Photoinduced intramolecular proton transfer of phenol-containing ligands and their zinc complexes

Article abstract of DOI:10.1021/jp053662p

N,N′-Bis(salicylidene)hydrazine (L_I), a bis-2-hydroxybenzene-type ligand H₂L, its tert-butyl derivative (L_II), and the corresponding Zn²⁺ complexes of the type Zn₂(LH)₂L (Zn-I and Zn-II) were synthesized. The molecular structure of Zn-II was determined by X-ray crystallography at -170°C. The photoreactions of the four compounds in solution were studied by time-resolved UV-vis spectroscopy using nanosecond laser pulses. A weak but strongly Stokes shifted fluorescence signal of the ligands L_I or L_II is suggested to be due to excited-state intramolecular proton transfer (ESIPT) from the phenolic hydroxy group to the nitrogen of the methine bond in analogy to the fast enol → keto tautomerization of other 2-hydroxybenzenes. A transient with the maximum at 480 nm, bleaching at 370 nm, and a lifetime of 0.01-0.3 ms is attributed to the trans-keto tautomer, formed via internal conversion. The decay occurs via trans → cis isomerization and proton back-transfer to the enol form. Quenching by water indicates a proton-catalyzed reaction. To account for similar fluorescence and transient properties in the cases of the Zn²⁺ complexes, a photoinduced tautomerism at one of the two free phenolic hydroxy groups is proposed. The rapid ESIPT followed by a relatively slow relaxation process is reversible.

Full text of DOI:10.1021/jp053662p

J. Phys. Chem. A 2006, 110, 2587-2594
2587
ARTICLES
Photoinduced Intramolecular Proton Transfer of Phenol-Containing Ligands and Their
Zinc Complexes
Helmut Go1rner,* Sumit Khanra, Thomas Weyhermu1ller, and Phalguni Chaudhuri*
Max-Planck-Institut fu¨r Bioanorganische Chemie, D-45413 Mu¨lheim an der Ruhr, Germany
ReceiVed: July 5, 2005; In Final Form: January 10, 2006
N,N′-Bis(salicylidene)hydrazine (LI), a bis-2-hydroxybenzene-type ligand H2L, its tert-butyl derivative (LII),
and the corresponding Zn2+ complexes of the type Zn2(LH)2L (Zn-I and Zn-II) were synthesized. The
molecular structure of Zn-II was determined by X-ray crystallography at -170 °C. The photoreactions of
the four compounds in solution were studied by time-resolved UV-vis spectroscopy using nanosecond laser
pulses. A weak but strongly Stokes shifted fluorescence signal of the ligands LI or LII is suggested to be due
to excited-state intramolecular proton transfer (ESIPT) from the phenolic hydroxy group to the nitrogen of
the methine bond in analogy to the fast enol f keto tautomerization of other 2-hydroxybenzenes. A transient
with the maximum at 480 nm, bleaching at 370 nm, and a lifetime of 0.01-0.3 ms is attributed to the trans-
keto tautomer, formed via internal conversion. The decay occurs via trans f cis isomerization and proton
back-transfer to the enol form. Quenching by water indicates a proton-catalyzed reaction. To account for
similar fluorescence and transient properties in the cases of the Zn2+ complexes, a photoinduced tautomerism
at one of the two free phenolic hydroxy groups is proposed. The rapid ESIPT followed by a relatively slow
relaxation process is reversible.
Introduction
in aqueous solutions have been summarized.11 Photodeactivation
of sterically hindered phenols occurs by internal conversion.
Widespread occurrence of tyrosyl radicals in metalloproteins
involved in oxygen-dependent enzymatic radical catalysis1,2 has
sparked and fueled the interest of chemists in using phenol-
containing ligands to study model compounds that contain
coordinated phenolate and its one-electron oxidized phenoxyl
radical.3 In a series of papers4 we have already demonstrated
the catalytic activity of phenol/phenoxyl-containing transition
metal complexes for aerial oxidation of organic substrates such
as alcohols, amines, and catechols; these are rare examples of
functional models for metalloenzymes such as galactose oxidase,
amine oxidases, and catechol oxidases. Additionally, one-
electron oxidation of a hydrogen-bonded phenol can occur by
concerted proton-coupled electron transfer.5 We have prepared
a phenol-containing ligand N,N′-bis(salicylidene)hydrazine (LI),
its tert-butyl derivative (LII) and, because the Zn2+ ion is redox-
inactive, their corresponding zinc complexes (Zn-I and Zn-
II). It appeared attractive to examine the photochemistry of these
ligands and complexes.
