Two-photon absorption properties of four new pentacoordinated diorganotin complexes derived from Schiff bases with fluorene

Article abstract of DOI:10.1016/j.jorganchem.2017.12.014

In this paper we report the synthesis and characterization of four novel pentacoordinated diorganotin complexes, obtained through a methodology that involves a multicomponent reaction of 4-([9H-fluorene-2-yl]ethynyl)-2-hydroxybenzaldehyde, 2-amino-5-nitrophenol and diphenyl or dibutyl-tin oxides. Diorganotin complexes 8–11 were obtained in high yields (70–80%) and were fully characterized by solution NMR (¹H, ¹³C and ¹¹⁹Sn), high resolution mass spectrometry (ESI-TOF) and ATR-FTIR. The optical properties were investigated by UV/Vis spectroscopy and Two-Photon Excitation Fluorescence (TPEF). The One-Photon Absorption (OPA) spectra shows two bands located around 400 and 500 nm, additionally, in the Two-Photon Absorption (TPA) spectra there is one main band located around 750 nm characterized by maximum values of TPA cross section (σ_TPA) in the range 450–500 GM with a secondary band located at 1000 nm with maximum σ_TPA values of 70 GM; the maximum brightness was observed for 2,2-Dibutyl-6-aza-1,3-dioxa-11-([9,9-dihexyl-9H-fluorene-2-yl]ethynyl)-16-nitro-2-stanna-[d,h]dibenzocyclononene with a value of 150 GM. A theoretical approach within the framework of DFT was applied to study the electronic transitions, showing that the 500 nm band is mainly associated to a HOMO→LUMO transition, and the band at 400 nm to a HOMO-1→LUMO transition. As main highlights the quantum efficiency of fluorescence of these excited states is as large as 0.43 due to the presence of the tin atom which favors molecular rigidity, these compounds possess TPA responses which are significantly enhanced with respect to related organotin chromophores previously reported.

Full text of DOI:10.1016/j.jorganchem.2017.12.014

Accepted Manuscript
Two-photon absorption properties of four new pentacoordinated diorganotin
complexes derived from Schiff bases with fluorene
Alejandro Enríquez-Cabrera, Alberto Vega-Peñaloza, Violeta Álvarez-Venicio,
Margarita Romero-Avila, Pascal G. Lacroix, Gabriel Ramos-Ortiz, Rosa Santillan,
Norberto Farfán
PII:
S0022-328X(17)30699-X
10.1016/j.jorganchem.2017.12.014
JOM 20209
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 4 October 2017
Revised Date: 8 December 2017
Accepted Date: 9 December 2017
Please cite this article as: A. Enríquez-Cabrera, A. Vega-Peñaloza, V. Álvarez-Venicio, M. Romero-
Avila, P.G. Lacroix, G. Ramos-Ortiz, R. Santillan, N. Farfán, Two-photon absorption properties of
four new pentacoordinated diorganotin complexes derived from Schiff bases with fluorene, Journal of
Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2017.12.014.
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ACCEPTED MANUSCRIPT
Two-photon absorption properties of four new pentacoordinated
diorganotin complexes derived from Schiff bases with fluorene.
Alejandro Enríquez-Cabrera,a Alberto Vega-Peñaloza,a Violeta Álvarez-Venicio, †a,b
Margarita Romero-Avila,a Pascal G. Lacroix,c Gabriel Ramos-Ortiz,b Rosa Santillan,d
Norberto Farfána*
a
Facultad de Química, Departamento de Química Orgánica, Universidad Nacional
Autónoma de México, Cd. Universitaria, Ciudad de México No. 04510, México. E-mail:
norberto.farfan@gmail.com
b
Centro de Investigaciones en Óptica, CIO, Apdo., Postal 1-948, 37000 Leon Gto, México
c
Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, F-31077
Toulouse, France
d
Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN,
CINVESTAV, Apdo., Postal 14-740, Ciudad de México, 07000, México
† From February 01, 2013 to January 31, 2015 V.A.-V. had a postdoctoral fellowship at
UNAM and from February 01, 2015 to July 31, 2016 at CIO
ABSTRACT
In this paper we report the synthesis and characterization of four novel pentacoordinated
diorganotin complexes, obtained through a methodology that involves a multicomponent
reaction of 4-([9H-fluorene-2-yl]ethynyl)-2-hydroxybenzaldehyde, 2-amino-5-nitrophenol
and diphenyl or dibutyl-tin oxides. Diorganotin complexes 8-11 were obtained in high
13
yields (70-80%) and were fully characterized by solution NMR (1H, C and 119Sn), high
resolution mass spectrometry (ESI-TOF) and ATR-FTIR. The optical properties were
investigated by UV/Vis spectroscopy and Two-Photon Excitation Fluorescence (TPEF).
The One-Photon Absorption (OPA) spectra shows two bands located around 400 and 500
nm, additionally, in the Two-Photon Absorption (TPA) spectra there is one main band
located around 750 nm characterized by maximum values of TPA cross section (σTPA) in

