Effects of water on the phosphorescence spectra of aromatic carbonyl compounds. Part 1.—Dual phosphorescence of benzophenone at 77 K in 2,2 2-trifluoroethanol–water
Abstract:From measurements of the phosphorescence spectrum and its decay at 77 K, the lowest triplet state of benzophenone (BP) at low concentrations (ca. 1 x mol dm-3) has been assigned to be of nn* character even in a strong hydrogen-bonding solvent such as 2,2,2-trifluoroethanol (TFE). At high sample concentrations (ca. 1 x mol dm-3), another nn*-type phosphorescence due to aggregated BP is observed. Addition of water (H,O or D,O, 1-4 vol.%) to BP-TFE solutions causes dual phosphorescence with lifetimes of 3.6 and 1… Show more
“…In the case of m- HOBP in ethanol the 0−0 band is considerably weakened and blue shifted as compared to that of BP. Similar types of changes in phosphorescence spectrum of BP have also been observed in ethanol−water and trifluoroethanol (TFE)−water mixtures as we observed for p -HOBP and m -HOBP, respectively, in ethanol glass . The triplet energies estimated from the positions of the highest energy peak or shoulder and the phosphorescence lifetimes are given in Table .…”
Time-resolved studies in the pico- and nanosecond time domain have
been performed to characterize the
triplet states of monohydroxy-substituted benzophenones, namely,
para- (p-), meta- (m-), and
ortho- (o-)
hydroxybenzophenones (HOBP). Due to a very fast intersystem
crossing (ISC) process, only the triplet states
have been detected in the subnanosecond time domain. Spectral
characteristics and lifetimes of the triplet
states of HOBP have been seen to be extremely sensitive to the position
of the OH group in the phenyl ring
as well as the solvent characteristics. In case of
m-HOBP and p-HOBP, the excited triplet state in
non-hydrogen-bond-forming solvents has an nπ* configuration and is capable
of abstracting a hydrogen atom
from another unexcited molecule to form ketyl and phenoxy type
radicals. But in hydrogen-bond-forming
solvents, the triplet state, which is strongly associated with the
solvent molecules as a hydrogen-bonded
complex, is very short-lived due to fast nonradiative relaxation via
hydrogen-stretching vibrations in
intermolecular hydrogen bonds with the solvents and is capable of
abstracting a hydrogen atom neither from
the solvent molecule nor from another unexcited HOBP molecule. In
the case of o-HOBP, due to strong
intramolecular hydrogen bonding, the internal conversion in the excited
singlet state is very efficient and
hence the yield of the triplet state is low (<15%) and also the
triplet state is very short-lived. However, in
methanol and DMSO, due to disruption of the intramolecular hydrogen
bond, the triplet yield is higher and
also the generation of the phenolate ion via excited-state proton
transfer is a significant process. Deprotonation
reactions probably taking place from both the excited singlet state as
well as the triplet state of the other two
derivatives also have been found to be significant in polar
solvents.
“…In the case of m- HOBP in ethanol the 0−0 band is considerably weakened and blue shifted as compared to that of BP. Similar types of changes in phosphorescence spectrum of BP have also been observed in ethanol−water and trifluoroethanol (TFE)−water mixtures as we observed for p -HOBP and m -HOBP, respectively, in ethanol glass . The triplet energies estimated from the positions of the highest energy peak or shoulder and the phosphorescence lifetimes are given in Table .…”
Time-resolved studies in the pico- and nanosecond time domain have
been performed to characterize the
triplet states of monohydroxy-substituted benzophenones, namely,
para- (p-), meta- (m-), and
ortho- (o-)
hydroxybenzophenones (HOBP). Due to a very fast intersystem
crossing (ISC) process, only the triplet states
have been detected in the subnanosecond time domain. Spectral
characteristics and lifetimes of the triplet
states of HOBP have been seen to be extremely sensitive to the position
of the OH group in the phenyl ring
as well as the solvent characteristics. In case of
m-HOBP and p-HOBP, the excited triplet state in
non-hydrogen-bond-forming solvents has an nπ* configuration and is capable
of abstracting a hydrogen atom
from another unexcited molecule to form ketyl and phenoxy type
radicals. But in hydrogen-bond-forming
solvents, the triplet state, which is strongly associated with the
solvent molecules as a hydrogen-bonded
complex, is very short-lived due to fast nonradiative relaxation via
hydrogen-stretching vibrations in
intermolecular hydrogen bonds with the solvents and is capable of
abstracting a hydrogen atom neither from
the solvent molecule nor from another unexcited HOBP molecule. In
the case of o-HOBP, due to strong
intramolecular hydrogen bonding, the internal conversion in the excited
singlet state is very efficient and
hence the yield of the triplet state is low (<15%) and also the
triplet state is very short-lived. However, in
methanol and DMSO, due to disruption of the intramolecular hydrogen
bond, the triplet yield is higher and
also the generation of the phenolate ion via excited-state proton
transfer is a significant process. Deprotonation
reactions probably taking place from both the excited singlet state as
well as the triplet state of the other two
derivatives also have been found to be significant in polar
solvents.