From the effects of alkyl substitution of phenols, steric
hindrance, solvent polarity and a balance of the deactivation
channels of the first excited singlet state, Brede and co-workers
have concluded that 2,6-di-tert-butylphenol is the most efficient
light quencher.12 The photochemistry of phenols has recently
been reviewed.13,14
When an appropriate H-atom acceptor group is substituted
in the ortho-position to a phenol, an intramolecular proton
transfer may play a decisive role.15-19 Some 2-hydroxyphenyl
compounds exhibit strongly red-shifted fluorescence under
conditions where hydrogen bonding of the phenolic hydroxyl
group to a nearby nitrogen atom is possible. The reason for the
large Stokes shift is excited-state intramolecular proton transfer
(ESIPT), which converts the normal enol into the keto form.
ESIPT has been documented for 2-(2′-hydroxyphenyl)benzox-
azole (HBO), 2-(2′-hydroxyphenyl)benzothiazole (HBS), (2-
hydroxyphenyl)benzazole, and various related 2-hydroxyben-
zenes.15-29 The photoconversion of these model compounds
from the enol form into the cis- and trans-keto forms was found
to be extremely fast.15
Here, the photoreactions of the bis-salicylaldazine ligands LI
and LII as well as two corresponding Zn2+ complexes were
studied in solution by time-resolved UV-vis spectroscopy. The
observed transient upon excitation at 308 nm is attributed to a
trans-keto tautomer that is photochemically produced by in-
tramolecular proton transfer from the hydroxy group to the
Details of a few photoreactions of Zn2+ complexes with
phenolate ligands have been published.6-10 The knowledge from
radiation chemistry and the properties of the phenoxyl radicals
* Corresponding authors. E-mail: H.G., goerner@mpi-muelheim.mpg.de;
P.C., chaudhuri@mpi-muelheim.mpg.de.
10.1021/jp053662p CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/08/2006

2588 J. Phys. Chem. A, Vol. 110, No. 8, 2006
Go¨rner et al.
TABLE 1: Crystal Data and Structure Refinement for
nitrogen of the methine bond. The decay properties via the cis-
keto to the enol form were compared with those of HBO and
HBS.
Zn-II
empirical formula
C91H130N6O6Cl2Zn2
1605.65
100(2)
0.710 73
monoclinic
P21/c, No. 14
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
unit cell dimensions
a (Å)
Experimental Section
Synthesis of LI was analogous to that described elsewhere.30
Synthesis of LII was as follows: 3,5-Di-tert-butylsalicylaldehyde
(2.34 gm, 10 mmol) and hydrazine hydrate (0.24 mL, 5 mmol)
were dissolved in 30 mL of methanol to yield a yellowish clear
solution, which was refluxed for 30 min. The resulting
precipitate was yellow microcrystalline solid. This was filtered,
washed with hexane, and then air-dried. The ligand was
recrystallized from dichloromethane solution. Yield: 180 mg
(70%). Anal. Calcd for C30H42N2O2: C, 77.88; H, 9.15; N, 6.05.
Found: C, 77.8; H, 9.1; N, 6.1. 1H NMR (CD2Cl2, 400 MHz):
δ 1.34 (18H, s, t-Bu), 1.46 (18H, s, t-Bu), 7.19 (2H, s, Ar),
7.46 (2H, s, Ar), 8.76 (2H, s, azomethine), 11.86 (2H, s, OH-
phenol). IR (KBr, cm-1): 3448, 2959, 2869, 1623, 1591, 1456,
1438, 1250, 1229, 1172, 1025, 963, 715. ESI-MS (m/z): 464.
Synthesis of Zn-I was carried out using the same protocol
as that for Zn-II, which is as follows: Bis-salicylaldazine ligand
(LH2) (0.23 gm, 0.5 mmol) was dissolved in CH2Cl2 (15 mL).
Then a 15 mL methanol solution of 0.13 g of Zn(CH3COO)2‚4
H2O (0.5 mmol) was added to the ligand solution, followed by
Et3N (0.13 mL, 1 mmol). The resulting solution was refluxed
for 30 min under argon, and after cooling, a deep yellow
microcrystalline solid precipitated out. This was filtered, washed
with diethyl ether, and then air-dried. Suitable X-ray quality
single crystals were grown from CH2Cl2-CH3CN solution of
the complex. Yield: 145 mg (40%). Anal. Calcd for C91H131N6O6-
Cl2Zn2: C, 68.02; H, 8.22; N, 5.23; Zn, 8.14. Found: C, 68.7;
17.4953(9)
b (Å)
c (Å)
17.5283(9)
30.010(2)
â (deg)
96.766(5)
9138.9(9); 4
1.167
0.635
volume (Å3); Z
density (cal) (Mg/m3)
absorp coeff (mm-1
F(000)
)
3440
crystal size (mm3)
0.14 × 0.13 × 0.04
θ range for data collection (deg)
reflections collected
2.97-22.50
80163
independent reflections
absorption correction
11916 [R(int) ) 0.0894)]
Gaussian, face indexed
11916/37/989
1.166
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
R1 ) 0.0814, wR2 ) 0.1664
R1 ) 0.0999, wR2 ) 0.1744
distillation. The absorption spectra were monitored on a UV/
vis spectrophotometer (HP, 8453). The molar absorption coef-
ficients of LI, LII, and Zn-I in MCH are ꢀ293 ) 3 × 104, ꢀ294
) 5 × 104, and ꢀ305 ) 9 × 104 M-1 cm-1, respectively. An
excimer laser (Lambda Physik, EMG 200) with a pulse width
of 20 ns and an energy <100 mJ was used for excitation at 308
nm. For a few experiments λexc ) 248 nm was used. As a
measure of the yield, Φꢀ was determined for optically matched
solutions using the TT absorption of benzophenone in aceto-
nitrile at 525 nm, Φꢀ525 ) 6 × 103 M-1 cm-1. The IR difference
spectra were recorded in a 0.5 mm CaF2 cell on a FTIR
spectrometer (Bruker IFS66). For excitation, another excimer
laser (Radiant Dyes, EXC-100) was applied. The emission
spectra were measured on a spectrofluorometer (Eclipse, Cary).
