Facile synthesis, spectroscopic and electrochemical properties, and theoretical calculations of porphyrin dimers with a bridging amide-bonded xanthene moiety

Journal of Porphyrins and Phthalocyanines

J. Porphyrins Phthalocyanines 2015; 19: 1–11

DOI: 10.1142/S1088424615500492

Published at http://www.worldscinet.com/jpp/

Facile synthesis, spectroscopic and electrochemical

properties, and theoretical calculations of porphyrin dimers

with a bridging amide-bonded xanthene moiety

Xu Liang^a,b, Li Xu^a, Minzhi Li^a, John Mack*^c◊, Justin Stone^c, Tebello Nyokong^c◊,

Yu Jiang^a, Nagao Kobayashi*^b◊and Weihua Zhu*^a◊

^aSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China

^bDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

^cDepartment of Chemistry, Rhodes University, Grahamstown 6140, South Africa

Received 22 December 2014

Accepted 13 February 2015

ABSTRACT: A free base porphyrin dimer bridged by a ﬂexible amide-bonded xanthene moiety and its

binuclearzinc(II)complexzinc(II)complexweresynthesizedandcharacterized.Structuralcharacterization

by MS and ¹H NMR spectroscopy conﬁrmed the bridged porphyrin dimer structure. The properties of the

dimers were characterized by IR, UV-visible absorption, ﬂuorescence and magnetic circular dichroism

(MCD) spectroscopy, and electrochemistry studies. Theoretical calculations were carried out to analyze

the electronic structures of porphyrin dimers with a bridging amide-bonded xanthene moiety.

KEYWORDS: xanthene, porphyrin dimers, spectroscopy, TD-DFT calculations, electrochemistry.

synthetic dyes in a wide range of applications, and their

spectroscopic properties have been extensively studied

[6]. Xanthene moieties have usually been introduced to

porphyrin arrays through direct C–C bonds at the meso-

INTRODUCTION

Porphyrin oligomers with macrocyclic rings that are

covalently linked in a cofacial conﬁguration have received

considerable attention in recent years due to their unique

positions and the electronic structures, and optical and

electronic structures and novel optical properties [1].

magnetic properties of these compounds have been studied

Several types of conjugated porphyrin oligomers have

in depth [7]. In contrast, porphyrin xanthenes with ﬂexible

been reported such as, doubly- or triply-fused co-planar

bridges such as alkyl chains and amines have received

porphyrins [2], alkyne-bridged porphyrin strands [3], and

considerably less attention [8], and their spectroscopic

p-phenylene-bridged twisted/planar porphyrin dimers

properties and electrochemical properties have not been

[4]. Interest of porphyrin dimers with linking moieties,

reported. Given the progress that has been made in

especially those that adopt a face-to-face manner has been

xanthene-bridged cofacial porphyrin dimers, the goal of

increasing in recent years. Porphyrin dimers with face-to-

this study was to synthesize new compounds with ﬂexible

face conformations exhibit an increased ability to bind two

amide bridging units and to investigate their spectroscopic

metalsionsinasuitablegeometry,openingupthepossibility

and electrochemical properties to develop an in-depth

of mimicking the activity of heme and nonheme iron and

understanding of their electronic structures.

copper-binding bioproteins such as oxidase, oxygenase,

and oxygen transport proteins, which have binuclear

active sites [5]. Xanthenes, have found widespread use as

EXPERIMENTAL

^◊SPP full member in good standing

*Correspondence to: Weihua Zhu, email: sayman@ujs.edu.

cn, tel/fax: +86 511-8879-1928; John Mack, email: j.mack@

Chemicals

Spectral grade o-dichlorobenzene for electro-

chemical measurements was purchased from the

ru.ac.za, fax: +27 46-622-5109; Nagao Kobayashi, email:

nagaok@m.tohoku.ac.jp, fax: +81 22-795-7719.

