Peripherally octamethyl zinc(II) phthalocyanines with various axial substituents

DOI: 10.1016/j.poly.2021.115024

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Polyhedron (2021)

Update date:2022-08-17

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Janczak, Jan

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Article abstract of DOI:10.1016/j.poly.2021.115024

A series of zinc(II) phthalocyanine complexes peripherally octa-substituted by methyl groups and with axially ligated N-donor ligands, (Zn(Me)₈Pc-L, where L is pyridine (4), 3-methylpyridine (5), 3,4-lutidine (6) and 3,5-lutidine (7)), was synt

Full text of DOI:10.1016/j.poly.2021.115024

Polyhedron 197 (2021) 115024

Contents lists available at ScienceDirect

Polyhedron

journal homepage: www.elsevier.com/locate/poly

Peripherally octamethyl zinc(II) phthalocyanines with various axial

substituents

Jan Janczak ¹

Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Okólna 2, 50-422 Wrocław, Poland

a r t i c l e i n f o

a b s t r a c t

Article history:

A series of zinc(II) phthalocyanine complexes peripherally octa-substituted by methyl groups and with

Received 30 November 2020

Accepted 31 December 2020

Available online 16 January 2021

axially ligated N-donor ligands, (Zn(Me)

Pc-L, where L is pyridine (4), 3-methylpyridine (5), 3,4-lutidine

Pc-L complexes 4–7 were obtained in the crys-

(6) and 3,5-lutidine (7)), was synthesized. All the Zn(Me)

talline form, suitable for X-ray single crystal analysis. The effect of the peripheral substituents as well as

the ligation effect of the axial ligands on the structure were analyzed. In all the here studied zinc(II)

phthalocyanine derivatives, the interaction of the metal center with the N atom of the axial ligand results

in its displacement by ~ 0.4 Å from the N -isoindole plane of phthalocyaninate macrocycle that adopts a

saucer-shaped form. The X-ray geometry and stereochemistry of the zinc(II) phthalocyanine derivatives

were compared with that obtained by DFT methods. Analysis of the Hirshfeld’s surface shows a reduction

Keywords:

Zinc(II) phthalocyanine derivatives

Crystal structure

Peripherally octa-substituted Zn(II)

phthalocyanine complexes

DFT and TD DFT

of the

actions in the investigated complexes 4–7. Partial MO energy diagrams and the calculated absorption

spectra of the Zn(Me) Pc-L derivatives 4–7 exhibit a bathochromic shift of the maximum absorption

wavelength (kmax) in comparison with the spectrum of Zn(Me) Pc by about 20 nm. Complexes 4–7 have

pÁÁÁ

interactions with the simultaneous increase of the CAHÁÁÁN, CAHÁÁÁN, HÁÁÁH and CAHÁÁÁ

p inter-

UV–Vis

a strong absorption band in the therapeutic window (600–900 nm) and the HOMO-LUMO energy gap is

sufﬁcient to excite the ground state of oxygen and therefore they can be tested as potential photosensi-

tizers for PDT.

. Introduction

view of their strong intense absorption in the therapeutic window,

namely in the red region of visible light [12]. An inﬂuential photo-

Phthalocyanines (Pcs) composed of four isoindole units con-

nected by azamethine bridges are planar macrocyclic compounds

containing an extended 18 electron conjugated aromatic system

Scheme 1) [1]. Due to their fascinating and various properties,

sensitizer for PDT application should be characterized with a high

singlet oxygen generation capacity, high triplet state quantum

yields and long triplet life-time. The limited solubility of large pla-

nar metallophthalocyanines and their aggregation behavior in bio-

logical systems is a very common phenomenon in this family of

(

as well as their high thermal and chemical stability, they are a very

important class of compounds that offer varied and diverse appli-

cations. Because of their intense blue-violet colour they were ini-

tially used in industry as dyes and pigments [2]. A still increasing

interest in phthalocyanines is due to their unique electronic and

optical properties [3] as well as unexceptional extraordinary pho-

tophysical, photochemical and electrochemical properties [4] that

give opportunities for their application as gas sensors [5], solar

cells [6], non-linear optics [7], optical data storage devices [8],

near-IR imaging agents [9], active matrix displays [10] and organic

light-emitting diodes (OLEDs) [11]. Metallophthalocyanines,

including zinc phthalocyanine and its derivatives, with low toxicity

have conﬁrmed to be promising as photosensitizers for PDT, in

compounds due to

p-p interactions between the molecules with

extended -electron delocalized systems, which decreases their

ﬂuorescence quantum yields and shortens their triplet lifetime,

and thus reduces their photosensitizing efﬁciency [13].