The fluorescence excitation spectra of the ligands and complexes
are similar to their absorption spectra concerning the long
wavelength band. The fluorescence excitation spectra of the
ligands and complexes deviate slightly from the absorption
spectra in the shorter-wavelength region, e.g., when the samples
are not diluted enough. The fluorescence lifetimes (τf) were
determined from decay kinetics by a single-photon counting
fluorometer (Edinburgh Instr. F900). The quantum yield of
fluorescence (Φf) was determined using 9,10-diphenylanthracene
in deoxygenated ethanol as reference, Φf ) 0.9. Irradiation was
performed using the 366 nm line of a 1000 W Xe-Hg lamp
and a monochromator. Photolysis of LII or Zn-II in acetonitrile
by 366 nm irradiation does not cause discernible absorption
changes, in contrast to the complexes in dichloromethane, where
decomposition was observed.
The crystallographic data for Zn-II are summarized in Table
1. Graphite monochromatic Mo KR radiation, λ ) 0.710 73 Å
was used for Zn-II. Yellow crystals of Zn-II were fixed with
perfluoropolyether onto glass fibers and mounted on a Nonius
Kappa-CCD diffractometer equipped with a cryogenic nitrogen
cold stream, and intensity data were collected at -173 °C. Final
cell constants were obtained from a least-squares fit of the setting
angles of all integrated reflections. Intensity data were corrected
for Lorentz and polarization effects. The data set for Zn-II was
corrected for absorption. The Siemens SHELXTL software
package (G. M. Sheldrick, Universita¨t Go¨ttingen) was used for
solution refinement and artwork of the structures; the neutral
atom scattering factors of the program were used.
1
H, 8.2; N, 5.2; Zn, 8.2. H NMR (CD2Cl2, 400 MHz): δ 1.34
(54H, s, t-Bu), 1.46 (54H, s, t-Bu), 7.19 (6H, s, Ar), 7.47 (6H,
s, Ar), 8.37(2H, s, azomethine), 8.50 (2H, s, azomethine), 8.79
(2H, d, azomethine), 11.86 (2H, s, OH-phenol). IR (KBr, cm-1):
3448, 2956, 2906, 1614,1591, 1529, 1461, 1426, 1250, 1229,
1166, 1026, ESI-MS (m/z): 1057 [L(LH)(Zn2+)2]+, 1520
[(Zn2+)2L(LH)2‚CH2Cl2]+.
In the Zn-II complex 6-aldimine protons (HCdN resonance)
are observed in the range 8.37-8.79 ppm, both as singlets and
as doublets. The 1H NMR spectrum clearly shows three different
sets of aldimine protons at 8.37, 8.50, and 8.79 ppm with
distinctly different chemical environments, in accordance with
the X-ray structure. When the imine carbon carries one proton,
the spectrum clearly shows hyperfine splitting of the imine
protons due to coupling with the proton on nitrogen, which is
also evidenced in the X-ray structure by the presence of strong
hydrogen bonding of the phenolic group with the nitrogen. The
downfield shift of the aldimine protons due to complexation is
consistent with the literature values reported earlier.30 We have
measured temperature-dependent 1H NMR spectra of the Zn-
II complex in dichlomethane solution from +25 to -75 °C with
a 10 deg interval, and they were found to be consistent in the
said temperature range. The complex is fairly stable for at least
24 h. (We note that there is a possibility of the self-exchange-
equilibrium, e.g., between a 2:1 complex and free ligand, as
we have seen with prolonged time only a singlet of the aldimine
1
protons in the H NMR spectra). The appearance of intramo-
lecular H-bonded phenolic protons at δ 11.86 ppm is consistent
with the H-bonded phenolic protons shift in the case of the free
ligands.
