Solvent-dependent excited state intramolecular proton transfer (ESIPT) pathways from phenol to carbon in 2,5-dihydroxyphenyl arenes

Photochemical &

Photobiological Sciences

View Article Online

View Journal

PAPER

Solvent-dependent excited state intramolecular proton

transfer (ESIPT) pathways from phenol to carbon in

2,5-dihydroxyphenyl arenes†

Cite this: DOI: 10.1039/c3pp50091h

Yu-Hsuan Wang and Peter Wan*

The ESIPT of three 2,5-dihydroxyphenyl-substituted arenes 9–11 was studied in various solvent systems,

to investigate the direction of the proton transfer from the phenol to the respective carbons of naphthyl,

phenanthrenyl and anthryl aromatic rings. In neat CH₃CN, 9–11 undergo direct ESIPT from the phenolic

OH to the ipso-position of the corresponding aromatic carbon acceptors, via an intramolecular charge

transfer state (S_1,ct), giving rise to observable zwitterions, ZIs 35, 25, 27, respectively. Surprisingly, the gene-

rated ZI in 9 proceeds via a 1,2-phenyl migration followed by re-aromatization to aﬀord 16 (a structural

isomer of 9) in quantitative yield. In 10 and 11, the corresponding ZIs proceed via electrocyclic ring

closure to furnish 20 and 28, respectively. In the case of 10, another intrinsic ESIPT pathway takes place

to the 10-position of a phenanthrenyl ring, giving QM 26 in high quantum eﬃciency (Φ_ex= 0.72). In

aqueous solution, 9 undergoes formal ESIPT to the more distal 2’- and 7’-positions of the naphthalene

ring, delivering QMs 18 and 38, which either revert back to the starting material or proceed via electro-

cyclic ring closure, respectively. In 11 in aqueous solution, formal ESIPT to the 10-position of the anthra-

cene ring takes place delivering QM 29, which readily aromatizes to regenerate starting material.

Received 18th March 2013,

Accepted 15th April 2013

DOI: 10.1039/c3pp50091h

www.rsc.org/pps

process can be further classified into direct ESIPT and formal

(solvent-mediated) ESIPT.⁹

Introduction

Excited state intramolecular proton transfer (ESIPT) has been

ESIPT to carbon atoms under neutral conditions usually

the subject of intensive investigations¹since the pioneering takes place with very low quantum eﬃciency.¹⁰However, devel-

work was reported by Weller.²This area is still of growing oping new carbon photoacids or photobases to initiate new

interest, mainly due to its fundamental nature as well as wide chemistry is still of particular interest to us not only due to

applications in systems such as photostabilizers,³fluorescent their potential industrial applications¹¹but also for gaining

chemosensors,⁴scintillators,⁵solar collectors,⁶and laser new insights into mechanistic details of excited state acid–

dyes.⁷In general, an ESIPT mechanism involves a proton base chemistry.¹²Our group has discovered and documented

transfer from the donor (acidic site) to the acceptor (basic site) new ESIPT systems where the proton transfer is from the phe-

moiety caused by a simultaneous enhancement of both acidity nolic OH to carbon atoms of adjacent aromatic moieties

and basicity in an electronic excited state. The best known including phenyl,¹³naphthyl,^14,17binaphthyl,¹⁵and anthryl¹⁶

ESIPT donor is the phenolic OH, which is much more acidic in rings. These ESIPT reactions all require a reactive twisted con-

the singlet excited state (pK_a* < pK_a),⁸whereas the basic group formation of the biaryl compound in the ground state, which

is usually a heteroatom, such as a carbonyl oxygen or a hetero- allows for the overlap between the s-orbital of phenol OH and

cyclic nitrogen atom. Depending on the distance between the the adjacent aromatic π-system. In these systems, the

proton donor and the acceptor in the molecule, the ESIPT enhanced acidity of phenolic proton in S₁is suﬃcient to facili-

tate the intramolecular proton transfer.

Department of Chemistry, University of Victoria, Box 3065, Victoria,

British Columbia V8W 3V6, Canada. E-mail: pwan@uvic.ca; Fax: +1 (250)721-7147;

Tel: +1 (250)721-8976

†Electronic supplementary information (ESI) available: Copies of the ¹H, ¹³C

NMR and 2D-spectra of all prepared and isolated compounds, UV-vis and fluo-

rescence spectra of compounds 9–11, fluoresence decay profile, transient absorp-

tion spectra obtained by LFP and computational data. See DOI:

ð1Þ

10.1039/c3pp50091h

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

For example, ESIPT from the phenolic OH to the 2′- and whether the solvent can play a key role to direct the

7′-naphthyl carbon atoms in 1-(2-hydroxylphenyl)naphthalene proton transfer from phenolic OH of the 2,5-dihydroxyphe-

(1-OD) led to formation of two quinone methides (QMs) 2 and nyl group to an adjacent aromatic carbon atom. Compound

3 with quantum yields of 0.14 (Φ_ex) and 0.2 (Φ_cycfor formation 9, compared with the previously reported parent compound

of 4), respectively (eqn (1)). QM 2 undergoes reverse proton 1, bears an additional hydroxyl group. As for 10, a phenan-

transfer to regenerate the deuterated starting material 1-2′D, threne unit can be regarded as

a benzonaphthalene

while QM 3 undergoes an electrocyclic ring closure to give isol- moiety.¹⁹Furthermore, compound 11 with an extended

able dihydrobenzoxanthene 4. It was observed that ESIPT in 1 phenyl ring at the 10-position, enhancing the conjugation,

to both 2′- and 7′-positions requires water mediation.¹⁴We was chosen to investigate the potentially more stabilized ZI

have recently reported a very eﬃcient ESIPT, from phenol to intermediate that could be best observed by laser flash

aromatic carbon in 2-phenyl-1-naphthol that has the highest photolysis (LFP). We show in this study that in addition to

quantum eﬃciency (Φ_ex

=

0.7) to date, via

a

conical the anticipated formal ESIPT to 2′- and 7′-positions of an

intersection.¹⁷

accepting naphthyl ring in the presence of water, a direct

In other developments, we reported a solvent-dependent ESIPT in 9 to the ipso-position was observed in aprotic sol-

ESIPT reaction in 9-(2,5-dihydroxyphenyl)anthracene (5): in vents, bringing about a rearrangement from a 1,2-phenyl

aprotic solvents, the direct ESIPT to the 9-position of the ring shift, to give a photoisomerization product in relatively

anthracene moiety takes place to generate zwitterion (ZI) 6, high yield. DHPAs 10 and 11 are also photochemically

which leads to cyclised product 8; whereas in protic solvents, reactive in both aprotic and protic solvents, leading to

the anticipated formal ESIPT to the 10-position occurs, to diﬀerent ESIPT processes. This is the first systematic study

aﬀord QM 7, which then regenerates the starting material 5 or of ESIPT between ArOH and aromatic carbon in which the

5-10D.¹⁸This study showed that in the excited state only the nature of the solvent directs the destination site of the

2,5-dihydroxyphenyl chromophore (neither 2,4- nor 2,6-di- proton transfer.

hydroxyphenyl) exhibits this unique intramolecular charge trans-

fer (ICT) emissive state, facilitating the proton transfer to the

9-position of the anthracene ring in aprotic solvents. That is,

by utilizing the appropriate solvent, we can direct where the

proton is transferred in the ESIPT. Comparing the ESIPT

process in 5 with the parent compound 9-(2-hydroxyphenyl)

anthracene,^16afor the latter, only a water-mediated relay mech-

anism takes place, and no ESIPT process was observed in

aprotic solvents. Therefore, special attention is paid to the

employment of the 2,5-dihydroxyphenyl group serving as the

proton donor upon excitation, which facilitates the formation

of a significant CT excited state, leading to diﬀerent ESIPT

processes.

Results and discussion

Synthesis

The desired DHPA derivatives (9–11) were readily prepared by

BBr₃demethylation in high yields from the corresponding

dimethoxy compounds 12, 13, and 15, which were in turn

synthesized according to slight modifications of the reported

procedures²⁰via Suzuki coupling reactions as shown in eqn

(3) and (4). Additional details are provided in the ESI.† The

ð2Þ

synthetic route also provides the corresponding methyl ether

compounds 12, 13, and 15, which were directly compared

with the photochemistry of the target DHPAs, to prove the

necessity of the phenolic OH group in the observed ESIPT

processes.

To explore whether the above ESIPT is applicable for

other 2,5-dihydroxyphenyl arenes (DHPA), we considered

derivatives for which naphthyl and phenanthrenyl groups

are the proton accepting carbon bases. Thus, three new

compounds 9–11 were synthesized to examine the influence

of diﬀerent accepting chromophores naphthyl, phenanthre-

nyl and anthryl rings on the eﬃciency and regioselectivity

ð3Þ

of ESIPT in both protic and aprotic solvents. That is,

Photochem. Photobiol. Sci.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

View Article Online

Photochemical & Photobiological Sciences

Paper

ð4Þ

UV-vis spectroscopy

To probe photoreactivity and the ability to form stable photo-

products, 9–11 were initially examined by irradiations in argon-

purged neat CH₃CN and H₂O–CH₃CN solvent systems in quartz

cuvettes. After each exposure to 300 or 350 nm light (16 lamps),

the UV-vis spectrum was taken (see ESI†). Photolyses of 9 in

both neat CH₃CN and H₂O–CH₃CN (∼10⁻⁵M, λ_ex= 300 nm) led

to dramatic changes in the absorption spectra with sharp iso-

sbestic points, suggesting that 9 under both conditions reacts

cleanly to give a diﬀerent chromophore (Fig. 1). The character-

istic absorption bands of 9 with maxima at 220 and 280 nm

upon irradiation decreased in intensity with the concomitant

growth of new absorption bands at 252, 300 and 371 nm in

H₂O–CH₃CN, whereas a broadening of characteristic absorption

bands in neat CH₃CN was detected. These two absorption

spectra are clearly diﬀerent, indicating that diﬀerent photo-

products were formed in protic vs. aprotic solvents. It is worth

noting that the photoproduct in Fig. 1(b) has finely structured

new absorption bands with a significant red-shift relative to that

of 9, indicating that an enhanced conjugation between phenyl

and naphthyl rings exists in the product.

Fig. 1 UV-vis spectra showing the conversion of 9 in (a) neat CH₃CN, (b) 1 : 1

H₂O–CH₃CN (both argon saturated) with irradiation. Photolysis was carried out

in a Rayonet photoreactor at 300 nm (16 lamps). Representative traces are

shown with the corresponding irradiation times as given in the ﬁgure. The long

wavelength region above 250 nm was expanded by 6-fold.

Irradiation of 10 in neat CH₃CN (∼10⁻⁵M, λ_ex= 300 nm) led

to drastic changes in the absorption spectra with well-defined

isosbestic points at 272 and 320 nm and new absorption

maximum at 375 nm, suggesting a clean transformation to give

an enhanced rigid structure and conjugation. However, less

eﬃcient changes in the absorption spectra of 10 were observed

on irradiation in H₂O–CH₃CN without a significant new band

growing at longer wavelength. DHPA 11 possesses an intense

anthracene absorption band at 260 nm and a weak but typical

absorption band between 320 and 420 nm. Irradiation of 11 in

neat CH₃CN or organic aprotic solvents such as diethyl ether,

cyclohexane and THF (∼10⁻⁵M, λ_ex= 350 nm) gave rise to a

rapid disappearance of the anthracene chromophore (Fig. 2),

indicating the loss of the anthracene ring. The presence of

sharp isosbestic points at 221, 305 and 313 nm suggests that a

clean transformation is likely being formed without occurrence

of secondary reactions. Presumably, in all of these solvent

systems listed above, 11 undergoes the same photoreaction to

Fig. 2 UV-vis spectra showing the conversion of 11 in neat CH₃CN (argon satu-

rated) with irradiation. Photolysis was carried out in a Rayonet photoreactor at

350 nm (16 lamps). Representative traces are shown with the corresponding

irradiation times as given in the ﬁgure. The long wavelength region above

give the same product, which is evidenced by later product 280 nm was expanded by 6-fold.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

studies. In 1 : 1 H₂O–CH₃CN and 2,2,2-trifluoroethanol (TFE),

To monitor the ESIPT process involving the naphthalene as

the UV-vis spectra of 11 did not exhibit obvious changes of the a carbon basic site for 9 in both aqueous and neat CH₃CN, we

anthracene absorption. We also examined another protic analyzed the deuterium incorporation into the recovered start-

solvent, CH₃OH, and found a gradual decrease of anthracene ing material and the photoproducts. Irradiations were typically

chromophore absorbance with the recovery rate (several carried out in both 0.001% and 10% D₂O–CH₃CN, followed by

minutes) of absorbance at 370 nm after photolysis, indicating washing with H₂O after irradiations. Low concentration of D₂O

either more eﬃcient photoreaction in CH₃OH than TFE and is employed to supply only suﬃcient deuterium content to

water or more long-lived photoproducts formed in CH₃OH that exchange the phenolic OH (to OD), and to avoid altering the

needs longer time to revert back to the starting material. These reaction pathway. The extent and the position of deuteration

findings suggest that photolysis of 11 in protic solvents gives were determined by NMR and MS. Photolysis of 9 (0.001%

very short-lived photoadducts (trapped by water or CH₃OH), D₂O) for 1 h furnished 28% conversion of the starting material

which quickly revert back to starting material (or that no photo- to give the corresponding deuterated photoisomerised product

products are formed). No notable absorption changes were 16-1′D. The disappearance of H_1′at δ 7.98 of 16-1′D in ¹H NMR

observed on irradiation of the methyl ethers 12, 13 and 15 in is evidence that the phenolic OD was indeed transferred to the

both neat and aqueous CH₃CN, consistent with the absorption 1′-position. No indication of deuterium incorporation at other

changes coming from an ESIPT reaction for 9–11 that require positions was observed. We can ascertain that the photoi-

the phenolic OH as the proton donor.