ACCEPTED MANUSCRIPT
the range 450 – 500 GM with a secondary band located at 1000 nm with maximum σTPA
values of 70 GM; the maximum brightness was observed for 2,2-Dibutyl-6-aza-1,3-dioxa-
11-([9,9-dihexyl-9H-fluorene-2-yl]ethynyl)-16-nitro-2-stanna-[d,h]dibenzocyclononene
with a value of 150 GM. A theoretical approach within the framework of DFT was applied
to study the electronic transitions, showing that the 500 nm band is mainly associated to a
HOMO→LUMO transition, and the band at 400 nm to a HOMO-1→LUMO transition. As
main highlights the quantum efficiency of fluorescence of these excited states is as large as
0.43 due to the presence of the tin atom which favors molecular rigidity, these compounds
possess TPA responses which are significantly enhanced with respect to related organotin
chromophores previously reported.
Keywords: Diorganotin complexes, NonLinear Optics, Two-Photon Absorption, DFT
1. INTRODUCTION
The development of new materials for nonlinear optics (NLO) has increased during the last
decades due to their potential applications in optoelectronics. In particular, materials based
on organic compounds with extended conjugated π-systems between electron-donor and
electron-withdrawing groups result of high interest due to their high molecular
hyperpolarizability, efficient and tunable electronic response, chemical versatility and
accessibility at low cost. Until the mid-90’s, most of the efficient NLO candidates were
based on the benchmark stilbazole skeleton,[1],[2] nevertheless various heteroatoms and
metals have gradually been incorporated into organic chromophores to develop molecules
of greater electronic complexity, and enhanced capabilities.[3] Within this field, organotin
complexes are well-known scaffolds that can be used to construct NLO compounds.[4] In
addition to the well known biological activities[5–8], the π-conjugated structure of

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organotin complexes have proved to be suitable for NLO properties,[9–15] and
optoelectronic applications such as OLEDS[16] (Organic Light Emitting Diodes).
The TPA phenomenon has become one of the most promising NLO properties, due to their
applications in photolithography,[17] microfabrication,[18] 2D[19] or 3D[20] data storage
and in biophotonics. Recently two-photon microscopy (TPM) has become an important tool
in biomedical research[21–23], because it offers high spatial resolution and deep tissue
penetration using laser radiation at wavelength within biomedical window (700 – 950
nm).[24,25] Kim and Cho[26] suggested that a suitable chromophore for TPM should
have a two-photon brightness larger than 50 GM in order to avoid photodamage of the
sample at laser powers commonly used in this technique, for which brightness[27] is
defined as the product (σ TPA = σTPA×φ) where φ is the fluorescent quantum yield.
'
Few examples of organotin complexes with TPA properties have been explored in the
literature.[9–12] They exhibit nonlinear absorption at the infrared wavelength of 750 – 800
nm with σTPA values ranging from 10 to 2500 GM (Figure 1)
In continuation of our study of the third-order NLO properties in organotin complexes, and
in a continuous effort aimed at extending their range of applications, herein we report the
multicomponent synthesis and characterization of four novel pentacoordinated diorganotin
complexes starting from 4-([9H-fluorene-2-yl]ethynyl)-2-hydroxybenzaldehyde, 2-amino-
5-nitrophenol and diphenyl or dibutyl-tin oxides. Complexes were designed using Schiff-
bases functionalized with ethynylfluorene in para position to the C=N group as ligands to
increase the TPA response, fluorene was introduced because it is well known that it
provides a better TPA response and favor efficient luminescence properties.[28–36]

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Figure 1.- Organotin complexes with TPA properties reported in the literature. The wavelength of the incident laser to measure σTPA is
written in parentheses.