Many of these applications are closely related to the solubility

of the photosensitizers, which results directly from their structures

and the nature of the intermolecular interactions in the solid. The

great architectural ﬂexibility of metallophthalocyanines can be

exploited to tune the molecular properties in pursuit of optimized

features, like as color, aggregation and optical absorption proper-

ties. There are several ways to improve their solubility and reduce

their ability to aggregate in solutions. One way to improve their

solubility is adding steric substituents in the peripheral and/or

non-peripheral positions of the phthalocyanine macrocycle.

Another way is to modify the metal center complexation by

ORCID: 0000-0001-9826-5821.

E-mail address: j.janczak@intibs.pl

https://doi.org/10.1016/j.poly.2021.115024

J. Janczak

Polyhedron 197 (2021) 115024

EDAX Pegasus XM4 spectrometer with an SDD Apollo 4D detector

mounted on a FEI Nova NanoSEM 230 microscope. In addition, the

elemental analysis was carried out with a Perkin-Elmer 240 ele-

mental analyzer. A thermogravimetric study (TGA) for the investi-

gated compound was performed over the temperature range 20–

00 °C using a Perkin Elmer TGA 4000. The sample weight was

ca. 25 mg. The heating speed rate was 5 °C/min. Pure nitrogen

gas as a dynamic ambient atmosphere was used. The Fourier trans-

À1

form infrared spectra were recorded between 4000 and 450 cm

on a Bruker IFS 113 V FTIR in KBr pellets (Fig. S1). Measurements

of the UV–Vis spectra were carried out at room temperature using

an Agilent UV–Vis/NIR Cary 5000 spectrometer. The UV–Vis spec-

À6

tra were recorded in pyridine (c = 10

(

mol/l) and toluene

À7

c = 5 Â 10 mol/l) solutions. Diffuse Reﬂectance Spectroscopy

DRS) experiments were conducted on a Cary-Agilent 5000 spec-

trometer with a Praying Mantis diffuse reﬂectance attachment.

The powder samples were loaded in a holder with a quartz window

2 3

and were measured in the region 300–1100 nm. A powder Al O

Scheme 1. The ways of possible modiﬁcations of metallophthalocyanines, showing

in red the 18-p electron aromatic macrocycle.

reﬂectance standard was used as the baseline. To minimize the

effects of regular reﬂection and particle size, the sample was

diluted with a non– or weak absorbing colorless standard of

. The concentration of the studied Zn(Me)

Pc-L derivatives

coordinating additional axial ligand(s). The third way to modify

and increase the solubility of MPcs is to use both methods simul-

taneously, i.e. H-substitution in the Pc ring and axial ligation of

the central metal of the MPc macrocycles (Scheme 1). All above

mentioned structural modiﬁcations of metallophthalocyanines

were found to be able to prohibit close self-organization of the

macrocycle, even in the solid state, and lead to improvement of

in the used Al

2 3

diluent was ~ 5% by weight. The corresponding

diluent was also used as the baseline standard. In addition, the

same support was also used as the diluent as well as the standard

to examine the effect of the oxide support contribution to the over-

all DRS spectra. The DRS spectra of the samples were recorded

under ambient conditions.

their solubility due to decreasing of the

p-p interactions between

the highly-conjugated 18- electron aromatic macrocycles.