somerised product 16 comes from a direct ESIPT to the 1′-posi-

tion of the naphthalene ring, followed by a 1,2-phenyl shift

and aromatization. In addition, irradiation of 9 (10% D₂O)

under the same conditions yielded a mixture of 9-2′D and 19-7′

D (eqn (7)), which were isolated on silica gel and analyzed by

¹H NMR. The recovered starting material (9-2′D) showed deu-

terium incorporation at the 2′-position (58%) of the naphtha-

lene ring, while the cyclised product 19-7′D (yield ∼ 52%)

showed exactly 50% of deuterium exchange of methylene

hydrogens at the 7′-position similar to the mechanism pro-

posed previously.¹⁴This finding indicates that one of the

methylene hydrogens comes from either water (D₂O) or the

phenolic OD, consistent with an intrinsic or water-mediated

ESIPT from the phenol to the 7′-position responsible for the

formation of 19. The quantum yields of photogeneration of

9-2′D and 19-7′D in 10% D₂O–CH₃CN were determined by com-

parison with the deuterium incorporation of 1 as a secondary

actinometer (Φ_ex= 0.14; Φ_cyc= 0.20)¹⁴to be 0.09 0.01 and

Product studies

In order to identify stable photoproducts, preparative scale

irradiations were carried out for compounds 9–11 in various

solvent systems (∼10⁻³M, deoxygenated, 300 or 350 nm, 16

lamps). The structure of each photoreaction product was deter-

mined by NMR characterization after isolation from the photo-

lysate. Initial irradiation of 9 was performed in neat CH₃CN (1 h,

300 nm) giving rise to the primary photoisomerized photopro-

duct 16 in 37% conversion and a trace amount of benzofuran

17 (<3%) (eqn (5)). Prolonged irradiation of 9 for 10 h allowed

quantitative formation of 16 along with the minor 17 isolated

in 60% and 9% yields by preparative TLC on silica gel, respect-

ively, without other observable side products. The structures of

products 16 and 17 were determined and assigned by 1D and

2D NMR analyses (see ESI†). The same photoproducts were

observed when irradiations were carried out in diethyl ether

and cyclohexane, indicating that 9 proceeds via the same

pathway in aprotic organic solvents. On the other hand, pho-

tolysis of 9 in H₂O–CH₃CN (1 : 9 (v/v)) under the same con-

ditions gave a single isolable cyclization product 19 in 18%

conversion. As reported previously (eqn (1)),¹⁴it is envisioned

that 9 undergoes a water-mediated (formal) ESIPT from pheno-

lic OH to the 7′-position to give QM 18, followed by an electro-

cyclic ring closure to form dihydrobenzoxanthene 19 (eqn (6)).

0.012

0.01, respectively. Also, the quantum yield for for-

mation of 16 in neat CH₃CN was measured to be 0.06 0.01

(Table 1).

ð7Þ

It is worth noting that the water concentration plays an

important role in determining the ESIPT pathways of 9,

leading to either direct ESIPT or formal ESIPT processes.

Three competing photochemical pathways found in 9 were

determined by water content. Thus, to investigate the eﬀect of

water content on these proposed processes of 9, solutions con-

taining varying amounts of D₂O in CH₃CN were irradiated

(Fig. 3). At very low water concentration (0–0.1 M D₂O), the for-

mation of photoisomerized product 16 takes place without

indication of deuterated 9-2′D, showing that the reaction

pathway is dominated by direct ESIPT to the 1′-position to give

16-1′D. Increasing the D₂O content beyond the nominal

ð5Þ

ð6Þ

Photochem. Photobiol. Sci.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

View Article Online

Photochemical & Photobiological Sciences

Paper

Table 1 Quantum yields for deuterium incorporation^aand cyclization^b

via a direct ESIPT while generation of 9-2′D and 19-7′D is via a

water-mediated ESIPT mechanism, in keeping with our

expectation.

Compound

Φ_ex

Φ_cyc

Photolysis of 10 was carried out in neat CH₃CN (∼10⁻³M,

300 nm, 16 lamps, 0.5 h) furnishing 20 (8%), 21 (<2%) and a

trace amount of phenanthrene (eqn (8)). Prolonged irradiation

(2 h) led to a significant amount of decomposition as noted by

1

5

9

0.14 0.02^c

0.03 0.01^d

0.012 0.01

0.20 0.02^c

0.12 0.03

0.09 0.01

0.06 0.01^e

f

10

11

0.72 0.1

—

0.18 0.03^g

0.13 0.03

0.78ⁱ

1

a broader aromatic region in H NMR spectra and increase of

h

—

isolated phenanthrene, suggesting that 20 is unstable.

Attempts to isolate pure 20 were unsuccessful due to similar

polarity with other side photoproducts and fast decomposition

on silica gel. However, the structure of 20 was supported by

^aQuantum yields of deuterium incorporation photolysed in 10% D₂O–

CH₃CN at 300 nm for 9 and 10 whereas at 350 nm for 5 and 11. The

secondary actinometer employed for 9 and 10 was 1 (Φ_ex= 0.14);¹⁴for

11 was 9-(2-hydrophenyl)anthracene (Φ_ex= 0.09).^{16a b}Quantum yields

of cyclization photolysed in 10% D₂O–CH₃CN at 300 nm for 9 and in

neat CH₃CN at 350 nm for 5 and 11. ^cFrom ref. 14. ^dFrom ref. 18.

^eQuantum yield (Φ_MeCN) for the photogeneration of 16 in neat CH₃CN.

^fQuantum yield of cyclization in neat CH₃CN was too low to be

measured. ^gQuantum yield (Φ_MeOH) for the photochemical formation

of 24 in neat CH₃OH. ^hNo observable deuterium exchange was

detected. ⁱPhotolysed in diethyl ether.

1

the H NMR spectrum, where a clear set of AB quartet signals

(two benzylic hydrogens) was observed at δ 5.90 and δ 4.64 (J =

8.8 Hz) along with three hydrogens at the connecting phenol

ring at δ 6.48 (dd, H_b), 6.60 (d, H_c), 6.65 (d, H_a). This finding

suggests that 20 is formed via ESIPT from phenolic OH to the

ipso-position of a phenanthrene ring, to give ZI 25, followed by

an intramolecular ring closure delivering 20.

ð8Þ

ð9Þ

Fig. 3 Dependence of yields of 9-2’D, photoisomerized product 16-1’D, and

photocyclization product 19-7’D present in photolysate following photolysis of

9 in varying D₂O concentration in CH₃CN (∼10⁻³M, 300 nm, 16 lamps, 5 min).

Yields in these runs were determined by ¹H NMR.

Irradiation of 10 (2 h) in 1 : 1 H₂O–CH₃CN gave rise to

photoproduct 23 in 13% yield after chromatographic isolation

from the photolysate (eqn (9)), whose structure was identified

by NMR characterization. It is likely that 22 is the first formed

photoproduct that could readily be oxidized to 23, or revert

back to starting material during the work-up or purification on

silica gel. NMR analysis indicated that the crude reaction

mixture was primarily a mixture of starting material and 22,

exhibiting a clear set of AB quartet signals at δ 5.07 and δ 4.63

(J = 7.0 Hz) with three hydrogens at the connecting phenol

ring at δ 6.17 (d, H_a), 6.56 (dd, H_b), 6.69 (d, H_c), which are con-

sistent with expectation for 22. However, after chromatography,

only 23 was isolated, supporting the assertion that 23 is

formed from 22 during the work-up stage

amount required for the exchange of the phenolic OH from 9

to 9-OD led to a decrease in forming cyclization product 16-1′

D. No measurable deuteration incorporation at the 2′-position

of 9 and cyclised product 19 was observed until about 1 M D₂O

was reached, with 16-1′D (8%), 9-2′D (5%) and 19-7′D (33%) co-

existing. Presumably, this occurs when 9 was fully deuterated

(to 9-OD) and the remaining D₂O mediated the ESIPT to 2′-

and 7′-positions, to give 9-2′D and 19-7′D, respectively, with

concomitant loss of proton transfer to form 16-1′D. As more

D₂O is added, the eﬃciency of formation of both 9-2′D and 19-

7′D rose and reached a maximum at ∼5 M and ∼2.5 M, respect-

ively, confirming that these photoproducts do not arise via

intrinsic ESIPT. Gradual decrease in yields of 9-2′D and 19-7′D

at higher water concentration (beyond 5 M D₂O) may be ration-

alized by the operation of competing ESPT to water to yield the

phenolate 9⁻. These results suggest that formation of 16-1′D is

ð10Þ

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

Photolysis of 10 was repeated (0.5 h), but the solvent much more eﬃciently than ESIPT to the ipso-position of the

changed to neat methanol. Methanol was chosen because it is phenanthrene ring, causing such low quantum eﬃciency for

a better nucleophile than water and better able to trap the generating 20 and 22.

anticipated QM or ZI intermediates,²¹which could possibly

provide a chemically stable methyl ether photoadduct for NMR

analyses. Indeed, a single photoproduct was isolated in rela-

tively high yield (68%) and identified as methyl ether 24 by

NMR analyses (eqn (10)). Extending the irradiation time to 3 h

ð12Þ

allowed quantitative formation of 24 (ratio of trans- and cis-

isomer is 12 : 1 based on the integral of methoxy peaks at

δ 3.34 and δ 3.35 from the crude ¹H NMR spectrum). The same

photolysis of 10 was carried out in CD₃OD (0.5 h), where deu-

terium incorporation at the ipso-position of the phenanthrene

ring was observed by the complete disappearance of the peak

at δ 4.85. The photochemistry of 10 in aprotic and protic

solvent systems appears to be similar, where a direct ESIPT

from the phenolic OH to the ipso-position of the phenanthrene

ð13Þ

ring delivering ZI 25 occurs. This intermediate is then either

attacked intramolecularly by phenolate to give 20 or intermole-

cularly by solvents (water or methanol) to give 22 and 24,

respectively.

Irradiation of 11 (∼10⁻³M, 350 nm, 16 lamps, 5 min) in

neat CH₃CN led to cyclised product 28 in 46% conversion (eqn

(12)). It is envisioned that 11 undergoes direct ESIPT to the

9-position generating ZI 27, followed by ring closure to give

ð11Þ

cyclised product 28. As previously reported by our group,¹⁸

photolysis of 5 in diethyl ether resulted in the higher quantum

eﬃciency. Indeed, various aprotic solvents were tested and

diethyl ether gave the highest quantum eﬃciency (Φ_cyc= 0.78).

