Excited-state dynamics and two-photon absorption cross sections of fluorescent diphenyl-tinIV derivatives with schiff bases: A comparative study of the effect of chelation from the ultrafast to the steady-state time scale

Article abstract of DOI:10.1021/jp904784b

Schiff bases bearing an intramolecular hydrogen bond are known to undergo excited-state intramolecular proton transfer and E-Z isomerization, which are related to their thermochromism and solvatochromism properties. In this study, we explored these ultrafast photoinduced processes for two doubly hydroxylated Schiff bases, salicylidene-2-aminophenol and 2-hydroxynaphthylmethylidene-2- aminophenol. From comparisons with our previously reported results for the parent monohidroxylated Schiff base salicylideneaniline, we were able to establish the lack of an effect of a second intramolecular hydrogen bond in the excited-state intramolecular proton-transfer process. Moreover, we synthesized and studied the photophysics of 14 diphenyl-tin^IV derivatives with Schiff bases with the same framework as the former two. In these organometallic compounds, we observed an increase of more than 50 times in the excited-state decay times in comparison with those of the free ligands. This finding is attributed to the coordination with the metallic center, which restricts the fluctuations of the geometry of the organic Schiff base skeleton. The emission bands of these complexes can be easily tuned through substitutions at the Schiff base ligand and can be made to be centered well above 600 nm. The much enhanced emissive behavior of all diphenyl-tin^IV derivatives allowed the study of several properties of their electronically excited states, including the effects of different substituents on their femtosecond and picosecond dynamics. Considering potential applications, we also performed transient absorption experiments to assess the wavelength interval for stimulated emission of this type of compound. Finally, we determined their two-photon absorption cross sections in the 760-820-nm range by measuring their two-photon induced fluorescence excitation spectra. Mainly, our results illustrate that the diphenyl-tin^IVmoiety, thanks to its size and its coordination mode with a single Schiff base, can be coordinated to this versatile framework to obtain tunable optical properties wherein the emissive states can have lifetimes on the nanosecond time scale.

Full text of DOI:10.1021/jp904784b

704
J. Phys. Chem. A 2010, 114, 704–714
Excited-State Dynamics and Two-Photon Absorption Cross Sections of Fluorescent
Diphenyl-TinIV Derivatives with Schiff Bases: A Comparative Study of the Effect of
Chelation from the Ultrafast to the Steady-State Time Scale
Jimena S. Zugazagoitia,† Mauricio Maya,† Carlos Damia´n-Zea,† Pedro Navarro,†
Hiram I. Beltra´n,‡ and Jorge Peon*,†
Instituto de Qu´ımica, UniVersidad Nacional Auto´noma de Me´xico, Circuito Exterior, Ciudad UniVersitaria,
04510 Me´xico D.F., Me´xico, and Departamento de Ciencias Naturales, UAM-Cuajimalpa, AVenida Pedro
Antonio de los Santos 84, 11850 Me´xico D. F., Me´xico
ReceiVed: May 21, 2009; ReVised Manuscript ReceiVed: NoVember 22, 2009
Schiff bases bearing an intramolecular hydrogen bond are known to undergo excited-state intramolecular
proton transfer and E-Z isomerization, which are related to their thermochromism and solvatochromism
properties. In this study, we explored these ultrafast photoinduced processes for two doubly hydroxylated
Schiff bases, salicylidene-2-aminophenol and 2-hydroxynaphthylmethylidene-2-aminophenol. From compari-
sons with our previously reported results for the parent monohidroxylated Schiff base salicylideneaniline, we
were able to establish the lack of an effect of a second intramolecular hydrogen bond in the excited-state
intramolecular proton-transfer process. Moreover, we synthesized and studied the photophysics of 14 diphenyl-
tinIV derivatives with Schiff bases with the same framework as the former two. In these organometallic
compounds, we observed an increase of more than 50 times in the excited-state decay times in comparison
with those of the free ligands. This finding is attributed to the coordination with the metallic center, which
restricts the fluctuations of the geometry of the organic Schiff base skeleton. The emission bands of these
complexes can be easily tuned through substitutions at the Schiff base ligand and can be made to be centered
well above 600 nm. The much enhanced emissive behavior of all diphenyl-tinIV derivatives allowed the study
of several properties of their electronically excited states, including the effects of different substituents on
their femtosecond and picosecond dynamics. Considering potential applications, we also performed transient
absorption experiments to assess the wavelength interval for stimulated emission of this type of compound.
Finally, we determined their two-photon absorption cross sections in the 760-820-nm range by measuring
their two-photon induced fluorescence excitation spectra. Mainly, our results illustrate that the diphenyl-tinIV
moiety, thanks to its size and its coordination mode with a single Schiff base, can be coordinated to this
versatile framework to obtain tunable optical properties wherein the emissive states can have lifetimes on the
nanosecond time scale.
Introduction
weak because of their fast excited-state decay processes,1–8 the
inclusion of a metallic center can dramatically enhance their
fluorescence (with the exception of some compounds bearing
heavymetals,becausetheycan,inprinciple,promotesinglet-triplet
mixing, also known as the “heavy-atom effect”).20,31
Schiff bases bearing an intramolecular hydrogen bond have
been extensively used as model systems to study fundamental
photophysical processes such as excited-state intramolecular
proton transfer (ESIPT), photochromism, solvatochromism, and
thermochromism.1–13 Among these compounds, salicylidene-
ortho-aminophenols and related Schiff bases have been further
targeted and studied because, in addition to the above-mentioned
characteristics, they also have the ability to act as versatile
chelating ligands.14 Some derivatives with this type of Schiff
base have found important applications, as they have been
considered as molecular switches,15 as materials with nonlinear
optical properties,16–19 and as components in the construction
of organic light-emitting devices (OLEDs).20–24
There are several examples of fluorescent derivatives of
similar systems (where two aromatic groups in the ligand are
joined through a section that includes a double bond14,20–30,32,33).
However, more studies are needed to determine the specific
effects of different and more versatile metallic centers in the
luminescent properties. This is important because the chelating
center will determine the degree of charge-transfer character of
the electronic excitation, as well as the extent of the electronic
delocalization in the molecule. In this context, we have carried
out a systematic study aimed to determine how the bonding to
a tin metal center changes the photophysical properties of the
basic salicylidene-ortho-aminophenol organic framework. The
present analysis also describes the contrast between the pho-
tophysics of the two doubly hydroxylated Schiff bases (“free
ligands”) salicylidene-2-aminophenol (1) and 2-hydroxynaph-
thylmethylidene-2-aminophenol (2), and the derivatives they
form when bis-chelating a diphenyl-tinIV unit (3 and 4 series,
respectively; see Scheme 1). As will be shown, the diphenyl-
Upon chelation, the photophysical features of the metallic
derivatives are vastly different from those of the free ligands,
where the most notorious, in some cases, is an improvement of
their fluorescent character.14,25–30 Here, it is worth emphasizing
that, although the fluorescence from Schiff bases is extremely
* To whom correspondence should be addressed. E-mail: jpeon@
servidor.unam.mx.
† Universidad Nacional Auto´noma de Me´xico.
‡ UAM-Cuajimalpa.
10.1021/jp904784b  2010 American Chemical Society
Published on Web 12/28/2009

Excited-State Dynamics of Diphenyl-TinIV Complexes
J. Phys. Chem. A, Vol. 114, No. 2, 2010 705
SCHEME 1: (a) Structures and Atom Numbering of the
Free Schiff Bases (1 and 2) and Corresponding
Diphenyl-TinIV Derivatives (3a-g and 4a-g) of This
Study and (b) Plane Angle Measurement of Structurally
Available Schiff Bases
us to distinguish the spectral region where the S1 f S0
stimulated emission can be observed, in contrast to excited
singlet-to-singlet state absorption (Sn r S1). Finally, taking
advantage of their highly emissive character, we measured the
two-photon absorption cross sections (TPACSs) of the 3- and
4-series compounds, by monitoring their two-photon excitation
fluorescence (TPEF).
