Time-Resolved Absorption Studies on the Photochromic Process of 2H-Benzopyrans in the Picosecond to Submillisecond Time Domain

Article abstract of DOI:10.1021/jp002032e

Picosecond to submillisecond photochromic reactions of 2,4-diphenyl-2H-benzopyran and 2,2,4-triphenyl-2H-benzopyran have been investigated by time-resolved absorption spectroscopy. The C-O bond cleavage of the benzopyrans (closed forms) occurs via the first excited singlet state within 2 ps to produce vibrationally excited open forms in the ground electronic state. In the subnanosecond to submillisecond time domain, several decay components with almost the same spectral profiles are observed. These components are assigned to respective stereoisomers with respect to two double bonds and one single bond of the open enone forms. From the pump-laser power dependencies of the yields of the open forms, it is suggested that the photocleavage gives at first only the open forms revertible to the closed form by a single-bond rotation, and that the photoexcitation of the first generated open forms gives rise to other open forms which need a double-bond rotation for reversion to the closed form. The photochromic reactions of a series of 2H-benzopyrans bearing substituents on the pyran ring have also been studied using nanosecond time-resolved absorption spectroscopy. The size of a substituent in the 4-position fairly affects the rate constants of the thermal reversion of the open form to the closed form.

Full text of DOI:10.1021/jp002032e

11478
J. Phys. Chem. A 2000, 104, 11478-11485
ARTICLES
Time-Resolved Absorption Studies on the Photochromic Process of 2H-Benzopyrans in the
Picosecond to Submillisecond Time Domain
Yoichi Kodama,†,‡ Takakazu Nakabayashi,‡ Katsunori Segawa,† Emi Hattori,†
Masako Sakuragi,§ Nobuyuki Nishi,*,‡ and Hirochika Sakuragi*,†
Department of Chemistry, UniVersity of Tsukuba, Tennodai, Tsukuba, 305-8571, Japan, Institute for Molecular
Science, Myodaiji, Okazaki 444-8585, Japan, and National Institute of Materials and Chemical Research,
Higashi, Tsukuba 305-8565, Japan
ReceiVed: June 5, 2000; In Final Form: August 17, 2000
Picosecond to submillisecond photochromic reactions of 2,4-diphenyl-2H-benzopyran and 2,2,4-triphenyl-
2H-benzopyran have been investigated by time-resolved absorption spectroscopy. The C-O bond cleavage
of the benzopyrans (closed forms) occurs via the first excited singlet state within 2 ps to produce vibrationally
excited open forms in the ground electronic state. In the subnanosecond to submillisecond time domain,
several decay components with almost the same spectral profiles are observed. These components are assigned
to respective stereoisomers with respect to two double bonds and one single bond of the open enone forms.
From the pump-laser power dependencies of the yields of the open forms, it is suggested that the photocleavage
gives at first only the open forms revertible to the closed form by a single-bond rotation, and that the
photoexcitation of the first generated open forms gives rise to other open forms which need a double-bond
rotation for reversion to the closed form. The photochromic reactions of a series of 2H-benzopyrans bearing
substituents on the pyran ring have also been studied using nanosecond time-resolved absorption spectroscopy.
The size of a substituent in the 4-position fairly affects the rate constants of the thermal reversion of the open
form to the closed form.
1. Introduction
compounds have not been clarified because of the difficulties
in assignment of the open forms involved. Their structural
features arise from the heteroatoms included and the complicated
molecular structure: (1) the open forms can take the ortho-
quinoidal, zwitterionic, and hybrid structures,15 and (2) the open
forms can take stereoisomeric configurations with respect to
the CdC double bonds.10
Photochromic materials, which undergo light-induced trans-
formation between two states exhibiting distinguishably different
absorption spectra, have received great attention in recent years
due to their potential applications in high-density optical storage
and optical switching.1-4 Among the most important classes of
organic photochromic materials, spiropyrans and their analogues
have been most extensively studied and best documented.1,2,5-19
Spiropyrans consist of a pyran moiety and another moiety
containing conjugated rings which are held orthogonal by a
common spiro-carbon atom. The photochromic process in the
spiropyran compounds features the dissociation of a C-O bond
to produce a distribution of isomeric open forms (merocyanines),
which are thermally and/or photochemically revertible to the
original closed form. Extensive conjugation of the π-electron
system in the open form leads to the appearance of the strong
absorption in the visible region where the closed form does not
absorb. The rate and mechanism of the reaction steps of some
spiropyrans and their analogues have been examined with
continuous absorption spectroscopy,15,17 transient absorption
spectroscopy,5,7,9-14,18,19 and transient Raman spectroscopy.6,8,11,18,19
Despite these efforts, the detailed dynamics of spiropyran
Since both the constituent π-electron systems do not interact
significantly in the spiro form, the absorption spectrum of the
spiropyran compound is approximately the superposition of the
spectra of the two individual moieties. The C-O bond is cleaved
by irradiation of UV light in the region where the pyran moiety
absorbs. Furthermore, it is known that only 2H-benzopyrans
(chromenes) and their analogues, which constitute the spiropyran
compounds as the pyran moiety, also show photochromic
behavior on UV irradiation.16,20-39 From the absorption spec-
troscopy at low temperatures, Becker et al. have indicated that
photoexcitation of the colorless benzopyran compounds proceeds
through the C-O bond cleavage, resulting in a distribution of
the colored open forms.20 The generated open forms were shown
to revert back to the original closed form either thermally or
photochemically.20 These results suggest that the photochromism
of benzopyrans can be treated as the simplified model of that
of spiropyrans. Some theoretical calculations have been carried
out for the ring-opening reactions of 2H-benzopyrans,16,25,32 in
order to ultimately understand the photochromism of spiropy-
rans. 2H-Benzopyran analogues have now been utilized as
* Corresponding authors. E-mail: nishi@ims.ac.jp; sakuragi@staff.chem.