Several structures of metal(II) phthalocyanines, including ZnPc

with various axial ligands, obtained by metal center complexation

are reported [14]. This work is aimed at the synthesis and crystal-

lization of modiﬁed zinc phthalocyanine derivatives, in which the

modiﬁcation occurs both on the peripheral carbon atoms of the

phthalocyanine macrocycle as well as the coordination sphere on

the central Zn atom in ZnPc. The inﬂuence of both modiﬁcation

effects of ZnPc, i.e. the introduction of methyl group substituents

at each peripheral position of the phthalocyanine macrocycle as

well as modiﬁcation of the central zinc atom, on the self-assembly

2.2. Synthesis of 1,2-dimethyl-4,5-dibromobenzene (2)

O-xylene (100 mL, 828 mmol) was placed in a 500 mL round-

bottom ﬂask and then bromine (96.5 mL, 1.86 mol) was added

gradually dropwise over about 3 h, maintaining the temperature

around 0 °C (ice bath) and with stirring (magnetic stirrer). The

resulting solid cake was then allowed to stand at room tempera-

ture for 24 h. The resulting product was dissolved in 500 mL

diethyl ether and washed with 300 mL 2-molar KOH and 200 mL

deionized water by shaking. The ether solution was dried over

and

the solid state as well as their inﬂuence on the solubility have

not been studied systematically so far. Trends in the interac-

interactions between the modiﬁed macrocyclic rings in

anhydrous MgSO , ﬁltered and the organic phase was concentrated

on a rotary evaporator to afford a faintly pink coloured oil which

solidiﬁed upon standing. Recrystallization of the crude product

from MeOH gave a white crystalline product in a yield of ~ 70%

(152.7 g, 578.5 mmol), m.p. 88–89 °C. A selected crystal was

checked on a single crystal X-ray diffractometer. The lattice param-

eters obtained were: a = 9.385(2), b = 7.920(2) and c = 12.616(3) Å

and b = 109.06(2) °, which are consistent with the literature data

[15]. The purity of the obtained crystalline product was also

checked with a powder diffractometer. The experimental X-ray

powder pattern is consistent with the calculated one (Fig. S2 in SI).

tions between the investigated peripherally methyl octa-substi-

tuted zinc(II) phthalocyanine molecules were analyzed using

Hirshfeld surface (HS) and 2D-ﬁngerprint plots. In addition, the

inﬂuence of such a structural modiﬁcation of the phthalocyanine

core as well as modiﬁcation of the coordination sphere of the cen-

tral Zn atom of on the change of the absorption position of the

most intense Q band and the molar absorption coefﬁcient was

investigated. The experimental results are supported by DFT and

TD DFT calculations.

. Experimental

2.3. Synthesis of 4,5-dimethylphthalonitrile (3)

2.1. Materials and methods

A mixture of 4,5-dibromoxylene (26.4 g, 0.1 mol) and CuCN

(

42 g, 0.47 mol) in DMF (200 mL) was reﬂuxed for 6 h. After cool-

ing, the solution was poured into 1 L of a solution of FeCl . The pre-

cipitate was ﬁltered off, washed with water and extracted with

CHCl (600 mL). The extract was dried with CaCl . The solvent

O-xylene, iodine, bromine, chloroform, dimethylformamide,

pyridine, 3-methylpyridine, 3,4-lutidine, 3,5-lutidine and the

metal salts were obtained from Sigma-Aldrich. All solvents were

puriﬁed prior to use by standard procedures. Metal salts were kept

before syntheses in a vacuum drying oven for 5 h at 110 °C. The

composition of the obtained crystals was checked with a Perkin-

Elmer 2400 elemental analyzer and with energy dispersive spec-

troscopy (EDS). EDS spectra were acquired and analyzed using an

was distilled off on a rotary evaporator and the resulting product

was puriﬁed by recrystallization from benzene, yielding a light-

yellow solid in the crystalline form. The solid 4,5-dimethylph-

thalonitrile was obtained in a yield of 71% (11.10 g, 71 mmol), m.

p. 171–172 °C.

J. Janczak

Polyhedron 197 (2021) 115024

2.4. Synthesis of the zinc phthalocyanine complexes 4–7

2.6. Powder X-ray diffraction (PXRD)

A mixture of 4,5-dimethylphthalonitrile 3 (0.2 g, 1.282 mmol)

The purity of the obtained peripherally octamethyl zinc(II)

phthalocyanines with various axial substituents (4–7) were

checked by powder X-ray diffraction on a PANanalytical X’Pert

and zinc acetate (0.08 g, 0.372 mmol) in a glass ampoule was cov-

ered with 10 mL pyridine (3-mepy, 3,4-lutidine or 3,5-lutidine) and

a few drops of DBU as a catalyst. Next the ampoule was degassed

and sealed under reduced pressure. The ampoule was heated at a

temperature of about 10 °C below the boiling point of pyridine

or its corresponding derivative for about 20 h. After such processes,

well-developed single crystals suitable for single-crystal X-ray

analysis were obtained. The crystals were separated by ﬁltration,

washed with acetone and diethyl ether, and dried in air.

diffractometer equipped with

Cu-K

radiation source

(k = 1.54182 Å). The diffraction data were recorded in the range

5–50 ° at room temperature. The powder diffraction patterns of

4–7, together with the simulated ones, are included in the support-

ing information (Fig. S3).