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2. EXPERIMENTAL SECTION
2.1 General Considerations
Melting points were measured in a Barnstead Electrothermal 9300 apparatus and are
uncorrected. ATR-FTIR experiments were registered in a PerkinElmer Spectrum 400
spectrometer. Absorption spectra were obtained with a Perkin Elmer spectrofluorometer
Lambda 900 UV/Vis/NIR. Emission spectra were obtained with an Edinburgh Instruments
spectrofluorometer FS5, with an excitation source of 405 nm; the fluorescence quantum
yields were measured also with FS5 using an integrating sphere. The values of σTPA were
obtained through the two-photon excited fluorescence technique (TPEF) using a
1
femtosecond laser Chameleon UltraTM tunable in the wavelength 700-1050 nm. H-NMR
13
spectra were recorded using a Varian MR-400 operating at 400 MHz; C-NMR spectra
were recorded at 100.58 MHz; 119Sn spectra were recorded using a Jeol 270 instrument
operating at 100.74 MHz; chemical shifts are relative to the residual non deuterated solvent
signals, fixed on δ δ
=7.26 ppm for 1H and = 77.00 ppm for 13C. HRMS data was acquired
in an Agilent Technologies MS TOF spectrometer. The elemental analyses were obtained
in a Perkin Elmer 2400 for CHNS. Starting materials were purchased from commercial
sources and used without further purification.
2.2. Syntheses
In order to obtain the four Schiff bases, it was necessary to prepare the two different
fluorene aldehyde derivatives shown in Scheme 1. In turn, 4-formyl-3-hydroxyphenyl
trifluoromethanesulfonate,[37] 2-bromo-9,9-dihexyl-9H-fluorene (1),[38] 2-ethynyl-9H-
fluorene (4) and 2-ethynyl-9,9-dihexyl-9H-fluorene (5)[39] were prepared as described in
the literature.

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Scheme 1. Synthesis of the starting materials 6 and 7.
2.2.1 General procedure for the synthesis of diorganotin complexes
Diorganotin complexes 8-11 were synthesized as shown in Scheme 2, using the following
general procedure; in a round-bottom flask were placed the aldehyde 6 or 7, 2-amino-5-
nitrophenol and dibutyltin or diphenyltin oxide in an equimolar ratio. The reaction mixture
was heated to reflux and vigorously stirred. After 4 h the solution was cooled to room
temperature and the mixture was decanted. Finally, the precipitated solid was filtered under
high vacuum and washed with cold methanol to yield the pure complexes in high yields.
The features of the multicomponent methodology provide not only high yield but a
simplified purification and the ability to synthesize diverse molecules with several degrees
of structural variety.
Scheme 2. Synthesis of diorganotin complexes 8-11, with the corresponding atom
numbering.

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4-([9H-fluorene-2-yl]ethynyl)-2-hydroxybenzaldehyde (6)
In a round-bottom flask were placed 168 mg (0.88 mmol) of 4, 200 mg (0.74 mmol) of the
aldehyde, 38 mg (0.055 mmol) of Pd(PPh3)2Cl2, 7 mg (0.037 mmol) of CuI under nitrogen
atmosphere followed by addition of 15 ml of anhydrous THF and 0.62 ml (4.44 mmol) of
anhydrous DIPA and the solution was refluxed for 4 hrs. The reaction was allowed to reach
room temperature and quenched with a saturated solution of NH4Cl. The organic phase was
extracted with ethyl acetate, dried using Na2SO4 and evaporated under vacuum followed by
purification over silica gel to yield 144 mg (53%) of the desired product. 1H-NMR (300
MHz, CDCl3) δ11.08 (s, OH), 9.89 (s, CHO), 7.85 – 7.72 (m, 3H), 7.63 – 7.52 (m, 3H),
7.47 – 7.29 (m, 2H), 7.21 – 7.13 (m, 2H), 3.94 (s, 2H). HRMS (ESI+-TOF) for C22H15O2
(calcd. 311.1067): m/z (%) = 311.1067 (M+H)+, error 0.01 ppm.
C22H14O2: C 85.14, H 4.55; found: C 82.73, H 4.78.
Anal. calcd. for
4-([9,9-dihexyl-9H-fluorene-2-yl]ethynyl)-2-hydroxybenzaldehyde (7)
The procedure is the same as described above for 6 using compound 5 as a starting
1
material. H-NMR (300 MHz, CDCl3) δ11.08 (s, OH), 9.89 (s, CHO), 7.76 – 7.64 (m, 2H),
7.58 – 7.50 (m, 3H), 7.42 – 7.29 (m, 3H), 7.22 – 7.16 (m, 2H), 1.98 (t, J = 8.1 Hz, 4H), 1.20
– 0.95 (m, 13H), 0.76 (t, J = 6.9 Hz, 5H), 0.69 – 0.52 (m, 4H). HRMS (ESI+-TOF) for
C34H39O2 (calcd. 479.2945): m/z (%) = 479.2945 (M+H)+, error 0.04 ppm. Anal. calcd. for
C34H38O2: C 85.31, H 8.0; found: C 85.05, H 8.05.
2,2-Dibutyl-6-aza-1,3-dioxa-11-([9H-fluorene-2-yl]ethynyl)-16-nitro-2-stanna-
[d,h]dibenzocyclononene (8)