tsukuba.ac.jp.
† University of Tsukuba.
‡ Institute for Molecular Science.
§ National Institute of Materials and Chemical Research.
10.1021/jp002032e CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/17/2000

Photochromic Process of 2H-Benzopyrans
J. Phys. Chem. A, Vol. 104, No. 49, 2000 11479
TABLE 1: Absorption Maxima of the Closed (λclosed) and Open Forms (λopen), Lifetimes (τopen), Activation Energy (Ea), and
Frequency Factors (A) for Disappearance of the Major Open Forms
a
λopen
compound
R1
R2
R3
R4
R5
λ
closed/nm
λA/nm
λB/nm
τopen
Ea/kJ mol-1
log(A/s-1
)
BP
4-MeBP
4-PhBP
2-PhBP
4-Me-2-PhBP
2,4-Ph2BP
2,2-Ph2BP
4-Me-2,2-Ph2BP
2,2,4-Ph3BP
2,2-Me2BP I
2,2-Me2BP II
2,2-Me2BP III
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Me
Me
Me
H
H
H
H
H
H
H
H
H
H
H
MeO
MeO
H
H
H
H
H
H
H
H
265 311
258 309
270 311
266 312
259 312
265 309
266 310
264 309
273 311
282 304
278 325
278 325
430
420
435
445
435
455
485
470
490
430
430
430
>1 ms
276 µs
194 µs
>1 ms
75.6 µs
48 µs
ndb
41.2
40.7
nd
35.8
34.3
nd
33.6
28.9
nd
nd
Me
Ph
H
Me
Ph
H
Me
Ph
H
10.8
10.9
nd
10.4
10.3
nd
10.3
10.0
nd
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
375 395
385 405
384 408
385 405
>1 ms
36 µs
H
12 µs
MeO
MeO
EtO
>1 ms
>1 ms
>1 ms
H
H
nd
nd
nd
nd
a λA and λB are absorption maxima of A and B bands, respectively. b nd stands for “not determined”.
ophthalmic glasses,33 and their photochromic reactions have also
been investigated for practical applications.28,31,33,38
7-Methoxy-2,2-dimethyl-2H-benzopyran (2,2-Me2BP I), 6,7-
dimethoxy-2,2-dimethyl-2H-benzopyran (2,2-Me2BP II), and
7-ethoxy-6-methoxy-2,2-dimethyl-2H-benzopyran (2,2-Me2BP
III) were commercially available from Aldrich. All experiments
were carried out in cyclohexane under argon at room temper-
ature (298 K). Sample solutions were prepared in concentrations
of 2 × 10-4 mol dm-3 in quartz cuvettes with 1 cm path length.
Solvent cyclohexane (Dojin, Spectrograde) was used as received.
Recently, Delbaere et al. have identified the structures of
geometrical isomers involved in the photochromism of 3H-
naphthopyrans with lifetimes in the second to minute range by
means of UV and NMR spectroscopy.36 However, very little is
known about the time developments of 2H-benzopyrans and
their analogues immediately after photoexcitation. Knowledge
on the dynamics in all the time-region involved is essential for
a detailed understanding of these photochromic reactions. In
the present paper, we report on the time-resolved absorption
study on the photoexcitation dynamics of 2,4-diphenyl-2H-
benzopyran (2,4-Ph2BP) and 2,2,4-triphenyl-2H-benzopyran
(2,2,4-Ph3BP) from picoseconds to submilliseconds. The dy-
namics of the ring-opening and the subsequent isomerization
of the open forms are discussed on the basis of the results
obtained. Nanosecond time-resolved absorption measurements
on the dynamics for the thermal ring closure of a series of
benzopyrans bearing substituents on the pyran ring are also
made, to reveal structural factors controlling the dynamics of
the thermal reversion of the open form to the closed form.