2.7. Hirshfeld surface analysis

Zinc(II)(pyridine)-2,3,9,10,16,17,23,24-octamethylphthalocya-

nine (4). Yield: 40.5% (0.100 g, 0.130 mmol). Analysis found: Zn,

Hirshfeld surface analysis and 2D ﬁngerprint plots, as well as

(

.42; C, 70.55; N, 16.44; H, 4.59%; calculated for C40

C H N): Zn, 8.50; C, 70.26; N, 16.39; H, 4.85%. IR/KBr (cm ):

5 5

32 8

H N Zn

percentage contributions for various intermolecular contacts in

the investigated crystals, were calculated using the Crystal

Explorer Ver. 3.1 program package [20].

À1

400w, 3287w, 2915w, 2216w, 1615 m, 1564w, 1517 s, 1498 s,

446 m, 1394 m, 1372 m, 1330w, 1303 s, 1181w, 1134w,

101vs, 1024 s, 1010vs, 878 m, 838 m, 781w, 775 s, 730w, 689 s.

Zinc(II)(3-methylpyridine)-2,3,9,10,16,17,23,24- octamethylph-

2.8. Theoretical calculations

thalocyanine (5). Yield: 37.7% (0.095 g, 0.121 mmol). Analysis

found: Zn, 8.30; C, 70.78; N, 16.00; H, 4.92%; calculated for C40

Molecular orbital calculations with full geometry optimization

À1

of the zinc phthalocyanine complexes 4–7 were performed with

the Gaussian16 program package [21]. All calculations were car-

ried out using the DFT method (Becke3-Lee-Yang-Parr exchange

correlation functional B3LYP) [22] with the 6-31G basis set [23],

assuming the geometry resulting from the X-ray diffraction study

as the starting structures. As convergence criterions, the threshold

limits of 0.00025 and 0.0012 a.u. were applied for the maximum

force and the displacement, respectively. The three-dimensional

molecular electrostatic potential (3D MESP) maps were obtained

on the basis of the DFT (B3LYP/6-31G) optimization. The calculated

8 6

Zn(C H

N): Zn, 8.35; C, 70.54; N, 16.09; H, 5.02%. IR/KBr (cm ):

394w, 3344w, 2360w, 2336w, 1617w, 1562w, 1486 m; 1450,

399 m, 1378 m, 1308 s, 1239 m 1202w, 1178 m, 1132w,

099vs, 1020 s, 988 m, 877 m, 855 m, 804w, 790w, 749 s, 711 s,

68 m, 646 m, 572w, 502w.

Zinc(II)(3,4-dimethylpyridine)-2,3,9,10,16,17,23,24-

octamethylphthalocyanine (6). Yield: 38.3% (0.098 g, 0.123 mmol).

Analysis found: Zn, 8.10; C, 71.08; N, 15.68; H, 5.14%; calculated for

À1

Zn(C

7 9

H N): Zn, 8.20; C, 70.81; N, 15.81; H, 5.18%. IR/KBr

(

cm ): 3382w, 3338w, 1614 m, 1485 s, 1458 m, 1398 m,

(

D MESP was mapped onto the total electron density isosurface

374w, 1332 m, 1308 s, 1240 m, 1203 m, 1174w, 1133w,

098vs, 1018 s, 988 m, 874 m, 857 m, 824w, 805w, 748 s, 714 s,

10w, 572w, 523w, 502w.

À3

0.008 e Å ) for each molecule. The colour code of the MESP maps

À1

is in the range À0.05 (red) to 0.05 e Å (blue). After the geometry

optimization, time-dependent (TD) DFT calculations [24] were per-

formed to evaluate the absorption spectrum employing the same

level and basis sets. All stationary points were optimized without

any symmetry assumptions and characterized by normal coordi-

nate analysis at the same level of theory.

Zinc(II)(3,5-dimethylpyridine)-2,3,9,10,16,17,23,24-

octamethylphthalocyanine (7). Yield: 38.6% (0.099 g, 0.124 mmol).