2

X. LIANG ET AL.

Aladdin Reagent Company of Shanghai. Other

chemicals and solvents were of analytical pure grade

and were obtained from the Shanghai Guoyao Co. All

solvents were dried and distilled prior to use, and 5,10,-

15,20-tetraphenylporphyrin was synthesized according

to standard literature procedures [9].

chromatography (eluent: CH₂Cl₂/hexane = 2:1) to give the

free base porphyrin dimer 4 as a dark-red solid.Yield 65 mg

(68%). IR (KBr): n, cm^-13405 (s, br), 3300 (s), 3043 (m),

2960 (s), 2350 (s), 1798 (w), 1666 (s), 1590 (s), 1506 (vs),

1471 (vs), 1437 (vs), 1395 (m), 1346 (s), 1221 (s), 1166

(m), 1110 (w), 1055 (w), 964 (vs), 783 (vs), 735 (vs), 700

(vs), 651 (w), 512 (w), 415 (w). ¹H NMR (CDCl₃, 298 K):

d_H, ppm 9.27 (2H, s), 8.74 (4H, d, J = 6.0 Hz), 8.70 (4H, d,

J = 6.0 Hz), 8.36 (4H, s), 8.25 (8H, dd, J₁= 8 Hz, J₂= 12

Hz), 8.17 (4H, d, J = 8.0 Hz), 8.14 (2H, d, J = 4.0 Hz), 7.88

(4H, d, J = 4.0 Hz), 7.77~7.73 (10H, m), 7.02 (10H, d, J =

15 Hz), 6.67 (8H, t, J₁= 8 Hz, J₂= 16 Hz), 1.86 (6H, s),

1.49 (18H, s), -3.01 (4H, s). MS (MALDI-TOF): m/z

1635.64 (calcd. [M + H]⁺1635.00).

Zn(II)-2,7-di-tert-butyl-9,9-dimethyl-4-[10,15,20-

triphenylporphyrin-10-(N-4-phenylamido-acyl)]-5-

xanthenecarboxylic acid amide (5). 10 mL of a MeOH

solution of zinc acetate (44 mg, 0.2 mmol, 10 eq.)

was slowly added to a 50 mL CHCl₃solution of the

xanthene-bridged free base porphyrin dimer 4 (32.6 mg,

0.02 mmol), and the reaction mixture was stirred and

reﬂuxed at 60°C for 1 h. After the removal of the organic

solvent, the residue was puriﬁed by silica gel column

chromatography with CH₂Cl₂as the eluent to provide the

Zn(II)-porphyrin dimer 5 as a green-purple solid. Yield

35.2 mg (92.9%). IR (KBr): n, cm^-13372 (s), 30052 (m),

2955 (s), 2356 (s), 1807 (w), 1675 (s), 1585 (s), 1508 (vs),

1476 (vs), 1435 (vs), 1393 (m), 1327 (s), 1230 (s), 1112

(w), 1070 (m), 1070 (m), 1000 (vs), 798 (s), 743 (s), 701

(s), 656 (w), 529 (w), 438 (w). ¹H NMR (CDCl₃, 298 K):

d_H, ppm 9.25 (2H, s), 8.90 (4H, d, J = 6.0 Hz), 8.80 (4H,

d, J = 6.0 Hz), 8.53 (4H, d, J = 6.0 Hz), 8.25 (8H, dd, J₁=

8 Hz, J₂= 12 Hz), 8.17 (4H, d, J = 8.0 Hz), 8.13 (2H, d,

J = 4.0 Hz), 8.02 (4H, d, J = 4.0 Hz), 7.77~7.69 (8H, m),

7.28 (8H, d, J = 12 Hz), 6.93 (4H, d, J = 15 Hz), 6.80 (8H,

t, J₁= 8 Hz, J₂= 16 Hz), 1.88 (6H, s), 1.47 (18H, s). MS

(MALDI-TOF): m/z 1761.53 (calcd. [M + H]⁺1761.70).

Synthesis

5-p-Nitrophenyl-10,15,20-triphenylporphyrin (2)

[10]. Nitrosonitric acid (3.2 mL) was slowly added

to a solution of 300 mL CH₂Cl₂and 5,10,15,20-

tetrahenylporphyrin (3.00 g, 4.88 mmol) and the mixture

was stirred at 0~5°C in an ice-bath for 4 h. The reaction

mixture was neutralized with ammonia solution to ca.

pH = 7.0, and the organic layer was washed with brine

and dried with anhydrous MgSO₄. After removal of the

solvent, the residue was recrystallized from CH₂Cl₂and

MeOH and ﬁnally puriﬁed by Al₂O₃gel chromatography

(eluent: CH₂Cl₂/hexane = 2:1) to give 5-p-nitrophenyl-

10,15,20-triphenylporphyrin as a purple solid.Yield 2.0 g

(62.6%). ¹H NMR (CDCl₃, 298 K): d_H, ppm 8.89 (2H, d,

J = 4.0 Hz), 8.86 (4H, s), 8.73 (2H, d, J = 4.0 Hz), 8.62

(2H, d, J = 8.0 Hz), 8.38 (2H, d, J = 8.0 Hz), 8.21 (6H, d,

J = 8.0 Hz), 7.83-7.77 (9H, m), -2.74 (2H, s).