2.2. Nanosecond Laser-Flash Photolyses. Nanosecond laser-
flash photolyses were performed using an excimer laser
(Lambda Physik LPX-100, XeCl, 308 nm, 100 mJ/pulse, 20-
ns) and a pulsed xenon arc (Ushio UXL-150, 150 W) as a
monitoring light source. The monitoring beam obtained from
the xenon lamp was oriented perpendicularly to the exciting
laser beam, passed through a sample cell and a grating
monochromator (JASCO CT-25C), and detected with a photo-
multiplier (Hamamatsu Photonics R928). The amplified signal
was recorded as the time profile of a transmittance change on
a storage oscilloscope (Iwatsu TS-8123), and accumulated for
3-5 times to be averaged. The system was computer-controlled
and the decay curves were analyzed by this computer system.
2.3. Picosecond Laser-Flash Photolyses. A beam from a
frequency-doubled cw mode-locked Nd:YAG laser (Coherent
Antares 76S, wavelength 532 nm, repetition rate 76 MHz,
average power 1.5 W, pulse duration 70 ps) was used to excite
a dual jet hybridly mode-locked dye laser (Coherent 702-1).
Light pulses with 590 nm wavelength, 3 ps duration, and 1 nJ
pulse energy were obtained at a repetition rate of 76 MHz. The
gain dye and saturable absorber used were Rhodamine 6G and
DODCI, respectively. Pulses from the dye lasers passed through
a dye amplifier (Quantel PTA-10) and amplified pulses with
pulse energies of 0.8 mJ were obtained at a repetition rate of
10 Hz. The dye amplifiers were excited by the second harmonic
of the output from a pulsed Nd:YAG regenerative amplifier
(Quantel RGA60-10, 532 nm, 10 Hz, 200 mW, 70 ps) seeded
by the cw mode-locked Nd:YAG laser. Kiton Red in water was
used as the amplifying dye solution in this study.
2. Experimental Section
2.1. Materials. 2H-Benzopyran (BP) and its derivatives
employed in this study are listed in Table 1. All the compounds
except methoxy derivatives were prepared as follows.
2H-Benzopyran40 was prepared by dehydration of 4-chro-
manol.
4-Methyl-2H-benzopyran (4-MeBP)41 and 4-methyl-2-phenyl-
2H-benzopyran (4-Me-2-PhBP)41,42 were prepared by dehydra-
tion of the products of the Grignard reactions of methyl iodide
with 4-chromanone and flavanone, respectively.
4-Phenyl-2H-benzopyran (4-PhBP)40,41 and 2,4-diphenyl-2H-
benzopyran (2,4-Ph2BP)40,41 were prepared by dehydration of
the products of the Grignard reactions of bromobenzene with
4-chromanone and flavanone, respectively.
2,2-Diphenyl-4-methyl-2H-benzopyran (2,2-Ph2-4-MeBP) was
prepared by dehydration of the product of the Grignard reaction
of bromobenzene with 4-methylcoumarin,42 which was prepared
by a reaction of phenol and ethyl acetoacetate in nitrobenzene
in the presence of aluminum chloride.43
2-Phenyl-2H-benzopyran (2-PhBP),44 2,2-diphenyl-2H-ben-
zopyran (2,2-Ph2BP),44,45 and 2,2,4-triphenyl-2H-benzopyran
(2,2,4-Ph3BP)46 were prepared by condensation of phenol with
1-phenyl-2-propyn-1-ol, 1,1-diphenyl-2-propyn-1-ol, and 1,1,3-
triphenyl-2-propyn-1-ol, respectively.
The second-harmonic of the output from the dye amplifier
was used as the pump beam (295 nm), and the remaining visible
radiation was focused into a cell containing water to generate
picosecond continuum. The generated continuum was passed
through a blue glass filter to produce the probe light in the 390-
500 nm region, and was split into two parts, one for a probe
beam and the other for a reference. After passing through fixed
(for the pump beam) and variable (for the probe beam) optical
delay lines, the pump and probe beams were focused noncol-
linearly into the sample cell and the transmitted light intensities

11480 J. Phys. Chem. A, Vol. 104, No. 49, 2000
Kodama et al.
Figure 2. Picosecond time-resolved absorption spectra of 2,4-Ph2BP
on the time sale from 10 to 80 ps excited at (A) 295 and (B) 266 nm.
Thin solid line, 10 ps; dotted line, 15 ps; dash-dotted line, 20 ps; dashed
line, 50 ps; thick solid line, 80 ps.
Figure 1. Nanosecond transient absorption spectra of (A) 2,4-Ph2BP
and (B) 2,2,4-Ph3BP in cyclohexane excited at 308 nm. Delay time,
(A) 185 and (B) 312 ns.
were detected by multichannel photodiodes (Hamamatsu Pho-
tonics C2326-22A) and collected at a 10 Hz repetition rate
synchronized with the light source. The instrumental response
function was determined by temporal rises of the excited-state
absorption of trans-stilbene, 1,4-diphenylbutadiene, and naph-
thalene. It was estimated to be a Gaussian function with a full
width at half-maximum of 4.5 ps.