Analysis found: Zn, 8.08; C, 61.12; N, 15.66; H, 5.145%; calculated

for C40

À1

Zn(C

7 9

H N): Zn, 8.20; C, 70.81; N, 15.81; H, 5.18%. IR/

KBr (cm ): 3380w, 3336w, 1615 m, 1487 s, 1458 m, 1399 m,

376w, 1334 m, 1304 s, 1240w, 1173w, 1134w, 1098vs, 1019 s,

87 m, 878 m, 858 m, 804 m, 748 s, 714 s, 667w, 574w, 548w,

02 m.

3. Results and discussion

3.1. Synthesis

,5-Dimethylphthalonitrile was obtained from o-xylene in a

.5. X-ray single crystal measurement

two-step reaction using standard procedures [25], as shown in

Scheme 2. The octa-substituted zinc phthalocyanine complexes

4–7 were prepared starting from 4,5-dimethylphthalonitrile 3

and zinc acetate, as shown in Scheme 3. The reaction was carried

out by the solvothermal method using a suitable solvent that also

played the role of an axial ligand as well as the role of the crystal-

lization medium. In addition, a few drops of 1,8-diaza[5.4.0]bicy-

cloundec-7-ene (DBU) were added to the reaction mixture as a

catalyst. The solvothermal reactions were carried out in glass

ampoules under reduced pressure at about 10 °C below the boiling

point of respective solvents (pyridine or its corresponding deriva-

tive) for about 20 h and then they were cooled to room tempera-

ture. Under the solvothermal conditions, the tetramerization of

nitrile 3 with the simultaneous incorporation of the Zn²⁺ion into

the central hole of the formed octamethyl-substituted phthalo-

cyaninate(2-) macrocycle takes place. The formed octamethyl sub-

stituted zinc phthalocyanine then interacts with pyridine or its

respective derivative yielding the axially ligated complexes 4–7.

The direct thermal reaction of 4,5-dimethylphthalonitrile with zinc

in the powdered form without pyridine or other solvents was also

The obtained single crystals of 4–7 were used for data collection

on a four-circle KUMA KM4 diffractometer equipped with a two-

dimensional CCD area detector. Graphite monochromatized Mo-

Ka radiation (k = 0.71073 Å) and the x-scan technique (Dx = 1°)

were used for data collection. Lattice parameters were reﬁned by

least-squares methods on all reﬂection positions. One image was

monitored as a standard after every 40 images for a control of

the stability of the crystal. Data collection and reduction along with

absorption correction were performed using the CrysAlis software

package [16]. The structures were solved by direct methods using

SHELXT [17], giving positions of almost all non-hydrogen atoms.

Initially, the structures were reﬁned using SHELXL-2018 [18] with

anisotropic thermal displacement parameters. Hydrogen atoms of

the phthalocyanine moiety as well as of the axial ligands were

reﬁned as rigid. Visualizations of the structures were made with

the Diamond 3.0 program [19]. Details of the data collection

parameters, crystallographic data and ﬁnal agreement parameters

are collected in Table 1.

J. Janczak

Polyhedron 197 (2021) 115024

Table 1

Crystal data and ﬁnal reﬁnement parameters.

Formula

N Zn

47 41 9

C H N Zn

Molecular weight

Crystal system

Space group

a (Å)

769.20

triclinic

P-1

783.23

triclinic

P-1

797.26

triclinic

P-1

797.26

triclinic

P-1

11.6888(5)

12.6385(6)

13.3262(7)

99.113(4)

104.705(4)

102.214(4)

1813.64(16)

11.9300(10)

12.3287(12)

14.2326(15)

102.251(6)

109.408(8)

102.738(9)

1830.7(3)

11.9023(7)

12.4117(8)

14.2480(9)

102.310(7)

108.180(10)

101.440(10)

1872.6(2)

12.1688(11)

12.1848(12)

14.5816(14)

106.139(8)

102.340(8)

102.526(8)

1939.0(3)

b (Å)

c (Å)

( )

b ( )

( )

V (Å )

F(000)

800

816

832

Temperature (K)

295(2)

1.409

0.725

0.21 Â 0.19 Â 0.15

multi-scan

0.9324 / 1.000

25,430 / 9175 / 4922

0.0748

295(2)

1.421

0.720

0.34 Â 0.21 Â 0.15

multi-scan

0.8947 / 1.000

25,556 / 9159 / 6597

0.0290

295(2)

1.414

0.705

0.29 Â 0.27 Â 0.14

multi-scan

0.9262 / 1.000

24,055 / 8505 / 4549

0.0725

295(2)