To probe other possible ESIPT processes available for 10, Photolysis of 11 in 1 : 1 H₂O–CH₃CN did not give rise to any

we analyzed the deuterium exchange in the recovered starting isolable photoproducts; the starting material was recovered

material. Surprisingly, irradiation of 10 (300 nm, 8 lamps) in even under the long irradiation times (18 h), which is consist-

10% D₂O–CH₃CN for 1 min eﬃciently led to deuterium incor- ent with the results of UV-vis trace studies. Similar photolyses

poration at the 10-position (10-10D) (34%) of the phenan- carried out in methanol and 10% D₂O–CH₃CN also resulted in

threne ring. Also, irradiation of 10 at very low D₂O no isolable photoproduct with observable deuterium exchange

concentration (0.05 M) also gave rise to 10-10D in quantitative in the recovered starting material (detectable by NMR). It is

yield, indicating that ESIPT is intrinsic. Presumably, direct most likely that 11 undergoes ESIPT to the 10-position to give

ESIPT from phenolic OH to the 10-position would give the QM 29 (eqn (13)), which readily reverts back to starting

deuterated QM intermediate 26 which then undergoes reverse material.

proton transfer to give deuterated starting material 10-10D

(eqn (11)). The quantum yield of 10-10D was determined by

comparison with the deuterium incorporation of 1 (Φ_ex

=

Fluorescence measurements

0.14)¹⁴and 2-phenyl-1-naphthol (31) (Φ_ex= 0.7)¹⁷as secondary

actinometers to give the quantum yield of 0.72 0.1 (Table 1). ESIPT reactions of phenol derivatives involve the singlet

Both actinometers gave similar results for the quantum yields excited state,^13–18which can be studied by using steady state

of deuterium incorporation, with the mean value within the and time-resolved fluorescence, to provide additional mechan-

error of the measurements. To the best of our knowledge, this istic information. In particular, the eﬃcient quenching of fluo-

eﬃcient ESIPT to carbon is the only one that is comparable rescence emission by addition of water would provide evidence

with the ESIPT of 2-phenyl-1-naphthol (Φ_ex= 0.7) to date. that the water-mediated ESIPT proceed from S₁.^1g,22All fluo-

Irradiation of 10 in 1 : 1 D₂O–CH₃CN (2 h) was also examined, rescence spectra were recorded in neat CH₃CN, aq. CH₃CN,

furnishing a mixture of deuterated 10-10D and 22-9D10D. The and other selected organic solvents. Quantum yields of fluo-

complete disappearance of peaks at δ 5.07 and δ 4.63 in the rescence were determined by using secondary reference

crude ¹H NMR spectrum in 22 supports the assertion that quinine bisulfate (Φ = 0.55)²³in 1.0 N H₂SO₄. Fluorescence

ESIPT from phenolic OH to both 9- and 10-positions upon lifetimes of 9–11 were obtained by single photon counting.

irradiation at higher water content. This finding could also The results are compiled in Table 2 and more details are given

imply that a direct ESIPT to the 10-position happened first and in ESI.†

Photochem. Photobiol. Sci.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

View Article Online

Photochemical & Photobiological Sciences

Paper

Table 2 Photophysical and photochemical parameters for 9–11 and 15^a

c

d

Compound

λ_em^b/nm

Φ_ex

Φ_f

τ_f^e(ns)

9

308

298

414

415

0.069 0.003

0.021 0.001

0.008 0.001

0.079 0.008

2.7 (55%), 1.2 (25%, 19 (20%)

0.72 (92%), 3.3 (8%)

1.3 (93%), 8.5 (7%)

10

11

15

257, 297, 354, 373, 393

294 (sh), 335 (sh), 350, 368, 389

410, 425, 500 (sh)

406, 425

2.2 (92%), 1.6 (8%)

^aMeasurements were performed in neat CH₃CN. ^bMaxima in the absorption spectra. ^cMaxima in the emission spectra. ^dFluorescence quantum

yield measured by using quinine bisulfate (Φ_f= 0.55) in 1.0 N H₂SO₄.²³^eFluorescence lifetimes in neat CH₃CN measured by time correlated

single photon counting (SPC). Estimated error is 0.1 ns.

Based on the previous study of DHPA 5 (Φ_f= 0.003),¹⁸we

assume that 2,5-dihydroxylphenyl derivatives may exhibit very

weak fluorescence due to formation of an intramolecular

charge transfer (ICT) state, leading to an eﬃcient operation of

ESIPT in aprotic solvents upon irradiation. Indeed, excitations

of 9 and 10 in neat CH₃CN led to relatively weak fluorescence

emissions with maxima at 414, 415 nm, respectively (Fig. 4).

The observed Stokes shifts of 9 and 10 are quite large, indicat-

ing that there is a significant twisting about the biaryl bond

toward planarity upon excitation to S₁. The high degree of

twisting would allow for strong charge transfer from the hydro-

xyphenyl group to the naphthyl or phenanthrenyl ring in the

excited state. Switching carbon bases or acceptors between

naphthyl and phenanthrenyl rings does little to influence their

emissive states. The fluorescence quantum yield for 10 in

CH₃CN is 3 times lower than 9, which could be rationalized by

the operation of more eﬃcient ESIPT in aprotic solvents for

10. Compared to the structurally similar naphthyl derivatives

30 (Φ_f= 0.04)²⁴and 31 (Φ_f= 0.03),¹⁷10 exhibits essentially the

same fluorescence quantum eﬃciency. This finding is consist-

ent with the assertion that the intrinsic ESIPT deactivation

pathway is operative from S₁to give QM 26 or ZI 25.

Fig. 4 Representative traces of 9 (top) and 10 (bottom) showing quenching of

Unlike the typically strong and structured anthracene emis-

sion observed for most anthracenes, excitation of 11 in neat

the ﬂuorescence emission in neat CH₃CN by addition of water. Excitation wave-

length λ_ex= 330 nm. Concentrations are shown in the ﬁgure.

CH₃CN led to dual emission consisting of a fine-resolved emis-

sion at 425 nm and a structureless broad shoulder band at

longer wavelengths (450–650 nm), attributed to the local position of the phenol ring in 11 (Φ_f= 0.008) leads to ∼100

excited (LE) and ICT states, respectively (Fig. 6). In contrast to fold reduction in fluorescence quantum eﬃciency. These

9 and 10, fluorescence of 11 showed a relatively small Stokes observations are consistent with a diﬀerent photochemical

shift, indicating small perturbation of the geometry for the pathway for 11 vs. 32 in neat CH₃CN (viz., facile photocycliza-

fluorescent states. The introduction of a phenyl ring at the 10- tion for 32).

position of the anthracene moiety compared to the parent

Addition of small amounts of water (∼1 M) to the CH₃CN

compound 5 (Φ_f= 0.003)¹⁸resulted in a slight increase of fluo- solution of 9 resulted in an initial bathochromic shift of the

rescence quantum yield to 0.008. Moreover, compared to pre- emission to 429 nm, along with a dramatic quenching of the

viously reported anthracene derivative 32 (Φ_f= 0.93, τ_f= 8.7 fluorescence intensity (Fig. 4). The decrease in fluorescence

ns),^16bthe introduction of extra phenolic OH at the para- intensity with added water is consistent with an increase in

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

Fig. 6 Fluorescence emission spectra of 11 in various solvents (10⁻⁵M). Exci-

tation wavelength λ_ex= 360 nm. Emission wavelength λ_em= 420 nm.

Fig. 5 Stern–Volmer plots of ﬂuorescence quenching vs. water concentration

in neat CH₃CN for 9 and 10.

the singlet excited state. In a non-polar solvent, such as cyclo-

hexane, 11 gives a single and fine-structured band with a

maximum at 407 nm (Φ_f= 0.25), assigned to be the LE state

(Fig. 6). With increasing solvent polarity, a dramatic decrease

in the emission intensity accompanied with the growth of a

new broad band at 500 nm was observed for 11, indicating

product formation. On addition of a small amount of water

(∼1 M) to the CH₃CN solution of 10, the fluorescence intensity

increases two fold along with an initial bathochromic shift of

the emission to 440 nm. It is assumed that water blocks the

non-radiative deactivation pathway of ESIPT to the 10-position

of the phenanthrene ring. Similar to the results found for 30

and 31,^17,24this behaviour can be rationalized by a very

eﬃcient intrinsic ESIPT pathway aﬀording QM 26 that is partly

quenched by addition of protic solvents. Addition of more

water (>2.8 M) to 10 quenches the fluorescence as well as shift-

ing the emission spectra hypsochromically at high water con-

centration (∼12 M). Both results suggest that water quenching

mainly comes from water-mediated ESIPT to form the corres-

ponding QMs rather than the transfer of proton to the water

(ESPT) to produce the excited state phenolates.

Stern–Volmer plots of the fluorescence quenching vs. water

concentration in CH₃CN for 9 and 10 (Fig. 5) show typical non-

linear behavior as previously observed in other direct and

water-mediated ESIPT processes.¹⁷Compound 11 is not shown

in this plot due to its weak emission in CH₃CN being unreli-

able for quenching studies. In 9, at low water content (<7.5 M),

quenching can be regarded as a polynomial function, which is

consistent with an ESIPT where water clusters are involved in

the deactivation of the singlet state, as reported in similar

water-mediated ESIPT systems.^18,21,25This finding could imply

that fluorescence quenching within water content lower than

7.5 M results from the increasing eﬃciency of the formal

ESIPT reaction to generate QMs. At a higher water concen-

tration (>10 M), the quenching eﬃciency levels oﬀ. For 10,

with a small amount of water (<1.5 M), unlike the results

found in 9, a drop of fluorescence quenching with water con-

centration was shown, suggesting that the direct ESIPT is oper-

that the relative contributions of the LE (S₁) to the ICT (S_1,ct

)

states are controlled by solvent polarity. This observation is in

agreement with the previous report for 5,¹⁸where the dual

fluorescence is rationalized by the proximity of S_1,ctand S₁

states, giving rise to two emissive states (π,π* and ICT). The

finding is also in accord with higher eﬃciency of photocycliza-

tion upon photolyses of 11 in non-polar solvents given the

proton transfer quenching of the ICT state. Based on the

results from fluorescence and product studies, 11 most likely

reacts (giving cyclization products) via the ICT state (S_1,ct).

Fluorescence lifetime for 9 was obtained by the three-expo-

nential decay function, suggesting three emissive states

(Table 2). The measured decays in neat CH₃CN are 2.7 (55%),

1.2 (25%) and 19 ns (20%) at a shorter wavelength of 400 nm,

whereas 2.2 ns (92%) and 15 ns (8%) at a longer wavelength of

470 nm, implying that the shortest lifetime (1.2 ns) could be

attributed to the ICT state. Fluorescence decay lifetimes of 10

and 11 were obtained by the two-exponential decay function,

probably attributed to the presence of two excited singlet

states, LE and ICT. In the case of 11, two decays in neat

CH₃CN were measured to be 1.3 (93%) and 8.5 ns (7%),

assigned to ICT and LE states, respectively. Similar to the

parent compound 5 with reported decay lifetimes to be 1.6 ns

(ICT) and 3.4 ns (LE), the extended conjugation onto an

anthracene chromophore in 11 does little to influence the life-

time of the ICT state.

Laser flash photolysis (LFP)

ative. At water concentrations above 1.5 M, the quenching Although studying photoreactions by UV-vis spectroscopy can

increases linearly with the water concentration until 7 M, indicate intermediacy of long-lived transient species

where the quenching eﬃciency levels oﬀ.

(>seconds), detecting short-lived zwitterions or carbocations

Fluorescence measurements of 11 in various solvents were requires nanosecond LFP. Thus, in order to characterize the

also examined to obtain more insights into the properties of proposed ZI and QM intermediates in the photochemistry of

Photochem. Photobiol. Sci.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

View Article Online

Photochemical & Photobiological Sciences

Paper

Fig. 8 Transient absorption spectra observed for 11 in O₂-purged CH₃CN solu-

Fig. 7 Transient absorption spectra observed for 10 in O₂-purged CH₃CN solu-

tion. (Inset: decay at 420 nm.)

tion. (Inset: decay at 480 nm.)

9–11, LFP was performed in neat CH₃CN and H₂O–CH₃CN (see

ESI†). Our group has been able to successfully characterize a

large variety of ZI and QM intermediates by LFP from ESIPT

and other reactions.²⁶LFP of 9 in N₂-purged solution CH₃CN

gave rise to a strong transient absorption between 320 and

580 nm with a maximum at 460 nm. This transient decayed

with k = 6.7 × 10⁵s⁻¹(τ = 1.5 μs), and was strongly quenched

by O₂with k = 1.6 × 10⁷s⁻¹(τ = 61 ns). Based on the similarity

with spectra detected for other naphthylphenols,^24,27and

quenching by O₂, we assigned this transient to the phenol

triplet–triplet absorption. The proposed ZIs 35 or 37 were not

observed due to its anticipated weak signal and short lifetime.