For the measurements with the excitation at 266 nm, a
regeneratively amplified output of Ti:sapphire laser was used
as a light source. This laser system was based on a mode-locked
Ti:sapphire laser (Spectra Physics Tsunami 3950, 800 nm, 82
MHz, 1.0 W, 3 ps) pumped by a CW argon ion laser (Spectra
Physics BeamLok 2060). The oscillator was seeded into a
regenerative amplifier (Spectra Physics TSA-10), which was
pumped by the second harmonic output from a Q-switched Nd:
YAG laser (Quanta-Ray GCR 150-10, 532 nm, 10 Hz, 1.5 W,
6-7 ns). The output pulses from the regenerative amplifier had
3 mJ pulse energy at a repetition rate of 10 Hz. The third-
harmonic of the output from the regenerative amplifier was used
as the pump beam, and the residual fundamental radiation was
focused into a water-cell to generate picosecond continuum. The
detection system was the same as described above.
3. Results
3.1. Time-Resolved Absorption of Benzopyrans. All the
benzopyrans (closed form) used in this study exhibit two
absorption bands around 270 and 310 nm as listed in Table 1.
On UV excitation, these molecules exhibit absorption spectra
in the visible region. Figure 1 shows nanosecond transient
absorption spectra of 2,4-Ph2BP and 2,2,4-Ph3BP in cyclohexane
solution. The spectrum from 2,4-Ph2BP exhibits a structured
band with two sharp peaks around 400 nm (A band) and a broad
band around 450 nm (B band), while that from 2,2,4-Ph3BP
gives only the broad B band around 490 nm. The spectral and
decay profiles are not affected in the presence of oxygen. The
absence of any oxygen effects on the formation and the
nanosecond-induced spectra of benzopyrans have already been
reported together with the lack of any observable triplet states.26
Therefore, the spectra in Figure 1 can be attributed to the
ground-state open forms generated via the excited singlet states
incomparisonwiththereportedspectraforsomebenzopyrans.20-23,26
Figure 2A shows spectral changes in the delay time of 10-
80 ps on excitation of 2,4-Ph2BP at 295 nm; a broad spectrum
Figure 3. Time-resolved absorption signals probed at 410 nm for 2,4-
Ph2BP in cyclohexane in various time regions. Dots show observed
data; solid curves are best-fits simulated to the experimental points by
convolution of the instrumental function with rise (τ1) and/or decay
(τ2 and τ3) exponential functions. (A) Pumped at 295 nm; τ1, 20 ps; τ2,
800 ps. (B) Pumped at 295 nm; τ1, 20 ps; τ2, 800 ps; τ3, 730 ns. (C)
Pumped at 308 nm; τ2, 730 ns; τ3, 48 µs. (D) Pumped at 308 nm; τ2,
48 µs; τ3, >1 ms.
at 10 ps delay time changes to a sharp one at 80 ps. However,
on excitation at 266 nm, the spectrum seems to change in a

Photochromic Process of 2H-Benzopyrans
J. Phys. Chem. A, Vol. 104, No. 49, 2000 11481
Figure 5. Time-resolved absorption spectra of 2,4-Ph2BP excited at
308 nm in cyclohexane in the microsecond time range. The delay time
is given on the right of each spectrum.
Figure 4. Time-resolved absorption signals probed at 470 nm for 2,2,4-
Ph3BP in cyclohexane in various time regions. Dots show observed
data; solid curves are best-fits simulated to the experimental points by
convolution of the instrumental function with rise (τ1) and/or decay
(τ2 and τ3) exponential functions. (A) Pumped at 295 nm; τ1, 20 ps; τ2,
12 µs. (B) Pumped at 295 nm; τ1, 20 ps; τ2, 12 µs. (C) Pumped at 308
nm; τ2, 12 µs; τ3, >1 ms. (D) Pumped at 308 nm; τ2, 12 µs; τ3, >1 ms.
slight different way as shown in Figure 2B. On 295 nm
excitation the spectrum around 425 nm decreases in intensity
in this time range, but on 266 nm excitation the spectrum
remains almost unchanged; the absorbance increments around
410 and 460 nm are smaller on 295 nm excitation, but larger
on 266 nm excitation. Both the features of the spectra at 80 ps
delay time are similar to those of the spectra observed with a
nanosecond delay time in the region of 400-470 nm (Figure
1A).
Figures 3 and 4 show the time developments of the spectra
monitored at 410 and 470 nm for 2,4-Ph2BP and 2,2,4-Ph3BP,
respectively. All the observed time-resolved curves can be
satisfactorily reproduced by the convolution of the instrument
response function with rise and/or decay exponential functions.