Experimental Methods
2-Hydroxy-1-naphthaldehyde was supplied from Aldrich and
recrystallized from ethanol. All other reactants were purchased
from Aldrich and used without further purification. The solution
NMR characterization experiments were performed on a Bruker
1
300 spectrometer at room temperature. The H NMR spectra
were recorded at 300.13 MHz using a spectral width of 6200
Hz and an acquisition time of 2.64 s. In all cases, the spectra
were obtained using 16384 data points, a 45° pulse width, and
16 scans. The 13C NMR spectra were recorded at 75.468 MHz
using a spectral width of 17360 Hz and an acquisition time of
2.50 s, with 16384 data points and at least 1024 scans. The
119Sn spectra were recorded at 100.726 MHz using a spectral
width of 10080 Hz, an acquisition time of 0.4 s, 16384 data
points, and at least 1024 scans. The direct observation of 2D
NMR spectra was performed through (13C-1H) heteronuclear
correlation (HETCOR), (1H-1H) correlation spectroscopy (COSY)
experiments using standard Bruker pulse sequences. The chemi-
cal shifts (ppm) are relative to TMS [δ(1H) ) 0.00 and δ(13C)
) 0.0 ppm] and SnMe4 [δ(119Sn) ) 0.0 ppm], and coupling
constants are quoted in hertz. Infrared spectra were recorded as
KBr pellets on a Bruker Tensor 27 FT-IR spectrophotometer.
Melting points were measured on a Fisher-Johns apparatus and
were not corrected. Mass spectrometry determinations through
the fast atom bombardment (FAB+) technique were made on a
JEOL SX-102A spectrometer using a meta-nitrobenzylic alcohol
matrix.
tinIV moiety is a chelation center with a very appropriate size
for a three-center interaction with a single Schiff base, making
it an adaptable system for designing multichromophoric systems.
In particular, diphenyl-tinIV chelation allows the tuning of the
position of the emission band in an easy-to-synthesize system
through changes in the substitutions at the Schiff base skeleton.
We also show that, although the large nuclear charge of the tin
atom could, in principle, accelerate the complexes’ S1 decay
due to intersystem crossing,20,31 these molecules have dramati-
cally increased fluorescence lifetimes in comparison with those
of the free bases because of the restrictions of their structural
fluctuations.
Synthesis of 1 and 2: General Procedure. Equimolar
amounts of salicylaldehyde (for 1) or 2-hydroxy-1-naphthalde-
hyde (for 2) and 2-aminophenol were refluxed in methanol for
1.5 h with azeotropic removal of water. Compounds 1 and 2
were obtained as solid precipitates and recrystallized twice from
a methylene chloride-methanol solution; their purities were
verified by thin-layer chromatography on Silufol UV sheets
using ethyl acetate, methylene chloride, and methanol as mobile
phases. 1H NMR structural characterization and physical proper-
ties were compared with those reported elsewhere.23,42
Synthesis of 3a-g and 4a-g: General Procedure. Equimo-
lar amounts of diphenyl-tinIV oxide, salicylaldehyde (for 3a-g)
or 2-hydroxy-1-naphthaldehyde (for 4a-g), and the correspond-
ing ortho-aminophenol (a, 2-aminophenol; b, 2-amino-4-nitro-
phenol; c, 2-amino-5-nitrophenol; d, 2-amino-para-cresol; e,
6-amino-meta-cresol; f, 2-amino-4-tert-butylphenol; g, 2-amino-
4-chlorophenol) were maintained in ethanol reflux for 12 h under
azeotropic removal of water. All compounds were isolated as
solids after reduced-pressure distillation of the ethanol; recrys-
tallized from a 1:5 mixture of dichloromethane and ethanol;
and then characterized by 1H, 13C, 2D (HETCOR and COSY),
and 119Sn NMR spectroscopies. For 3a-g, all data resemble
those reported previously.35,43 The yields, melting points, IR,
NMR, mass spectrometry (FAB+), and elemental analysis data
for 4a-g are reported in the following paragraphs. Purities were
verified by thin-layer chromatography on Silufol UV254 sheets
using ethanol as the mobile phase, as well as by comparison
between steady-state fluorescence excitation and absorption
spectra.
We chose a diphenyl-tinIV moiety based on the fact that, in
contrast to what happens with dimethyl-tinIV, di-n-butyl-tinIV,
and other nonaromatic diorganil-tinIV groups that usually lead
to the formation of dimeric and other supramolecular coordina-
tion species, the compounds resulting from chelation of a
diphenyl-tinIV fragment by Schiff bases have the desirable
feature that they remain as monomers, thus facilitating their
spectroscopic and physicochemical analysis.16,34–41 The consid-
eration of a chelation center with two extra organometallic bonds
(to phenyl groups in this case) also indicates the possibility of
using these extra positions for combined effects in future
multichromophoric systems (synthetically, the tinIV center is
ideal for this purpose).
To fully compare the photophysics of the ligands with those
of the derivatives, we performed both steady-state and time-
resolved experiments. With the steady-state measurements, we
analyzed the influence that different substituents placed at the
aminophenol (aniline) ring have on the electronic transitions
of the derivatives (3 and 4 series). With time-resolved fluores-
cence up-conversion experiments, we were able to determine
that the improved rigidity of the Schiff base framework in the
3- and 4-series diphenyl-tinIV compounds produces a remarkable
increase (of more than 50 times) in the fluorescence lifetimes
with respect to the analogous free Schiff bases. Thinking about
future possible applications such as the construction of organic
light-emitting devices (OLEDs), we also examined the excited-
state dynamics through femtosecond transient absorption for one
representative tinIV derivative (4d). Such measurements allowed

706 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Zugazagoitia et al.
2,2-Diphenyl-6-aza-1,3-dioxa-2-stanna[d]benzo[1,2-h]naph-
thocyclononene (4a). Red crystals, yield 88%, mp 208-210
°C. IR (ν) (KBr): 3051, 1593 (CdN), 1538, 1471, 1396, 1306,
1175, 1073, 982, 830, 739, 699, 613 (SnsC), 558 (SnsO), 449
(SnsN) cm-1. 1H NMR (δ, ppm, CDCl3): 9.39 (1H, H7), 7.94
(4H, Hortho), 7.42 (1H, H16), 7.78 (1H, H11), 7.58 (1H, H13),
7.38 (4H, Hmeta), 7.37 (1H, H15), 7.36 (2H, Hpara), 7.31 (1H,
H20), 7.22 (1H, H18), 7.18 (1H, H10), 7.17 (1H, H14), 7.12
(1H, H21), 6.71 (1H, H19). 13C NMR (δ, ppm, CDCl3): 172.6
(C9), 158.6 (C7), 156.0 (C4), 139.5 (Cipso), 138.9 (C11), 136.4
(Cortho), 134.1 (C17), 132.6 (C5), 130.6 (Cpara), 129.5 (C13),
129.4 (C20), 128.7 (Cmeta), 128.5 (C15), 127.3 (C12), 124.8
(C14), 123.6 (C10), 118.7 (C21), 118.6 (C16), 117.0 (C19),
114.7 (C18), 109.1 (C8). 119Sn NMR (δ, ppm): -325.7 (CDCl3),
-420.2 (DMSO-d6). MS-FAB m/z (%): 535 M+[120Sn] (20),
533 M+[118Sn] (15), 531 M+[116Sn] (7), 458 M+[120Sn] - Ph
(5), 456 M+[118Sn] - Ph (4), 454 M+[116Sn] - Ph (3), 391 (10),
380 M+[120Sn] - C12H11 (5), 378 M+[118Sn] - C12H11 (3), 376
M+[116Sn] - C12H11 (1), 307 (35), 289 (17), 154 (100), 136
(61), 107 [C7H7O+] (17), 89 [C4H9O2+] (15), 77 [C6H5+] (13).
Elemental analysis. Found (%): C, 65.15; H, 3.88; N, 2.66.
Calculated for C29H21NO2Sn (%): C, 65.20; H, 3.96; N, 2.62.