LFP of 10 in CH₃CN under N₂gave rise to a strong transient

absorption in region of 380–550 nm with a maximum at

480 nm (Fig. 7). The transient absorption band at 480 nm

decayed by second-order kinetics, giving lifetimes of 2.2 and

53 μs: the former was quenched by O₂(τ = 51 ns), assignable to

the triplet state; whereas the latter was not aﬀected by O₂,

probably corresponding to the QM 26. Compared to the pub-

lished spectra of similar structures,²⁷we tentatively assigned

this long-lived species (τ = 53 μs) to QM 26. To verify if the

observed longer-lived transient corresponds to QM 26, LFP of

its methoxy derivative 13 that cannot generate QM was also

examined. In the transient spectra of 13 taken in CH₃CN

under N₂, a signal that corresponds to the triplet was detected

absorbing between 380 and 440 nm with a maximum at

420 nm, decaying in 1.6 μs (under O₂, τ = 40 ns). No other

detectable longer-lived species was found at 480 nm, further

providing a firm assignment of the long-lived transient to QM

26 observed by LFP of 10.

LFP of 11 in neat CH₃CN under N₂led to a transient between

280 and 500 nm with a maximum at 420 nm, decaying in

61 μs, along with a bleaching signal at 390 nm (Fig. 8). This

bleaching signal resulted from the absorption of the laser

flash by the substrate. The recovery of the bleaching to the

baseline after longer time scale delays indicates the good rever-

sibility of the process initiated by the laser flash. Saturating

the solution with oxygen quenched the lifetime to 63 ns. This

transient is assigned to the triplet state of 11 based on simi-

larity to known T–T absorption detected for other anthracene

derivatives.¹⁶However, the transient at 420 nm decayed by

second-order kinetics, giving lifetimes of 63 ns and 2.8 μs. For

the corresponding methoxy derivative 15, LFP in neat CH₃CN

under O₂yielded a similar transient at 420 nm, decaying in a

single-exponential fashion (τ ∼ 50 ns), assignable to the triplet

state of 15. Unlike the transient observed in 11, no other

detectable longer-lived species was detected in 15 at 420 nm

except for triplets, providing further evidence that the other

decay observed in 11 could be assignable to the trityl cation

27. The anticipated ZI 27 has a trityl cation as a chromophore,

and should absorb around 420 nm.^26,28,29Therefore, we tenta-

tively assign the long-lived species (τ ∼ 2.8 μs) to ZI 27, which

has a similar decay lifetime to ZI 34.

Attempts to detect the ZI 33 for the parent compound 5

were unsuccessful perhaps due to its anticipated short lifetime

(<20 ns). By introducing a phenyl ring at the 10-position of the

anthracene ring, the anticipated ZI 27 can be stabilized by an

extended conjugation system. Structurally similar to the antici-

pated ZI 27, Basarić et al. recently reported the LFP of the

photogenerated trityl ZI intermediate 34 giving rise to a transi-

ent at 420 nm with a lifetime of 7.5 μs.²⁷

In both N₂and O₂-purged H₂O–CH₃CN solutions, LFP of all

9, 10 and 11 led to relatively weak transient absorptions, which

precluded further study. The proposed intermediates QMs 18

and 38, structurally similar to QMs 2 and 3, should absorb at

360–560 nm,¹⁴where the T–T absorption was usually detected.

As for QM 29, in addition to the spectral interference from the

triplet signals, substrate bleaching of the anthracene chromo-

phore obscured its transient spectra. Hence, due to their short

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

lifetimes, overlapping with the triplets, as well as their very low

quantum eﬃciency of formation their detection was not

possible.

Photochemical reaction mechanisms

Based on the above data from product studies, deuterium

exchange, fluorescence measurements, and LFP, the mechan-

ism of photoreactions of 9–11 can now be proposed.

Excitation of 9 to S₁in aprotic solvents such as CH₃CN or

diethyl ether (or with low H₂O content) resulted in initial

direct ESIPT delivering ZI 35, as demonstrated by deuterium

exchange observed at the 1′-position of photoisomerized

product 16 (Scheme 1). ZI 35 proceeds via either rotation of

the phenyl–naphthyl bond (blue arrow) to facilitate ring

Scheme 2

closure, to give possible cyclised product 36, or via protonation positions of the phenanthrenyl ring giving rise to ZI 25 and

to revert back to the starting material. However, 36 was not QM 26, respectively (Scheme 3). The fate of ZI 25 depends on

observed. A faster process apparently is a phenyl shift (red the solvent system. In neat CH₃CN, ZI 25 undergoes a rotation

arrow) to form a more stable benzylic carbocation 37 instead. (blue arrow) about the phenyl–phenanthrene ring bond to

ZI 37 then rapidly undergoes proton transfer aﬀording the facilitate intramolecular ring closure giving rise to 20.

major observed photoproduct 16, which on further photolysis However, 20 is unstable and is readily oxidized to 21. In

can lead to 17 as a minor product. 17 could also form directly CH₃OH and aqueous solution, ZI 25 undergoes nucleophilic

from 37. Although both 2,5-dihydroxyphenyl naphthyl deriva- quenching by CH₃OH or H₂O (red arrow), delivering 24 or 22,

tives (9 and 16) are well known compounds,^28,30this kind of respectively. Photoadduct 22 is unstable and can readily elim-

photo-initiated transformation of two regioisomers has not inate water to regenerate starting material or oxidized to give

been reported before.

23. The quantum yields for photogeneration of 20 and 22 are

Excitation of 9 in the aqueous media leads to formal ESIPT relatively low. This is explained by a competing ultrafast intrin-

(Scheme 2), as indicated by fluorescence quenching with sic ESIPT pathway from phenolic OH to the 10-position, deli-

added water and deuterium exchange at 2′- and 7′-positions of vering QM 26 which upon tautomerization regenerates

the naphthyl ring. As shown in Scheme 2, a water-mediated deuterated starting material 10-10D (Φ_ex= 0.72) in the pres-

ESIPT in 9-OD from phenol to the 2′- and 7′-carbon atoms on ence of D₂O. QM 26 could also be trapped by CH₃OH (or

the naphthyl ring gives rise to the QM intermediates 38 and water) to give photoadduct 39, however, it could also rapidly

18, respectively. The former undergoes tautomerization to revert back to starting material so that we were unable to

furnish deuterated starting material 9-2′D, whereas the latter detect it. Evidence for QM 26 formation was supported by for-

proceeds via electrocyclic ring closure to give cyclised 19-7′D. mation of 10-10D and LFP studies of 10 (τ = 53 μs). In addition,

Measured quantum yields for deuterium incorporation and deuterium incorporation was observed at both 9- and 10-posi-

cyclization indicate that ESIPT to the 7′-carbon is an order of tions in photoadducts 22 and 24 when photolyses were carried

magnitude more eﬃcient than to the 2′-carbon. This mechan- out in 1 : 1 D₂O–CH₃CN and CD₃OD, respectively. Since a rela-

ism is consistent with the previous report for the parent com- tively high eﬃciency of direct ESIPT to the 10-position over the

pound 1.¹⁴Excitation of 10 in both aprotic and protic solvent ipso-position of the phenanthrene ring was observed in 10, it

systems resulted in direct ESIPT to both 9 (ipso)- and 10- is most likely the formation of QM 26 occurs first to regenerate

Scheme 1

Photochem. Photobiol. Sci.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

View Article Online

Photochemical & Photobiological Sciences

Paper

Scheme 3

10-10D, followed by the photoaddition by CH₃OH or water via singlet excited state surface as previously reported in studies of

ZI 25. Consequently, we conclude that upon irradiation of 10 structurally similar naphthols 30 and 31.^17,24Furthermore,

in either protic or aprotic solvents, there are two direct ESIPT considering the possible syn or anti conformers of 10 in the

pathways competing: the major one is to the 10-position of the ground state population equilibrium as discussed for naphthol

phenanthrene ring delivering QM 26; the minor is to the 9 derivative 31,²⁴it is assumed that one of the syn conformers

(ipso)-position giving rise to ZI 25.

(S10-1) dominates favouring the direct ESIPT to the 10-position

whereas the other syn conformer (S10-2) is unfavorable due to

being hindered by a connected phenyl ring of the phenan-

threne moiety as shown. Based on B3LYP/6-311 calculations,

S10-1 is lower in energy than S10-2. That is, in 10, the ground

state conformational equilibrium eﬀectively facilitates direct

ESIPT to the 10-position of the phenanthrene ring, leading to a

very high quantum eﬃciency of deuterium incorporation of 10.

Similar to 5, the reaction mechanism of 11 in aprotic sol-

vents such as CH₃CN and diethyl ether is direct ESIPT from

the phenol to the 9-position to generate ZI intermediate 27,

The reason for the observed high quantum eﬃciency (Φ_ex

=

0.72) in 10 may be due to the possible conical intersections which upon bond rotation and ring closure gives cyclised

with the S₀dominating the deactivation pathway from the product 28 (Scheme 4). As demonstrated in fluorescence

Scheme 4

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

studies, the ESIPT is via an ICT emissive state. Evidence for experiments were performed in a Rayonet photochemical

intermediate 27 was provided by LFP studies which showed reactor equipped with 16 lamps of 300 nm or 350 nm as indi-

formation of a triaryl methyl carbocation with a lifetime ∼ cated. During irradiations, solutions were purged with a

2.8 μs at λ_max= 420 nm, consistent with that reported in the lit- stream of argon and cooled by a tap water cold finger conden-

erature.^28,29Very high quantum eﬃciency of photocyclization ser. Acetonitrile for photolysis was of HPLC grade. Other sol-

was observed in diethyl ether (Φ_cyc= 0.78). A water-mediated vents (ACS grade) used for synthesis were purchased from

ESIPT process could also be initiated upon irradiation of 11 in Aldrich and used as received. Preparative TLC was carried out

the presence of protic solvents, delivering QM 29, which is in on silica gel GF Uniplates (20 cm × 20 cm) and purchased

accord with the previous report of the related systems.¹⁸from Analtech. CDCl₃, D₂O and CD₃CN were purchased from

Nucleophilic attack of QM 29 could take place at the 9-position Cambridge Isotope Laboratory.

by water or methanol, but these products could not be isolated

presumably due to their facile reversion back to starting

Materials

material.

1-Bromonaphthalene, 9-bromophenanthrene, 9,10-dibromoan-

thracene, 2,5-dimethoxyphenyl boronic acid, phenylboronic

acid, Pd(PPh₃)₄, and BBr₃(1.0 M solution in CH₂Cl₂) were pur-

chased from Aldrich and used upon received. DHPA derivatives

9–11 were readily obtained from the corresponding dimethoxy

Conclusions

2,5-Dihydroxyphenyl derivatives 9–11 were synthesized and derivatives 12, 13 and 15 by employing BBr₃demethylation

their photochemistry and photophysics investigated in various reaction. 12, 13 and 15 were then prepared from Suzuki coup-

solvent systems. This work demonstrates that the solvent plays ling reactions according to the described procedures.^12b,17

a key role in determining details of proton transfer from phe-

General procedure for Suzuki coupling reaction

nolic OH of the 2,5-dihydroxyphenyl ring to the anticipated

positions of the carbon bases in 9–11. In an aqueous solvent, A mixture of 1 equiv. of bromo compound and 1.1 equiv. (or

upon excitation to the S₁, 9 and 11 undergo water-mediated 1.0 equiv. whereas indicated) of boronic acid was dissolved in

ESIPT from phenolic OH to the specific carbons of the con- a mixture of toluene (15 mL) and MeOH (5 mL) in a 100 mL

necting aromatic ring, delivering QM intermediates, which round-bottom flask. To the reaction mixture was added 2

could either revert back to the starting materials or proceed via equiv. of K₂CO₃and 3 mol% of tetrakis-triphenylphosphine

cyclization. In aprotic solvents, 9–11 all undergo direct ESIPT palladium Pd(PPh₃)₄and the mixture was heated at reflux for

from phenolic OH to the ipso-positions via intramolecular 24 h under an N₂atmosphere. The reaction mixture was then

charge transfer states (S_1,ct), giving rise to the corresponding cooled to room temperature and to the mixture was added

ZIs 35, 25, 27, respectively. Interestingly, the generated ZI in 9 H₂O (20 mL). The two layers were separated and the aqueous

was followed by a 1,2-phenyl shift, furnishing the photoisome- layer was extracted with CH₂Cl₂(3 × 10 mL). The combined

rization product 16 in quantitative yield. In 10 and 11, the organic layers were washed with brine and dried over anhy-

corresponding ZIs proceed via ring closure (collapse of the ZIs) drous MgSO₄. After filtration, concentration under reduced

to furnish 20 and 28, respectively. Two or three competing pressure aﬀorded the crude product. Purification of the

photochemical pathways for 9–11, formal ESIPT vs. direct residue was achieved by flash column chromatography on

ESIPT, are controlled by the selection of solvent and water silica gel using ethyl acetate in an n-hexane mixture (EtOAc–

concentration.

hexane gradient, 0% to 10%) as an eluent.