In the picosecond regions, the rise times of the induced
absorption from both 2,4-Ph2BP and 2,2,4-Ph3BP are determined
by the instrumental-limited rise component having a lifetime
longer than hundreds of picoseconds and the small amplitude
of the slower rise component with a rise constant of 20 ps. The
ratio of the amplitude of the slow component to that of the very
fast component is 0.22 for 2,4-Ph2BP (Figure 3A) and 0.25 for
2,2,4-Ph3BP (Figure 4A). The slower rise component arises from
the spectral change occurring in tens of picoseconds as shown
in Figure 2. The instrumental-limited rise indicates that the
generation of the observed species is much faster than the
Figure 6. Pump-laser power dependence of time-resolved absorption
signals for (A) 2,4-Ph2BP and (B) 2,2,4-Ph3BP in cyclohexane in the
subnanoseconds to submilliseconds time range. The approximate values
of pump-laser power density are (a) 160 mJ cm-2, (b) 73 mJ cm-2, (c)
24 mJ cm-2. (A) pump, 308 nm; probe, 410 nm. (B) pump, 308 nm;
probe, 470 nm.
temporal resolution of the present experimental system (≈2 ps).
The rise times are not affected by monitoring wavelength and
by light polarization within the experimental accuracy. Several
decay components are observed in the time range of subnano-
seconds to submilliseconds, although the change in the spectral
profile with the delay time is not observed clearly (Figures 1A
and 5). For 2,4-Ph2BP, four components are observed with decay
time constants of (1) several hundreds ps, (2) 730 ns, (3) 48
µs, and (4) >1 ms, and for 2,2,4-Ph3BP, two components with
decay time constants of (1) 12 µs and (2) >1 ms. The
components with lifetimes of 48 µs for 2,4-Ph2BP and 12 µs
for 2,2,4-Ph3BP seem to be the major species induced by the
UV irradiation (Figures 3D and 4D).
Figure 6 shows the pump-laser power dependence of the time-
resolved absorption of 2,4-Ph2BP and 2,2,4-Ph3BP in the time
regions of subnanoseconds to submilliseconds. The yields of
the major components with lifetimes of 48 µs for 2,4-Ph2BP
and 12 µs for 2,2,4-Ph3BP are linearly dependent on the pump-
laser power, while those of the minor components with longer

11482 J. Phys. Chem. A, Vol. 104, No. 49, 2000
Kodama et al.
SCHEME 1
from a transition state to a metastable merocyanine were
estimated to be approximately 700 and 470 fs, respectively.13
The fast rise is followed by a change with a time constant of
about 20 ps. In this time region, a broad spectrum changes in
profile to a sharp one, as seen in Figure 2. Immediately after
the ring-opening reaction, vibrationally excited open forms in
the S0 state should be generated because a large amount of
excess vibrational energy [in this case, difference between
electronic energies of the photoexcited closed form and the S0
state of the open forms] is expected. A relaxation process from
vibrationally excited states to a thermodynamically favorable
state may be hence observed spectroscopically. Generally
speaking, it takes several to tens of picoseconds for vibrationally
excited large molecules to reach a thermal equilibrium with
surrounding solvent molecules, during which the spectral profile
generally changes from a broad one to a sharp one.47,48 Thus
we may assign the spectral change in tens of picoseconds to
the vibrational relaxation of the S0 open forms toward a thermal
equilibrium with solvents.
The vibrational relaxation behavior in the S0 open forms
seems to depend on the pump wavelength as in Figure 2A,B.
This means that the amount of the excess energy deposited in
the S0 open forms depends on the pump-wavelength used to
excite the S0 closed form. If the ring opening takes place from
the lowest excited singlet state (S1) of the closed molecules after
they reach a thermal equilibrium with surrounding solvent
molecules through dissipation of the excess energy to solvents,
the S0 open form would be formed with the same excess
vibrational energy and the spectral change would be independent
of the pump wavelength, 295 or 266 nm. Therefore, the pump-
wavelength dependence of the spectral change suggests that the
C-O cleavage does not occur from the S1 state of the closed
form in a thermal equilibrium with solvent molecules. The two
pump wavelengths (266 and 295 nm) may excite the different
optical transitions of the closed form as listed in Table 1. The
266 nm pump photon creates a higher electronically excited
(Sn) state of the closed form, which should very rapidly relax
to the vibrationally excited S1 closed form through internal
conversion. As mentioned above, the ring opening of the closed
form occurs within 2 ps and the C-O cleavage does not occur
from the thermal equilibrium in the S1 closed form. Thus the
Sn f S1 internal conversion may compete with the ultrafast
C-O bond cleavage.
Figure 7. Plot of the transient absorption signals for (A) 2,4-Ph2BP
and (B) 2,2,4-Ph3BP in cyclohexane in the submillisecond time range
against the pump-laser power density on a log-log scale. (A) pump,
308 nm; probe, 410 nm; delay time, 250 µs. (B) pump, 308 nm; probe,
470 nm; delay time 100 µs. The lines are best-fits to the observed data.
The slopes of the fitted lines are (A) 2.3 and (B) 2.1.
lifetimes (>1 ms) increase in the approximately second order
of the pump-laser power as shown in Figure 7. This result
indicates that the major components are produced through a
one-photon process whereas the minor long-lived components
are produced through a two-photon process within a nanosecond
pump-laser pulse.