5-p-Aminophenyl-10,15,20-triphenylporphyrin

(3) [11]. SnCl₂∙2H₂O (2.0 g, 1.62 mmol) was added to a

100 mL solution concentrated HCl and 5-p-nitrophenyl-

10,15,20-triphenylporphyrin (916 mg, 0.400 mmol).

The mixture was vigorously stirred in a preheated oil

bath 70°C for 2 h, and then neutralized with ammonia

solution to ca. pH = 8.0. The reaction mixture was

quenched by 50 mL ice-water and the water phase was

extracted with ethylacetate (3 × 100 mL). The combined

organic layers were dried with anhydrous MgSO₄. After

removal of the organic solvent, the residue was puriﬁed

through recrystallization by adding MeOH to the CH₂Cl₂

solution to afford pure 5-p-aminophenyl-10,15,20-

triphenylporphyrin 3 as a purple solid. Yield 767 mg

(87.3%). ¹H NMR (CDCl₃, 298 K): d_H, ppm 8.93 (2H, d,

J = 4.0 Hz), 8.84 (6H, s), 8.20 (2H, d, J = 8.0 Hz), 7.98

(2H, d, J = 8.0 Hz), 7.74~7.80 (9H, m), 7.06 (2H, d, J =

8.0 Hz), 4.02 (2H, s), -2.76 (2H, s).

2,7-Di-tert-butyl-9,9-dimethyl-4-[10,15,20-tri-

phenylporphyrin-10-(N-4-phenylamido-acyl)]-5-

xanthenecarboxylic acid amide (4). A mixture of

2,7-di-tert-butyl-9,9-dimethyl-4,5-xanthene-dicarboxylic

acid (41 mg, 0.10 mmol) and SOCl₂(5 mL) was reﬂuxed

for 2 h. After excess SOCl₂was removed, the xanthene

acyl chloride was dried and dry CH₂Cl₂(5 mL) was added.

The mixture was stirred at room temperature and a mixture

of 5-p-aminophenyl-10,15,20-triphenylporphyrin (3)

(57 mg, 0.10 mmol) in CH₂Cl₂(15 mL) and 2 drops of

Et₃N was then added dropwise to the resulting light yellow

solution and a white vapor was observed to form in the

reaction ﬂask. The mixture was stirred for 2 h at 0°C. The

crude product was puriﬁed directly by silica gel column

Materials and equipment

Cyclic voltammetry was performed in a three-electrode

cell using a BiStat or Chi-730C electrochemistry station.

A glassy carbon disk electrode was utilized as the working

electrode while a platinum wire and a saturated calomel

electrode (SCE) were employed as the counter and

reference electrodes, respectively. An “H” type cell with

a fritted glass layer to separate the cathodic and anodic

sections of the cell was used during bulk electrolysis.

The working and counter electrodes were made from

platinum mesh and the reference electrode was an SCE.

The working and reference electrodes were placed in one

compartment while the counter electrode was placed in

the other. UV-visible absorption spectra were recorded

with a HP 8453A diode array spectrophotometer. All

of the electrochemical measurements were carried

out under a nitrogen atmosphere. Magnetic circular

dichroism (MCD) spectra were measured with a JASCO

J. Porphyrins Phthalocyanines 2015; 19: 2–11

FACILE SYNTHESIS, SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES, AND THEORETICAL CALCULATIONS

3

J-820 equipped with a 1.6 T (tesla) permanent magnet

by using both the parallel and anti-parallel ﬁelds. The

conventions of Piepho and Schatz are used to describe

MCD intensity and the Faraday terms [12, 13]. MALDI-

TOF mass spectra (MS) were collected using Bruker

Daltonics autoﬂexII MALDI-TOF MS spectrometer.