1-(2,5-Dimethoxyphenyl)naphthalene (12).^20a,30bFollowing

the general procedure, a mixture of 650 mg (3.14 mmol) of

1-bromonaphthalene and 629 mg (3.45 mmol) of 2,5-dimethoxy-

phenylboronic acid in the presence of K₂CO₃(867 mg,

6.28 mmol) and Pd(PPh₃)₄(109 mg, 0.09 mmol) was converted

Experimental section

General

All NMR spectra were recorded on Bruker AC300 (300 MHz) to 646 mg (78%) of the product 12 as a colorless crystal. Mp

and Avance 500 (500 MHz) instruments. For the structural 91–92 °C (Lit.^30bmp = 94 °C); ¹H NMR (500 MHz, CDCl₃)

assignment of signals, 2D homonuclear COSY and NOESY and δ 7.87 (d, J = 8.5 Hz, 1 H), 7.85 (d, J = 8.3 Hz, 1 H), 7.60 (d, J = 8.6

heteronuclear HSQC and HMBC techniques were used. Hz, 1 H), 7.51 (dd, J = 8.3, 8.0 Hz, 1 H), 7.45 (td, J = 7.6, 1.2 Hz,

Melting points were determined using a Gallenkamp melting 1 H), 7.41–7.36 (m, 2 H), 6.97 (d, J = 8.8 Hz, 1 H), 6.94 (dd, J =

point apparatus (and were not corrected). UV-Vis spectra were 8.8, 2.8 Hz, 1 H), 6.86 (d, J = 2.8 Hz, 1 H), 3.78 (s, 3 H, OMe),

taken on a Varian Cary 1 spectrophotometer at rt. IR spectra 3.62 (s, 3 H, OMe); ¹³C NMR (125 MHz, CDCl₃) δ 153.5, 151.5,

were recorded on a spectrophotometer in KBr. Accuracy mass 136.8, 133.4, 132.0, 130.5, 128.1, 127.7, 127.2, 126.4, 125.7,

analysis was carried out at the UVIC Genome BC Proteomics 125.6, 125.3, 117.6, 113.7, 112.4, 56.3, 55.8.

Centre. A mg mL⁻¹solution of the analyte was diluted by a

1-(2,5-Dimethoxyphenyl)phenanthrene

(13).^20aFollowing

factor of 10–100 and injected by liquid infusion at ∼300–500 the general procedure, a mixture of 500 mg (1.94 mmol) of

nL min⁻¹by a syringe pump through a nano-ESI source, and 9-bromophenanthrene and 389 mg (2.14 mmol) of 2,5-dimethoxy-

all solvents used were of MS grade. All the irradiation phenylboronic acid in the presence of K₂CO₃(535 mg,

Photochem. Photobiol. Sci.

This journal is © The Royal Society of Chemistry and Owner Societies 2013

View Article Online

Photochemical & Photobiological Sciences

Paper

3.88 mmol) and Pd(PPh₃)₄(67.3 mg, 0.06 mmol) was converted was purified by flash column chromatography on silica gel

to 501 mg (82%) of the product 13 as a colorless crystal. Mp (CH₂Cl₂–hexane gradient, 0% to 10%) to aﬀord 526 mg (92%)

1

150–152 °C; H NMR (300 MHz, CDCl₃) δ 8.75 (d, J = 8.5 Hz, 1 of the product 9 as an oﬀ-white solid. Mp 108–110 °C (Lit.^30b

1

H), 8.72 (d, J = 8.6 Hz, 1 H), 7.88 (dd, J = 7.8, 1.4 Hz, 1 H), 7.68 mp = 83–84 °C); H NMR (500 MHz, CDCl₃) δ 7.91 (d, J = 8.4

(s, 1 H), 7.67–7.57 (m, 5 H), 7.53–7.47 (m, 1 H), 6.99–6.97 (m, 2 Hz, 2 H, H-2, H-8), 7.67 (d, J = 8.7 Hz, 1 H, H-5), 7.55 (dd, J =

H), 6.95–6.93 (m, 1 H), 3.81 (s, 3 H, OMe), 3.63 (s, 3 H, OMe); 8.4, 8.35, 1 H, H-3), 7.51 (dd, J = 8.4 Hz, 8.1 Hz, 1 H, H-7),

¹³C NMR (75 MHz, CDCl₃) δ 153.7, 151.8, 135.7, 131.7, 131.3, 7.47–7.44 (m, 2 H, H-4, H-5), 6.93 (d, J = 8.7 Hz, 1 H, H-11),

130.6, 130.25, 130.2, 128.7, 127.7, 127.2, 126.6, 126.5, 126.34, 6.84 (dd, J = 8.7 Hz, 3.1 Hz, 1 H, H-12), 6.75 (d, J = 3.1 Hz, 1 H,

126.3, 122.7, 122.6, 117.6, 113.8, 112.4, 56.4, 55.8.

H-14), 4.55 (s, 1 H, OH_a), 4.45 (s, 1 H, OH_b); ¹³C NMR

9-Bromo-10-(2,5-dimethoxyphenyl)anthracene (14). Follow- (125 MHz, CDCl₃) δ 149.0 (s, C-10), 147.1 (s, C-13), 133.9 (s,

ing the general procedure, a mixture of 800 mg (2.38 mmol) of C-1), 133.7 (s, C-15), 131.6 (s, C-11), 128.9 (d, C-2/C-8), 128.4 (d,

9,10-dibromoanthracene and 433 mg (2.38 mmol) of 2,5- C-2/C-8), 128.0 (d, C-4), 127.0 (s, C-9), 126.8 (d, C-6), 126.4 (d,

dimethoxyphenylboronic acid in the presence of K₂CO₃C-7), 125.7 (d, C-3), 125.5 (d, C-5), 117.5 (d, C-11), 116.4 (d,

(647 mg, 4.76 mmol) and Pd(PPh₃)₄(82.5 mg, 0.07 mmol) was C-12/C-14), 116.3 (d, C-12/C-14); IR (CHCl₃, cm⁻¹) 3381, 3075,

converted to 709 mg (76%) of the product 14 as a yellowish 3046, 2360, 1650, 1491, 1437, 1203, 735.

solid, mp 147–148 °C; ¹H NMR (500 MHz, CDCl₃) δ 8.59 (d, J =

1-(2,5-Dihydroxyphenyl)phenanthrene (10). From 500 mg

8.8 Hz, 2 H), 7.64 (d, J = 8.6 Hz, 2 H), 7.57 (ddd, J = 8.8, 8.7, 1.1 (1.59 mmol) of 1-(2,5-dimethoxyphenyl)phenanthrene (13) and

Hz, 2 H), 7.37 (ddd, J = 8.8, 8.7, 1.1 Hz, 2 H), 7.06 (d, J = 1.8 a solution of BBr₃, the reaction gave the crude product which

Hz, 2 H), 6.83–6.82 (t, J = 1.8 Hz, 1 H), 3.78 (s, 3 H, OMe), 3.53 was purified by recrystallized from methylene chloride to

(s, 3 H, OMe); ¹³C NMR (125 MHz, CDCl₃) δ 153.8, 152.4, aﬀord 455 mg (96%) of the product 10 as an oﬀ-white crystal,

1

134.5, 131.2, 130.5, 128.1, 127.4, 127.1, 125.7 123.0, 118.3, mp 188–190 °C; H NMR (500 MHz, CD₃CN) δ 8.82 (d, J = 8.3

114.6, 112.9, 56.6, 56.0; IR (CHCl₃, cm⁻¹) 3853, 3744, 2934, Hz, 1 H, H-4/H-5), 8.79 (d, J = 8.4 Hz, 1 H, H-4/H-5), 7.95 (d, J =

2831, 1497, 1423, 1224, 1046, 903, 760.

8.1 Hz, 1 H, H-1), 7.72–7.63 (m, 4 H, H-3, H-6, H-2, H-7), 7.70

9-(2,5-Dimethoxyphenyl)-10-phenylanthracene (15). Follow- (s, 1 H, H-10) 7.56 (dd, J = 8.4, 8.0 Hz, 1 H, H-8), 6.84 (d, J = 8.8

ing the general procedure, a mixture of 671 mg (1.71 mmol) of Hz, 1 H, H-13), 6.80 (dd, J = 8.8, 2.8 Hz, 1 H, H-12), 6.74 (d, J =

14 and 230 mg (1.88 mmol) of phenylboronic acid in the pres- 2.8 Hz, 1 H, H-11), 6.03 (s, 1 H, OH), 4.65 (s, 1 H, OH); ¹³C

ence of K₂CO₃(472 mg, 3.42 mmol) and Pd(PPh₃)₄(57.8 mg, NMR (125 MHz, CD₃CN) δ 151.2, 148.7, 136.1, 132.8, 132.2,

0.05 mmol) was converted to 627 mg (94%) of the product 15 131.4, 131.1, 129.7, 129.2, 128.8, 128.1, 127.94, 127.9, 127.8,

as an oﬀ-white solid, mp 201–203 °C; ¹H NMR (300 MHz, 127.75, 124.0, 123.7, 118.7, 117.6, 116.9; IR (CHCl₃, cm⁻¹

)

CDCl₃) δ 7.71–7.67 (m, 4 H), 7.63–7.47 (m, 5 H), 7.37–7.30 (m, 3701, 3675, 3566, 3470, 1655, 1455, 1203, 725; MS (ESI) m/z

4 H), 7.13 (d, J = 8.9 Hz, 1 H), 7.09 (dd, J = 8.9, 2.7 Hz, 1 H), 285 (M⁺− H); HRMS, calcd for C₂₀H₁₄O₂: 285.09211 (M⁺− H);

6.91 (d, J = 2.7 Hz, 1 H), 3.82 (s, 3 H, OMe), 3.61 (s, 3 H, OMe); Found 285.09186.

¹³C NMR (75 MHz, CDCl₃) δ 153.8, 152.5, 139.2, 137.1,

9-(2,5-Dihydroxyphenyl)-10-phenylanthracene

(11). From

133.6,131.5, 131.4, 130.0, 129.9, 128.7, 128.4, 127.4, 127.1, 654 mg (1.68 mmol) of 9-(2,5-dimethoxyphenyl)-10-phenylan-

126.9, 125.0, 124.96, 118.3, 114.3, 112.8, 56.6, 55.8; IR (CHCl₃, thracene (15) and a solution of BBr₃, the reaction gave the

cm⁻¹) 3055, 2933, 2825, 1495, 1428, 1269, 1222, 1045, 766; MS crude product which was purified by flash column chromato-

(ESI) m/z 391 (M⁺+ 1), 390 (M⁺); HRMS, calcd for C₂₈H₂₂O₂: graphy on silica gel (EtOAc–hexane gradient, 20% to 30%) and

391.16925 (M⁺+ H); Found 391.17047.

recrystallized from methylene chloride and hexanes to aﬀord

516 mg (85%) of the product 11 as a colorless crystal, mp

220–221 °C; H NMR (500 MHz, CDCl₃) δ 7.74 (d, J = 8.6 Hz, 2

General procedure for BBr₃demethylation reaction

1

To a solution of the methoxy compound in CH₂Cl₂(20 mL) at H, H-1/H-4), 7.70 (d, J = 8.3 Hz, 2 H, H-1/H-4), 7.62–7.53 (m, 3

0 °C was added 5 equiv. of BBr₃(1 M in CH₂Cl₂) drop-wise H, H-6 and H-7), 7.47–7.45 (m, 2 H, H-5), 7.41–7.33 (m, 4 H,

under N₂. The ice bath was removed and the reaction mixture H-2 and H-3), 7.06 (d, J = 8.8 Hz, 1 H, H-10), 6.97 (dd, J = 8.8,

was stirred at room temperature for an additional 3 h under 3.2 Hz, 1 H, H-9), 6.80 (d, J = 3.2 Hz, 1 H, H-8), 4.65 (s, 1 H,

N₂. The reaction mixture was quenched by CH₃OH (10 mL) OH), 4.29 (s, 1 H, OH); ¹³C NMR (125 MHz, CDCl₃) δ 149.3,

and stirred for 10 min. Followed by addition of water, the 148.0, 138.7, 138.6, 131.2, 131.1, 130.3, 130.1, 129.4, 128.5,

mixture was extracted with CH₂Cl₂(3 × 10 mL) and the com- 127.7, 127.3, 126.1, 126.05, 125.4, 125.1, 118.5, 116.8, 116.6; IR

bined organic layers were dried over anhydrous MgSO₄. After (CHCl₃, cm⁻¹) 3670, 3619, 3470, 3356, 1653, 1456, 1198, 913,

filtration, the solvent was removed on a rotary evaporator. The 748; MS (ESI) m/z 361(M⁺− H); HRMS, calcd for C₂₀H₁₄O₂:

crude residue was then purified by flash column chromato- 361.12341 (M⁺− H); Found 361.12274.

graphy on silica gel using 0–10% of CH₂Cl₂in ethyl acetate as

an eluent and further purified by crystallization from methyl-

General procedure for preparative photolyses

ene chloride and n-hexanes.

