Remarkable Influence of Phenyl/Arylethynylation on the Photochromism of 2,2-Diphenylbenzopyrans (Chromenes)

Article abstract of DOI:10.1002/ejoc.201501076

Phenyl/arylethynylation of 2,2-diphenylbenzopyrans (i.e., chromenes) permits modulation of the spectral properties of photogenerated colored o-quinonoid intermediates, and brings about remarkable stabilization of the latter, rendering the phenomenon of ph

Full text of DOI:10.1002/ejoc.201501076

DOI: 10.1002/ejoc.201501076
Full Paper
Photochromism
Remarkable Influence of Phenyl/Arylethynylation on the Photo-
chromism of 2,2-Diphenylbenzopyrans (Chromenes)
Arindam Mukhopadhyay,[a] Vijay Kumar Maka,[a] and Jarugu Narasimha Moorthy*[a]
Abstract: Phenyl/arylethynylation of 2,2-diphenylbenzopyrans the 6-position of the chromene nucleus leads to more red-
(i.e., chromenes) permits modulation of the spectral properties shifted absorptions as well as to increased lifetimes of the col-
of photogenerated colored o-quinonoid intermediates, and ored species when compared with functionalization at the 7-
brings about remarkable stabilization of the latter, rendering position. Given that photochromism is observable in the parent
the phenomenon of photochromism observable readily at chromene at only low temperatures, phenyl/arylethynylation,
room temperature. Modification of the spectrokinetic proper- which precludes steric inhibition of resonance, is demonstrated
ties of the o-quinonoid intermediates are a consequence of res- to render bleaching kinetics of the colored intermediates that
onance effects, with the substituent effects being relayed nicely approach those reported for industrially relevant diarylnaphtho-
from one end of the molecule to the other. Functionalization at pyrans.
Introduction
Our own interest has been focused on the photochromism
exhibited by chromenes and on modulation of the spectro-
kinetic properties of the photogenerated colored intermediates
derived from chromenes.[7] As mentioned earlier, aromatic an-
nulation,[7a] electronic effects transmitted by arylation[7b–7d] and
fluorenylation,[8] toroidal conjugation extant to the systems
based on hexaphenylbenzenes,[7e] etc. have been shown to in-
fluence the photochromic phenomenon of chromenes. We re-
cently showed that helicity, as a steric force, allows unprece-
dented stabilization of the otherwise fleeting photogenerated
colored o-quinonoid intermediates.[7h] The influence of
through-space electronic effects has also been demonstrated
in chromenes with cofacially oriented aryl rings[7f] and also in
paracyclophanes grafted with chromenes.[7g,7i] Compelling evi-
dence has emerged from our studies that strong mesomeric
effects influence the spectral as well as kinetic behavior of the
photogenerated colored o-quinonoid intermediates to permit
their observation at room temperature. In a continuation of
these investigations, we were motivated to examine how aryl-
ethynylation of chromenes modifies spectral attributes as well
as stabilization of the photogenerated colored o-quinonoid in-
termediates. Unlike in arylchromenes, the ethynyl spacer should
be expected to completely offset steric inhibition of resonance
in addition to bringing about a bathochromic shift of the ab-
sorptions of the photogenerated colored intermediates. We,
therefore, designed and synthesized 5-/6-/7-phenylethynyl-
chromenes (5-/6-/7-PhE) and their diphenylamino-functional-
ized analogues, 5-/6-/7-(N,N-diphenylamino)phenylethynyl-
chromenes (5-/6-/7-DpaPhE; Figure 1), to explore the phenome-
non of photochromism.
The design and synthesis of molecular systems that respond to
light and heat as external stimuli constitutes an important area
in the development of organic photochromic materials, which
are of paramount importance in variable-transmission glasses,
molecular switches, nanoscale machines, high-density optical
storage, imaging devices, smart windows and biological phe-
nomena such as photomorphogenesis and vision process.[1,2]
Among the known photochromic systems,[1,2] photochromic di-
arylnaphthopyrans, which belong to the class of chromenes
(i.e., 2,2-diaryl-2H-1-benzopyrans), are of particular importance
because of their practical utility in ophthalmic lenses.[3] In gen-
eral, photoirradiation of colorless pyrans (closed forms) results
in heterolytic cleavage of the C(sp3)–O bond in their singlet
excited states, leading to formation of colored o-quinonoid in-
termediates (open forms).[4] The latter may revert to the initial
colorless closed forms either thermally or photochemically with
visible light. The parent chromene (i.e., 2,2-diphenyl-2H-1-
benzopyran) without any benzo-annulation or other modifica-
tions is known to exhibit photochromism only at very low tem-
peratures (173–263 K).[4,5] Indeed, reversion of the open forms
to the closed forms in simple benzopyrans is too fast to allow
detection of the photogenerated quinonoid intermediates at
room temperature. Modulation of the spectrokinetic properties
of o-quinonoid intermediates and the associated photochromic
phenomenon has been a subject of extensive investigations.[6]
[a] Department of Chemistry, Indian Institute of Technology,
Kanpur 208016, India
E-mail: moorthy@iitk.ac.in
Herein, we report that simple phenyl/arylethynylation of the
parent chromene brings about remarkable stabilization of
photogenerated colored intermediates to the extent that their
http://home.iitk.ac.in/~moorthy/jnm.html
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201501076.