2,2-Diphenyl-6-aza-1,3-dioxa-15-methyl-2-stanna[d]ben-
zo[1,2-h]naphthocyclononene (4d). Red powder, yield 94%,
mp 205-208 °C. IR (ν) (KBr): 3052, 3028, 1602 (CdN), 1534,
1493, 1471, 1291, 1193, 1150, 840, 813, 757, 735, 701 (SnsC),
1
536 (SnsO), 443 (SnsN) cm-1. H NMR (δ, ppm, CDCl3):
9.47 (1H, H7), 7.98 (1H, H16), 7.93 (4H, Hortho), 7.84 (1H, H11),
7.66 (1H, H13), 7.48 (1H, H15), 7.38-7.37 (6H, Hmeta, Hpara),
7.28 (1H, H10), 7.22 (1H, H14), 7.19 (1H, H21), 7.04 (2H,
H18, H20), 2.32 (3H, H34). 13C NMR (δ, ppm, CDCl3): 172.5
(C9), 156.4 (C4), 155.7 (C7), 139.5 (Cipso), 138.7 (C11), 136.5
(Cortho), 134.1 (C17), 132.1 (C5), 130.6 (C20), 130.3 (Cpara),
129.5 (C13), 128.7 (Cmeta), 128.5 (C15), 127.4 (C12), 126.4
(C19), 124.9 (C14), 123.6 (C10), 118.8 (C16), 118.4 (C21),
115.0 (C18), 109.2 (C8), 20.9 (C34). 119Sn NMR (δ, ppm):
-324.4 (CDCl3), -399.0 (DMSO-d6). MS-FAB m/z (%): 549
M+[120Sn] (21), 547 M+[118Sn] (16), 545 M+[116Sn] (7), 472
M+[120Sn] - Ph (5), 470 M+[118Sn] - Ph (3), 468 M+[116Sn]
- Ph (2), 394 M+[120Sn] - C12H11 (5), 392 M+[118Sn] - C12H11
(4), 390 M+[116Sn] - C12H11 (1), 391 (8), 307 (26), 289 (13),
154 (100), 136 (61), 107 [C7H7O+] (18), 89 [C4H9O2+] (17),
77 [C6H5+] (15). Elemental analysis. Found (%): C, 64.89; H,
4.81; N, 2.46. Calculated for C30H23NO2Sn· · ·C2H5OH (%): C,
64.67; H, 4.92; N, 2.36.
2,2-Diphenyl-6-aza-1,3-dioxa-15-nitro-2-stanna[d]benzo[1,2-
h]naphthocyclononene (4b). Red powder, yield 93%, mp
234-236 °C. IR (ν) (KBr): 3070, 1597 (CdN), 1536, 1501,
1430, 1386, 1304, 1164, 1080, 991, 834, 746, 698 (SnsC), 594
2,2-Diphenyl-6-aza-1,3-dioxa-16-methyl-2-stanna[d]ben-
zo[1,2-h]naphthocyclononene (4e). Red powder, yield 80%,
mp 177-179 °C. IR (ν) (KBr): 3064, 2917, 1606 (CdN), 1539,
1491, 1454, 1432, 1397, 1366, 1307, 1183, 1073, 979, 862, 828,
1
(SnsO), 488, 450 (SnsN) cm-1. H NMR (δ, ppm, CDCl3):
1
9.57 (1H, H7), 8.36 (1H, H18), 8.19 (1H, H16), 8.07 (1H, H20),
7.97 (1H, H11), 7.89 (4H, Hortho), 7.74 (1H, H13), 7.61 (1H,
H15), 7.45-7.40 (6H, Hmeta, Hpara), 7.39 (1H, H14), 7.25 (1H,
H10), 7.12 (1H, H21). 13C NMR (δ, ppm, CDCl3): 174.1 (C9),
164.8 (C4), 157.8 (C7), 140.9 (C11), 138.1 (Cipso), 137.9 (C19),
136.2 (Cortho), 133.9 (C17), 132.5 (C5), 130.8 (Cpara), 129.7
(C13), 129.3 (C15), 129.0 (Cmeta), 127.4 (C12), 125.3 (C20),
124.6 (C10), 124.4 (C14), 119.0 (C16), 118.1 (C21), 111.5
(C18), 109.4 (C8). 119Sn NMR (δ, ppm): -316.9 (CDCl3),
-417.1 (DMSO-d6). MS-FAB m/z (%): 580 M+[120Sn] (9), 578
M+[118Sn] (7), 576 M+[116Sn] (3), 503 M+[120Sn] - Ph (3), 501
M+[118Sn] - Ph (2), 499 M+[116Sn] - Ph (1), 391 (6), 307 (27),
735, 698 (SnsC), 574 (SnsO), 482, 448 (SnsN) cm-1. H
NMR (δ, ppm, CDCl3): 9.29 (1H, H7), 7.95 (4H, Hortho), 7.81
(1H, H16), 7.73 (1H, H11), 7.53 (1H, H13), 7.39 (1H, H15),
7.42-7.38 (6H, Hmeta, Hpara), 7.16 (1H, H10), 7.15 (H18), 7.14
(1H, H14), 6.94 (1H, H21), 6.49 (1H, H19). 13C NMR (δ, ppm,
CDCl3): 172.0 (C9), 158.3 (C4), 154.9 (C7), 139.8 (C20), 139.6
(Cipso), 138.4 (C11), 136.5 (C5), 136.4 (Cortho), 134.0 (C17),
130.2 (Cpara), 129.3 (C13), 128.6 (Cmeta), 128.3 (C15), 127.5
(C12), 124.7 (C10), 123.4 (C14), 119.0 (C21), 118.7 (C16),
118.0 (C19), 114.2 (C18), 109.0 (C8), 21.4 (C34). 119Sn NMR
(δ, ppm): -325.9 (CDCl3), -421.1 (DMSO-d6). MS-FAB m/z
(%): 549 M+[120Sn] (23), 547 M+[118Sn] (17), 545 M+[116Sn]
(9), 472 M+[120Sn] - Ph (5), 470 M+[118Sn] - Ph (3), 468
M+[116Sn] - Ph (2), 394 M+[120Sn] - C12H11 (5), 392 M+[118Sn]
- C12H11 (5), 390 M+[116Sn] - C12H11 (1), 391 (20), 307 (32),
289 (14), 154 (100), 136 (67), 107 [C7H7O+] (21), 89 [C4H9O2 ]
+
+
(21), 77 [C6H5 ] (19). Elemental analysis. Found (%): C, 59.99;
H, 3.49; N, 4.81. Calculated for C29H20N2O4Sn (%): C, 60.14;
H, 3.48; N, 4.84.
+
289 (18), 154 (100), 136 (66), 107 [C7H7O+] (23), 89 [C4H9O2 ]
+
(23), 77 [C6H5 ] (23). Elemental analysis. Found (%): C, 64.75;
2,2-Diphenyl-6-aza-1,3-dioxa-16-nitro-2-stanna[d]benzo[1,2-
h]naphthocyclononene (4c). Red powder, yield 90%, mp > 290
°C. IR (ν) (KBr): 3048, 1602 (CdN), 1568, 1539, 1515, 1475,
1432, 1384, 1336, 1315, 1278, 1145, 1073, 873, 835, 816, 741,
699 (SnsC), 578 (SnsO), 452 (SnsN) cm-1. 1H NMR (δ, ppm,
CDCl3): 9.60 (1H, H7), 8.08 (1H, H16), 8.00 (1H, H11), 7.89
(4H, Hortho), 7.87 (1H, H18), 7.76 (1H, H13), 7.65 (1H, H19),
7.58 (1H, H15), 7.57 (1H, H20), 7.56 (1H, H21), 7.42-7.40
(6H, Hmeta, Hpara), 7.39 (1H, H14), 7.24 (1H, H10). 13C NMR
(δ, ppm, CDCl3): 174.0 (C9), 157.8 (C7), 155.1 (C4), 147.1
(C20), 140.4 (C11), 138.1 (Cipso), 135.6 (Cortho), 134.2 (C17),
133.8 (C5), 130.0 (Cpara), 129.1 (C13), 128.6 (C15), 128.3
(Cmeta), 127.1 (C12), 124.1 (C10), 123.8 (C14), 118.4 (C16),
114.5 (C21), 112.4 (C18), 111.6 (C19), 109.1 (C8). 119Sn NMR
(δ, ppm): -322.5 (CDCl3), -414.6 (DMSO-d6). MS-FAB m/z
(%): 580 M+[120Sn] (11), 578 M+[118Sn] (8), 576 M+[116Sn] (4),
503 M+[120Sn] - Ph (4), 501 M+[118Sn] - Ph (2), 499 M+[116Sn]
- Ph (1), 391 (21), 307 (24), 289 (14), 154 (100), 136 (73),
107 [C7H7O+] (23), 89 [C4H9O2+] (24), 77 [C6H5+] (24).