The other compounds, HBO and HBS (Aldrich), and the
solvents (Merck, Fluka) were used as received, e.g., acetonitrile
(Uvasol quality) or methylcyclohexane (MCH) purified by

Proton Transfer of Phenol-Containing Ligands
J. Phys. Chem. A, Vol. 110, No. 8, 2006 2589
Figure 2. Fluorescence emission (right, λexc ) 380 nm) and excitation
(left, λem ) 550 nm) spectra of (a) Zn-II, (b) Zn-I, and (c) LI in
MCH at 25 °C.
and angles. Such distortions are most probably due to the steric
requirements of the dinucleating ligand. The average Zn-O
bond length is around 1.875 Å, whereas the average Zn-N bond
distance is 2.045 Å. The average trans O-Zn-O bond angle is
123.3°, whereas the trans N-Zn-N angle is 121.1°. Further-
more, the O-Zn-N angle lies in the range between 93.8° and
114.2°. Recently, dinuclear Fe3+ and Ru3+ complexes with the
unsubstituted ligand have been reported.32
Ground-State Properties in Solution. The ligand LII in
cyclohexane at room temperature has an absorption spectrum
with peaks at 290 and 360 nm, but no band above 400 nm.
This is similar for both ligands in acetonitrile. The absorption
spectrum of the complex Zn-II in cyclohexane has peaks at
310, 380 and 440 nm. Thermally, the absorption at 310 and
380 nm increases slightly with time and the 440 nm band
decreases. The spectrum is initially also the same in carbon
tetrachloride or dichloromethane, but the peak at 440 nm
disappears faster, the half-life is a few hours. Zn-II in
acetonitrile shows two peaks at 310 and 380 nm, no band above
400 nm and no thermal change. The 440 nm band of complex
Zn-I also remains unchanged in acetonitrile or ethanol, in
contrast to Zn-II. The stable complex in the cases of Zn-I in
any solvent and of Zn-II in nonpolar solvents is due a to metal
to ligand charge transfer (CT). For Zn-II in dichloromethane
or solvents of larger polarity, we propose replacement of one
ligand by these solvents and/or water. No spectral change was
found for LII in ethanol on addition of water (1-30%).
Fluorescence Properties. The emission of a complex in
cyclohexane or MCH (λexc ) 360 or 440 nm) shows a broad
spectrum with maximum at λfem ) 580 nm for Zn-II (Figure
2a) and at λfem ) 530 nm for Zn-I (Figure 2b). The lifetime
(τf) is shorter than 0.3 ns for the main component. Therefore,
phosphorescence is excluded and the emission is attributed to
fluorescence. An example of the fluorescence emission and
excitation spectra is shown for LI in MCH (Figure 2c). The
fluorescence excitation spectra of the ligands and complexes
in MCH, acetonitrile or ethanol are similar to their absorption
spectra concerning the long wavelength band. Therefore, in the
cases of the complexes, a CT to metal transition as the possible
origin of the fluorescence can be excluded. Oxygen does not
markedly change the emission intensity, which goes along with
the relatively short fluorescence lifetime. Addition of water
(0.01-1 M) to LII in ethanol has only a minor effect.
Figure 1. ORTEP view of the Zn-II complex.
TABLE 2: Selected Bond Lengths (Å) and Angles (deg) for
Zn-II
Zn(1)‚‚‚Zn(2)
Zn(1)-O(41)
Zn(1)-O(1)
Zn(1)-N(8)
Zn(1)-N(48)
5.06
1.874(4)
1.873(4)
2.044(5)
2.075(5)
Zn(2)-O(56)
Zn(2)-O(81)
Zn(2)-N(88)
Zn(2)-O(49)
1.879(4)
1.885(4)
2.023(4)
2.036(5)
O(41)-Zn(1)-O(1) 122.4(17) O(56)-Zn(2)-O(81) 124.11(17)
N(8)-Zn(1)-N(48) 123.36(18) N(88)-Zn(2)-N(49) 118.78(18)
O(41)-Zn(1)-N(8) 115.94(19) O(56)-Zn(2)-N(88) 114.21(18)
O(1)-Zn(1)-N(8)
93.79(17) O(81)-Zn(2)-N(88) 95.45(18)
O(41)-Zn(1)-N(48) 93.90(18) O(56)-Zn(2)-N(49) 95.89(17)
O(11)-Zn(1)-N(48) 109.79(17) O(81)-Zn(2)-N(49) 110.18(18)
Results
Solid-State Structure of Zn-II. The molecular geometry
and atom labeling scheme of the neutral molecule in Zn-II are
shown in Figure 1. The structure of the complex molecule
consists of a discrete neutral dinuclear unit and one dichlo-
romethane molecule as solvent of crystallization. Selected bond
lengths and angles are listed in Table 2. The X-ray structure
confirms that a dinuclear Zn2+ complex has indeed been formed
in such a way that the distorted tetrahedral geometry for each
Zn2+ metal ion in the molecule is present. Each ligand binds
two Zn metal atoms on each side of the N-N bond via one
phenolate-O and one imine-N atom. The second ligand binds
similarly, thus leaving one of the phenolate-O atoms, O(16) and
O(96), in each of these two ligands protonated.