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Figure 1. Molecular structures of the aryl- and arylethynylchromenes.
thermal bleaching kinetics approach those observed for com- ventional Claisen rearrangement route using p-toluenesulfonic
mercially relevant naphthopyrans.
acid (PTSA) as a catalyst. The resultant 5-/6-/7-iodochromenes
were subjected to Sonogashira/“sila”-Sonogashira cross-coupling
reactions[9] under Pd0-catalyzed conditions to afford the target
chromenes in excellent isolated yields (Scheme 1).
Results and Discussion
Synthesis of Arylethynyl-Substituted Chromenes
Spectrokinetic Properties of the Photochromic
Arylethynyl-Substituted Chromenes
The synthesis of target chromenes 5-/6-/7-PhE and 5-/6-/7-Dpa-
PhE, is shown in Scheme 1. To begin with, 3-/4-iodophenol was The absorption spectra of all phenyl/arylethynyl-substituted
annulated with 1,1-diphenylpropargyl alcohol following the con- chromenes 5-/6-/7-PhE and 5-/6-/7-DpaPhE in toluene
Scheme 1. Synthesis of arylethynylchromenes.
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Table 1. Absorption properties and spectrokinetic data of chromenes and their respective photogenerated colored intermediates.
Chromene
Absorption properties[a,b]
Decay rates [s–1 [c]
]
Before hν [nm] (ε)
After hν [nm]
k1
k2
[f]
2H-Chromene[d]
5-Ph[g]
310
410, 510[e]
416 (br)
414 (br), 522 (br)
422, 530 (sh)
484
424, 564 (br)
474
422 (br)
421 (br), 545 (br)
440
488 (br)
424, 577 (br)
497
[h]
317 (br, 4026)
322 (br, 2669)
325 (12740)
313 (28435)
322 (39944)
355 (33237)
300 (br, 25965)
302 (br, 28525)
310 (36550)
360 (30640)
350 (42390)
364 (46285)
6-Ph[g]
0.06
0.002
0.002
7-Ph[g]
0.14
[h]
5-DpaPh[g]
6-DpaPh[g]
7-DpaPh[g]
5-PhE
0.07
0.011
0.008
0.11
[h]
6-PhE
7-PhE
5-DpaPhE
6-DpaPhE
7-DpaPhE
0.014 (76)
0.005 (24)
0.006 (1)
0.023 (99)
[h]
0.013 (75)
0.021 (99)
0.005 (25)
0.006 (1)
[a] Based on absorption spectra of phenyl/arylethynyl-substituted chromenes in toluene solutions (4 × 10–5
M). [b] br: broad and sh: shoulder. [c] Unless
otherwise mentioned, the decays for the photogenerated colored intermediates were monitored at 288 K after steady-state irradiation for 3–5 min. The
amplitudes (%) of the fast (k1) and slow (k2) decaying components are shown in the parenthesis. [d] Based on absorption spectra of 2,2-diphenyl-2H-1-
benzopyran in ethanol at 295 K, see ref.[5] [e] Obtained from microsecond laser flash experiments using a Q-switched Nd-YAG laser with excitation at 266 nm,
see ref.[5] The absorption band at 410 nm is the strong band. [f] The lifetime of the colored species is in the microsecond time scale, see ref.[5] [g] From ref.[7b]
at 288 K. [h] The decay was too fast to allow monitoring.
(4 × 10–5
M) are shown in Figure 2. The long-wavelength ab- blue. The UV/Vis absorption spectra of all chromenes before
sorption maximum (λmax) and the molar extinction coefficient and after photoirradiation are shown in Figure 3 and in the
(ε) at λmax for each case are collected in Table 1. Analysis of Supporting Information. Whereas photoirradiation of 6-PhE led
the spectra reveals that the absorptions corresponding to π,π* to purple-violet coloration with absorption maximum at ca.
transitions taper off well below 390 and 425 nm for PhE and 545 nm, that of 6-DpaPhE led to dark-blue coloration with λmax
DpaPhE series, respectively. As evident from Figure 2, the spec- at ca. 577 nm (Table 1). In stark contrast, irradiation of 5- and
tral attributes are different for the differently-substituted ana- 7-substituted chromenes 5-PhE/5-DpaPhE and 7-PhE/7-DpaPhE,
logues of chromenes. The absorptions of 7-arylethynyl-substi- led to yellow (λmax = 422/488 nm) and orange-red coloration
tuted chromenes are considerably redshifted when compared (λmax = 440/497 nm), respectively. It is noteworthy that the col-
with those of the 5-/6-substituted analogues.