“…The shorter lifetime (2.6 ms) might be associated with the n π* triplet state, whereas the longer lifetime (55 ms) might be associated with the ππ* triplet state. This kind of dual phosphorescence has already been reported for a few of the aromatic carbonyl compounds, including benzophenone. 2b,26b, In ethanol, however, the unsymmetrically charge-distributed ππ* or CT triplet state is the only emitting state, as, in this matrix, the energy level of the T 1 state of ππ* or CT configuration is much lower than that of the triplet state of n π * configuration. 2 Phosphorescence spectra of p -ABP in (a) MCH and (b) ethanol matrixes.…”
Section: Resultssupporting
confidence: 57%
“…The energy levels for the lowest triplet excited states, as determined from the onset of the phosphorescence spectra, are at 66.6 kcal mol -1 in MCH and 65.6 kcal mol -1 in ethanol, as compared to those at 68.5 kcal mol -1 for BP and 67.5 kcal mol -1 for p -HOBP. , The phosphorescence decay of p -ABP is biexponential in MCH with lifetimes of 2.6 ± 0.3 and 55 ± 5 ms, whereas in ethanol matrix, the phosphorescence emission decays as a single exponential with a lifetime of 80 ± 5 ms. The ketones with T 1 states of n π* configuration have phosphorescence lifetimes not longer than a few milliseconds, and those with the T 1 states of ππ* configuration have lifetimes of more than a few tens of milliseconds and, in many cases, up to several hundred milliseconds. ,, Hence, in the case of p -ABP, the double-exponential decay in MCH indicates the presence of two kinds of triplet states, namely, the n π* and ππ* states, energetically very close to each other. The shorter lifetime (2.6 ms) might be associated with the n π* triplet state, whereas the longer lifetime (55 ms) might be associated with the ππ* triplet state.…”
The photophysical properties of the singlet (S 1 ) and triplet (T 1 ) excited states of p-aminobenzophenone (p-ABP) have been investigated in various organic solvents using steady-state as well as transient absorption spectroscopy with picosecond and subpicosecond time resolution. p-ABP is weakly fluorescent in benzene as well as in polar aprotic solvents, acetonitrile, DMSO, and DMF, but nearly nonfluorescent in cyclohexane and methanol. In cyclohexane, the S 1 state has the nπ* configuration and is short-lived [τ(S 1 ) ∼ 12 ps]. In methanol, a polar and protic solvent, the S 1 state is much shorter-lived [τ (S 1 ) < 1ps, and hence, we have not been able to detect any transient, even in subpicosecond time scale] because of the formation of an intermolecular hydrogen-bonded complex with the solvent. In all other solvents, the S 1 state has a ππ* or CT configuration and, hence, is much longer-lived (>100 ps). The triplet yield is much higher in nonpolar solvents than in polar solvents but the lifetime shows the reverse trend. In nonpolar solvents, the T 1 state is an equilibrium mixture of the states having nπ* and ππ* configurations because of their close proximity in energy, and it is photochemically reactive toward hydrogen-atom-abstraction reactions. In polar solvents, the T 1 state is unreactive because of its ππ* or CT character. A comparison has also been made among the photophysical properties of benzophenone (BP), p-hydroxybenzophenone (p-HOBP), and p-ABP.
“…It is well-known that solvent polarity affects the electronic structure of the excited triplet states of carbonyl compounds. The T 1 state characters of the carbonyl molecules with small energy separation between the 3 nπ*− 3 ππ* states are significantly sensitive to the matrix polarity. − On the other hand, for benzophenone, the complex formation with water is essential to change the T 1 state character from 3 nπ* to mixed 3 nπ*− 3 ππ*. , The energy levels of 3 nπ* states are destabilized more than those of 3 ππ* by the solvation with polar molecules or hydrogen bond formation with protic solvents.…”
Phosphorescence and TREPR spectra have been measured for
2-methyl-1,4-naphthoquinone (MNQ), 2,3-dimethyl-1,4-naphthoquinone (DMNQ), and 2-methoxynaphthoquinone (MeONQ)
in several matrices at low
temperatures. The |D| value of ZFS parameter
decreases with increasing the electron-donating character
of
the substituent group. For MNQ, the T1 state is
clearly assigned to be 3nπ* in character. The
broad
phosphorescence spectra and minor effects of the matrix polarity on the
ZFS parameters lead to the conclusion
that the character of the T1 states are mainly
3ππ* in DMNQ and MeONQ. The T1 states
of the present
naphthoquinone derivatives have more or less mixed character of the
3nπ* and 3ππ* states.
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