The short O(16)‚‚‚N(9) and O(96)‚‚‚N(89) separations of
2.617 and 2.628 Å, respectively, clearly indicate the occurrence
of strong hydrogen bond interactions, shown by dotted lines in
Figure 1, between the said atoms, suggesting protonated
uncoordinated O(16) and O(96). Indeed, a difference Fourier
analysis in the latter refinement stages did reveal peaks
assignable to protons, and they were included in the final
refinement cycle. The presence of two protons per dinuclear
unit is in complete accord with the charge balance considerations
of the neutral complex Zn-II. Thus the tetracoordinated metal
ions have N2O2 coordination spheres with the salicylaldimine
moiety in the facial mode with the Zn‚‚‚Zn separation of ca.
5.06 Å. All the salicylaldimine fragments are satisfactorily
planar. To accommodate two Zn ions, one of the ligands is
twisted along the N-N bond. Large distortions from the ZnN2O2
tetrahedra are clearly evident, particularly from the bond lengths
The fluorescence at room temperature is generally weaker
for the ligands than for the complexes. The quantum yields are
between Φf ) 0.003 and 0.02 (Table 3). For HBO or HBS in
acetonitrile the values are, to a certain extent, comparable, e.g.,
λfem ) 500 and 520 nm and Φf ) 0.012 and 0.015, respectively.

2590 J. Phys. Chem. A, Vol. 110, No. 8, 2006
Go¨rner et al.
TABLE 3: Fluorescence Maxima and Quantum Yield of the
Ligands and Complexesa
TABLE 4: Transient Properties of the Ligands and
Complexesa
parameter
λa (nm)
compd
LI
solvent
λfex (nm)
λfem (nm)
Φf
solvent
MCH
acetonitrile 480 480 480
ethanol 480 480 480
acetonitrile 370 370 370
acetonitrile 0.4 0.4 0.6
LI LII Zn-I Zn-II
HBS
MCH
acetonitrile
ethanol
310, 380
300, 380
300, 390
300, 380
300, 380
300, 370
320, 440
320, 420
320, 420
320, 440
320, 430
320, 430
550
550
0.001 (0.15)b
0.0003
480 480 480
470
480
490
375
0.6
0.3
0.5
0.1
430 [440]b
550
0.0004 (0.2)
0.001 (0.12)
0.0005
0.0004 (0.18)
0.002 (0.17)
0.02
0.005 (0.15)
0.001 (0.14)
0.003
LII
MCH
acetonitrile
ethanol
MCH
acetonitrile
ethanol
MCH
acetonitrile
ethanol
560 (550)b
540
λb (nm)
yieldc
550
530 (530)
530
520 (530)
580 (540)
570
kw (105 M-1s-1
)
tert-butanol 1.5
acetonitrile 0.5 0.4 0.8
ethanol 1.8 1.6 1.4
2
1.2
Zn-I
Zn-II
0.2 [0.1]
a In argon- or air-saturated solution at 25 °C, λexc ) 308 nm. b Values
in brackets refer to HBO. c Relative value Φꢀ/(Φꢀ)ref using benzophe-
none in acetonitrile as reference.
570
0.003 (0.16)
a Using λexc ) 380 nm and λem ) 550 nm at 25 °C. b Values in
parentheses refer to -196 °C.
TABLE 5: Lifetime τK (µs) of the Keto Tautomer of the
Ligands and Complexesa
solvent
LI
LII
Zn-I
Zn-II
HBS
cyclohexane/MCH
dichloromethaneb
tert-butanol
+waterb
15
100
20
1.6
150
1.2
16
1.5
10
60
30
1
120
1
30
50
10
1.6
300
120
200
300
6
acetonitrile
1200
30 [1]c
80
+waterb
ethanol
15
1
20
60
+waterb
a In argon- or air-saturated solution at 25 °C, λexc ) 308 nm. b In
the presence of 5 M H2O. c Value refers to HBO.
Figure 3. Transient absorption spectra of Zn-II in (a) MCH and (b)
acetonitrile at 20 ns (O), 1 µs (4), 0.1 ms (b), 1 ms (2), and 10 ms
(9) after the 308 nm pulse. Insets: decay at 460 nm (upper) and 370
nm (lower).
The fluorescence intensity strongly increases on going to rigid
media: Φf ) 0.1-0.2 for Zn-I or Zn-II in glassy MCH or
ethanol at -196 °C (Table 3). The lifetime also increases, τf )
3-5 ns at -196 °C, whereas the spectra have comparable band
maxima (λfex and λfem) for the ligands and complexes in fluid
and glassy media.