ored photogenerated intermediates derived from 6-phenyl/aryl-
ethynyl-substituted chromenes displayed highly redshifted ab-
sorptions (ca. 80–105 nm) as compared with those of the spe-
cies derived from the corresponding 5- and 7-substituted regio-
isomers (Table 1). Furthermore, among the differently substi-
tuted chromenes, 7-phenyl/arylethynyl-substituted derivatives
Figure 2. UV/Vis absorption spectra of 5-/6-/7-PhE (a) and 5-/6-/7-DpaPhE (b)
in toluene (4 × 10–5
M).
As shown in Table 1, the phenyl/arylethynyl-substituted
chromenes 5-/6-/7-PhE and 5-/6-/7-DpaPhE are found to exhibit
remarkably higher molar extinction coefficients when com-
pared with those of the corresponding aryl-substituted chrom-
ene analogues 5-/6-/7-Ph and 5-/6-/7-DpaPh. Furthermore, the
diphenylaminophenylethynyl-substituted chromenes 5-/6-/7-
DpaPhE exhibit significantly redshifted absorption maxima (ca.
50 nm) and considerably higher molar extinction coefficients
when compared with those of phenylethynyl-substituted ana-
logues 5-/6-/7-PhE.
Their photochemical behavior was examined in toluene (ca.
2 × 10–3
M M) at 288 K (see the Supporting
as well as 5 × 10–4
Figure 3. UV/Vis absorption spectra (ca. 2 × 10–3
PhE (b), 6-DpaPhE (c) and 7-DpaPhE (d) before (black) and after photoirradia-
tion (red). Note the changes in the colors of the solutions after steady-state
photoirradiation.
M in toluene) of 6-PhE (a), 7-
Information). Upon exposure to UV irradiation (λex ca. 350 nm),
the colorless solutions of the chromenes were found to un-
dergo ready coloration, which varied from orange-yellow to
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were found to exhibit better colorability (see the Supporting pared with those of 6-PhE (k1 = 0.014 s–1) and 6-DpaPhE (k1 =
Information).
0.013 s–1). Remarkably slower rates of decay (ca. 4–6 times) are
observed for 6-/7-PhE and 6-/7-DpaPhE when compared with
those of the arylchromene analogues 6-/7-Ph and 6-/7-DpaPh.
This attests to the considerable influence of the ethynyl spacer
on the spectrokinetic properties of the photogenerated colored
intermediates. Furthermore, coloration-reversion cycles for two
representative chromenes, namely, 6- and 7-DpaPhE, which
were carried out to examine their stabilities, did not reveal any
indication of degradation,[10] which attests to their respectable
stabilities upon photoirradiation.
The photogenerated colored intermediates of phenyl/aryl-
ethynyl-substituted chromenes were found to revert to the col-
orless forms within 2–3 min on standing in the dark at room
temperature. The thermal decay kinetics of the colored species,
with the exception of those of 5-phenyl/arylethynyl-substituted
chromenes, were followed by monitoring changes in the ab-
sorbance with time at their respective long-wavelength absorp-
tion maxima; the thermal reversion of the colored intermediates
of 5-phenyl/arylethynyl-substituted chromenes was found to be
too fast to allow their decay kinetics to be monitored at 288 K.
Thus, solutions of the chromenes in toluene were exposed to
UV radiation until the photostationary state (PSS) was reached.
Subsequently, the change in the absorbance at the absorption
maximum of the colored intermediate with time was followed
at 288 K for each case (Figure 4). The duration of irradiation for
attainment of the PSS was determined from a coloration plot
in each case (see the Supporting Information).
Influence of the Ethynyl Spacer on the Photochromic
Behavior
Mechanistic details of the photochromism of chromenes have
been extensively investigated.[6] Suffice it to say that the TC and
TT isomers shown in Scheme 2 are mainly responsible for the
observed coloration upon UV irradiation of chromenes, which
results in heterolysis of the C(sp3)–O bond in the singlet excited
state. The TT isomer is generated by a two-photon absorption
of the chromene through initial formation of the TC isomer. Due
to the requirement of two-photon absorption, the TT isomer is
likely to be formed only minimally when the chromene is sub-
jected to UV irradiation for short durations. Thus, at shorter
durations of irradiation, it is the TC isomer that is largely respon-
sible for the observed color.[11] The parent chromene, i.e., 2,2-
diphenyl-2H-1-benzopyran, is known to exhibit photochromism
only at low temperatures (173–263 K).[4,5] Reversion of the o-
quinonoid intermediates (open forms) to the precursor closed
form occurs too rapidly to allow detection of the former at
room temperature.