Elemental analysis. Found (%): C, 60.34; H, 3.51; N, 4.92.
Calculated for C29H20N2O4Sn (%): C, 60.14; H, 3.48; N, 4.84.
H, 4.87; N, 2.48. Calculated for C30H23NO2Sn· · ·C2H5OH (%):
C, 64.67; H, 4.92; N, 2.36.
2,2-Diphenyl-6-aza-1,3-dioxa-15-tert-butyl-2-stanna[d]ben-
zo[1,2-h]naphthocyclononene (4f). Red powder, yield 86%,
mp 236-240 °C. IR (ν) (KBr): 3049, 2954, 1600 (CdN), 1538,
1491, 1458, 1427, 1396, 1362, 1303, 1279, 1192, 1098, 1072,
830, 739, 699 (SnsC), 557 (SnsO), 496, 446 (SnsN) cm-1
.
1H NMR (δ, ppm, CDCl3): 9.49 (1H, H7), 7.95 (4H, Hortho),
7.94 (1H, H16), 7.82 (1H, H11), 7.65 (1H, H13), 7.48 (1H,
H15), 7.38 (1H, H18), 7.37-7.34 (6H, Hmeta, Hpara), 7.29 (1H,
H20), 7.27 (1H, H14), 7.21 (1H, H10), 7.10 (1H, H21), 1.36
(9H, H35, H36, H37). 13C NMR (δ, ppm, CDCl3): 172.4 (C9),
156.3 (C4), 155.6 (C7), 140.0 (C19), 139.4 (Cipso), 138.7 (C11),
136.5 (Cortho), 134.1 (C17), 131.3 (C5), 130.2 (Cpara), 129.5
(C13), 128.7 (Cmeta), 128.5 (C15), 127.4 (C12), 126.8 (C20),
124.9 (C10), 123.6 (C14), 118.6 (C16), 118.1 (C21), 111.2
(C18), 109.2 (C8), 34.3 (C34), 31.6 (C35, C36, C37). 119Sn
NMR (δ, ppm): -321.9 (CDCl3), -391.8 (DMSO-d6). MS-FAB
m/z (%): 591 M+[120Sn] (21), 589 M+[118Sn] (15), 587 M+[116Sn]
(7), 514 M+[120Sn] - Ph (4), 512 M+[118Sn] - Ph (3), 510

Excited-State Dynamics of Diphenyl-TinIV Complexes
J. Phys. Chem. A, Vol. 114, No. 2, 2010 707
M+[116Sn] - Ph (1), 422 M+[120Sn] - C13H13 (4), 420 M+[118Sn]
- C13H13 (3), 418 M+[116Sn] - C13H13 (2), 391 (6), 307 (29),
sample solution. Fluorescence was collected with a pair of
parabolic mirrors and refocused to another 0.5-mm BBO crystal,
where it was crossed at its focal point with the remaining and
temporally delayed fundamental gate beam. The resultant
frequency-mixing signal was finally focused into a double
monochromator (Oriel) and detected with a photomultiplier tube.
The instrument response function was determined by time-
resolving the Raman scattering of the solvent and was found to
be Gaussian with full widths at half maxima of 190 ( 5, 210
( 5, and 220 ( 5 fs for setups resolving fluorescence at 422,
390, and 385 nm, respectively.
+
289 (13), 154 (100), 136 (62), 107 [C7H7O+] (17), 89 [C4H9O2 ]
+
(16), 77 [C6H5 ] (13). Elemental analysis. Found (%): C, 67.22;
H, 4.86; N, 2.43. Calculated for C33H29NO2Sn (%): C, 67.14;
H, 4.95; N, 2.37.
2,2-Diphenyl-6-aza-1,3-dioxa-15-chloro-2-stanna[d]ben-
zo[1,2-h]naphthocyclononene (4g). Brown powder, yield 82%,
mp > 290 °C. IR (ν) (KBr): 3054, 3032, 1596 (CdN), 1535,
1471, 1427, 1393, 1363, 1295, 1197, 1074, 914, 832, 794, 739,
1
699 (SnsC), 555 (SnsO), 523, 493, 454 (SnsN) cm-1. H
For transient absorption experiments, the pulsed beam at the
output of the Ti:sapphire oscillator was passed through a
regenerative amplification system to yield 0.3 mJ pulses at a
repetition rate of 1 kHz. Following a previously reported
procedure,45 the pump at 400 nm was generated in a 0.5-mm
ꢀ-BBO crystal and separated from the remaining fundamental
with a dichroic mirror (CVI), after which it was passed through
a half-wave plate to adjust to magic angle conditions, and finally
focused at the sample with a 15-cm fused-silica lens. A portion
of the remaining fundamental was passed through a 1-mm Z-cut
sapphire plate to generate a white light continuum probe beam
that was sent to a delay stage before it was focused in the
sample. The relative spot sizes of the probe and pump beams
(200 and 20 µm, correspondingly) ensured that the focused
probe light was homogeneously overlapped with the excitation
pulses for the full spectral range of our experiments. Our setup
used single-channel detection where a double 10-cm mono-
chromator was set to a particular wavelength to acquire the
transient transmittance change through the use of a diode, an
optical chopper in the path of the pump beam, and a lock-in
amplifier. The probe light intensity (I0) was determined in a
separate measurement with the pump beam blocked and with
the chopper directly modulating the intensity of the probe beam.
The instrument response function for transient absorption
measurements was found to be Gaussian with a 400-fs full width
at half-maximum.45
NMR (δ, ppm, CDCl3): 9.28 (1H, H7), 7.90 (4H, Hortho), 7.87
(1H, H16), 7.81 (1H, H11), 7.59 (1H, H13), 7.43 (1H, H15),
7.39-7.35 (4H, Hmeta, Hpara), 7.26 (1H, H18), 7.21 (1H, H14),
7.17 (1H, H10), 7.13 (1H, H20), 7.03 (1H, H21). 13C NMR (δ,
ppm, CDCl3): 173.2 (C9), 157.1 (C4), 156.4 (C7), 139.6 (C11),
139.1 (Cipso), 136.4 (Cortho), 134.0 (C17), 133.3 (C5), 130.4
(Cpara), 129.5 (C13), 129.0 (C20), 128.8 (Cmeta), 128.7 (C15),
127.4 (C12), 124.8 (C10), 123.9 (C14), 121.6 (C19), 119.5
(C21), 118.8 (C16), 114.9 (C18), 109.2 (C8). 119Sn NMR (δ,
ppm): -321.5 (CDCl3), -406.3 (DMSO-d6). MS-FAB m/z (%):
569 M+[120Sn] (40), 567 M+[118Sn] (30), 565 M+[116Sn] (14),
492 M+[120Sn] - Ph (10), 490 M+[118Sn] - Ph (8), 488
M+[116Sn] - Ph (4), 415 M+[120Sn] - Ph2 (9), 413 M+[120Sn]
- Ph2 (8), 411 M+[118Sn] - Ph2 (4), 391 (12), 307 (25), 289
+
(14), 154 (100), 136 (68), 107 [C7H7O+] (22), 89 [C4H9O2
]
+
(23), 77 [C6H5 ] (20). Elemental analysis. Found (%): C, 61.20;
H, 3.65; N, 2.52. Calculated for C29H20ClNO2Sn (%): C, 61.25;
H, 3.55; N, 2.46.