Transient Absorption Properties. Transients were observed
(within the excitation pulse) for the four compounds at ambient
temperatures in all solvents examined. The transient of Zn-II
in MCH (Figure 3a), acetonitrile (Figure 3b) or ethanol shows
a broad absorption spectrum with a maximum (λa) at 480 nm
and a bleaching (λb) at 370 nm, λexc ) 248 nm. The transient
absorption is similar, when Zn-II is excited at 308 nm, but the
bleaching appears only at a sufficiently low concentration, e.g.,
corresponding to A350 <1. The decay at 480 nm follows first-
order kinetics at lower laser intensity and matches the bleaching
recovery (inset of Figure 3a). The time-resolved UV-vis spectra
of Zn-I, LII, and LI in acetonitrile (Figure 4a-c) and various
other media are similar in the respect that the transient is formed
within 10 ns is rather long-lived and has a maximum at λa )
470-490 nm (Table 4). The main decay component has a
lifetime (τK) ranging from 10 to 120 µs for LII in nonaqueous
solvents and from 0.06 to 1 ms for Zn-II (Table 5). The
lifetimes for LI and Zn-I at ambient temperatures are more or
less analogous. Exclusion of oxygen has no effect on τK, as
checked for the four compounds in cyclohexane or acetonitrile.
The transient species is attributed to the trans-keto tautomer
(see Discussion). The presence of the 440 nm peak of Zn-II
in nonpolar solvents or the absence of this band, due to exchange
Figure 4. Transient absorption spectra of (a) Zn-I, (b) LII, and (c) LI
in acetonitrile at 20 ns (O), 1 µs (4), 0.1 ms (b), and 1 ms (2) after
the 308 nm pulse. Insets: decay at 480 nm (upper) and 370 nm (lower).
of a ligand in polar solvents, does not affect the properties of
the observed trans-keto tautomer.
Increasing the temperature results in a shorter lifetime for
all cases in MCH, acetonitrile or ethanol. The activation energy,
obtained from linear Arrhenius plots (Figure 5), is Et-c ) 15,
14, and 14 kJ mol-1 for LII in MCH, acetonitrile, and ethanol,
respectively, and about 1 kJ mol-1 larger for Zn-II. The main
effect is a variation of the preexponential factor, being smallest
for Zn-II in acetonitrile, where τK is largest. The relative yield
of the complexes is larger than for the ligands, and the product
Φꢀ480 was compared to the reference benzophenone (Table 4).
The photocycle is fully reversible because repeated flashing of
any of the four compounds did not reveal products in a
measurable degree.
The transient was also detected in the IR using the step-scan
technique. Photolysis of LI in dichloromethane shows a bleach-

Proton Transfer of Phenol-Containing Ligands
J. Phys. Chem. A, Vol. 110, No. 8, 2006 2591
was likewise found to become faster on addition of water, the
rate constants are kw ) 1 × 104 and 2 × 104 M-1 s-1
,
respectively. It is noteworthy that for 2-hydroxybenzenes such
a linear dependence has not been reported as yet.
Discussion
Origin of the Fluorescence. The fluorescence in the 500-
600 nm range is suggested to be due to a proton transfer from
the hydroxy group to the nitrogen of the methine bond upon
excitation of the four compounds examined. Analogous pho-
toprocesses are well-known for HBO or HBS15-21 and water-
soluble 2-hydroxyphenylbenzazoles.8 A simplified model of the
ESIPT mechanism of HBO/HBS via enol f keto tautomerism
is illustrated in Scheme 1. The excited singlet enol state (1*E)
Figure 5. Plots of 1/τK vs 1/T for LII (open) and Zn-II (full) in MCH
(circles), acetonitrile (triangles), and ethanol (squares).
leads via the fluorescing excited singlet cis-keto state (1*Kcis)
to the trans-keto form.21,24,25 The corresponding
K
trans
state
1*
as source of fluorescence has been excluded.15-21 Electronic
coupling between acidic and basic centers of a 2-hydroxy-N-
methyl Schiff base and the intramolecular hydrogen bonding
and proton-transfer processes play a role. The trans f cis
isomerization (rate: kt-c) is rate determining in the thermal
relaxation process followed by proton back-transfer (rate:
k-p).16,17,21 Φf changes when the amount of water for HBO in
dioxane or the hydrogen bonding ability is increased.27 For
4-benzothiazole and 4-benzazole-type 2-yl-3-hydroxyphenoxy-
acetic acids8 in ethanol, the Stokes shift is much smaller than
for HBO or HBS in nonpolar solvents.15,27
The ESIPT mechanism, now suggested for LI or LII, is
illustrated in Scheme 2. The intramolecular hydrogen bonding
and proton transfer processes are probably analogous to HBO/
HBS or related 2-hydroxybenzenes. Excitation of the ligand enol
1*
form leads to the fluorescing
K
cis
state. Fluorescence from
1*
the
Φf on going from fluid to rigid media (Table 3). Internal
conversion 1*
cis f Ktrans has to be concluded, if also for LI or
K
trans
state is not compatible with the strong increase in
Figure 6. Plots of 1/τK vs [H2O] for LI (4), LII (O), Zn-I (2), and
Zn-II (b) in ethanol; insets: decay at 460 nm for 2 M water (upper
to lower, respectively).