Figure 4. Comparative normalized thermal decay profiles of the photogener-
ated colored intermediates of 6-/7-PhE (a) and 6-/7-DpaPhE (b) in toluene at
288 K. The wavelengths at which the decays were monitored are given in
parentheses.
Our primary objective in investigating the photochromic be-
havior of arylethynyl-substituted chromenes was to establish
The decay in each case was best fitted to a biexponential the extent to which the ethynyl spacer mediates π-conjugation
function (Table 1). Comparison of the data shows that faster between phenyl/aryl and benzopyran rings, and modifies ab-
thermal decays are observed for the colored intermediates of sorption as well as the lifetimes of the o-quinonoid intermedi-
7-PhE (k1 = 0.023 s–1) and 7-DpaPhE (k1 = 0.021 s–1) when com- ates generated by photoexcitation. Of the three substituted
Scheme 2. Mechanistic paradigm for the photochromism of 2H-chromene.
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chromenes, weak photochromism was generally observed with chromic shift of ca. 13–23 nm is observed for the absorption of
5-substituted derivatives in the sense that the photogenerated colored intermediates of 6-/7-DpaPhEs when compared to
colored species revert too rapidly; electronic effects are trans- those of 6-/7-DpaPhs. Clearly, the mesomeric effects operate
mitted poorly at this position, whereas the steric bulk preclude differently and significantly for the chromenes functionalized
mesomeric effects.[7b,7d] We shall, therefore, limit ourselves to the with phenyl/arylethynyl groups.
photochromic phenomenon observed with 6- and 7-substi-
The thermal reversion kinetics followed in each case by mon-
tuted chromenes. The fact that strong mesomeric effects are itoring change in the absorbance of the colored intermediate
operative between the phenyl/arylethynyl moiety and the at long wavelength absorption maximum show remarkable in-
benzene ring of the benzopyran is evident from the shifts in crease in the lifetimes (1/k) of the colored o-quinonoid interme-
the absorptions of the closed as well as open forms generated diates, as revealed by the decay rate constants k1 and k2. In
upon photolysis. As far as phenylethynylchromenes are con- general, the fast-decaying component, i.e., k1, is predominant
cerned, the mesomeric effect is manifested in hypsochromic and is largely responsible for the observed color. As mentioned
shifts of the shorter wavelength absorption maxima and very earlier, this component is attributed to the TC isomer
high molar absorptivities relative to those of 6- and 7-phenyl- (Scheme 2), whereas k2 is attributed to the minor TT isomer,
chromenes. In contrast, bathochromic shifts are observed in the which is generated by two-photon absorption; the latter may
absorption maxima of their corresponding photogenerated o- be assumed to be unimportant for spectrokinetic analyses.
quinonoid intermediates. Highly pronounced effects are ob- Thus, the o-quinonoid intermediates of 6-PhE and 6-DpaPhE are
served for 6- and 7-diphenylaminophenylethynyl-substituted found to decay ca. 4-fold slower relative to those of 6-Ph and
chromenes. Bathochromic shifts of 9–28 nm are observed for 6-DpaPh. Likewise, the colored species of 7-PhE and 7-DpaPhE
the absorptions of the closed forms and 13–23 nm for absorp- are found to decay ca. 5-fold slower than those of 7-Ph and 7-
tion of the ring-opened forms, relative to the corresponding DpaPh; the decay rates of the o-quinonoid intermediates of 6-
diphenylaminophenyl-substituted chromenes (Table 1). Here PhE and 6-DpaPhE are similar to those of 7-PhE and 7-DpaPhE.
also, the molar absorptivities are almost double for the closed For the two different substitutions, i.e., at the 6- and 7-positions,
forms, attesting to the transmission of the effect due to the the colored species in the case of 6-phenyl/arylethynylchrome-
electron-releasing diphenylamino substituent. The positional nes are found to decay ca. 1.5-fold slower than those of 7-
dependence of the absorption maximum, particularly for the phenyl/arylethynylchromenes. It turns out that the observed
photogenerated o-quinonoid intermediates, is noteworthy; a spectrokinetic properties of 6-/7-phenyl/arylethynyl chromenes
more redshifted absorption is observed for functionalization at are remarkably better than those of a large number of photo-
the 6-position compared with that for functionalization at the chromic chromenes reported to date. In fact, the stabilities of
7-position for a given substituent. This shift is as much as the photogenerated colored intermediates due to phenyl/aryl-
105 nm for 6-/7-PhEs and 80 nm for 6-/7-DpaPhEs. Furthermore, ethynylation are much higher than those achieved with hetero-
the introduction of C≡ C between the phenyl and diphenyl- cyclic substitution,[6a] hetero-fusion,[8] aromatic annulation,[7a]
benzopyran as in 6-/7-PhEs brings about a bathochromic shift toroidal conjugation,[7e] or complexation with metal ions,[2d]
in the absorptions of the colored intermediates by 18–23 nm, etc. The decay rate constants at room temperature for the pho-
when compared with those of 6-/7-Phs. Likewise, a batho- togenerated colored intermediates of some of the commercially
Figure 5. Structures of some commercially relevant and well-known naphthopyrans. Thermal decay rate constants at room temperature for their photogener-
ated colored intermediates are given in parentheses.
