W. Zhao and E. M. Carreira, Org. Lett., 2003, 5, 4153.
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Scheme 2 Naphthopyran 12 and propenone 13.
assignment; the differences being within the expected error
bars for DFT NMR simulations.
In order to assess the influence of an aryl ring on the
properties of the pyran–merocyanine dye interconversion the
reaction between 4-methoxy-1-naphthol and the propynol 8b
was next undertaken. The crude reaction product resulting
from this combination afforded two components after purifica-
tion, the naphthopyran 12 (79%) and the propenone 13 (9%);
no merocyanine dye analogous to 10 was observed (Scheme 2).
The propenone 13, formed by a Meyer–Schuster rearrangement
of 8b,16 was characterised by the presence of a signal at d 192
for the CQO function in the 13C NMR spectrum and a singlet
at d 7.0 for the alkene proton in the 1H NMR spectrum. The
naphthopyran 12 displayed no discernible photochromic res-
ponse at ambient temperature however; a red-purple colour was
discernible when a toluene solution chilled with solid CO2 was
irradiated, but the colour faded immediately upon termination
of irradiation. It is apparent that the presence of the 4-tolyl
substituent favours the pyran 12, presumably as a consequence
of increased steric interactions destabilising the merocyanine
species.
In summary, this report constitutes the first example of the
isolation and full characterisation of a merocyanine dye derived
from a 2H-naphtho[1,2-b]pyran. Furthermore, examination of
the absorption spectra of the merocyanine together with a
sample of irradiated naphthopyran revealed that each isomeric
merocyanine has a different absorption maximum as a con-
sequence of the different geometry; a feature which is supported by
TD-DFT calculations. The presence of low concentrations of acid
favours the pyran ring-opening to afford the (E)-photomerocyanine.
10 L. M. Carvalho, A. M. S. Silva, C. I. Martins, P. J. Coelho and
A. M. F. Oliveira-Campos, Tetrahedron Lett., 2003, 44, 1903.
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13 Single crystal X-ray diffraction data was collected on a Bruker
Venture diffractometer equipped with a graphite monochromated
Cu(Ka) radiation source and a cold stream of N2 gas. Selected crystal
data for 10: C28H24O4, M = 424.47, monoclinic, a = 17.9087 (7) Å, b =
13.6467 (6) Å, c = 17.9157 (7) Å, b = 94.4847 (19)1, V = 4365.1 (3) Å3,
T = 150 K, space group C2/c, Z = 8, m = 0.687 mmꢁ1, 17 375 measured
reflections, 4119 independent reflections (Rint = 0.0555). The final R1
values were 0.0532 (I 4 2s(I)). The final wR(F2) values were 0.1554
(I 4 2s(I)). The final R1 values were 0.0657 (all data). The final wR(F2)
values were 0.1668 (all data). The goodness of fit on F2 was 1.062.
Largest peak and hole 0.267 and ꢁ0.291 e Åꢁ3. See CCDC 1000252.
The foregoing features have implications for the study of the fading 14 C. D. Gabbutt, J. D. Hepworth, B. M. Heron, S. M. Partington and
D. A. Thomas, Dyes Pigm., 2001, 49, 65.
kinetics and applications of photochromic compounds in com-
mercial ophthalmic systems.
15 This reasonably fits the experimental value of 496 nm and corre-
sponds to an error of 0.21 eV, typical of TD-DFT. See a recent review
We thank the EPSRC for access to the National Mass
Spectrometry Service, Swansea. A. C.-E. and D. J. acknowledge
on TD-DFT benchmarks: A. D. Laurent and D. Jacquemin, Int.
J. Quantum Chem., 2013, 113, 2019.
16 (a) C. D. Gabbutt, B. M. Heron, C. Kilner and S. B. Kolla, Org. Biomol.
Chem., 2010, 8, 4874; (b) D. A. Engel and G. B. Dudley, Org. Biomol.
Chem., 2009, 7, 4149.
´
the European Research Council (ERC) and the Region des Pays
de la Loire for financial support in the framework of a Starting
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