All preparative photolyses were carried out in a Rayonet photo-

1-(2,5-Dihydroxyphenyl)naphthalene (9).^30bFrom 640 mg reactor at 300 and 350 nm (16 lamps). 100 mL of sample solu-

(2.42 mmol) of 1-(2,5-dimethoxyphenyl)naphthalene (12) and tions (c ∼ 10⁻³M) were prepared in a quartz tube and purged

a solution of BBr₃, the reaction gave the crude product that with argon for 20 min prior to and continuously during the

This journal is © The Royal Society of Chemistry and Owner Societies 2013

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

irradiation. Cooling was achieved by a tap water cold finger. C-3, C-1, C-4; H-2 and C-4, C-6, C-7, C-5; H-6 and C-4, C-2; OH

After irradiation, 100 mL of water was added and extracted by and C-2, C-6, C-1.

CH₂Cl₂(3 × 50 mL). The combined organic layers were dried

NOE interactions: H-13 and H-12; H-5 and H-6; H-10 and

over anhydrous MgSO₄and the solvent was removed under H-11; H-8 and H-9; H-2 and H-8.

reduced pressure. If photolyses were carried out in a neat

Photolysis of 9 in 1 : 9 H₂O–CH₃CN. A solution of 30 mg

organic solvent, the extraction step was not performed. The (0.127 mmol) of 9 in 100 mL of 5% H₂O–CH₃CN was irradiated

residue was then purified if necessary by preparative TLC at 300 nm for 1 h under argon purge. The crude residue was

plates and then characterized by NMR and MS.

analyzed by ¹H NMR which showed a mixture of starting

Photolysis of 1-(2,5-dihydroxyphenyl)naphthalene (9) in material 9 and single photoproduct 19 (∼71% conversion). The

CH₃CN. A solution of 30 mg (0.127 mmol) of 9 in 100 mL of crude residue was chromatographed on a thin layer of silica

neat CH₃CN was irradiated for 10 h at 300 nm under argon using 35% ethyl acetate in hexanes as an eluent to aﬀord

purge. The crude residue was chromatographed on a thin layer photoproduct 19 as a single product.

of silica using 25% ethyl acetate in hexanes as an eluent to

aﬀord the following photoproducts:

6,6a-Dihydrobenzo[kl]xanthen-10-ol (19). 16 mg (53%); pale

yellow oil; ¹H NMR (500 MHz, CDCl₃) δ 7.48 (d, J = 7.5 Hz,

1 H), 7.33 (t, J = 7.7 Hz, 1 H), 7.22 (d, J = 2.9, 1 H), 7.18 (t, J = 7.7

Hz, 1 H), 6.24–6.18 (m, 2 H), 5.31–5.29 (m, 1 H), 4.56 (s, 1 H,

OH), 3.47–3.37 (m, 2 H); ¹³C NMR (125 MHz, CDCl₃) δ 150.6

(s), 149.6 (s), 132.0 (s), 130.3 (s), 128.6 (s), 128.0 (d), 127.4 (d),

127.3 (d), 125.3 (d), 124.8 (s), 120.3 (d), 118.3 (d), 116.3 (d),

109.9 (d), 69.7 (d), 29.6 (t). IR (CHCl₃, cm⁻¹) 3651, 3620, 3046,

2360, 1650, 1488, 1203, 745.

Photolysis of 9 in 1 : 9 D₂O–CH₃CN. A solution of 15 mg

(0.064 mmol) of 9 in 100 mL of 1 : 9 D₂O–CH₃CN was irra-

diated at 300 nm for 1 h under argon purge. The crude residue

was analyzed by ¹H NMR, which showed a mixture of 9-2′D

and 19-7′D in a 48 : 52 ratio. The area of the peak corres-

ponding to the 2′-position of 9 (δ 7.91) was reduced by 60%,

indicating a 40 : 60 ratio of 9 to 9-2′D present in the sample.

Runs at diﬀerent D₂O concentrations were performed in the

same way. Results from these runs are shown in Fig. 3.

2-(2,5-Dihydroxyphenyl)naphthalene (16).^30b18 mg (60%);

colorless crystals; mp: 171–173 °C (Lit.^30bmp = 173 °C); ¹H

NMR (500 MHz, CD₃CN) δ 7.98 (d, J = 1.5 Hz, 1 H, H-1),

7.90–7.88 (m, 3 H, H-4, H-5 and H-8), 7.66 (dd, J = 8.7, 1.5 Hz,

1 H, H-3), 7.51–7.49 (m, 2 H, H-6 and H-7), 6.83 (d, J = 3.1 Hz,

1 H, H-14), 6.79 (d, J = 8.7 Hz, 1 H, H-11), 6.68 (dd, J = 8.7, 3.1

Hz, 1 H, H-12), 6.55 (s, 1 H, OHb), 6.42 (s, 1 H, OHa); ¹³C NMR

(125 MHz, CD₃CN) δ 150.7 (s, C-13), 147.2 (s, C-10), 136.3 (s,

C-2), 133.6 (s, C-15), 132.6 (s, C-16), 129.2 (s, C-9), 128.1 (d,

C-8), 127.9 (d, C-3/1) 127.7 (d, C-5/4), 127.6 (d, C-5/4), 126.3 (d,

C-6/7), 126.1 (d, C-6/7), 117.2 (d, C-11/14), 117.1 (d, C-11/14),

115.5 (d, C-12).

Photolysis of 10 in CH₃CN. A solution of 30 mg

(0.105 mmol) of 10 in 100 mL of neat CH₃CN was irradiated

for 2 h at 300 nm under argon purge. The crude residue was

HMBC interactions: H-1 and C-3, C-9, C-16; H-3 and C-1,

C-16; H4/5/8 and C-6/7, C-4/5, C-16, C-15, C-2; H-6 and C-8,

C-16; H-7 and C-5, C-15; H-14 and C-1, C-2, C-10; H-11 and

C-9, C-10, C-13; H-12 and C-14, C-10; OHb and C-13, C-14;

OHa and C-9, C-11, C-10.

1

analyzed by H NMR which showed a mixture of 10, 20 (∼8%

conversion), 21 (<2%) and a trace amount of phenanthrene.

Preparative TLC (25% ethyl acetate in hexanes) was used to

isolate minor product 21, whose structure was confirmed by

NOE interactions: H-1 and H-8; H-1 and H-14; H-3 and H-14.

1

comparison to the H NMR of the authentic material reported

in the literature.³²

Photolysis of 10 in CH₃OH. A solution of 30 mg

(0.105 mmol) of 10 in 100 mL of neat CH₃OH was irradiated

for 3 h at 300 nm under argon purge. The crude residue was

Naphtho[1,2-b]benzofuran-8-ol (17).³¹3 mg (9%); yellow chromatographed on a thin layer of silica using 40% ethyl

solid; ¹H NMR (500 MHz, CDCl₃) δ 8.41 (d, J = 8.2 Hz, 1 H, acetate in hexanes as an eluent to aﬀord 24 as a single

H-13), 7.96 (d, J = 8.2 Hz, 1 H, H-10), 7.91 (d, J = 8.6, 1 H, H-8), photoproduct:

7.73 (d, J = 8.6 Hz, 1 H, H-9), 7.62 (t, J = 7.8 Hz, 1 H, H-12),

2-(10-Methoxy-9,10-dihydro-phenanthren-9-yl)-benzene-1,4-

7.56–7.53 (m, 2 H, H-5 and H-11), 7.40 (d, J = 2.4 Hz, 1 H, H-2), diol (24). 23 mg (68%); light yellow oil; ¹H NMR (500 MHz,

6.96 (dd, J = 8.7 Hz, 2.4 Hz, 1 H, H-6), 4.86 (s, 1 H, OH); ¹³C CDCl₃) δ 7.92 (d, J = 7.6 Hz, 1 H), 7.89 (d, J = 7.8 Hz, 1 H), 7.42

NMR (125 MHz, CDCl₃) δ 153.2 (s, C-16), 151.8 (s, C-1), 151.1 (dt, J = 7.6, 1.6, 1 H), 7.39 (dt, J = 7.6, 1.6 Hz, 1 H), 7.30–7.18

(s, C-4), 133.3 (s, C-15), 128.6 (d, C-10), 126.6 (d, C-12), 126.4 (m, 4 H), 6.64 (d, J = 8.5 Hz, 1 H), 6.48 (dd, J = 8.5, 3.1 Hz,

(d, C-11), 126.1 (s, C-3), 124.1 (d, C-9), 123.2 (s, C-14), 121.7 (d, 1 H), 5.84 (d, J = 3.1 Hz, 1 H), 4.88 (d, J = 3.3 Hz, 1 H), 4.75 (s,

C-13), 121.2 (s, C-7), 119.3 (d, C-8), 118.7 (d, C-8), 114.6 (d, 1 H, OH), 4.50 (d, J = 3.3 Hz, 1 H), 4.12 (s, 1 H, OH), 3.34 (s, 3 H,

C-6), 112.5 (d, C-5), 106.0 (d, C-2).

OMe); ¹³C NMR (125 MHz, CDCl₃) δ 149.5 (s), 147.3 (s), 134.1

HMBC interactions: H-13 and C-11, C-15, C-16; H-10 and (s), 133.1 (s), 130.6 (d), 130.5 (d), 129.4 (d), 128.7 (d), 128.0 (d),

C-13, C-9, C-12; H-8 and C-15, C-3, C-9, C-7; H-9 and C-7, C-14, 127.6 (d, C-16), 124.2 (d), 123.8 (d), 116.8 (d), 116.4 (d), 114.3

C-10, C-15; H-12 and C-14, C-10; H-11 and C-13, C-15; H-5 and (d), 81.1 (d, C-10), 77.4 (d), 56.8 (q, OMe), 44.1 (d, C-1).

Photochem. Photobiol. Sci.

View Article Online

Photochemical & Photobiological Sciences

Paper

Photolysis of 10 in 1 : 1 H₂O–CH₃CN. A solution of 30 mg

NOE interactions: H-7 and H-1; H-7 and H-8; H-3 and H-4;

(0.105 mmol) of 10 in 100 mL of 1 : 1 H₂O–CH₃CN was irra- H-10.

diated for 2 h at 300 nm under argon purge. The crude residue

Photolyses of 12, 13 and 15 in CH₃CN. All the methyl ether

was analyzed by ¹H NMR which showed a mixture of 10, 22 precursors 12, 13 and 15 were irradiated under similar con-

(∼21% conversion), and a trace amount of 23. Preparative TLC ditions, no observable photoproducts were formed, suggesting

(35% ethyl acetate in hexanes) was used to isolate 23 as the that all the above photoreactions require the phenolic OH to

only photoproduct, whose structure was confirmed by com- proceed through proton transfer for the indicated reactions.