Steady-State Spectroscopy. Stationary absorption spectra
were recorded with a Cary-50 spectrophotometer (Varian).
Steady-state fluorescence emission and excitation spectra were
measured on a Cary Eclipse spectrophotometer (Varian) and
were always performed on solutions with optical densities less
than or equal to 0.05. All measurements were performed at room
temperature (20 ( 1 °C) in HPLC- (high-performance-liquid-
chromatography-) quality solvents.
Two-Photon Absorption Cross Sections (TPACSs). To
determine TPACSs, we followed the two-photon-induced
fluorescence measurement technique,46–48 using a constantly
refreshed 10 µM solution of recrystallized RhB in HPLC-grade
methanol solution as the reference. To produce TPEF, we used
a Ti:sapphire oscillator pumped by a 5-W Verdi V5 continuous
laser (Coherent Inc.) to generate a 83 MHz, femtosecond pulsed
train. The obtained 750-890-nm tunable pulsed beam was
passed through a fused-silica compressor and a half-wave plate,
followed by a calcite Glan-Taylor polarizer before it was split
by a neutral-density filter. The less intense branch was passed
through a chopper and then focused into a silicon diode
connected to an oscilloscope (Agilent) to continuously check
the intensity of the beam. To determine the correspondence
between the voltage and the power of the beam, we made a
linear regression study at every excitation wavelength used.
Power values were taken for intensities at which the power-
meter resolution was accurate, and then its linear behavior with
respect to voltage was verified.
Fluorescence Quantum Yields. To avoid self-absorption of
emitted fluorescence and ensure linear behavior, diluted solutions
(about 4 µM) of diphenyl-tinIV derivatives with Schiff bases in
HPLC-grade ethanol were used to record steady-state fluores-
cence emission spectra at two different excitation wavelengths
(355 and 490 nm). Fluorescence quantum yields (φ) were
estimated with reference to rhodamine B (RhB) in methanol.44
The RhB standard was purchased from Aldrich and recrystal-
lized from methylene chloride. Steady-state fluorescence emis-
sion spectra of 0.44 µM solutions of RhB in methanol were
acquired at the same two excitation wavelengths as the 3 and 4
series. The fluorescence quantum yield of RhB in methanol (φR)
was taken as 0.70.44 Measurements of fluorescence at each
excitation wavelength and optical densities of all compounds
were recorded in triplicate using a 1-cm quartz cell. We found
typical percentage errors in φ of 10%. Fluorescence quantum
yields of compounds 1 and 2 were measured in a similar way,
by direct excitation of the E-OH form (355 nm).
Time-Resolved Spectroscopy. Our experimental up-conver-
sion technique has been previously reported.7 Briefly, a pulsed
beam with a repetition rate of 100 MHz, generated in a Ti:
sapphire oscillator pumped by a 5-W Verdi V5 laser (Coherent
Inc.), was passed through a fused-silica prism compressor before
it was frequency-doubled in a 0.5-mm BBO (BaB2O4) crystal.
The second harmonic was passed through a half-wave plate to
control its polarization to magic-angle conditions and then
focused into a 1-mm-path-length flow quartz cell containing the
The remaining beam branch was focused into the center of a
stoppered 1-cm-path-length quartz cell containing the sample.
To ensure homogeneity of the solutions and avoid erroneous
measurements due to photobleaching, a small magnetic stirrer
was introduced into the cell so the samples were constantly
stirred. Because of this, small amounts of dust had to be avoided
because they could cause an abnormally high value of the
detected TPEF. To achieve this goal, we filtered every solution

708 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Zugazagoitia et al.
SCHEME 2: Energy Diagram and Electronic Transitions
Involved in the Photoinduced Cycle of the Free Schiff
Bases under Studya,b
a The ε and hν labels indicate absorption and fluorescence emission,
respectively. b The S0 state of the Z-NH (Z-ketoenamine) tautomer is
represented by two different lines: The upper solid line indicates
solvent-solute combinations in which the Z-NH isomer is less stable
at room temperature and its steady-state absorption band at ∼450 nm
is rather small (e.g., 1 in methylene chloride), whereas the lower dashed
line refers to the cases in which a ground-state equilibrium between
the E-OH (E-enolimine) and Z-NH forms can be reached at room
temperature (e.g., 1 in methanol).
Figure 1. Stationary absorption (solid lines) and fluorescence spectra
(dashed line) of (a) 1 and (b) 2 in different solvents. Fluorescence
spectra were recorded upon excitation at 355 nm.
through 0.45-µm nylon filters (Millipore) supported in 2.5-cm-
diameter swinnex filter holders (Millipore). The resulting TPEF
was collimated and continuously collected by a UV-vis portable
spectrophotometer (Ocean Optics) with variable integration time,
situated perpendicular to the incident beam. Because TPACSs
were measured relative to a standard, every sample measurement
was taken between two measurements of RhB reference
solution; each time, five values were recorded, and they were
all measured by recording the intensity of the TPEF spectra at
the fluorescence maximum. To estimate typical percentage error,
four randomly chosen diphenyl-tinIV derivatives were measured
in duplicate. We found typical percentage errors in TPACSs of
15%. To ensure a biphotonic excitation process, we verified
the TPEF squared dependence on beam intensity for four
randomly chosen derivatives.
because the tautomeric shift toward the Z-NH form is not
necessarily determined by a high dielectric constant of the
solvent. Instead, the solvatochromic effect on Schiff bases must
be explained by means of both intramolecular and specific
solvent-solute interactions. Hence, in 1, the intramolecular
hydrogen bond in the E-OH tautomer is sufficiently stable to
predominate in all solvents. Only the alcohol solvents allow, to
a small extent, the formation of the Z-NH isomer by stabilizing
it with a particular hydrogen-bond network. In contrast, the
Z-NH form of 2 is more easily formed. The overall preference
for the E-OH form of 1 versus the preference for the Z-NH
isomer of 2 presumably lies in the fact that the loss of
aromaticity when going from the E-OH to the Z-NH form in 2
is compensated by the other aromatic ring of the naphthalene
system.10,13
The steady-state emissions of 1 and 2 are largely Stokes-
shifted with respect to the E-OH absorption bands (Figure 1),
even upon direct excitation of this last isomer. In agreement
with other Schiff bases, this supports the fact that the dominant
fluorescence signal comes from the Z-NH isomer that is formed
after an ESIPT from the E-OH local excited state.1–9,11–13 We
estimated the fluorescence quantum yields of 1 and 2 in ethanol
as 0.003 ( 0.001 and 0.005 ( 0.001, respectively, under direct
excitation of the E-OH form. These values are consistent with
the typical low yields of Schiff bases with intramolecular
hydrogen bonds,6,8,13 accounting for fast depletion channels
occurring in the excited state.
Photodegradation of the samples was monitored by stationary
absorption spectra taken both before and after TPEF was
induced. Compounds 3a-g and 4a-g were dissolved in HPLC-
grade ethanol, and the resulting solutions had concentrations
of around 0.1 mM. Exact molarities were calculated with the
stationary absorption spectra.
Results and Discussion
Steady-State Spectroscopy. The stationary absorption spectra
of the dihydroxylated free Schiff bases 1 and 2 in different
solvents are shown in Figure 1, where we also include their
steady-state emission spectra in ethanol. From comparison with
similar systems,1–9,11–13 for both Schiff bases, the absorption band
at shorter wavelengths can be assigned to the S1 r S0 transition
of the E-enolimine isomer (E-OH; see Scheme 2 for isomer
visualizations). In 1, solvatochromism is noticeable only in protic
solvents such as methanol and ethanol, where the shoulder
centered at 450 nm indicates the S1 r S0 absorption of the
Z-ketoenamine form (Z-NH).1–9,11–13 In contrast, 2 exhibits a
more marked solvatochromic effect. Here, the band at 380 nm
corresponding to the E-OH form is prominent only in carbon
tetrachloride and methylene chloride, and it decreases in more
polar solvents with the simultaneous increase of absorption at
wavelengths above 400 nm, indicating the shift of the tautomeric
equilibrium toward the Z-NH isomer.