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Aldehyde 6 (76 mg, 0.244 mmol), 2-amino-5-nitrophenol (37 mg, 0.244 mmol) and
dibutyltin oxide (61 mg, 0.244 mmol) were used following the general procedure.
Compound 8 was obtained as a red solid (116 mg, 70 %). M.p.: decomposes at 186 °C.
FTIR-ATR (
ν
, cm-1): 2956, 2921, 2200, 1598 (C=N), 1586 (C=N), 1560, 1495, 1334(NO2),
1
3
1285(NO2). H-NMR (400 MHz, CDCl3)
(m, 2H), 7.73 (s, 1H), 7.65 (d,
Hz, 1H), 7.39 (d, = 9.0 Hz, 1H), 7.35 (td,
6.99 (d, = 1.5 Hz, 1H), 6.90 (dd,
1.60 – 1.49 (m, 4H), 1.35 (hex,
MHz, CDCl3)
δ
8.67 (s, J(Sn-H) = 41.7 Hz, 1H), 7.84 – 7.75
J
= 2.5 Hz, 1H), 7.61 – 7.53 (m, 3H), 7.41 (td,
J = 7.4, 1.2
J
J
= 7.4, 1.2 Hz, 1H), 7.25 (d, = 8.2 Hz, 1H),
J
J
J
= 8.2, 1.5 Hz, 1H), 3.93 (s, 2H), 1.72 – 1.58 (m, 4H),
J
= 7.3 Hz, 4H), 0.87 (t, J
= 7.3 Hz, 6H). 13C-NMR (100
δ
170.3, 163.3, 159.9, 148.6, 143.8, 143.4, 142.9, 141.0, 137.5, 135.7, 133.5,
131.1, 128.6, 127.6, 127.2, 125.5, 125.3, 120.7, 120.6, 120.5, 120.1, 117.7, 115.0, 113.4,
111.5, 96.0, 89.4, 36.9, 27.1, 26.8, 22.7, 13.7. 119Sn-NMR (100 MHz, CDCl3)
δ
-176.0.
HRMS (ESI+-TOF) for C36H35N2O4Sn (calcd. 679.1613): m/z (%) = 679.1609 (M+H)+,
error -0.59 ppm. Anal. calcd. for C36H34N2O4Sn: C 63.83, H 5.06, N 4.14; found: C 63.32,
H 4.81, N 4.34.
2,2-Dibutyl-6-aza-1,3-dioxa-11-([9,9-dihexyl-9H-fluorene-2-yl]ethynyl)-16-nitro-2-stanna-
[d,h]dibenzocyclononene (9)
Aldehyde 7 (250 mg, 0.522 mmol), 2-amino-5-nitrophenol (81 mg, 0.522 mmol) and
dibutyltin oxide (130 mg, 0.522 mmol) were used following the general procedure.
Compound 9 was obtained as a red solid (370 mg yield 81%). M.p.: 143-144 °C FTIR-
1
ATR (
ν
, cm-1): 2954, 2924, 2199, 1587 (C=N), 1561, 1496, 1335 (NO2), 1285 (NO2). H-
NMR (400 MHz, CDCl3)
δ
8.69 (s, 3J(Sn-H) = 41.6 Hz, 1H), 7.74 – 7.66 (m, 2H), 7.65 (d,