K
LII the rate kt-c for trans f cis isomerization is much smaller
than the rate k-p for proton back-transfer. The solvent polarity
does not have a large influence on the fluorescence properties.
The Stokes shift, here simply based on the excitation and
emission maxima (Table 3), of ca. 5000 cm-1 for the two Zn
complexes is quite large and even larger, 8100-8800 cm-1, in
the cases of the ligands. The fluorescence properties are
comparable with those of HBO or HBS. A much smaller Stokes
shift of ca. 1500 cm-1 has been reported for a water-soluble
fluorinated HBO in methanol.7 The values for a Zn complex
ing at 1627 cm-1 and two absorption peaks at 1520 and 1650
cm-1 within the 308 nm pulse. The spectrum is similar for LII,
and both transients decay with τK of ca. 10 µs. The shorter
lifetime with respect to τK ) 60-100 µs (Table 5) is ascribed
to a quenching process, taking into account that the substrate
concentration, which is required for the IR measurements, is
ca. 20 times larger than under UV-vis conditions.
Interestingly, the decay of the transient becomes faster on
addition of water. From initially linear plots of 1/τK vs [H2O]
in ethanol (Figure 6), rate constants of kw ) 1.8 × 105, 1.6 ×
105, 1.4 × 105, and 0.2 × 105 M-1 s-1 were obtained at pH 7
for LI, LII, Zn-I, and Zn-II, respectively. The transient
absorption spectra remain the same as without water. Similar
results, in particular the lowest kw value for Zn-II, were found
in mixtures of acetonitrile or tert-butyl alcohol with water (Table
4). The decay of the transient of HBO and HBS in acetonitrile
with a 1,2-benzoquinone diimine ligand in water are λfem
)
576 nm, τf ) 1.6 ns and Φf ) 0.026.6 Novel fluorescent Zn
sensors, e.g., 3-hydroxy-3-phenyl-1-(o-carboxyphenyl)triazene9
and fluorescein-base10 Zn2+ complexes, were reported in the
literature, but their Stokes shifts are small. For a water-soluble
fluorinated HBO in methanol, the exchange of the phenolic
proton by Zn2+ causes a marked enhancement in Φf.7
SCHEME 1

2592 J. Phys. Chem. A, Vol. 110, No. 8, 2006
Go¨rner et al.
SCHEME 2
SCHEME 3
Nature of the Transient of the Ligands. The transient of
LI or LII (parts c and b of Figure 4) could, in principle, be (1)
a triplet state, (2) a radical in nature,33 (3) a solvated electron,
(4) a zwitterion, (5) an isomer due to rotation about the CdN
double bond or (6) an enol-keto tautomerism. Possibility 1 is
excluded because a triplet state reacts with oxygen, but oxygen
does not quench the observed transient. Photoionization of
phenols and formation of a phenoxyl radical has been
reported,12-14 but possibility 2 of a radical is also unlikely
because no reversibility in a first-order reaction would be
expected after homolytic cleavage. A second-order decay
component was generally present at higher laser intensity, but
this could mainly be avoided at a low enough intensity. Radical
ions could be formed by photoinduced electron transfer and
disappear by electron back-transfer, but the lack of any effect
of solvent polarity is not in line with this hypothesis. No solvated
electron was observed upon 20 ns 248 nm laser photolysis of
the ligands in tert-butyl alcohol or methanol, i.e., under
conditions where the solvated electron is not scavenged within
a few nanoseconds, as is the case in acetonitrile. A minor
contribution of solvated electrons could be detected only for
Zn-I at higher laser intensity. The absence of photoionization
for Zn-II under these conditions indicates slightly different
ionization potentials of the two complexes. We therefore
conclude that photoejection of an electron (3) does not occur.
Possibility 4 is a common photoprocess of heteroaromatics, e.g.,
spiropyrans,34 but unlikely for our cases. The isomerization (5)
could be hypothesized because of the two CdN double bonds
in the ligand. However, the shift in the maximum to λa ) 480
nm is too large compared to other cases, e.g., merocyanines, in
the literature.33 We are left with possibility 6 of an enol-keto
tautomerism, Scheme 2.