In contrast with the free Schiff bases, the diphenyl-tinIV
derivatives (series 3 and 4) have significantly higher fluorescence
quantum yields. Table 1 summarizes the fluorescence quantum
yields, absorption maxima, molar extinction coefficients, and
Stokes shifts of ethanol solutions of the metallic compounds.
Figure 2 depicts steady-state spectra of ethanol solutions of three
representative derivatives: 3a, 4a, and 4d (respective information
for the remaining compounds is provided in the Supporting
Information). From Table 1, it can be seen that, in addition to
the expected red shifting of the emission and absorption bands
of all 4-series compounds with respect to the corresponding
derivative of the 3 series, there is a noticeable variation of the
As has been discussed elsewhere,9,12 solvent polarity is not a
unique parameter to explain these solvatochromic effects,

Excited-State Dynamics of Diphenyl-TinIV Complexes
J. Phys. Chem. A, Vol. 114, No. 2, 2010 709
(nm), Molar Extinction Coefficients ε (M-1 ·cm-1), Stokes Shifts (cm-1), and Fluorescence
abs
TABLE 1: Absorption Maxima λ
max
Quantum Yields O of 3- and 4-Series Compounds
ε/104
Stokes shift
φa
compound
λ
max
ε/104
Stokes shift
φa
abs
abs
compound
λ
max
3a
445
316
441
350
306
464
323
454
318
449
320
451
318
453
313
1.4
0.8
1.2
1.1
1.2
2.0
1.7
1.1
0.7
1.4
0.7
1.3
0.8
1.1
0.7
4990
14160
3480
9380
13490
3370
12780
5390
14810
4700
13680
5530
14810
4950
0.15
4a
470
347
467
353
1.5
0.7
1.3
1.2
4470
12010
2790
0.06
3b
0.19
4b
0.15
9710
3c
3d
3e
3f
0.05
0.04
0.19
0.05
0.11
4c
4d
4e
4f
489
357
476
347
475
346
475
347
477
349
1.7
0.7
1.0
0.5
1.5
0.7
1.6
0.8
1.3
0.6
2590
10150
21010
12660
4270
12120
4770
12530
4300
0.10
0.02
0.08
0.03
0.06
3g
4g
14830
11990
a φ values were calculated as follows:15,60 φ ) φRF(λ)ODR(λ)η2/(FR(λ)OD(λ)ηR2), where F(λ) is the integrated area of the emission obtained by
excitation at wavelength λ, OD(λ) is the optical density at the same wavelength, η is the refractive index of the solvent, and the subscript R
indicates the reference molecule.
our case). On the other hand, the absorption and emission bands
appear at somewhat shorter wavelengths when meta-orienting
groups (sNO2) are at the N-meta position.
The existence of these substituent effects is a clear signature
of a high conjugation in the system. However, as we elaborate
below, for the 3 and 4 series, the aniline and salicylidene rings
are not necessarily in the same plane: In the crystal phase, the
structures of salicylidene-ortho-aminophenols (free ligands) are
not far from complete planarity, showing small deviations
between the salicylidene (nine atoms taken for plane calcula-
tions) and aniline moieties (eight atoms taken for plane
calculations; see Scheme 1 for plane measurement explanation).
From a series of reported X-ray structures,42,49–53 depending on
the nature and position of the substituents, these deviations range
only from 13.44° for the E-OH form of 149 to 2.24° when
compound 1 has a nitro substituent at the O-para position in
the salicylidene moiety and adopts a Z-NH geometry.53 On the
other hand, in the case of tinIV derivatives, the size of the tin
atom itself has been shown to be enough to distort planar
geometries, even if a fused ring system is constructed.16,34,37
From the X-ray structures of the diphenyl-tinIV derivatives of
the 3 series, it was possible to measure the following angles
between planes in the organic Schiff base skeleton: 17.75° (3a),54
25.57° (3a),55 10.84° (3c),35 17.38° (3d),35 25.3° (3e),35 and
19.41° (3f).35 Clearly, in the tinIV derivatives, the ring systems
(salicylidene and aniline moieties) do not exhibit a better
coplanarity than in the free Schiff bases, and therefore, the
conjugation in the overall organometallic system cannot be
ascribed simply to an increase in the degree of π-delocalization
along the ligand skeleton, but also probably to the participation
of the metalsand even the two organometallic phenyl groupssin
the energy of the electronic transitions, conferring them with a
charge-transfer character.
Time-Resolved Spectroscopy. As depicted in Scheme 2, the
generally agreed photoinduced cycle of free Schiff bases bearing
an intramolecular hydrogen bond starts with the excitation of
the E-OH form to an S1 (π,π*) state that undergoes an ultrafast
barrierless ESIPT process leading to a fluorescent S1 (π,π*)
species of the Z-NH tautomer.7,8 The decay of the latter isomer
toward its ground state yields the so-called phototautomer, which
can complete the cycle by going back to the E-OH form through
a ground-state proton transfer (PT). In addition to internal
conversion processes, another known depletion pathway that
Figure 2. Molar extinction coefficient (solid colored lines) and
emission (dashed lines) and excitation (circles) spectra of representative
derivatives in ethanol solution. Fluorescence spectra were recorded with
an excitation wavelength of 420 nm, and excitation spectra were
recorded at the fluorescence maximum for each compound.
emission maxima and quantum yields along each series of
compounds. Such differences can be summarized in terms of
the nature and position of the substituents placed at the aniline
fragment and are in full agreement with the tendency first noted
on the steady-state spectra by Morisige and originally reported
for beryllium, scandium, aluminum, and gallium derivatives with
substituted Schiff bases:26–28 Longer absorption and emission
wavelengths are associated with the inclusion of substituents
with an electron-donating capacity at the N-meta position [X
) sCl, sCH3 and sC(CH3)3 in Scheme 1] and by electron-
withdrawing substituents at the N-para position (Y ) sNO2 in

710 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Zugazagoitia et al.
Figure 3. Fluorescence up-conversion measurements of acetonitrile
solutions of 1. The pump beam was set at 385 nm, and the emission
was monitored at the wavelengths indicated in each plot. Solid thicker
lines are nonlinear least-squares fits to multiexponential decay models
convoluted with the instrumental response function.
partially accounts for the deactivation of the excited Z-NH form
implies its isomerization to the S0 state of the E-NH isomer
(photochromic tautomer), which rapidly decays toward the more
stable Z-NH ground state.1–8
Because the geometries of the E-OH forms of 1 and 2 force
the second hydroxyl group in the aniline ring to be near the
imine bond, a second intramolecular hydrogen bond can be
formed. In fact, according to refs 42 and 49, the solid-state
packing of the E-OH form of salicylidene-ortho-aminophenols
allows the hydroxyl group from the salicylidene moiety to form
hydrogen bonds with both the nitrogen and the hydroxyl group
in the aniline ring. This additional hydrogen bond might alter
ESIPT dynamics of 1 and 2 with respect to those of monohy-
droxylated Schiff bases. To gain insight into this query, we
performed time-resolved up-conversion measurements of 1 and
compared its excited-state dynamics with those of its extensively
studied monohydroxylated analogue salicylideneaniline.2–5,7,8
Following the procedure reported in a previous contribution,7
we used acetonitrile as the solvent and set the excitation beam
at 385 nm. For these experiments, the upconverted fluorescence
was monitored at 490 and 517 nm. From the experiments with
detection at 490 nm (see inset in Figure 3), we determined the
time constant for the ESIPT process as 50 ( 20 fs (τ1, from the
emission decay of the locally excited E-OH form). Once this
value was set, we adjusted the data obtained at 517 nm to a
multiexponential decay (main graph in Figure 3). The other
characteristic times are τ2 ) 370 ( 20 fs for the vibrational
relaxation and cooling after the proton transfer has occurred
(lifetime of a “hot” Z-NH isomer in its first electronically excited
state)1–3 and τ3 ) 9.6 ( 2 ps for the final depletion of the “cold”
Z-NH S1 state. The sub-100-fs decay component compares very
well with the rapid lifetime of the E-OH isomer of salicylide-
neaniline in acetonitrile,5,7 indicating the lack of effect of a
second hydroxyl group in ESIPT dynamics for systems in
solution.