3.2. Effects of Substituents on Photochromism of Ben-
zopyrans. The wavelengths of absorption maxima of the closed
and open forms are summarized in Table 1. Although no
remarkable differences are observed in the two absorption
maxima of the closed forms, the absorption spectra of the open
forms can be classified into two groups: one that shows both
the A and B bands such as 2,4-Ph2BP (Figure 1A), and the other
that shows only the B band such as 2,2,4-Ph3BP (Figure 1B).
The lifetimes of the major open forms at 298 K are also listed
in Table 1, ranging in the wide time region; for example, the
major component of 2,2,4-Ph3BP has a lifetime of 12 µs;
however, that of 2,2-Ph2BP has a lifetime longer than 1 ms.
For 4-MeBP, 4-PhBP, 4-Me-2-PhBP, 2,4-Ph2BP, 4-Me-2,2-Ph2-
BP, and 2,2,4-Ph3BP, the activation parameters for disappear-
ance of the major open forms are determined from the
temperature dependence of their lifetimes in the region of 293-
313 K (Table 1).
4. Discussion
4.1. Mechanism of the C-O Bond Cleavage of 2,4-Ph2BP
and 2,2,4-Ph3BP. For both benzopyrans, 2,4-Ph2BP and 2,2,4-
Ph3BP, the spectral rises are found to be faster than the
instrumental detection limit (≈2 ps), and the spectral profiles
are similar to those of the open forms observed in the time
regions of subnanoseconds to submilliseconds. These results
indicate that the ring opening of the closed forms occurs within
2 ps, producing vibrationally excited open forms in the ground
electronic state (S0). These observations are in agreement with
the behavior of spiropyrans and their analogues reported so
far;5,7,10,11,13,14,18,19 the C-O bond of a spiropyran was shown
to break in less than 100 fs to form a metastable species by
means of time-resolved absorption spectroscopy.14 For a spiroox-
azine the time constants for the C-O cleavage and the relaxation
4.2. Structures and Dynamic Behavior of the Open Forms
of 2,4-Ph2BP and 2,2,4-Ph3BP. The decay components fol-
lowing vibrational relaxation exhibit almost the same spectral
profiles within our experimental uncertainty (Figures 1 and 5);
they are distinguished only by the decay time constants. Thus
the components can be assigned to geometrical isomers of the
open forms, and the difference in lifetime reflects the difference
in the rate of the thermal reversion to the closed form. The open
forms of benzopyrans consist of eight isomers with respect to
two double bonds and one single bond as depicted in Scheme
2. The structures of the isomers are expressed by the geometries
around the C2-C3, C3-C4, and C4-C4a bonds of benzopyrans;
as an example, ZEE for an isomer with Z, E, and E geometries

Photochromic Process of 2H-Benzopyrans
J. Phys. Chem. A, Vol. 104, No. 49, 2000 11483
SCHEME 2
2), the eight open forms are formed on photocleavage of the
C-O bond. However, ZZZ and ZZE are hardly stabilized and
can be discarded, since they suffer from a large steric hindrance
between the 2-phenyl group and the conjugate enone skeleton.
Among the remaining six isomers, EZE, ZEE, and EEE need
the C4-C4a double-bond rotation on reverting to the closed form,
as was discussed for EEs in 2,2,4-Ph3BP and the naphthopyran.
The steric hindrance in ZEE of 2,4-Ph2BP is similar to that in
EE of 2,2,4-Ph3BP, since both benzopyrans have the phenyl
groups at the 2- and 4-positions at the side of the carbonyl
oxygen atom. This suggests that the lifetime of ZEE may be
comparable to that of EE of 2,2,4-Ph2BP. The EZE and EEE
isomers, which have no steric interaction between the phenyl
groups, are considered to be more stable than ZEE. From these
considerations, EZE, ZEE, and EEE can be assigned to the
residual components with lifetimes longer than 1 ms in Figure
3D.
SCHEME 3
The other three decay components are attributable to EZZ,
ZEZ, and EEZ. Among those three isomers, the EZZ isomer
can most easily revert to the closed form, and can be regarded
as the most unstable species from comparison with the results
of hexatriene isomers.49-52 Thus, the EZZ isomer is tentatively
assigned to the short-lived component with a lifetime of several
hundreds of picoseconds. This assignment is consistent with
the lack of a component with a lifetime of hundreds of
picoseconds in the spectrum of 2,2,4-Ph3BP, where s-cis
conformers are discarded on the large steric hindrance. Im-
mediately after photocleavage of 1,3-cyclohexadienes, the time
constants for formation of an s-trans conformer from an s-cis
one have been reported to be several picoseconds.49-52 As for
previtamine D generated from the ring opening of 7-dehydro-
cholesterol, the s-cis to s-trans isomerization has been shown
to occur with a time constant of 125 ps.53 The lifetime assigned
to the present EZZ isomer is longer than those for all-cis
conformers of trienes reported so far.49-53 This might be due
to hydrogen bonding between the carbonyl group and the
olefinic hydrogen in the present isomer.16 The ring closure is a
most plausible process for EZZ, though the isomerization to
EZE or EEZ is not completely excluded. The ZEZ and EEZ
isomers can revert to the closed form through a single-bond
rotation; however, the steric hindrance between the phenyl
groups at the 2- and 4-positions is larger in ZEZ than in EEZ.