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Scheme 3. Mesomeric effects in 6-/7-DpaPhEs and their respective o-quinonoid intermediates.
relevant and well-known naphthopyrans, namely, 3,3-diphenyl- phenyl/arylethynylchromenes as compared with those of 7-
3H-naphtho[2,1-b]pyran (NP1, also known as Reversacol Rush phenyl/arylethynylchromenes. The noteworthy feature of the
Yellow),[12a,12b] 2,2-diphenyl-2H-naphtho[1,2-b]pyran (NP2),[12a] ethynyl spacer is that the conjugation between two aryl rings
2,2-bis(4-methoxyphenyl)-5,6-dimethyl-2H-naphtho[1,2-b]- at the termini is freely facilitated without any sterically imposed
pyran (NP3, also known as Reversacol Berry Red),[12c] 2-(4′-piper- constraints because of the cynlidrical electronic distribution.
idinophenyl)-2-phenyl-5-methoxycarbonyl-9-dimethylamino-
2H-naphtho[1,2-b]pyran (NP4),[12d] and 2-(4′-dimethylamino-
Conclusions
phenyl)-2-(4′′-methoxyphenyl)-5-hydroxymethyl-9-pyrrolidino-
2H-naphtho[1,2-b]-pyran (NP5)[12d] are shown in Figure 5. The
It is shown that simple phenyl/arylethynyl-substituted chrom-
data presented in Table 1 show that the colored intermediates
of arylethynylchromenes exhibit bleaching rate constants much
better than those of NP1/NP4/NP5 and approach those of NP3,
namely, Reversacol Berry Red.
enes can be readily accessed by Sonogashira/“sila”-Sonogashira
coupling as a key reaction. Photoirradiation of all the aryl-
ethynylchromenes leads to readily observable color changes at
room temperature. with the color observed in the case of 5-
The remarkable changes in the spectrokinetic properties of and 7-substituted chromenes being yellow to orange-red and
the photogenerated colored o-quinonoid intermediates can be that in the case of 6-substituted analogues being purple-violet
understood from the resonating structures shown in Scheme 3. to dark-blue. Clearly, functionalization at the 5-, 6-, and 7-posi-
Accordingly, more extended mesomeric effect in the case of 7- tions results in varying degrees of mesomeric effects, which
substituted chromene than in the 6-substituted analogue allow modulation of the absorption properties of chromenes
should account for the observed redshift in the absorption and, more importantly, their photogenerated o-quinonoid inter-
maximum of the former. In a similar manner, the differences mediates; the colored intermediates of 5-substituted chrome-
in the absorption behaviors of the photogenerated quinonoid nes are found to be too short lived to allow their reversion
species can also be traced to differences in their mesomeric kinetics to be followed. In contrast, the intermediates of 6- and
structures. One observes an extended conjugation in the case 7-arylethynyl chromenes are found to be considerably longer
of the o-quinonoid intermediate of 6-DpaPhE, whereas the con- lived than those derived from analogous phenyl/aryl-substi-
jugation is truncated in the case of the intermediate of 7-Dpa- tuted chromenes. Indeed, the spectrokinetic properties of 6-/7-
PhE. In essence, the absorption maximum in the case of chrom- phenyl/arylethynyl chromenes are much better than many pho-
ene 6-DpaPhE is blueshifted compared with that of 7-DpaPhE, tochromic chromenes reported to date. The observed results
but the former gives rise to quinonoid intermediates with more are remarkable in view of the fact that simple phenyl/arylethy-
extended conjugation. The fact that the extended conjugation nylation of halo-substituted chromenes allows access to a series
is manifested in more stabilization is clearly borne out from of photochromic systems with tunable spectrokinetic proper-
slower decay kinetics of the colored species in the case of 6- ties at ambient temperatures. Clearly, the resonance effects of
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in CH2Cl2 (30 mL), washed with brine solution, and the organic
contents were extracted with CH2Cl2 (3 × 20 mL). The combined
extract was dried with anhydrous Na2SO4, filtered, and concentrated
to dryness under reduced pressure. The crude material was sub-
jected to silica gel column chromatography (CH2Cl2/hexane, 10 %)
to afford pure 6-(phenylethynyl)-2,2-diphenyl-2H-1-benzopyran
(0.42 g, 89 %) as a colorless solid.
varying magnitudes are nicely relayed through the ethynyl
group from one end of the molecular system to the other, with-
out any steric inhibition of resonance, permitting thermal rever-
sion kinetics that are comparable to the intermediates of the
commercially relevant naphthopyrans.