Fourier transform infrared (FT-IR) spectra were recorded

using Bruker Vector 22 FT-IR spectrometer (using KBr

electronic separation increases. This makes it more suitable

for studying compounds where there is signiﬁcant charge

transfer in the electronic excited states.

RESULTS AND DISCUSSION

Synthesis and characterization

1

pellets) in the 400–4000 cm^-1region. H NMR spectra

were recorded on a Bruker AVANCE 400 spectrometer

(operating at 400.13 MHz) using the residual solvent as

an internal reference for ¹H (d = 7.26 ppm for CDCl₃).

The synthetic pathway used to prepare porphyrin

dimers with a bridging amide-bonded xanthene moiety

is shown in Scheme 1. A nitro group is added at the

para-positions of one of the phenyl substituents of

tetraphenylporphyrin to form 1, which is then reduced

to form the amino group of 2. Reaction with xanthene

acyl chloride results in the formation of an amide-bonded

xanthene-bridged porphyrin dimer. The MALDI-TOF-

mass spectra reveals a strong parent peak at m/z = 1635.64

(calcd. [M + H]⁺= 1635.00) for the xanthene-bridged

free base porphyrin 4, providing direct evidence that the

target molecule was obtained. The parent peak of the

Zn(II) porphyrin dimer 5 at m/z = 1761.53 (calcd. [M +

H]⁺= 1761.70) was also observed. The ¹H NMR spectrum

of 4 contains several peaks in the aromatic region from

both the porphyrins and xanthene moieties (Fig. 1). The

assignment of the proton signal at -3.50 ppm to the inner

N–H protons of the porphyrin rings was further conﬁrmed

Theoretical calculations

The density functional theory (DFT) method was

used to carry out geometry optimizations for free base

5,10,15,20-tetraphenylporphyrin (H₂TPP) and its Zn(II)

complex (ZnTPP), and for 4 and 5 with the B3LYP

functional of the Gaussian09 program package [14] and

6-31G(d) basis sets. To simplify the calculations the t-butyl

groups on the xanthene bridging moieties were replaced

with methyls, since replacing one alkyl group with another

is unlikely to make a signiﬁcant difference in this context,

since there is no scope for steric interactions. The CAM-

B3LYP functional was used to calculate the electronic

absorption properties based on the time-dependent

(TD-DFT)method, sinceitincludesalong-rangecorrection

of the exchange potential, which incorporates an increasing

fraction of Hartree–Fock (HF) exchange as the inter-

1

by D₂O exchanged H NMR measurements. The proton

signals at d = 1.50, 1.85 were assigned to the -tBu and

–CH₃substituents on the xanthene bridging moiety,

N

NH

N

NH

N

(ii)

(i)

H₂N

O₂N

HN

N

1

2

3

+

Ph

tBu

N

HN

Ph

N

NH

N

Zn

Ph

HN

OH

O

Ph

(iv)

(iii)

O

Ph

O

N

NH

N

Ph

Zn

HN

OH

HN

Ph

tBu

Ph

5

4

Scheme 1. Synthesis of the amide-bonded xanthene-bridged porphyrin dimers 4 and 5. Synthetic procedure: (i) nitrosonitric acid,

CH₂Cl₂, 0~5°C, 4 h; (ii) SnCl₂∙2H₂O, HCl, 70°C, 2 h; (iii) SOCl₂, reﬂux, 2 h; CH₂Cl₂, Et₃N, 0°C, 2 h; (iv) Zn(OAc)₂∙2H₂O, CHCl₃/

MeOH, reﬂux, 1 h

J. Porphyrins Phthalocyanines 2015; 19: 3–11

4

X. LIANG ET AL.

Fig. 1. ¹H NMR spectra of 4 (top) and 5 (bottom) in CDCl₃(left) and for the D₂O exchange experiment for 4 (right)