The time-resolved emissions of ethanol solutions of 1 and 2
are shown in the main graphs of Figure 4. We included
experiments in this solvent to make direct comparisons between
the free bases and the diphenyl-tinIV derivatives, which were
studied in ethanol solution for solubility reasons. In this case,
the pump beam was set at 422 nm (direct excitation of the Z-NH
form), and the fluorescence signal was monitored at 517 nm.
The fast feature (“spike” at t ) 0) in these traces is due to self-
phase modulation and Raman scattering from the flow cell, as
shown by solvent-only scans taken back-to-back with the
solution measurements (see Supporting Information). Fluores-
cence lifetimes were obtained through single-exponential fits
excluding the points around t ) 0. By this procedure, we found
Figure 4. Fluorescence up-conversion measurements of ethanol
solutions of compounds 1 (top, main), 2 (bottom, main), 3a (top, inset),
and 4a (bottom, inset). The pump beam was set at 422 nm, and the
emission was monitored at 517 nm for 1 and 2 and at 580 nm for 3a
and 4a. Solid thicker lines are nonlinear least-squares fits to single-
exponential models of the fluorescence lifetimes. For comparison
purposes, fits for the 1 and 2 data are included in the insets.
time constants of 39 ( 1 ps for 1 and 17.5 ( 0.5 ps for 2. In
particular, the lifetime of 1 in ethanol is in excellent agreement
with the 40 ps reported by Barbara and co-workers through the
use of a Streak camera with 5-ps resolution.2
From the above results, it is interesting to note that the
fluorescence lifetime of 2 (17.5 ps) is only about one-half that
of 1 (39 ps). This suggests that the E-Z isomerization of the
quinoidal form (NH) through rotation around the C3sC4 bond
has a straightforward dependence on the motion of the aniline
group. However, effects such as the barrier size, the shape of
the substituents, and the specific and nonspecific (hydrodynamic
friction) interactions with the solvent can sensibly affect the
free-base keto-form excited-state decay.
For the participation of the Schiff base skeleton as a
chromophore in the metallic derivatives 3 and 4, we found much
longer fluorescence decay times in comparison with those of
the free bases. For 3a and 4a, the time-resolved fluorescence is
depicted in the insets of Figure 4, where we have also included
the fits of the decay curves of 1 and 2 for comparative purposes.
Fluorescence decay curves for ethanol solutions of 3b-e and
4b-e are shown in Figure 5. The pump beam was set at 422
nm for 3a and 4a and at 390 nm for 3b-e and 4b-e. For all
of the compounds, we monitored the upconverted fluorescence
signal at 580 nm. In addition to a “long decay” in the
fluorescence (see below), for all compounds, small-amplitude
modulations in the traces with time constants of up to a few
tens of picoseconds are present. These exponential components
of the emission decay trace are due to relaxation of the system
within the S1 state (vibrational relaxation and solvation; this is
further detailed in the transient absorption results). Table 2
summarizes the parameters for the emission decays for com-
pounds 3a-e and 4a-e, including the first singlet excited-state
lifetimes (long decay component) and the radiative rate con-
stants. From these data, it can be seen that, except for
compounds 3c and 4c, the excited-state lifetimes are larger for

Excited-State Dynamics of Diphenyl-TinIV Complexes
J. Phys. Chem. A, Vol. 114, No. 2, 2010 711
Figure 6. Transient absorption as a function of delay time for ethanol
solutions of 4d. For these experiments, the pump beam was set at 400
nm. The inset shows details of the early-time signal evolution for the
stimulated emission signals.
Figure 5. Fluorescence up-conversion measurements for ethanol
solutions of 3b-e and 4b-e. The pump beam was set at 390 nm, and
the emission was monitored at 580 nm. Solid thicker lines are least-
squares fits to single-exponential models of the fluorescence lifetimes.
All data were normalized.
Such studies indicate that the fluorescence quantum yield of
the ligands involved in series 3 and 4 is determined at least in
part, by competition between radiative decay, E-Z isomeriza-
tion, and direct internal conversion to the ground state. In the
complexes, the proximity of the S0 and S1 potential energy
surfaces is probably also related to the nonradiative S1 decay
channels, which, in turn, is sensitive to the extent of the aromatic
system (series 3 vs 4), as well as to the electron-withdrawing
or -donating character of the substituents.
TABLE 2: Up-conversion Fitting Parameters and Radiative
Decay Rate Constants kSr of Selected Compoundsa
b
compound
R1
τ1 (ps)
R2
τ2 (ns)
kSr (ns-1
)
3a
3b
3c
3d
3e
4a
4b
4c
4d
4e
0.095
-0.149
-0.152
0.158
-0.323
0.209
-0.748
-0.273
0.244
5.9
2.2
3.6
6.5
0.7
7.3
0.1
0.2
6.2
-
0.669
0.817
0.841
0.632
0.777
0.571
0.748
0.741
0.516
0.749
1.7
1.0
0.3
0.6
1.7
0.9
0.4
0.5
0.4
0.7
0.09
0.19
0.17
0.07
0.11
0.07
0.38
0.20
0.05
0.11
In addition to the time-resolved fluorescence experiments, we
studied one of the derivatives through transient absorption
measurements to ascertain the spectral regions where the
different types of transient signals appear. We chose 4d as
representative of the 3 and 4 series because of its solubility
and because its S1 lifetime (∼0.4 ns) allowed its excited-state
dynamics to be fully observed with our experimental setup.
Figure 6 shows the photoinduced absorbance change as a
function of time for two spectral intervals. As can be seen, for
the first case (upper graph, 510-575 nm), positive transient
signals are detected, whereas for the spectral range of the lower
graph (600-710 nm), the signals are due to stimulated emission
(negative transient absorption signal). Except for the data at 670
nm, all transient absorption signals were fitted to a double-
-
a Fitting function was set as R1e-t/τ + R2e-t/τ
.
b kSr values were
1
2
calculated as follows:31 kSr ) φ/τ, where φ is the fluorescence
quantum yield and τ is the fluorescence lifetime.
the 3 series than for the 4 series. This tendency is also followed
by the fluorescence quantum yields (Table 1). The radiative
decay rates (kSr ) are higher for 4-series compounds than for the
corresponding 3-series compounds, with the exception of
compounds 3b and 4b. For example, we found fluorescence
decay times of 1.70 ( 0.1 ns for 3a and 0.90 ( 0.1 ns for 4a.
Mainly, the remarkable increment in the fluorescent state
lifetime of more than 50 times when the Schiff bases 1 and 2
are coordinated to a diphenyl-tinIV moiety indicates that the
effect of chelation with a tinIV center reduces the rate of the
nonradiative channels and avoids possible E-Z isomerization
processes, dramatically enhancing the lifetime of the first
electronically excited state. It is worth noticing that this effect
is not exclusive of tin, because there are some other combina-
tions of Schiff bases with metals that are also highly fluorescent
or phosphorescent.14,20–28,30,32,33,56 In this respect, the chelating
center can be seen as the molecular feature that stands for the
stabilization of the S1 state by “holding” or restricting the organic
Schiff base skeleton and avoiding the loss of excitation energy
throughinternalconversiontothegroundstate,orisomerization.14,23
The variations of the decay times and quantum yields within
the complexes can be rationalized by considering recent
calculations of the potential energy surfaces of Schiff bases.8
-t/τ2
exponential function plus a constant term (R1e-t/τ + R2e
+
1
R3, best-fit parameter values are included in Table 3) convoluted
with the instrument response function. From a global analysis
considering all single-wavelength experiments, the time constant
of the slow component associated with the S1 lifetime of the
derivative was observed to be τ2 ) 380 ps, in excellent
agreement with the up-conversion result. The signal at 670 nm
had to be adjusted to a three-exponential function plus a constant
term (two “short” τ1 components and one slow τ2 component
set to 380 ps). The shorter time τ1 was assigned to solvation
and vibrational relaxation in the S1 state (see below),57,58 whereas
the constant term R3 that carries a small signal amplitude was
assigned to long-lived species formed from the fluorescent state,
most likely a phosphorescent triplet state. This last proposal is
based on the fact that heavy main-group metals are known to
strongly promote spin-orbital coupling.20,31 In particular,
phosphorescence emissions from SnIV-bearing compounds have
already been reported.20,59

712 J. Phys. Chem. A, Vol. 114, No. 2, 2010
Zugazagoitia et al.