Experimental Section
6-(Phenylethynyl)-2,2-diphenyl-2H-1-benzopyran (6PhE): Color-
less crystalline solid; m.p. 150–152 °C. IR (KBr): ν = 3051, 2921, 2201,
General Aspects: Anhydrous tetrahydrofuran (THF) was freshly dis-
tilled from sodium prior to use. All other solvents also were distilled
prior to use. 1H and 13C NMR spectra were recorded with a JEOL
(400 MHz and 100 MHz, respectively) spectrometer in CDCl3 as sol-
vent. IR spectra were recorded with a Bruker Vector 22 FTIR spectro-
photometer. The EI mass spectra were recorded with a Waters GCT
Premier QTOF machine. Melting points were determined with a
JSGW melting-point apparatus. UV/Vis absorption spectra were re-
corded with a SHIMADZU UV-1800 spectrophotometer. Photolysis
was carried out in a Luzchem photoreactor fitted with 350 nm UV
lamps (8 W, 14 lamps).
˜
1630, 1595, 1492, 1480, 1445, 1364, 1270, 1246, 1117 cm–1. 1H NMR
(CDCl3, 400 MHz): δ = 6.21 (d, J = 10.0 Hz, 1 H), 6.61 (d, J = 10.0 Hz,
1 H), 6.90 (d, J = 8.7 Hz, 1 H), 7.21 (d, J = 1.4 Hz, 1 H), 7.26–7.30 (m,
3 H), 7.31–7.36 (m, 7 H), 7.41–7.44 (m, 4 H), 7.46–7.51 (m, 2 H) ppm.
13C NMR (CDCl3, 100 MHz): δ = 83.1, 88.0, 89.2, 115.9, 116.7, 121.1,
122.8, 123.5, 126.9, 127.6, 127.9, 128.2, 128.3, 129.5, 129.8, 131.4,
133.0, 144.5, 152.7 ppm. MS (EI+): m/z calcd. for C29H20O [M]+
384.1514; found 384.1514.
The Sonogashira coupling reaction of a mixture of 5- and 7-iodo-
2,2-diphenyl-2H-1-benzopyran (0.5 g) with phenylacetylene as de-
scribed above led to a mixture of 5- and 7-(phenylethynyl)-2,2-di-
phenyl-2H-1-benzopyrans (0.398 g, 85 %). The two regioisomeric
chromenes were separated by silica gel column chromatography
(CH2Cl2/hexane, 5 %).
Synthesis of Precursor 5-/6-/7-Iodochromenes: The precursor 5-/
6-/7-iodochromenes were prepared by acid-catalyzed condensation
reaction between 3-/4-iodophenol and 1,1-diphenylpropargyl alco-
hol and subsequent Claisen rearrangement. A representative proce-
dure for the synthesis of the iodochromenes is described below.
7-(Phenylethynyl)-2,2-diphenyl-2H-1-benzopyran (7-PhE): Yield
0.257 g (55 %); colorless crystalline solid; m.p. 118–120 °C. IR (KBr):
A 50 mL two-necked, round-bottom flask was charged with 3-/4-
iodophenol (0.50 g, 2.26 mmol), 1,1-diphenylprop-2-yn-1-ol (0.57 g,
2.72 mmol), a catalytic amount of p-toluenesulfonic acid (0.04 mg,
0.22 mmol), and anhydrous CH2Cl2 (20 mL). The solution was stirred
at room temperature under nitrogen gas atmosphere for 4 h, then
washed with saturated Na2CO3 solution, and the mixture was ex-
tracted with CH2Cl2 (3 × 20 mL). The combined extract was dried
with anhydrous Na2SO4, filtered, and concentrated to dryness under
reduced pressure. Further purification of the crude material by silica
gel column chromatography (CH2Cl2/hexane, 8 %) afforded the io-
dochromene either as a colorless crystalline material or as a viscous
liquid in a respectable yield (65–68 %). Whereas 4-iodophenol
yielded exclusively 6-iodochromene as a colorless crystalline mate-
rial (0.61 g, 65 %), 3-iodophenol led to an inseparable mixture (col-
orless viscous liquid, 0.63 g, 68 %) of regioisomers, i.e., 5- and 7-
iodochromenes, in 1:1.8 ratio. The regioisomeric mixture was used
as such for the next step without further separation.