methine bond. This involves the trans-keto form, which is
observed within the pulse width. The back-transfer via the cis-
keto to the enol form is suggested to be slow trans f cis
isomerization and fast proton back-transfer. The lifetime of the
cis-keto form of 2-hydroxyarenes has been reported to be much
shorter than that of the trans-keto form.21-24 For HBO/HBS
the values related to the ground state are comparable to LI and
LII, e.g., λa ) 460 nm, τK ) 10-100 µs.17,18,21 However, the
reaction mechanism, in particular for HBO, is more complex,
owing to intersystem crossing and proton back-transfer via the
triplet entity.15-20 Moreover, a second-order reaction for re-
enolization has been observed for HBO in nonpolar sol-
vents.17,22,24,25 A small amount (a few millimolar) of water is
sufficient to open a pathway via proton-catalyzed re-enolization,
e.g., for HBO in a nonpolar solvent.25 On the other hand, the
effect of trace water is not significant for HBO in polar media.26
ESIPT of the Complexes. The presence of the 440 nm band
in the spectrum of the Zn-II complex in nonpolar solvents,
the decrease of the absorption at 440 nm in weakly polar
solvents such as carbon tetrachloride or dichloromethane, and
the absence in more polar solvents indicate an exchange of
binding of one ligand. The observed process could be formation
of a modified complex due to replacement of one ligand in Zn2-
(LH)2L by water or any solvent of large polarity. However, the
presence of the 440 nm peak for Zn-I in nonpolar as well as
polar solvents indicates a stable complex Zn2(LH)2L.
The transient of the complex could be attributed to a metal-
centered redox process, but this has to be excluded because Zn2+
ions are not redox-active. A phenol-related radical cation and
the solvated electron could be expected. However, this photo-
ejection of an electron (3) is not the case for Zn-II as no
solvated electron was observed upon 20 ns laser photolysis at
308 nm in tert-butyl alcohol or ethanol (see above). The Zn2-
(LH)2L structure shows that intramolecular hydrogen bonding
A proton is proposed to be rapidly transferred from the
phenolic hydroxy group of LI and LII to the nitrogen of the

Proton Transfer of Phenol-Containing Ligands
J. Phys. Chem. A, Vol. 110, No. 8, 2006 2593
or proton transfer from the phenolic hydroxy group to the
nitrogen of the methine bond is possible for the ligands
themselves. Only one part of the bis-chromophore of the
complex is involved. The ESIPT mechanism for the two Zn
complexes is illustrated in Scheme 3.
It is reasonable to conclude that the mechanism of transient
formation in all four cases is ESIPT. Intramolecular proton back-
transfer is consequently the mechanism of transient decay. The
lifetimes in nonaqueous solution, due to trans f cis isomer-
ization followed by proton back-transfer, are comparable but
not the same for the ligands and complexes in any given solvent
(Table 5). Longer lifetimes for the complexes than the ligands
are conceivable because the ligand flexibility in the complex is
strongly restricted. The preexponential factor, changing by more
than 1 order of magnitude, is sensitive to the solvent, whereas
the activation energy, Et-c ) 14-16 kJ mol-1, is essentially
constant (Figure 5).
For the ligands and complexes at -196 °C both viscosity
and temperature account for the large increases in Φf (Table 3)
and τf. The viscosity in MCH or ethanol increases by 10-12
orders of magnitude on going from +25 to -196 °C.35 This
has no marked influence on the fluorescence spectra but strongly
hinders radiationless deactivation steps competing with fluo-
rescence. The photophysical properties of the four compound
in a rigid environment are internal conversion and fluorescence
as dominant and minor processes, respectively.
The presence of water accelerates the proton back-transfer
(Figure 6), in particular, the enhancing effect of water on the
back-transfer is comparable for LI, LII, and Zn-I (Table 4).
Such a proton-catalyzed re-enolation has been observed for
HBO,20 but a linear dependence of 1/τK vs [H2O] has not been
reported as yet. For 4-benzothiazole and 4-benzazole-type 2-yl-
3-hydroxyphenoxyacetic acids in water, ESIPT is disrupted due
to intermolecular hydrogen bonding; metal coordination with
Zn2+ can also inhibit ESIPT.8
The absorption and fluorescence excitation spectra of the
complexes reveal the additional 440 nm band with respect to
the ligands, probably due to a metal to ligand CT transition.
The observed differences due to the ligand vs complex nature
concerning ESIPT and back-transfer are relatively small. One
could expect a slower back-transfer for the ligand in the complex
and this trend was indeed found (Table 5). The function of the
tert-butyl groups in LII and Zn-II seems to be due to both
electronic and steric reasons, but only in small quantity. For
example, the fluorescence is virtually not much affected by the
bulky groups (Table 2). The longest τK value for Zn-II in a
variety of solvents indicates a steric effect. On the other hand,
a much smaller kw value for Zn-II in tert-butyl alcohol or
ethanol (Figure 6) is not due to a shielding. The hydrophobic
tert-butyl groups may distort a water mediated proton-catalyzed
re-enolization reaction in the complex, but the four kw values
are more or less similar in the nonprotic acetonitrile (Table 4).
N,N′-bis(salicylidene)hydrazine type and the novel complexes
are believed to provide a wide application potential.
Acknowledgment. We thank Professor W. Lubitz for his
support and Mrs. H. Schucht, Mr. L. J. Currell and C. Lemsch
for technical assistance. Financial support from the DFG
(Molecular Magnetism, Ch 111/3-1) is gratefully acknowledged.
Supporting Information Available: X-ray crystallographic
files in CIF format for Zn-II, CCDC 279985. This material is
available free of charge via Internet at http://pubs.acs.org.
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