1

parison to the H NMR of the authentic material reported in

Quantum yields

the literature.³²

Naphtho[1,2,3-kl]xanthen-11-ol (23). ¹H NMR (500 MHz, Quantum yields of deuterium incorporation (Φ_ex) and (Φ_cyc

)

CDCl₃) δ 7.49–7.47 (m, 2 H), 7.35–7.33 (m, 2 H), 7.25–7.21 (m, for 9 and 10 were measured by comparison with the deuterium

4 H), 6.67 (d, J = 3.0 Hz, 1 H), 6.49 (dd, J = 8.6, 3.0 Hz, 1 H), exchange and cyclization of 1 as a secondary actinometer (Φ_ex

6.45 (d, J = 8.6 Hz, 1 H), 5.86 (s, 1 H), 4.47 (s, 1 H), 4.33 (s, 1 H, = 0.14; Φ_cyc= 0.20). A known amount of 9 and 10 was dissolved

OH).

in 10% D₂O–CH₃CN (50 mL), neat CH₃CN, or CH₃OH in a

Photolysis of 10 in 1 : 9 D₂O–CH₃CN. A solution of 5 mg of quartz tube and irradiated long enough to give a low conver-

10 in 50 mL of 1 : 9 D₂O–CH₃CN was prepared and each was sion (5 min for 9; 1 min for 10) in a Rayonet reactor at 300 nm

irradiated at 300 nm (8 lamps, 16 lamps) for 1 min to 5 min. (8 or 16 lamps). After irradiation, 50 mL of H₂O was added and

Deuterium incorporation into the substrate was observed by extractions with CH₂Cl₂(3 × 50 mL) were carried out. The

¹H NMR on the basis of comparison to the spectrum of an extracts were dried over anhydrous MgSO₄. In order to obtain

authentic material ¹H NMR spectrum.

Photolysis of 11 in CH₃CN. A solution of 30 mg actinometer 2-phenyl-1-naphthol (31) (Φ_ex

more accurate quantum yield for DHPA 10, another secondary

0.7)¹⁷was

=

(0.083 mmol) of 11 in 100 mL of neat CH₃CN was irradiated for employed. An equimolar amount of 1 or 31 was irradiated

1 h at 350 nm under argon purge. The crude residue was chro- under the same conditions. Each run, the system was main-

matographed on a thin layer of silica using 25% ethyl acetate in tained at ∼15 °C by an internal cooling finger and purged with

hexanes as an eluent to aﬀord 28 as a single product.

argon for 15 min prior to irradiation. After aqueous workup,

extraction and removal of the solvent, extent of the deuterium

incorporation or corresponding photochemical reactions were

determined by ¹H NMR. Integration from ¹H NMR was used to

determine the relative conversion of deuterium exchanges,

cyclization or corresponding photochemical reactions. All the

values are the results from at least three independent

irradiations. Alternatively, quantum yields for photochemical

cyclization of 11 were calculated by comparison with the deu-

terium exchange of 9-(2-hydroxyphenyl)anthracene as a sec-

6-Phenyl-6,11-dihydro-6,11-[1,2]benzenodibenzo[b,e]oxepin- ondary actinometer (Φ_ex

=

0.09).^16aFollowing the same

2-ol (28). 28 mg (95%); oﬀ-white solid; mp: 168–170 °C; ¹H procedures above, the irradiations were operated in a Rayonet

NMR (500 MHz, CDCl₃) δ 8.36 (d, J = 7.7 Hz, 1 H, H-14), 7.64 (t, reactor at 350 nm (16 lamps).

J = 7.7 Hz, 1 H, H-15), 7.48 (tt, J = 7.4, 1.3, 1 H, H-16), 7.42 (t,

UV-vis studies

J = 7.4 Hz, 1 H, H-17), 7.39 (dd, J = 7.4, 1.0 Hz, 2 H, H-8), 7.22

(td, J = 7.7, 1.3 Hz, 2 H, H-9), 7.10 (td, J = 7.7 Hz, 1.3 Hz, 3 H, Solutions (10⁻⁵M) in the indicated solvent system were pre-

H-10, H-18), 6.84 (d, J = 7.7 Hz, 2 H, H-11), 6.71 (d, J = 2.9 Hz, pared in 1.0 cm quartz cuvettes (3.0 mL) and purged with a

1 H, H-1), 6.65 (d, J = 8.6 Hz, 1 H, H-4), 6.53 (dd, J = 8.6, 2.9 stream of nitrogen gas by a syringe needle for 5 min prior to

Hz, 1 H, H-3), 4.61 (s, 1 H, H-7), 4.43 (s, 1 H, OH); ¹³C NMR measurements. Irradiations were carried out in a Rayonet 100

(125 MHz, CDCl₃) δ 149.0 (s, C-6), 146.6 (s, C-5), 143.4 (s, photoreactor at 300 or 350 nm (16 lamps). The cooling system

C-19), 139.4 (s, C-13), 137.8 (s, C-20), 129.8 (d, C-18), 128.4 (d, was achieved by fan. The irradiation was stopped at regular

C-9), 128.3 (d, C-15), 127.8 (d, C-11), 127.6 (d, C-16), 127.4 (d, time intervals and each UV-vis trace was recorded.

C-14), 126.9 (d, C-10 and C-17), 124.1 (d, C-8), 120.7 (d, C-4),

Steady state and time-resolved fluorescence measurements

115.5 (d, C-3), 113.5 (d, C-1), 81.9 (s, C-12), 52.3 (d, C-7); IR

(CHCl₃, cm⁻¹) 3381, 3075, 3046, 2360, 1650, 1491, 1437, 1203, Fluorescence spectra were not corrected for spectral response.

735; MS (ESI) HRMS, calcd for C₂₀H₁₄O₂: 361.12341 (M⁺− H); Since only relative changes were studied based on fluorescence

Found 361.12264.

emission from known chromophores, uncorrected spectra

HMBC interactions: H-7 and C-5; H-3 and C-1, C-5; H-4 and suﬃced for the mechanistic studies in this work. Steady-state

C-5, C-6; H-1 and C-7, C-3, C-5; H-11 and C-12, C-9; H-10 and fluorescence measurements were conducted on a Photon Tech-

C-8, C-20; H-18 and C-16; H-9 and C-19, C-8, C-11; H-8 and nology International (PTI) Quanta-Master luminescence

C-20, C-10; H-17 and C-13, C-15; H-16 and C-18, C-14; H-15 spectrometer. All the samples dissolved in CH₃CN with OD <

and C-17, C-13; H-14 and C-18.

0.1 at the excitation wavelength (310, 320, 330 or 340) were

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

purged with nitrogen for 15 min prior to analysis. The

measurements were performed at 20 °C. Fluorescence

quantum yields were determined by comparison of the inte-

gration of the emission band with that of quinine bisulfate (Φ_f

= 0.55 in 1.0 N H₂SO₄),²³followed by correction for the diﬀer-

ences in the refractive index. Measurements were recorded at

three diﬀerent excitation wavelengths and three quantum

yields were calculated and the mean value was reported. Fluo-

rescence lifetimes (τ_f) were measured on an instrument with

light emitting diode (excitation wavelength = 310 nm), utilizing

a time-correlated single photon counting technique in 1023

channels. Histograms of the instrument response functions

using LUDOX scatterer and sample decays were recorded until

they typically reached 3 × 10³counts in the peak channel.

excited state intramolecular proton transfer (ESIPT) in

o-hydroxyphenyl-(1,3,5)-triazine, J. Phys. Chem. A, 2005, 109,

7527–7537; (b) T. P. Smith, K. A. Zaklika, K. Thakur,

G. C. Walker, K. Tominaga and P. F. Barbara, Ultrafast

studies on proton transfer in photostabilizers, J. Photochem.

Photobiol., A, 1992, 65, 165–175.

4 (a) D. Y. Han, J. M. Kim, J. Kim, H. S. Jung, Y. H. Lee,

J. F. Zhang and J. S. Kim, ESIPT-based anthraquinonylcalix-

[4]crown chemosensor for In³⁺, Tetrahedron Lett., 2010, 51,

1947–1951; (b) F. F. Oliveira, D. C. Santos, A. A. Lapis,

J. R. Correa, A. F. Gomes, F. C. Gozzo, C. Fabio,

P. F. Moreira, V. C. de Oliveira, F. H. Quina and B. A. Neto,

On the use of 2,1,3-benzothiadiazole derivatives as selective

live cell fluorescence imaging probes, Bioorg. Med. Chem.

Lett., 2010, 20, 6001–6007.

Laser flash photolysis (LFP)

5 (a) J. M. Kauﬀman, Review of progress on scintillator fluors

for the detectors of the SSC, Radiat. Phys. Chem., 1993, 41,

365–371; (b) A. Pla-Dalmau, 2-(2′-Hydroxyphenyl)benzothia-

zoles, benzoxazoles, and benzimidazoles for plastic scintil-

lation applications, J. Org. Chem., 1995, 60, 5468–5473.

6 (a) F. Vollmer and W. Rettig, Fluorescence loss mechanism

due to large-amplitude motions in derivatives of 2,2′-bipyri-

dyl exhibiting excited-state intramolecular proton transfer

and perspectives of luminescence solar concentrators,

J. Photochem. Photobiol., A, 1996, 95, 143–155; (b) D.-Y. Chen,

C.-L. Chen, Y.-M. Cheng, C.-H. Lai, J.-Y. Yu, B.-S. Chen,

C.-C. Hsieh, H.-C. Chen, L.-Y. Chen, C.-Y. Wei, C.-C. Wu

and P.-T. Chou, Design and synthesis of trithiophene-

bound excited-state intramolecular proton transfer dye:

enhancement on the performance of bulk heterojunction

solar cells, ACS Appl. Mater. Interfaces, 2010, 2, 1621–1629.

7 (a) K. Sakai, M. Takemura and Y. Kawabe, Lead chloride-

based layered perovskite incorporated with an excited state

intramolecular proton transfer dye, J. Lumin., 2010, 130,

2505–2507; (b) M. Kasha, Proton-transfer spectroscopy and

proton-transfer lasers, Acta Phys. Pol., A, 1987, 71, 717–729;

(c) S. Park, O.-H. Kwon, S. Kim, S. Park, M.-G. Choi,

M. Cha, S. Y. Park and D.-J. Jang, Imidazole-based excited-

state intramolecular proton-transfer materials: synthesis

and amplified spontaneous emission from a large single

crystal, J. Am. Chem. Soc., 2005, 127, 10070–10074;

(d) D. A. Parthenopoulos, D. P. McMorrow and M. Kasha,

Comparative study of stimulated proton transfer lumines-

cence of three chromones, J. Phys. Chem., 1991, 95, 2668–

2674; (e) A. Douhal, F. Amat-Guerri, A. U. Acuňa and

K. Yoshihara, Picosecond vibrational relaxation in the

excited-state proton-transfer of 2-(3′-hydroxy-2′-naphthyl)

benz-imidazole, Chem. Phys. Lett., 1994, 217, 619–625.

8 B. Valeur, Molecular Fluorescence: Principles and Appli-

cations, Wiley-VCH Verlag GmbH, Weinheim, 2001, ch. 4,

pp. 99–106.

All LFP studies were conducted employing a YAG laser, with a

pulse width of 10 ns and an excitation wavelength of 266 nm.

Static cells (0.7 cm) were used and solutions were purged with

N₂and O₂for 20 min prior to the measurements. Absorbances

at 266 nm were ∼0.4.

Acknowledgements

We thank the Natural Sciences and Engineering Research

Council (NSERC) of Canada and the University of Victoria for

financial support.

Notes and references

1 (a) J. F. Ireland and P. A. H. Wyatt, Acid–base properties of

electronically excited states of organic molecules, Adv. Phys.

Org. Chem., 1976, 12, 131–221; (b) W. Klöpﬀer, Adv. Photo-

chem., 1977, 10, 311; (c) L. G. Arnaut and S. J. Formosinho,

Excited-state proton transfer reactions I. Fundamentals and

intermolecular reactions, J. Photochem. Photobiol., A, 1993,

75, 1–20; (d) S. J. Formosinho and L. G. Arnaut, Excited-

state proton transfer reactions II. Intramolecular reactions,

J. Photochem. Photobiol., A, 1993, 75, 21–48;

(e) S. M. Ormson and R. G. Brown, Excited state intramole-

cular proton transfer part 1: ESIPT to nitrogen, Prog. React.

Kinet., 1994, 19, 45–91; (f) D. Le Gourriérec, S. M. Ormson

and R. G. Brown, Excited state intramolecular proton trans-

fer part 2: ESIPT to oxygen, Prog. React. Kinet., 1994, 19,

211–275; (g) N. Agmon, Elementary steps in excited state

proton transfer, J. Phys. Chem. A, 2005, 109, 13–35;

(h) Hydrogen-transfer Reactions, ed. J. T. Hynes,

J. P. Klinman, H.-H. Limbach and R. L. Schowen, Wiley-

VCH, Weinheim, 2007.

2 A. Z. Weller, Innermolekularer protonenübergang im

angeregten zustand, Z. Elektrochem., 1956, 60, 1144–1147.