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J
= 2.5 Hz, 1H), 7.58 (dd,
J
= 8.8, 2.5 Hz, 1H), 7.55 – 7.50 (m, 2H), 7.40 (d, J = 8.8 Hz,
1H), 7.37 – 7.32 (m, 3H), 7.27 (d,
J
= 8.2 Hz, 2H), 7.02 (s, 1H), 6.93 (d,
J
= 8.2 Hz, 1H),
1.98 (t, = 8.2 Hz, 4H), 1.71 – 1.59 (m, 4H), 1.59 – 1.50 (m, 4H), 1.35 (h,
J
J = 7.3 Hz, 4H),
1.19 – 0.97 (m, 12H), 0.87 (t,
J
= 7.3 Hz, 6H), 0.77 (t,
J = 7.1 Hz, 6H), 0.68 – 0.55 (m, 4H).
13C-NMR (100 MHz, CDCl3)
δ
170.4, 163.3, 159.9, 151.3, 151.1, 148.6, 142.5, 140.3,
137.5, 135.7, 133.5, 131.2, 128.0, 127.1, 126.4, 125.5, 123.1, 120.7, 120.6, 120.3, 119.9,
117.7, 115.0, 113.4, 111.5, 96.4, 89.3, 55.3, 40.5, 31.7, 29.8, 27.1, 26.8, 23.9, 22.7, 22.7,
14.1, 13.7.
119Sn-NMR (100 MHz, CDCl3) -176.1. HRMS (ESI+-TOF) for
δ
C48H59N2O4Sn (calcd. 847.3491): m/z (%) = 847.3482 (M+H)+, error -1.09 ppm Anal.
calcd for C48H58N2O4Sn: C 68.17, H 6.91, N 3.31; found: C 68.43, H 7.17, N 3.69
2,2-Diphenyl-6-aza-1,3-dioxa-11-([9H-fluorene-2-yl]ethynyl)-16-nitro-2-stanna-
[d,h]dibenzocyclononene (10)
Aldehyde 6 (50 mg, 0.161 mmol), 2-amino-5-nitrophenol (25 mg, 0.161 mmol) and
diphenyltin oxide (47 mg, 0.161 mmol) were used following the general procedure.
Compound 10 was obtained as a red solid (81 mg yield 70%). M.p.: 194-196 °C. FTIR-
ATR (ν
, cm-1): 3049, 2197, 1596 (C=N), 1588 (C=N), 1564, 1500, 1340 (NO2), 1281
1
3
(NO2). H-NMR (400 MHz, CDCl3)
δ 8.71 (s, J(Sn-H) = 52.2Hz, 1H), 7.97 – 7.87 (m,
5H), 7.85 – 7.76 (m, 3H), 7.65 – 7.55 (m, 3H), 7.47 – 7.41 (m, 8H), 7.39 – 7.31 (m, 2H),
= 8.2, 1.5 Hz, 1H), 3.96 (s, 2H). 13C-NMR (100 MHz, CDCl3)
7.26 (m, 1H), 6.97 (dd,
J
δ
170.2, 163.3, 159.2, 148.6, 143.9, 143.5, 143.1, 141.0, 138.9, 136.9, 136.6, 136.0, 133.9,
131.1, 130.9, 129.1, 128.7, 127.7, 127.2, 125.7, 125.3, 121.3, 120.6, 120.4, 120.1, 117.7,
115.0, 114.0, 112.1, 96.6, 89.4, 36.9. 119Sn-NMR (100 MHz, CDCl3)
δ -320.6. HRMS

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(ESI+-TOF) for C40H27N2O4Sn (calcd. 719.0987): m/z (%) = 719.0992 (M+H)+, error
0.64ppm).
2,2-Diphenyl-6-aza-1,3-dioxa-11-([9,9-dihexyl-9H-fluorene-2-yl]ethynyl)-16-nitro-2-
stanna-[d,h]dibenzocyclononene (11)
Aldehyde 7 (250 mg, 0.522 mmol), 2-amino-5-nitrophenol (81 mg, 0.522 mmol) and
diphenyl oxide (151 mg, 0.522 mmol) were used following the general procedure.
Compound 11 was obtained as a red solid (393 mg yield 82%). M.p.: 211-212 °C. FTIR-
ATR (
ν
, cm-1): 2952, 2926, 2854, 2198, 1589 (C=N), 1563, 1497, 1337 (NO2), 1283 (NO2).
8.68 (s, 3J(Sn-H) = 52.1Hz, 1H), 7.94 – 7.89 (m, 4H), 7.75
1H-NMR (400 MHz, CDCl3)
δ
– 7.69 (m, 2H), 7.59 (dd,
J = 9.0, 2.6 Hz, 1H), 7.57 – 7.54 (m, 2H), 7.48 – 7.33 (m, 12H),
7.26 (d,
J
= 8.1 Hz, 1H), 6.97 (dd,
J = 8.1, 1.5 Hz, 1H), 2.00 (t, J = 8.2 Hz, 4H), 1.23 – 0.95
(m, 12H), 0.77 (t,
J
= 7.0 Hz, 6H), 0.71 – 0.54 (m, 4H). 13C-NMR (101 MHz, CDCl3)
δ
170.2, 163.2, 159.2, 151.3, 151.1, 148.7, 142.6, 140.3, 139.0, 136.9, 136.6, 136.0, 134.0,
131.2, 130.9, 129.1, 128.0, 127.1, 126.5, 125.6, 123.1, 121.3, 120.6, 120.3, 119.9, 117.7,
115.0, 114.0, 112.1, 97.0, 89.3, 55.4, 40.5, 31.7, 29.9, 23.9, 22.7, 14.2. 119Sn-NMR (100
MHz, CDCl3)
δ
-320.6. HRMS (ESI+-TOF) for C52H51N2O4Sn (calcd. 887.2865): m/z (%)
= 887.2870 (M+H)+, error 0.53 ppm) Anal. calcd for C52H50N2O4Sn: C 70.52, H 5.69, N
3.16; found: C 70.13, H 5.26, N 3.39
2.3 Computational details
The four compounds (8-11) were fully optimized without any constraints and symmetry
requirement using the Gaussian-09 program package[40] within the framework of the
Density Functional Theory (DFT).The hybrid functional B3PW91[41,42] was selected for