ν = 3056, 2923, 2852, 2210, 1635, 1607, 1540, 1500, 1447, 1324,
˜
1
1255, 1214, 1107 cm–1. H NMR (CDCl3, 400 MHz): δ = 6.20 (d, J =
10.0 Hz, 1 H), 6.61 (d, J = 10.0 Hz, 1 H), 6.97 (d, J = 7.8 Hz, 1 H), 7.02
(dd, J = 7.8, 1.3 Hz, 1 H), 7.10 (s, 1 H), 7.24–7.29 (m, 2 H), 7.30–7.36
(m, 7 H), 7.41–7.45 (m, 4 H), 7.49–7.53 (m, 2 H) ppm. 13C NMR (CDCl3,
100 MHz): δ = 82.7, 89.4, 89.8, 119.4, 121.4, 122.9, 123.2, 124.0,
124.8, 126.4, 126.9, 127.5, 128.1, 128.2, 128.3, 129.7, 131.6, 144.6,
152.2 ppm. MS (EI+): m/z calcd. for C29H20O [M]+ 384.1514; found
384.1516.
5-(Phenylethynyl)-2,2-diphenyl-2H-1-benzopyran (5-PhE): Yield
0.141 g (30 %); colorless viscous liquid. IR (KBr): ν = 3056, 2930,
˜
2850, 2215, 1635, 1607, 1540, 1488, 1417, 1324, 1256, 1214,
1107 cm–1. 1H NMR (CDCl3, 400 MHz): δ = 6.28 (d, J = 10.0 Hz, 1 H),
6.93 (d, J = 10.0 Hz, 1 H), 7.05 (m, 2 H), 7.15 (d, J = 10.0 Hz, 1 H),
7.26–7.30 (m, 2 H), 7.32–7.39 (m, 7 H), 7.43–7.47 (m, 4 H), 7.54–7.57
(m, 2 H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 82.5, 86.7, 93.6, 116.9,
120.2, 121.5, 122.4, 123.1, 124.9, 127.0, 127.5, 128.1, 128.4, 129.0,
129.7, 131.6, 144.6, 152.4 ppm. MS (EI+): m/z calcd. for C29H20O [M]+
384.1514; found 384.1514.
6-Iodo-2,2-diphenyl-2H-1-benzopyran: Colorless crystalline solid;
m.p. 134–136 °C. IR (KBr): ν = 3049, 3028, 1631, 1595, 1489, 1472,
˜
1
1244, 1213, 1128 cm–1. H NMR (CDCl3, 400 MHz): δ = 6.20 (d, J =
10.0 Hz, 1 H), 6.55 (d, J = 10.0 Hz, 1 H), 6.70 (d, J = 8.7 Hz, 1 H), 7.27
(d, J = 3.2 Hz, 1 H), 7.29–7.36 (m, 6 H), 7.38–7.42 (m, 5 H) ppm. 13C
NMR (CDCl3, 100 MHz): δ = 83.1, 118.8, 122.1, 123.5, 126.9, 127.7,
128.2, 129.9, 134.9, 138.0, 144.4, 152.4 ppm. MS (EI+): m/z calcd. for
C21H15OI [M]+ 410.0168; found 410.0168.
Synthesis of 4-Diphenylaminophenylethynylchromenes (5-/6-/
7-DpaPhE): These compounds were synthesized by “sila”-Sonoga-
shira coupling reaction of the precursor iodochromenes under Pd0-
catalyzed conditions. A representative procedure is described for
the synthesis of 6-DpaPhE.
Synthesis of Phenylethynylchromenes (5-/6-/7-PhE): These com-
pounds were synthesized by Sonogashira coupling reaction of
iodochromenes under Pd0-catalyzed conditions. A representative
procedure for the synthesis of 6-PhE is described below.
To a two-necked, round-bottom flask (50 mL) charged with 6-iodo-
2,2-diphenyl-2H-1-benzopyran (0.60 g, 1.46 mmol), N,N-diphenyl-4-
[(trimethylsilyl)ethynyl]aniline (1.0 g, 2.92 mmol), THF (20 mL), and
triethylamine (10 mL) were added tetrabutylammonium fluoride tri-
Into a two-necked, round-bottom flask charged with 6-iodo-2,2-di- hydrate (1.84 g, 5.84 mmol), Pd(PPh3)2Cl2 (0.10 g, 0.14 mmol), and
phenyl-2H-1-benzopyran (0.50 g, 1.22 mmol), phenylacetylene
(0.20 mL, 1.83 mmol), THF (20 mL), and triethylamine (10 mL), were
introduced Pd(PPh3)2Cl2 (0.09 g, 0.12 mmol) and CuI (0.03 g,
CuI (0.04 g, 0.22 mmol). The reaction mixture was stirred under
nitrogen gas atmosphere at 60 °C for 6 h. Upon completion of the
reaction as judged by TLC analysis, the reaction mixture was con-
0.18 mmol). The resultant reaction mixture was stirred at room tem- centrated to dryness under reduced pressure. The crude material
perature under nitrogen gas atmosphere for 5 h. Upon completion was subsequently dissolved in CH2Cl2 (30 mL), washed with brine,
of the reaction as judged by TLC analysis, the reaction mixture was and the organic matter was extracted with CH2Cl2 (3 × 20 mL). The
concentrated to dryness in vacuo. The crude material was dissolved combined extract was dried with anhydrous Na2SO4, filtered, and
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Full Paper
concentrated to dryness under reduced pressure. The crude residue
was subjected to silica gel column chromatography (CH2Cl2/hexane,
10 %) to give pure 6-(4-diphenylaminophenylethynyl)-2,2-diphenyl-
2H-1-benzopyran (0.68 g, 84 %) as a light-yellow crystalline solid.