Optical spectroscopy and TD-DFT calculations

UV-visible absorption spectra were recorded in CH₂Cl₂

at room temperature (Fig. 3). UV-visible absorption

spectroscopy is one of the most useful methods for

characterizing porphyrins and their analogues, due to the

presence of the forbidden and allowed Q and B-bands

of Gouterman’s 4-orbital model [15] in the 500–600

and 400–450 nm regions, respectively. A D_16hsymmetry

C₁₆H₁₆^2-species can be regarded as the parent hydrocarbon

perimeter for describing and rationalizing the optical

properties of tetrapyrrolic porphyrinoids. The p-system

contains a series of MOs arranged with M_L= 0, 1, 2,

3, 4, 5, 6, 7 and 8 nodal properties in ascending

energy terms based on the magnetic quantum number for

the cyclic perimeter, M_L. The frontier p-MOs have M_L=

4 and 5 nodal properties, respectively. The four spin-

allowed M_L= 4 → 5 excitations result in two orbitally

Fig. 2. Infrared spectra of 4 (bottom, red) and 5 (top, black).

Boxes are used to highlight peaks at ca. 3300 and 1000 cm^-1

conﬁrm the presence of the inner N-H protons of 4 and the

coordination of Zn(II) ions in the context of 5

1

degenerate E_uexcited states, due to the DM_L= 9, and

DM_L= 1 transitions. This results in the forbidden and

allowed Q and B-bands of Gouterman’s 4-orbital model

for porphyrins [15], since an incident photon can provide

only one quantum of orbital angular momentum. MCD

spectroscopy can be used to identify the main electronic

Q(0,0) and B(0,0) bands, due to the presence of intense

derivative-shaped Faraday A₁terms or, in the context of

lower symmetry compounds, coupled pairs of Gaussian-

shaped Faraday B₀terms [13]. The MCD spectra of 4

and 5 are very similar to those of H₂TPP and ZnTPP

[14a] (Fig. 3), since the relative energies of the frontier

p-MOs of the free base and Zn(II) complexes are very

similar. Derivative-shaped positive pseudo-A₁terms are

observed in the MCD spectrum of 5 for the Q(0,0), Q(0,1)

and B(0,0) bands at 597, 556 and 420 nm. It is noteworthy,

however, that the signal is signiﬁcantly less symmetrical

than in the B-band region than is typically observed

respectively. Similar proton peaks were observed in the

spectrum of the Zn(II) porphyrin complex 5. Solid-state

infrared spectra (IR) (Fig. 2) were measured to obtain

further information about the coordination environment

of these two compounds. The peak at 3300 cm^-1conﬁrms

the presence of an amide moiety. The 3320 cm^-1band

of 4, which is not observed in the IR spectrum of 5, can

be assigned to the inner N-H bonds of the macrocyclic

ring and is often used as a diagnostic peak for free base

porphyrins. The shift of the peaks at around 1000 cm^-1

also conﬁrms the coordination interaction between the

Zn(II) ion and porphyrin rings.

J. Porphyrins Phthalocyanines 2015; 19: 4–11

FACILE SYNTHESIS, SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES, AND THEORETICAL CALCULATIONS

5

Fig. 3. UV-visible absorption (bottom, solid line), ﬂuorescence spectra (bottom, dashed gray line), and MCD (top) spectra of H₂TPP

and ZnTPP in CHCl₃, and of 4 and 5 in CH₂Cl₂. TD-DFT spectra for the B3LYP optimized geometries were calculated by using

the CAM-B3LYP functional with 6–31G(d) basis sets (Table 1) and are plotted against a secondary axis. The Q and B-bands of

Gouterman’s 4-orbital model [13] are highlighted with dark gray diamonds, while bands associated with transitions between the two

porphyrin ring moieties are highlighted with light gray diamonds

for ZnTPP and radially symmetric analogues of Zn(II)

tetraphenylporphyrins [16]. When molecular modeling

calculations (Figs 4 and 5 and Table 1) were carried out it

soon became obvious that the ﬂexible amide bonds enable

access to conformations in which the two porphyrin rings

lie orthogonal to one another, so that there is considerably

less scope for the strong exciton interactions that are

observed for cofacial porphyrin dimers [17]. In TD-DFT

calculations this results in two sets of Q- and B-bands,

which are very similar in energy to those predicted for

the parent tetraphenylporphyrin monomers (Figs 3–5 and

Table 1). Michl’s a, s, -a and -s nomenclature (Fig. 5)