Therefore, the ZEZ isomer is considered to be faster in reversion
and is assigned to the 730 ns component, and thus, the EEZ
isomer to the 48 µs component. The photochromic processes
of 2,4-Ph2BP obtained in this study are depicted in Scheme 4.
around the C2-C3, C3-C4, and C4-C4a bonds. The eight
isomers can be classified into four groups, ZZ, EZ, ZE, and EE
on the geometries around the C3-C4 and C4-C4a bonds. In this
manner, the two isomers in a group can be distinguished by
different substituents, R1 and R2, at the 2-position; for R1 )
R2, the two isomers are reduced to an identical structure, and
the resulting four isomers are expressed as ZZ, EZ, ZE, and
EE.
Recently, Delbaere et al. have studied the photochromic
behavior of 3,3-diphenyl-3H-naphtho[2,1-b]pyran, which cor-
responds to the case of R1 ) R2, and identified the structures
of the open forms with lifetimes of seconds to minutes by
NMR.36 Among four possible isomers of this naphthopyran
molecule, two isomers ZZ and ZE with the Z geometry at the
C3-C4 single bond were discarded because of a large steric
hindrance between the 2-phenyl group and the conjugate enone
skeleton. The observed two components, a short-lived species
with a lifetime of 9 s and a long-lived species with a lifetime
of 2000 s, were identified as EZ and EE, respectively. It was
also revealed that the photocleavage of the C-O bond yielded
the two open forms in significantly different amounts, EZ as a
major component and EE as a minor one.
As shown in Figures 6 and 7, the nanosecond pump-laser
power dependence of the induced absorption signals indicates
that the short-lived major components with a lifetime of tens
of microseconds are produced by a one-photon process, whereas
the long-lived minor components are formed through a two-
photon process within a pump-laser pulse with 20 ns duration.
Since the photocleavage of the C-O bond takes place within 2
ps after photoexcitation, the yield of the further excitation of
the photoexcited closed form should be very small. Furthermore,
no long-lived components were observed by the picosecond
pump-laser pulse with a power density of 180 mJ cm-2, which
is higher than those used in the measurements of nanosecond
pump-laser power dependence. Therefore, it is concluded that
the long-lived open isomers are formed from photoexcitation
of the short-lived open isomers, but not from the closed form.
In other words, the C-O photocleavage gives at first only the
open forms which are revertible to the closed form by a single-
bond rotation, and their photoisomerization leads to other open
In the case of 2,2,4-Ph3BP (R1 ) R2 ) R3 ) Ph in Scheme
2), the situation is very similar to the above naphthopyran
molecule; the sterically unfavorable ZZ and ZE can be discarded.
Hence, from comparison with the naphthopyran results, the
major component with a lifetime of 12 µs in Figure 4C is
identified as EZ, and the remaining minor long-lived component
with a lifetime longer than 1 ms is attributable to EE. The large
difference in lifetime is rationalized in terms of the extent of
structural change in reversion to the original closed form. The
EZ isomer can easily revert to the closed form by the single-
bond rotation, whereas EE needs the double-bond rotation in
addition to the single-bond rotation. The obtained photochromic
processes of 2,2,4-Ph3BP are depicted in Scheme 3.
In the case of 2,4-Ph2BP (R1 ) R3 ) Ph, R2 ) H in Scheme

11484 J. Phys. Chem. A, Vol. 104, No. 49, 2000
Kodama et al.
SCHEME 4
forms which need a double-bond rotation for reversion to the
closed form. Since such a two-step two-photon behavior has
also been observed very recently in the photochromism of 1H-
benzopyran derivatives.54 It may be said that the concept of
two-step two-photon photochromism generating long-lived
isomers applies to other photochromic systems.
major components are assigned to be the isomers which can
revert to the closed form by a single-bond rotation: EEZ for
2-PhBP, 4-Me-2-PhBP, and EZ for BP, 4-MeBP, 4-PhBP, 2,2-
Ph2BP, 4-Me-2,2-Ph2BP, and 2,2-Me2BPs. The preexponential
factors (log(A/s-1) ) 10.0-10.9) for the major components of
some benzopyrans (Table 1) indicate a significant loss of
freedom (∆S‡×ff ) -(45-60) J K-1 mol-1) in the decay
process, reflecting the recyclization into the original closed form.