3 (a) M. J. Paterson, M. A. Robb, L. Blancafort and

9 M. Kasha, Proton-transfer spectroscopy: perturbation of the

tautomeriztion potential, J. Chem. Soc., Faraday Trans. 2,

1986, 82, 2379–2392.

A. D. DeBellis, Mechanism of an exceptional class of photo- 10 J. Wirz, Kinetics of proton transfer reactions involving

stabilizers: a seam of conical intersection parallel to

carbon, Pure Appl. Chem., 1998, 70, 2221–2232.

Photochem. Photobiol. Sci.

View Article Online

Photochemical & Photobiological Sciences

Paper

11 (a) N. A. O’Connor, A. J. Berro, J. R. Lancaster, X. Gu,

closure of quinone methides photogenerated from biaryl

S. Jockusch, T. Nagai, T. Ogata, S. Lee, P. Zimmerman,

systems, Can. J. Chem., 2005, 83, 1306–1323.

C. G. Willson and N. J. Turro, Toward the design of a 17 N. Basarić, N. Došlić, J. Ivković, Y.-H. Wang, M. Mališ and

sequential two photon photoacid generator for double

exposure photolithography, Chem. Mater., 2008, 20, 7374–

7376; (b) R. M. D. Nunes, M. Pineiro and L. G. Arnaut,

P. Wan, Very eﬃcient generation of quinone methides

through excited state intramolecular proton transfer to a

carbon atom, Chem.–Eur. J., 2012, 18, 10617–10623.

Photoacid for extremely long-lived and reversible pH- 18 Y.-H. Wang and P. Wan, Excited state intramolecular

jumps, J. Am. Chem. Soc., 2009, 131, 9456–9462;

(c) J. E. Kwon and S. Y. Park, Advanced organic opto-

proton transfer (ESIPT) in dihydroxyphenyl anthracenes,

Photochem. Photobiol. Sci., 2011, 10, 1934–1944.

electronic materials: harnessing excited-state intramolecu- 19 M. Lukeman, Excited state intramolecular proton transfer

lar proton transfer (ESIPT) process, Adv. Mater., 2011, 23,

3615–3642.

(ESIPT) to aromatic carbon, PhD Dissertation, University of

Victoria, Victoria B.C., Canada.

12 (a) H. Shizuka, Excited state proton-transfer reaction and 20 (a) I. A. Z. Al-ansari, Intramolecular charge transfer states

proton-induced quenching of aromatic compounds, Acc.

Chem. Res., 1985, 18, 141–147; (b) L. M. Tolbert and

K. M. Solntsev, Excited-state proton transfer: from con-

strained systems to “super” photoacids to superfast proton

transfer, Acc. Chem. Res., 2002, 35, 19–27; (c) T. Förster,

Pure Appl. Chem., 1970, 24, 443; (d) P. Wan and D. Shukla,

in a series of (2,5-dimethoxyphenyl)-arenes, J. Photochem.

Photobiol., A, 1993, 72, 15–21; (b) J. Yin, M. P. Rainka,

X.-X. Zhang and S. L. Buchwald, A highly active Suzuki cata-

lyst for the synthesis of sterically hindered biaryls: novel

ligand coordination, J. Am. Chem. Soc., 2002, 124, 1162–

1163.

Chem. Rev., 1993, 93, 571–584; (e) M. Lukeman and P. Wan, 21 (a) R. Krishnan, T. G. Fillingim, J. Lee and G. W. Robinson,

in Handbook of organic photochemistry and photobiology, ed.

W. Horspool and F. Lenci, CRC Press, Boca Raton, FL, 2nd

edn, 2004, ch. 39, pp. 1–19; (f) P. Wan, B. Barker, L. Diao,

M. Fischer, Y. Shi and C. Yang, 1995 Merck Frosst Award

Lecture. Quinone methides: relevant intermediates in

organic chemistry, Can. J. Chem., 1996, 74, 465–475.

Solvent structural eﬀects on proton dissociation, J. Am.

Chem. Soc., 1990, 112, 1353–1357; (b) G. W. Robinson,

Proton charge transfer involving the water solvent, J. Phys.

Chem., 1991, 95, 10386–10391; (c) N. Agmon, D. Huppert,

A. Masad and E. Pines, Excited-state proton transfer

to methanol–water mixtures, J. Phys. Chem., 1991, 95,

10407–10413; (d) K. M. Solntsev, D. Huppert, N. Agmon

and L. M. Tolbert, Photochemistry of “super”

photoacids. 2. Excited-state proton transfer in methanol/

water mixtures, J. Phys. Chem. A, 2000, 104, 4658–4669.

13 (a) P. Wan, D. W. Brousmiche, C. Z. Chen, J. Cole,

M. Lukeman and M. Xu, Quinone methide intermediates

in organic photochemistry, Pure Appl. Chem., 2001, 73,

529–534; (b) M. Lukeman and P. Wan, Excited state intra-

molecular proton transfer (ESIPT) in 2-phenylphenol: an 22 (a) K. M. Solntsev, D. Huppert, L. M. Tolbert and

example of proton transfer to a carbon of an aromatic ring,

Chem. Commun., 2001, 1004–1005; (c) M. Lukeman and

P. Wan, A new type of excited-state intramolecular proton

transfer: proton transfer from phenol OH to a carbon atom

of an aromatic ring observed for 2-phenylphenol, J. Am.

Chem. Soc., 2002, 124, 9458–9464; (d) N. Basarić and

P. Wan, Excited state proton transfer (ESPT) from phenol to

N. Agmon, Solvatochromic shifts of “Super” photoacids,

J. Am. Chem. Soc., 1998, 120, 7981–7982; (b) M. Fischer and

P. Wan, Nonlinear solvent water eﬀects in the excited-state

(formal) intramolecular proton transfer (ESIPT) in m-

hydroxy-1,1-diaryl alkenes: eﬃcient formation of m-

quinone methides, J. Am. Chem. Soc., 1999, 121, 4555–

4562.

nitrogen and carbon in (2-hydroxyphenyl)pyridines, Photo- 23 I. B. Berlman, in Handbook of fluorescence spectra of aro-

chem. Photobiol. Sci., 2006, 5, 656–664. matic molecules, Academic Press, New York, 1971, p. 363.

14 M. Lukeman and P. Wan, Excited-state intramolecular 24 N. Basarić, N. Došlić, J. Ivković, Y.-H. Wang, J. Veljković,

proton transfer in o-hydroxybiaryls: a new route to dihy-

droaromatic compounds, J. Am. Chem. Soc., 2003, 125,

1164–1165.

K. Mlinarić-Majerski and P. Wan, Excited state intramole-

cular proton transfer (ESIPT) from phenol to carbon in

selected phenylnaphthols and naphthylphenols, J. Org.

Chem., 2013, 78, 1811–1823.

15 M. Flegel, M. Lukeman and P. Wan, Photochemistry of 1,1′-

bi-2-naphthol (BINOL)-ESIPT is responsible for photorace- 25 (a) L. M. Tolbert and J. Haubrich, Photoexcited proton

mization and photocyclization, Can. J. Chem., 2008, 86,

161–169.

16 (a) M. Flegel, M. Lukeman, L. Huck and P. Wan, Photoaddi-

tion of water and alcohols to the anthracene moiety of 9-

(2′-hydroxyphenyl) anthracene via formal excited state

intramolecular proton transfer, J. Am. Chem. Soc., 2004,

transfer from enhanced photoacids, J. Am. Chem. Soc.,

1994, 116, 10593–10600; (b) M. Fischer and P. Wan, m-

Quinone methides from m-hydroxy-1,1-diaryl alkenes via

excited-state (formal) intramolecular proton transfer

mediated by a water trimer, J. Am. Chem. Soc., 1998, 120,

2680–2681.

126, 7890–7897; (b) N. Basarić and P. Wan, Competing 26 (a) D. Brousmiche, M. Xu, M. Lukeman and P. Wan, Photo-

excited state intramolecular proton transfer pathways from

phenol to anthracene moieties, J. Org. Chem., 2006, 71,

2677–2686; (c) Y. Shi and P. Wan, Solvolysis and ring

hydration and photosolvolysis of biphenyl alkenes and

alcohols via biphenyl quinone methide-type intermediates

and diarylmethyl carbocations, J. Am. Chem. Soc., 2003,

Photochem. Photobiol. Sci.

View Article Online

Paper

Photochemical & Photobiological Sciences

125, 12961–12970; (b) Y. Shi and P. Wan, Charge polariz- 29 (a) G. Hoﬀmann, A. Schoenbucher and H. Steidl, Fluo-

ation in photoexcited alkoxy-substituted biphenyls: for-

mation of biphenyl quinone methides, J. Chem. Soc., Chem.

Commun., 1995, 1217–1218; (c) N. Basarić, I. Žabčić,

K. Mlinarić-Majerski and P. Wan, Photochemical formation

and chemistry of long-lived adamantylidene-quinone

methides and 2-adamantyl cations, J. Org. Chem., 2010, 75,

102–116.

rescence yields of triphenylcarbonium derivatives, Z. Natur-

forsch., A, 1973, 28, 1136–1139; (b) A. Azarani,

A. B. Berinstain, L. J. Johnston and S. Kazanis, Electron

transfer reactions between excited diarylmethyl and triaryl-

methyl carbocations and aromatic donors, J. Photochem.

Photobiol., A, 1991, 57, 175–189; (c) M. H. Acar, Y. Yagci and

W. Schnabel, Formation of triphenylmethyl cation during

the photolysis of phenylazotriphenyl methane: a laser flash

photolysis study, Polym. Int., 1998, 46, 331–335.

27 N. Basarić, N. Cindro, D. Bobinac, L. Uzelac, K. Mlinarić-

Majerski and P. Wan, Zwitterionic biphenyl quinone

methides in photodehydration reactions of 3-hydroxybiphe- 30 (a) H. Kawafuchi and T. Inokuchi, Novel access to cyclo-

nyl derivatives: laser flash photolysis and antiproliferation

study, Photochem. Photobiol. Sci., 2012, 11, 381–396.

hexane-1,4-diones and 1,4-hydroquinones via radical 1,2-

acyl rearrangement on 2-(halomethyl)cyclopentane-1,3-

diones using cobaloxime-mediated electroreduction or tri-

butyltin hydride, Tetrahedron Lett., 2002, 43, 2051–2054;

(b) M.-E. Million, P. Mailliett, H. Chen, G. Bashiardes,

J. Boiziau, F. Parker, A. Commercon, B. Tocque,

B. P. Roques and C. Garbat, Synthesis and biological evalu-

ation of a new series of phenylhydroquinone derivatives as

inhibitors of EGF-R-associated PTK activity, Anti-Cancer

Drug Des., 1996, 11, 129–153.

28 (a) R. A. McClelland, C. Chan, F. L. Cozens, A. Modro and

S. Steenken, Laser flash photolysis generation, spectra, and

lifetimes of phenylcarbenium ions in trifluoroethanol and

hexafluoroisopropyl alcohol. On the UV spectrum of the

benzyl cation, Angew. Chem., Int. Ed. Engl., 1991, 30, 1337–

1339; (b) F. L. Cozens, V. M. Kanagasabapathy,

R. A. McClelland and S. Steenken, Lifetimes and UV-visible

absorption spectra of benzyl, phenethyl, and cumyl carbo-

cations and corresponding vinyl cations. A laser flash pho- 31 K. Schimmelschmidt, Synthesen in der Diphenylenoxyd-

tolysis study, Can. J. Chem., 1999, 77, 2069–2082; Reihe, Justus Liebigs Ann. Chem., 1950, 566, 184–206.

(c) R. A. McClelland, N. Banait and S. Steenken, Electrophi- 32 B. J. Morrison and O. C. Musgrave, Thiones as reactive

lic reactions of xanthylium carbocations produced by flash

photolysis of 9-xanthenols, J. Am. Chem. Soc., 1989, 111,

2929–2935.

intermediates in condensations of diketones with aro-

matics mediated by tetraphosphorus decasulfide, Phos-

phorus, Sulfur Silicon Relat. Elem., 2002, 177, 2725–2744.

Photochem. Photobiol. Sci.

Article Doi

Solvent-dependent excited state intramolecular proton transfer (ESIPT) pathways from phenol to carbon in 2,5-dihydroxyphenyl arenes

DOI: 10.1039/c3pp50091h

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c3pp50091h

Full text of DOI:10.1039/c3pp50091h

Products guided by the article

R&D Labs maybe for 178266-72-3

Relevant to this article

Hot Product