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the computations of the molecular geometries, in relation to its previous use in related tin
derivatives with Schiff base ligands.[43–46] The LANL2DZ pseudo-potential was used to
account for relativistic effects on the tin atom.[47] In order to provide a reliable comparison
with the experimental data, solvent effects were included by using the polarizable
continuum model (PCM) implemented in Gaussian09 for chloroform (ε = 4.7113). Owing
to the difficulties encountered to get geometry convergence in the case of molecules
containing long alkyl chains, the two hexyl chains present on the fluorenyl fragment of
compounds 9 and 11 were replaced by two ethyl chains in the actual geometries used for
the computations. However, as the dominant contribution to the transitions of interest is
restricted to the
π-delocalized electronic core only (vide infra), the properties of these
slightly modified species can readily account for those of 9 and 11. After each computation,
vibrational analysis was performed at the same level in order to establish the presence of a
minimum on the potential energy surfaces. The UV-visible spectra were computed with the
CAM-B3LYP[48] functional, which is usually efficient for computations involving long
range electronic effect, and which was found to reproduce the spectra with a better
accuracy than related B3LYP, B3PW91, and PBE0. Within this computational method, the
difference between experiment and computation is about 0.35 eV and 0.40 eV for the first
and second transition, respectively (Table S11). These values are a little large compared to
the average discrepancy around 0.25 - 0.30 eV encountered in medium size organic
molecules,[49] but fully acceptable in molecules of large size, with heavy metal atoms (e.g.
Sn) and long range charge transfer capabilities. Molecular orbitals were plotted with
GABEDIT 2.4.8.[50]

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3. RESULTS AND DISCUSSION
3.1. Chemical characterization
The synthesis of diorganotin complexes 8-11 was corroborated through solution NMR, 1H,
13C, HSQC, HMBC and COSY experiments. The 1H-NMR spectra show that the chemical
3
shifts for H7 (HC=N) lie in the range between
δ
=8.67-8.71 ppm; the J(119/117Sn-1H)
coupling constants for H7 are between 41.7 Hz and 41.6 Hz for the dibutyl tin, and 52.4 Hz
and 52.1 Hz for the diphenyl tin derivatives, which are in agreement with previous
reports.[43],[51] The 13C-NMR spectra show that the chemical shifts for C7 appear
between 163.2 and 163.5 ppm; this indicates an enhanced π−bridging between the ortho
-
aminophenol and the salicylidene moiety. The C=N fragment shows a characteristic
intense band in the IR spectra around 1590 cm-1 for the four complexes (see Table 1 and
Table S5-6 for the complete NMR data).
Table 1.- Selected spectroscopic data.
NMR (ppm)
IR (cm-1)
Compound
H-7
C-7
Sn
C=N NO(asym) NO(sym)
(J119/117Sn)a
8.67
163.3 -176.0
1586
1587
1588
1589
1334
1335
1340
1337
1285
1285
1281
1283
8
9
(41.7)
8.68
(41.6)
8.71
(52.4)
8.68
163.5 -176.1
163.3 -320.6
163.2 -320.6
10
11
(52.1)
a
J119/117Sn in Hz
3.2. One-photon absorption and photoluminescence properties
The electronic absorption spectra of the four diorganotin complexes show two bands with
peaks at ca 380 and 490 nm (Figure 2) that correspond to * transitions; while the
π-π
emission spectra have a peak located around 605 nm although with a red shifting effect in
complexes 8 and 9 compared with 10 and 11.

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8
9
10
11
8
9
10
11
6000
5000
4000
3000
2000
1000
0
1,0
0,8
0,6
0,4
0,2
0,0
325 350 375 400 425 450 475 500 525 550 575 600
450
500
550
600
650
700
750
800
λ (nm)
λ (nm)
Figure 2.- Absorption (left) and emission (right) spectra of 0.1mM 8-11 in CHCl3.
It is well known that the optical spectra of -conjugated chromophores substituted by
donor/acceptor groups are dominated by intense low-lying transitions having charge
transfer character within the -organic linkage. Along this line, the nature of the alkyl
π
π
substituent in the fluorene moiety (R1 = H in 8 and 10, and R1 = C6H13 in 9 and 11, as
shown in Scheme 2) is not expected to affect significantly the UV-Visible spectra of the
investigated species.
The presence of a metal within the conjugation pathway of a π-conjugated chromophore
has occasionally been reported to strongly affect the NLO properties of the Schiff-base
complexes, by virtue of a specific contribution of the d-orbitals along the conjugation
pathway.[52],[53] On the other hand, the possibility for charge transfer from a
π-conjugated
substituent (e.g. phenyl) towards a withdrawing fragment through the tin atom has not been
evidenced in previous investigations of “push-pull” organotin derivatives.[43]
The dominant low-lying transitions computed by DFT are shown in Table S10 and are
compared to the experimental data using chloroform as a solvent. The agreement between
computation and experiment appears satisfactory, with blue shifted transitions observed in
the DFT data. In particular, both approaches indicate that the electronic transitions are