The lifetimes of the major components of the open forms at
298 K are dependent on the molecular structure. The benzopy-
rans bearing no substituent at the 4-position (R3 in Table 1),
such as BP, 2-PhBP, 2,2-Ph2BP, and 2,2-Me2BPs, show a
lifetime longer than 1 ms. On the contrary, the lifetimes of those
with a substituent at the 4-position, such as 4-MeBP, 4-PhBP,
4-Me-2-PhBP, 2,4-Ph2BP, 4-Me-2,2-Ph2BP, and 2,2,4-Ph3BP,
are in the order of microseconds. The size of the 4-substituents
also affects the lifetimes of the main components: for 2,2-Ph2-
BP, 4-Me-2,2-Ph2BP, and 2,2,4-Ph3BP, which have a substituent
of H, Me, and Ph, respectively, the lifetimes are >1 ms, 36 µs,
and 12 µs. For the 2-substituents, increase in the number of the
phenyl groups reduces the lifetime as seen for 4-PhBP (194
µs), 2,4-Ph2BP (48 µs), and 2,2,4-Ph3BP (12 µs). Consequently,
the rate for the ring closure depends on the structure; in
particular, introduction of a substituent in the 4-position brings
about steric hindrance against the 2-substituent to accelerate the
ring closure involving a single-bond rotation.
The isomers are formed in very much different yields from
photocleavage of 2,4-Ph2BP and 2,2,4-Ph3BP; the conformers
which can revert to the closed form by a single-bond (C3-C4)
rotation, such as EZ in 2,2,4-Ph3BP and EEZ in 2,4-Ph2BP, are
produced in a larger amount than those which need a double-
bond (C4-C4a) rotation for reverting to the closed form, such
as EE in 2,2,4- Ph3BP and EZE in 2,4-Ph2BP. This difference
may be ascribed to the difference between the one-photon and
two-photon photoisomerization processes. In spiropyran systems
it has been proposed that thermal isomerization of the primary
ring-opening product, the internal temperature of which is very
high because of a large amount of excess vibrational energy
generated via the C-O cleavage, is responsible for a distribution
of merocyanines.10 The present study suggests that the photo-
isomerization of the open forms also plays a significant role in
the isomer distribution in some cases.
4.3. Effects of Substituents on Photochromism of Ben-
zopyrans. As mentioned in Subsection 3.1, the benzopyrans
are classified into two classes with respect to the spectrum of
the open forms. Among those with a phenyl group at the
2-position (R1, R2 in Table 1), 2-PhBP, 4-Me-2-PhBP, 2,4-Ph2-
BP, and 2,2-Ph2BP show the structured A bands; however, no
A band is observed for those with a sterically crowded structure,
such as 4-Me-2,2-Ph2BP and 2,2,4-Ph3BP. The B band tends
to be shifted with increasing the number of phenyl substituents;
for example, the band maxima appear at 430, 445, 485, and
490 nm for BP, 2-PhBP, 2,2-Ph2BP, and 2,2,4-Ph3BP, respec-
tively. Thus the B band is assigned to the conjugate systems of
enones, which is in agreement with the theoretical calculations
using an LCAO-MO-SCF-CI procedure.23 The origin of the A
band cannot be assigned in the present study, although it is noted
that the spectra at delay time 82.8 and 165 µs in Figure 5, which
are considered to mainly arise from the double-bond rotated
open forms of 2,4-Ph2BP, also exhibit the structured A band
around 400 nm.
5. Conclusion
The ring opening of benzopyrans, 2,4-Ph2BP and 2,2,4-Ph3-
BP, is completed within 2 ps, producing vibrationally excited
open forms in the S0 state, which relax to thermal equilibria
with a time constant of ≈20 ps. In 2,2,4-Ph3BP, two decay
components are detected; the major component is short-lived
and identified as the EZ isomer, which can revert to the closed
form by the single-bond rotation. The minor long-lived com-
ponent is assigned to the EE isomer; its reversion to the closed
form needs the double-bond rotation in addition to the single-
bond rotation. In 2,4-Ph2BP, among the observed four compo-
nents, the most short-lived one is assigned to EZZ, which can
most easily revert to the closed form. The long-lived components
with lifetimes longer than 1 ms are assigned to EZE, ZEE, and
EEE, all of which need the double-bond rotation to revert to
the closed form. The ZEZ and EEZ isomers revert to the closed
form through the single-bond rotation; however, the extent of
Disappearance of the major components observed in the
nanosecond time-resolved absorption in Figures 3 and 4 can be
identified to a thermal ring closure giving the original molecules.
According to the discussion of 2,4-Ph2BP and 2,2,4-Ph3BP, the

Photochromic Process of 2H-Benzopyrans
J. Phys. Chem. A, Vol. 104, No. 49, 2000 11485
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slower for EEZ.
The photocleavage of 2,4-Ph2BP and 2,2,4-Ph3BP gives at
first only the open forms revertible to the closed form by the
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2-position affects the rate of thermal reversion to the closed
form.
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