for the four frontier p-MOs of porphyrinoids with

differing symmetries [18] facilitate the comparison

with the spectra of H₂TPP and ZnTPP (Fig. 3 and

Fig. 4. Energies of the frontier p-MOs for the B3LYP geometries

Table 1). No bands associated with transitions involving

of H₂TPP, ZnTPP, 4 and 5 calculated with the CAM-B3LYP

MOs localized on the xanthene bridging moiety are

functional and 6–31G(d) basis sets. In each case, the HOMO-

predicted to have signiﬁcant intensity in the visible

LUMO gap is plotted against a secondary axis with a large gray

region (Fig. 3 and Table 1). The xanthene-bridged free

diamond. In the context of 4 and 5, MOs associated with the

base porphyrin dimer 4 has ﬂuorescence emission bands

at 656 and 717 nm, while the corresponding Zn(II)-

porphyrin complex has bands at 607 and 649 nm (Fig. 3).

two porphyrin ring moieties are offset to the left and right, while

those that are localized on the xanthene bridging moiety are not.

Small black diamonds are used to highlight occupied MOs

J. Porphyrins Phthalocyanines 2015; 19: 5–11

6

X. LIANG ET AL.

Fig. 5. Nodal patterns and energies of the frontier p-MOs of 4 and 5. H and L denote the HOMO and LUMO (bottom), respectively.

A and B superscripts are used to denote MOs localized on the two porphyrin moieties. Nodal patterns of the four frontier p-MOs

of zinc tetraazaporphyrin at an isosurface value of 0.04 a.u (top). Michl [19] introduced a, s, -a and -s nomenclature to describe the

four frontier p-MOs with M_L= 4 and 5 nodal patterns based on whether there is a nodal plane (a and -a) or an antinode (s and -s)

on the y-axis. This makes it easier to compare MOs of porphyrin complexes with differing symmetries (Table 1)

If the relative intensities of the two major bands are

ignored in each case, the ﬂuorescence spectra of 4 and

5 can be viewed as near mirror images of the emission

bands of the related porphyrin monomers, and there are

relatively small Stokes shifts of 9 and 11 nm, respectively,

as is normally the case with monomeric porphyrins [15],

which suggests that the S₁state is localized on a single

porphyrin ring moiety in each case.

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FACILE SYNTHESIS, SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES, AND THEORETICAL CALCULATIONS

7

Table 1. TD-DFT spectra of the B3LYP optimized geometries for H₂TPP, ZnTPP, 4 and 5 calculated with the CAM-B3LYP

functional and 6-31G(d) basis sets

b

Band^a

#

Calcd.^c

Exp^d

Wave function^e=

H₂TPP

—

Q_x

Q_y

B

1

—

Ground state

2

3

4

5

7

16.8

18.5

27.1

27.9

32.8

596

540

369

359

305

(0.01)

(0.03)

(1.26)

(1.66)

(0.62)

15.6

18.4

641

544

59% s → -a; 40% a → -s; …

58% s → -s; 41% a → -a; …

56% a → -s; 35% s → -a; …

59% a → -a; 43% s → -s; 9% H-3 → -a; …

80% H-3 → -a; 8% s → -a; …

24.0

—

416

—

ZnTPP

—

Q

1

2

3

4

5

—

Ground state

18.5

27.8

542

360

(0.01)

(1.50)

53% s → -a/-s; 47% a → -a/-s; …

53% a → -a/-s; 47% s → -a/-s; …

53% a → -a/-s; 46% s → -a/-s; …

53% s → -a/-s; 46% a → -a/-s; …

16.9

592

B

23.8

421

4

—

Q_x^A

1

2

—

Ground state

16.7

599

(0.02)

51% s^A→ -s^A; 34% a^A→ -a^A;

9% s^A→ -a^A; 6% a^A→ -s^A; …

15.4

648

Q_x^B

Q_y^A

Q_y^B

B_x^A

B_x^B

B_y^A

3

4

5

6

7

8

16.7

18.4

18.5

26.8

27.0

27.6

597

543

542

373

370

363

(0.01)

(0.04)

(1.67)

(1.87)

(1.57)

52% s^B→ -s^B; 34% a^B→ -a^B;

8% s^B→ -a^B; 5% a^B→ -s^B; …

50% s^A→ -a^A; 34% a^A→ -s^A;