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roughly the same for the four different species, with a general trend for a more intense
transition at longer wavelength.
Absorption and emission properties of complexes 8-11 were studied with different solvents
(hexane, dioxane, toluene, chloroform, tetrahydrofuran, acetone, methanol, acetonitrile and
dimethylsulfoxide (for detailed information see the supporting information). The results
show that the complexes can be grouped according to the tin substituent (R2 in Scheme 2),
as observed in Table S9, the absorption bands located around 490 nm for 8 and 9 are
bathochromically shifted about 10 nm compared to 10 and 11, and the band located around
380 nm is hypsochromically shifted around 10nm (except when using DMSO as solvent);
for the emission band there is also a bathochromic shift of about 10 nm when comparing 8
and 9 with 10 and 11 (except when using acetone and DMSO as solvent). In nonpolar
solvents, the quantum yield
φ
of fluorescence for the four complexes was in the range 0.23-
for 10 and 11 is approximately 20%
0.43. In these solvents (except hexane) the value of
φ
larger compared with 8 and 9. Such improvement in the photoluminescence can be partly
associated to the higher molecular rigidity introduced by phenyl substitution in the tin
atom. In polar solvents, the values of
φ tend to decrease (with the exception of DMSO).
The pentacoordinated diorganotin complexes were envisioned with a push-pull structure; in
order to detect possible charge-transfer properties, solvatochromism effects were analyzed
using the Lippert-Mataga theory[54] and the Catalán[55] scale to perform a more detailed
analysis (see supporting information), resulting in a lack of response towards the polarity of
the solvent.
To further analyze the origin of the optical properties of 8-11, the description of their low-
lying transitions at the orbital level is also provided (see Table S11). The data indicate the

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same origin for the low energy transitions (1
→
2 and 1
→ 3), arising from a dominant
combination of HOMO-1
→
LUMO and HOMO
→ LUMO excitations, in all cases.
The orbitals involved in the transitions are depicted in Figure 3. They appear to be very
similar for the four (8-11) derivatives. In any case, the electron density is delocalized on the
entire extent of the π-conjugated structure in the occupied HOMO-1 and HOMO levels;
and located on the metal organic rigid skeleton without any contribution of the fluorene
moieties in the LUMO level. Interestingly, there is no contribution of either the alkyl chains
(R1 in Scheme 2) of the fluorene or the butyl and phenyl ligands (R2 in Scheme 2) of the tin
atom, in either HOMO-1, HOMO, and LUMO levels, which strongly supports the idea of
closely related electron transitions, arising from the same π-electronic structures along the
8-11 series. This overall description brings a sizeable charge transfer character to the
electron transitions with the expectation of associated solvatochromic and NLO properties.
3.3 Two-photon absorption studies
We are currently interested in developing metal coordination compounds with high TPA
properties.[56–59] Thus the σTPA of the pentacoordinated diorganotin complexes were
measured by the TPEF technique in DMSO, THF and CHCl3. The σTPA values were
calculated through TPEF technique as follows:
ꢅ ꢇꢄꢈꢄꢉꢄ
ꢆ
ꢀꢁꢂꢃ = ꢀꢄ
ꢅꢄꢇꢆꢈꢆꢉꢆ
Where
is the fluorescence quantum yield and
stands for the sample or the dye used as reference (Rhodamine B).
ꢅ
is the integrated area of the emission band,
ꢇ
is the molar concentration (mol L-1),
ꢈ
ꢉ
is the refraction index, the subscript
S
or R
Figure 4 shows the values of σTPA as a function of wavelength for three different solvents.
As we can see, the solvent influences the optical nonlinearities exhibited by the organotin

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complexes. In the polar solvents such as THF and DMSO the maximum σTPAvalues are
between 200 and 240 GM at 750 nm, while in chloroform (nonpolar solvent) the maximum
values of σTPA increase almost by a factor of two. The enhancement of TPA response in
chloroform allows to observe a secondary peak at 1000 nm with a maximum σTPA ~ 70
GM. Considering the push–pull structure of the diorganotin complexes, the electronic
transitions corresponding to the absorption bands shown in Figure 2 are allowed through
optical excitation of one or two photons. Thus the TPA peaks at ca 750 and 1000 nm
correspond to OPA bands at 380 and 490 nm, respectively
3.4 Discussion
From the DFT results, the orbitals involved in the low-lying transitions of the UV-Vis
spectra are very similar for the four molecules (HOMO-1→LUMO; HOMO→LUMO), in
all cases the tin atom and its substituents (butyl or phenyl) are not involved in these
transitions, although they increase the structural rigidity, promoting a greater
π-
delocalization; the structural rigidity seems to be somewhat increased when phenyl is
employed as substituent in the tin atom favoring an increase in the fluorescence quantum
yield in agreement with the experimental results. In all complexes the transitions arise from
a very similar π-electronic structure, which is confirmed experimentally through the UV-
Vis due to the small change in the bands when switching from 8 or 9 (butyl) to 10 or 11
(phenyl) in the same solvent.