TABLE 3: Transient Absorption Fitting Parameters for
Compound 4da
wavelength
(nm)
R1
-0.035
0.015
-0.033
-0.019
-0.031
0.015
τ1(ps)
5.3
23.5
0.4
6.5
35.9
2.9
R2
R3
475
510
550
575
600
650
670b
710
760
0.068
0.074
0.141
-0.001
0.003
0.011
0.005
0.006
0.047
-0.044
-0.169 5 × 10-4
0.035, -0.034
0.002
0.002
5.3, 27.9 -0.12
2.7
0.9
0.002
-0.015 1 × 10-4
-0.007 2 × 10-4
a Fitting function was set as R1e-t/τ + R2e-t/τ + R3, where the
“slow” component τ2 was globally fitted to 380 ps. b For the
transient stimulated emission signal at 670 nm, the signal had to be
adjusted to a three-exponential function plus a constant term;
therefore, two coefficients and two “short” time constants are
indicated. The slow component at this wavelength was also set to
380 ps.
1
2
Figure 7. Two-photon absorption cross sections (TPACSs) for
representative 3-series compounds. To allow for easier visualization
of the data, the results for derivatives with enhanced TPACS values
with respect to 3a are plotted in the upper graph.
For the 475-575-nm region, it is clear that the signals are
dominated by excited-state absorption (Sn r S1), even consider-
ing that the first absorption band of the ground state (S1 r S0)
also occurs in this region. This indicates the larger oscillator
strength of the excited-state absorption transition with respect
to that of the absorption from the ground state (otherwise, a
negative, bleaching signal would be observed). More interest-
ingly, for wavelengths above 600 nm, the signal is dominated
by the stimulated transition. This further illustrates the poten-
tially useful optical properties of these derivatives, where the
restriction of the rapid decay channels in the organic skeleton
yields highly emissive compounds that can be synthesized from
very simple Schiff bases. The short-time signal evolution (see
Figure 6, inset) consists of small-amplitude changes in the
transient signals that reflect the time scale on which the
vibrational relaxation in the first excited singlet potential energy
surface takes place (up to a few tens of picoseconds; see τ1
values in Table 3). These observations indicate that, although
the molecule reaches its lowest vibrational energy in the
fluorescent state, the interaction with the metal center (bis-
chelation) restricts the ligand from exploring the potential energy
surface in regions where the molecules can undergo deactivation
(internal conversion to the ground state or isomerization) as in
the case of the free ligands.
Figure 8. Two-photon absorption cross sections (TPACSs) for
representative 4-series compounds. To allow for easier visualization
of the data, the results for derivatives with enhanced TPACS values
with respect to 4a are plotted in the upper graph.
Two-Photon Absorption Cross Sections (TPACSs). Be-
cause of their much improved fluorescent character and the
current interest in developing metal coordination compounds
with high two-photon absorption abilities and other nonlinear
optical properties,16–19,56 we measured the TPACSs of the 3 and
4 series of compounds, taking advantage of their emissive nature.
TPACSs (∂) were calculated as follows46–48
and φ(2) is taken to be equal to φ.60 Values for δR were taken as
reported elsewhere.44
In Figures 7 and 8, we have plotted the TPACSs of
representative derivatives of the 3 and 4 series, respectively.
The plots in each figure are divided into two groups to allow
an easier visual analysis of the data: In both figures, the upper
graphs display the TPACSs of those compounds with improved
values compared to 3a and 4a, whereas the bottom plots present
the results of those compounds that exhibit smaller values.
Results for the remaining compounds are provided as Supporting
Information. It can be clearly seen that, in both kinds of
diphenyl-tinIV derivatives, whether they have a Schiff base ligand
with a salicylidene (3 series) or a naphthalydene (4 series)
moiety, the same trend is followed: TPACSs are enhanced by
groups that have an electron-donating capability placed at the
N-meta position and electron-withdrawing groups at N-para
position.
F(2,λ)CRφ(R2)ηR
F(R2,λ)Cφ(2)η
(λ)
∂
) ∂(Rλ)
where F(2,λ) is the integrated area of the emission bands obtained
by two-photon excitation at the pump wavelength λ, C is the
concentration (in mol L-1), φ(2) is the two-photon excitation
fluorescence quantum yield, and the subscript R stands for the
respective values of the reference molecule (rhodamine B in
methanol). Because of its rapid relaxation, the two-photon
excitation fluorescence (TPEF) comes from the lowest vibra-
tional level of the S1 excited state, so the one-photon and two-
photon excitation fluorescence spectra are essentially identical

Excited-State Dynamics of Diphenyl-TinIV Complexes
J. Phys. Chem. A, Vol. 114, No. 2, 2010 713
Compounds 3 and 4 can be seen as a cyclic π-donor-acceptor
system (-π-D-A-π-), where the π-donor group is the
nitrogen and the π-acceptor is the organometallic group, both
bridged by the aromatic rings of the aniline and the salicylidene
(3) or naphthalidene (4) groups. It has been found that high
TPACS values are associated with the inclusion of several
electron-donor and -acceptor groups and to an extensive
π-delocalization.18,19,47,61 This is in agreement with the fact that
TPACS values of compounds with a naphthalene moiety (4),
which includes more π-conjugation, are higher than those of
their analogues with a salicylidene moiety (3). Further evidence
of the latter is found in the substituents’ effects, because the
trends that were found to increase TPACS values were also
found to cause a red shift of both absorption and emission bands
(see Steady-State Spectroscopy section). Although clear, the
differences in TPACSs caused by different substituents in the
aniline ring are not extreme, and we explain this by the fact
that the substituents are in one of the π-bridges and, as a result,
their electron-withdrawing and electron-donating capabilities
affect both the D and A groups in the cyclic -π-D-A-π-
pattern.
TPACS values of 3 and 4 are probably not high enough for
their direct use in applications based on nonlinear optic
processes. However, the recently reported high two-photon
absorbers based on other metallic compounds with Schiff bases
as ligands18,19,62 suggest that TPACSs can be improved by orders
of magnitude by changing the extension and conjugation of the
organic skeleton, as well as the nature of the chelating moiety.
The identification of the substituent effects on the two-photon
absorption ability of these derivatives presented herein consoli-
dates an interesting case study for the highly pursued design of
compounds with high two-photon absorption ability. Here, it is
worth emphasizing that their highly emissive nature makes it
possible to measure these nonlinear optical properties through
their fluorescence excitation spectra.
derivatives as important models to take into account for the
design of new molecular entities and materials with highly
nonlinear optical applications and the construction of light-
emitting devices.
Acknowledgment. We are thankful to Professor Ahmed H.
Zewail and the California Institute of Technology for the
donation of equipment used in this study. For financial support,
we thank Consejo Nacional de Ciencia y Tecnolog´ıa (CONA-
CyT, Grant 79494) and Universidad Nacional Auto´noma de
Me´xico (PAPIIT, Grant IN212907). We also thank M.Sc. Nieves
Zavala Segovia and M.Sc. Luis Velasco Ibarra for the acquisi-
tion of the NMR and mass spectra of the compounds of this
study.
Supporting Information Available: Steady-state spectra of
compounds 3b-g, 4b, 4c, and 4e-g; single-wavelength fluo-
rescence up-conversion traces for back-to-back experiments of
pure ethanol and ethanol solutions of 1 and 2 that show the
presence of a fast feature at early times; and two-photon
absorption cross section information for compounds 3a, 3f, 3g,
4a, 4f, and 4g. This material is available free of charge via the
Internet at http://pubs.acs.org.
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