9% s^A→ -s^A; 6% a^A→ -a^A; …

18.1

24.0

551

417

51% s^B→ -a^B; 35% a^B→ -s^B;

8% s^B→ -s^B; 5% a^B→ -a^B; …

26% a^A→ -a^A; 19% a^B→ -a^B; 15% s^A→ -s^A;

12% s^B→ -s^B; 12% a^A→ -s^A; 7% s^A→ -a^A; …

35% a^B→ -a^B; 21% s^B→ -s^B; 15% a^B→ -a^B;

9% s^A→ -s^A; 6% a^B→ -s^B; …

25% a^A→ -s^A; 18% a^B→ -s^B; 18% s^A→ -a^A;

16% a^A→ -a^A; 13% s^B→ -a^B; 11% s^A→ -s^A; …

B_y^B

—

9

16

17

20

16.7

32.2

32.6

33.2

599

311

307

302

(1.33)

(0.24)

(0.61)

(0.28)

34% a^B→-s^B; 24% s^B→-a^B; 21% a^A→-s^A; 15% s^A→-a^A; …

44% H-4^Xan→ -s^B; 29% H-7^B→ -s^B; …

62% H-11^A→ -s^A; 11% H-11^A→ -a^A; 6% s^A→ -a^A; …

—

32% H-7^B→ -s^B; 29% H-4^Xan→ -a^B;

16% H-4^Xan→ -s^B; 9% H-7^B→ -a^B; …

5

—

1

2

3

—

Ground state

Q_x^A

18.3

18.4

545

544

(0.01)

55% s^A→ -a/-s^A; 45% a^A→ -a/-s^A; …

53% s^A→ -a/-s^A; 46% a^A→ -a/-s^A; …

16.8

597

Q_y^B

4

5

18.5

542

541

(0.01)

52% s^B→ -a/-s^B; 46% a^B→ -a/-s^B; …

53% s^B→ -a/-s^B; 47% a^B→ -a/-s^B; …

(Continued)

J. Porphyrins Phthalocyanines 2015; 19: 7–11

8

X. LIANG ET AL.

Table 1. (Continued)

b

Band^a

#

Calcd.^c

367

Exp^d

Wave function^e=

B_x^A

B_y^A

B_x^B

B_y^B

6

27.3

27.6

27.8

27.9

(2.41)

(1.50)

(2.24)

(0.40)

35% a^A→ -a/-s^A; 28% s^A→ -a/-s^A;

17% a^B→ -a/-s^B; 16% s^B→ -a/-s^B; …

7

8

9

363

360

358

47% a^A→ -a/-s^A; 38% s^A→ -a/-s^A;

4% a^B→ -a/-s^B; 4% s^B→ -a/-s^B; …

23.8

420

41% a^B→ -a/-s^B; 36% s^B→ -a/-s^B;

12% a^A→ -a/-s^A; 9% s^A→ -a/-s^A; …

43% a^B→ -a/-s^B; 37% s^B→ -a/-s^B;

8% a^A→ -a/-s^A; 7% s^A→ -a/-s^A; …

—

15

16

31.0

31.2

322

321

(0.10)

(0.09)

—

93% H-12^Xan→ -a/-s^A; …

b

^aBand assignment described in the text. The number of the state assigned in terms of ascending energy within the TD-DFT

calculation. ^cCalculated band energies (10³.cm^-1), wavelengths (nm) and oscillator strengths in parentheses (f). ^dObserved

energies (10³.cm^-1) and wavelengths (nm) in Figs 3 and 4. ^eThe wave functions based on the eigenvectors predicted by TD-DFT.

One-electron transitions associated with the four frontier p-MOs of Gouterman’s 4-orbital model [13] are highlighted in bold.

H and L refer to the HOMO and LUMO, respectively. A and B superscripts are used to denote the two porphyrin rings, while

a Xan superscript is used to denote MOs that are localized primarily on the xanthene bridging moiety. Michl’s a, s, -a and -s

nomenclature for the four frontier p-MOs [18] is used to facilitate comparison of the transition of porphyrinoids with differing

symmetries.