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Figure 3.- Frontier orbitals involved in the low-lying transitions of 8-11, with relative energies.

ACCEPTED MANUSCRIPT
Figure 4.-σTPA at different wavelength in THF (upper left panel), DMSO(upper right panel)
and CHCl3 (lower panel)
The nonlinear absorption spectra show that the excited states corresponding to OPA band at
400 nm can be also produced by TPA activity, providing a maximum σTPA around 750 nm
when DMSO or THF are used as solvents. The OPA band at 490 nm has a weak TPA
response at 1000 nm (roughly observed in TPA spectra). In contrast, in chloroform, the
excited states corresponding to the band at 490 nm exhibits clearly enhanced TPA response
at 1000 nm, in addition to the band at 800 nm (400 nm of OPA). From the examination of
Figure 4, the role of the subtituents (butyl and phenyl) on the σTPA it seems to have a
significant effect in chloroform in the range from 700 to 800 nm with σTPA values of 8 and
9 being larger than 10 and 11, however in THF or DMSO there is not a substantial
difference between dibutyl and diphenyl substituent.

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Figure 5.- Organotin compounds synthesized in this work.
The need to develop chromophores that are highly soluble in organic solvents is achieved
by the introduction of the hexyl chains on the fluorene, with the expectation that this shall
not interfere significantly in the optical properties, however as can be seen in the Figure 4
this is not the case for σTPA, in DMSO when the hexyl chains are present (9 or 11) the TPA
is higher than without it (8 or 10), in chloroform it seems to be the opposite, and in THF
there is not a substantial difference
Comparison of the values obtained for 10 and 11 with those of a similar molecule, shows
that the σTPA for the molecules described in this work is approximately 10 times larger than
the value in a previous report (see Figure 6).[11] This enhancement of the TPA can be
attributed to an increased
moiety and the triple bond.
π conjugation as a result of the introduction of the fluorene

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Figure 6.- Comparison of σTPA for 10 and 11 (left), a previously reported molecule (right).
4.- CONCLUSIONS
We synthesized four novel pentacoordinated diorganotin compounds which were fully
characterized. A solvatochromic study showed no dependence on solvent polarity pointing
out to the existence of specific interactions and a small change in the dipole moment.
These complexes can be grouped depending on the substituent of the tin atom, either
diphenyl or dibutyl, due to the small changes in their optical properties attributed only to
the rigidity of the framework provided by the tin center, which is better for the phenyl
substituent than the butyl. The TPA spectra were evaluated in three different solvents, in
all cases, the band located around 400 nm in the OPA spectra was TPA active with a σTPA
up to 517 GM for 9 when using chloroform as solvent, the band located around 500 nm in
the OPA spectra was also active for the four complexes. Furthermore the maximum
brightness observed in our complexes was 150 GM that corresponds to 9 in chloroform,
which is ten times larger than the one measured by Peon et al[11] and almost twice as large
than the one measured by Tian et al[9] (95 GM in DMF). These results suggest that these
complexes can be used for Two-Photon Microscopy, due to their better brightness.
The new tin complexes show an enhancement in the TPA property probably related to an
increase in the
π conjugation pathway due to the presence of the fluorene (donor) and the

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nitro (acceptor) group. Both, the TPA and fluorescence properties were then improved
compared with other organotin compounds, also with the introduction of a dialkylated
fluorene the solubility of this type of compounds is greatly enhanced allowing for further
applications in solution.
5.- SUPPORTING INFORMATION
UV-vis/fluorescence spectra and solvatochromic analysis, 1H NMR, 13C NMR, 119Sn NMR
and infrared spectra for the compounds 8-11, molecular geometries and orbitals energies
obtained by DFT calculations.
6.- ACKNOWLEDGEMENTS
This work has been performed within the framework of the French-Mexican international
laboratory (LIA-LCMMC). The authors thank CNRS (France), PAIP, PAPIIT IN216616
(UNAM) and CONACyT (Mexico) for financial supports, A.E.C. (270200) thanks
CONACyT for the PhD grant, Violeta Álvarez. and Alberto Vega thanks UNAM-DGAPA.
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Four novel pentacoordinated diorganotin compounds were synthesized and fully characterized.
The Two-Photon absorption spectra was recorded in three different solvents resulting in a
maximum of 517GM and a brightness of 150GM.
An enhancement of the two-photon absorption properties are achieved thanks to the introduction
of a fluorene moiety.