Keywords: Photochromism · Photochemistry · Reactive
intermediates · Conjugation · Oxygen heterocycles
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IR (KBr): ν = 3034, 2923, 2204, 1634, 1587, 1507, 1485, 1331, 1282,
˜
1242, 1128 cm–1. 1H NMR (CDCl3, 400 MHz): δ = 6.19 (d, J = 10.0 Hz,
1 H), 6.59 (d, J = 10.0 Hz, 1 H), 6.88 (d, J = 8.2 Hz, 1 H), 6.96–7.00
(m, 2 H), 7.02–7.12 (m, 6 H), 7.17 (s, 1 H), 7.21–7.28 (m, 7 H), 7.29–
7.35 (m, 6 H), 7.39–7.43 (m, 4 H) ppm. 13C NMR (CDCl3, 100 MHz):
δ = 83.0, 88.2, 88.4, 116.2, 116.5, 116.7, 121.0, 122.5, 122.8, 123.4,
124.8, 127.0, 127.6, 128.1, 129.3, 129.4, 129.6, 132.3, 132.8, 144.5,
147.2, 147.6, 152.5 ppm. MS (EI+): m/z calcd. for C41H29NO [M]+
551.2249; found 551.2244.
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1489, 1446, 1331, 1278, 1217, 1108 cm–1. 1H NMR (CDCl3, 400 MHz):
δ = 6.19 (d, J = 10.0 Hz, 1 H), 6.61 (d, J = 10.0 Hz, 1 H), 6.90–7.02
(m, 4 H), 7.03–7.08 (m, 3 H), 7.09–7.14 (m, 4 H), 7.23–7.29 (m, 6 H),
7.30–7.37 (m, 6 H), 7.40–7.45 (m, 4 H) ppm. 13C NMR (CDCl3,
100 MHz): δ = 82.7, 88.7, 90.2, 116.0, 119.2, 121.1, 122.2, 122.9,
123.5, 124.4, 124.6, 124.9, 126.4, 126.9, 127.5, 128.1, 129.3, 129.4,
132.5, 144.7, 147.1, 147.9, 152.2 ppm. MS (EI+): m/z calcd. for
C41H29NO [M]+ 551.2249; found 551.2247.
5-(4-Diphenylaminophenylethynyl)-2,2-diphenyl-2H-1-benzo-
pyran (5-DpaPhE): Yield 0.251 g (31 %); light-yellow viscous liquid.
IR (KBr): ν = 3033, 2920, 2203, 1635, 1586, 1509, 1489, 1331, 1278,
˜
1218, 1108 cm–1. 1H NMR (CDCl3, 400 MHz): δ = 6.24 (d, J = 10.0 Hz,
1 H), 6.89 (d, J = 10.0 Hz, 1 H), 6.98–7.03 (m, 3 H), 7.04–7.09 (m, 3
H), 7.10–7.15 (m, 5 H), 7.24–7.29 (m, 6 H), 7.30–7.39 (m, 6 H), 7.41–
7.45 (m, 4 H) ppm. 13C NMR (CDCl3, 100 MHz): δ = 82.5, 86.0, 93.9,
115.9, 116.6, 120.6, 121.6, 122.2, 123.0, 123.6, 124.8, 124.9, 127.0,
127.5, 128.1, 128.9, 129.4, 129.5, 132.5, 144.7, 147.1, 148.0,
152.4 ppm. MS (EI+): m/z calcd. for C41H29NO [M]+ 551.2249; found
551.2247.
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Supporting Information: Synthesis of starting materials, solution-
state photolyses of arylethynylchromenes, coloration plots, photo-
1
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of all the compounds.
Acknowledgments
J. N. M. is thankful to the Board of Research in Nuclear Sciences,
Government of India (BRNS, Department of Atomic Energy,
sanction number 2012/37C/34/BRNS/1958) for generous fund-
ing. A. M. and V. K. M. sincerely acknowledge the Council of
Scientific and Industrial Research (CSIR), New Delhi for senior
and junior research fellowships, respectively. Mr. Sanjoy Sasmal
is thanked for his help during the initial stages of the project.
Received: August 19, 2015
Published Online: November 27, 2015
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