Table 2. Redox potentials vs. SCE (in V) for

4 and 5. The E values are taken from DPV

½

measurements

E^Ox

E^Red

2

1

2

½

4

5

+1.53

+1.22

+1.10

+0.89

-1.11

-1.26

-1.46

-1.60

Cyclic voltammetry measurements

Electrochemical measurements were carried out

in o-dichlorobenzene containing 0.1 M TBAP to gain

further insight into the electronic properties. Table 2

provides the redox potentials E values derived cyclic

½

and differential pulse voltammetry (CV and DPV)

measurements. The voltammograms for xanthene-

bridged free base porphyrin dimer 4 (Fig. 6) contain

two reversible one-electron reduction steps at -1.11 V

for [H₂Por]/[H₂Por]^-and at -1.46 V for [H₂Por]^-/

[H₂Por]^2-, respectively, under these experimental condi-

tions. Two reversible one-electron oxidation curves are

also observed at +1.10 and +1.53 V for the 1st and 2nd

oxidation steps. In the voltammograms of the xanthene-

bridged Zn(II) porphyrin dimer 5 (Fig. 6) two reversible

one-electron reduction processes were observed at -1.26

and -1.60 V for the [Zn(II)Por]/[Zn(II)Por]^-and [Zn(II)

Por]^-/[Zn(II)Por]^2-steps, respectively. Two oxidation

processes are also observed at +0.89 and +1.22 V

which are similar to those observed for monomeric

Zn(II) 5,10,15,20-tetraphenylporphyrins [19]. The i_p^Red

and i_p^Oxvalues, determined from CV measurements for

4 and 5 made at various scan-rates from 20–500 mV

Fig. 6. CV (bottom) and DPV (top) measurements of 4 (left)

and 5 (right) in o-dichlorobenzene

J. Porphyrins Phthalocyanines 2015; 19: 8–11

FACILE SYNTHESIS, SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES, AND THEORETICAL CALCULATIONS

9

(Fig. 7), provide an insight into the reversibility of the

system on an experimental time-scale. The good linear

½

correlations observed for plots of peak current vs. v

for 4 and 5 (Fig. 8) conﬁrm that all of the oxidation and

reduction processes are diffusion controlled.

CONCLUSION

A free base porphyrin dimer with a bridging amide-

bonded xanthene moiety and its binuclear Zn(II)

complex have been synthesized through a facile reaction

of 5-p-aminophenyl-10,15,20-triphenylporphyrin and

xanthene chloride. A detailed characterization by

IR, UV-visible absorption, MCD, and ﬂuorescence

spectroscopy was carried out and further insights into

the electronic structures were obtained from theoretical

calculations and various electrochemistry measurements.

Theopticalspectroscopyof4and5isverysimilartothatof

regular meso-tetraphenylporphyrin monomers, since the

ﬂexible amide-bonded xanthene bridging moiety enables

access to energetically stable conformations in which

the porphyrin rings lie in an orthogonal arrangement

with respect to each other. Since porphyrin dimers with

functional bridging units have potential applications in

the ﬁeld of energy conversion and catalytic chemistry,

the detailed analysis of their spectroscopic and redox

Fig. 7. Cyclic voltammetry measurements for 4 and 5 in

o-dichlorobenzene at different scan rates

½

Fig. 8. The dependence of square root of the scan-rate (v ) on the peak current (i_p) for the 1^stand 2^ndreduction and 1^stand 2^nd

oxidation steps during cyclic voltammetry measurements of 4 and 5

J. Porphyrins Phthalocyanines 2015; 19: 9–11

10

X. LIANG ET AL.

properties should assist the design and investigation

of other bridged porphyrin dimers. Further research is

currently being pursued in this regard.

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(c) Dogutan DK, Bediako DK, Teets TS, Schwalbe

M and Nocera DG. Org. Lett. 2010; 12: 1036–1039.

(d) Xu N, Powell DR and Richter-Addo GB. Nitric

Oxide 2014; 37: 61–65.

Acknowledgements

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Financial support was provided by the National

Scientiﬁc Foundation of China (No. 21171076). The

authors thank Prof. Zhen Shen of Nanjing University

for the MCD spectral measurement. The theoretical

calculations were carried out at the Centre for High

Performance Computing in Cape Town, South Africa.

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Article Doi

Facile synthesis, spectroscopic and electrochemical properties, and theoretical calculations of porphyrin dimers with a bridging amide-bonded xanthene moiety

DOI: 10.1142/S1088424615500492

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