1.366

0.681

0.28 Â 0.24 Â 0.15

multi-scan

0.9352 / 1.000

26,930 / 9644 / 4950

0.0813

À1

calc [g cm

]

À1

m (mm

)

Crystal size (mm )

Abs. correction

min/Tmax

Total / Unique /Obs reﬂs

int

(F )]^a

R [F > 2

0.0638

0.1015

0.0402

0.0915

0.0740

0.1213

0.0660

0.1012

wR [F²all reﬂs]

1.002

1.047

1.062

1.010

À3

max

min (e Å

)

0.419, À0.416

0.314, À0.361

0.508, À0.420

0.362, À0.411

CCDC No.

2027869

2027870

2027871

2027872

R =

wR={

||F

[w(F

|–|F

||/

) ]/

}^½; w^À¹

²(F

) + (aP)²+ bP where P = (F

+ 2F²

–F

)/3. The a and b parameters are 0.0294 and 0.0027 for 4, 0.0236 and 1.5630 for 5, 0.0024 and

.8995 for 6, and 0.0083 and 1.9641 for 7.

For a better understanding of the reaction formation of the zinc

phthalocyanine derivatives during the solvothermal processes as

well as the nature of the interaction between the reacting mole-

cules, three-dimensional molecular electrostatic potentials have

been calculated [27]. The molecular electrostatic potential (MESP)

is related to the electronic density in a molecule and is a very use-

ful tool in determining sites for electrophilic and nucleophilic reac-

tions as well as for intermolecular interactions and the

organization of molecules in the solid-state [28]. The three-dimen-

sional MESP maps of 4,5-dimethylphthalonitrile, that undergoes

Scheme 2. Synthetic route to 3,5-dimethylphthalonitrile.

cyclotetramerization in the presence of Zn ions with the forma-

tion of the zinc phthalocyanine derivative, as well as for the reac-

tion solvent molecules (pyridine, 3-picoline, 3,4-lutidine or 3,5-

lutidine) and the ﬁnal product molecules were calculated (Fig. 1).

Looking at the 3D MESP maps, it can be seen that the formed octa-

methyl-substituted zinc phthalocyanine, with a positive EP about

the central Zn(II) ion on both sides of the macrocyclic Pc ring, in

a solution of pyridine or its corresponding derivatives can interact

with the ring nitrogen atom of the solvent, which has a negative EP.

As a result of this interaction, an axial Zn-N donor–acceptor bond is

formed. The MESP maps calculated for the reaction product mole-

cules, i.e. the octamethyl-substituted zinc phthalocyanines, are

helpful in understanding the process of their nucleation and crystal

growth.

Scheme 3. Synthetic strategy used to prepare the complexes 4–7.

3.2. Thermal properties

In order to determine the thermal stability of the obtained com-

performed by the procedure used for the synthesis of normal

unsubstituted MPcs, described elsewhere [26], however all

attempts resulted in octamethyl substituted zinc phthalocyanine

in the powdered form. So, the pyridine or its respective derivatives

present in the reaction played a role not only as a reactive medium

but also as an axial ligand due to their coordination abilities, and

additionally as a medium for crystallization of the formed ZnPc-

complexes.

pounds in the crystalline form, thermal analysis was carried out on

samples of about 25 mg, with a heating rate of 5 °C/min. Thermo-

gravimetric analyses of the investigated compounds 4–7 are illus-

trated in Fig. 2. All the investigated complexes are relatively very

stable and show one step of weight loss during heating. The weight

loss is related to the breaking of the axial coordination Zn-N bond

and the release of the axial ligand (pyridine À 4, 3-picoline À 5,

3,4-lutidine À 6 and 3,5-lutidine À 7) at a respective temperature

J. Janczak

Polyhedron 197 (2021) 115024

À3

Fig. 1. The calculated 3D MESP is mapped onto the total electron density isosurface (0.008 e Å ) for 4,5-dimethylphthalonitrile (a), the octa-substitued Zn[(Me)

-Pc]

derivative (b), solvent molecules (L): pyridine, 3-methylpyridine, 3,4-lutidine and 3,5-lutidine (c) and the ﬁnal products of Zn[(Me)

the range –0.05 (red) to 0.05 e Å (blue). ((Colour online.))

-Pc]L (d). The colour code of the MESP is in

À1

8 8

1.33% for Zn(Me) Pc-py (4), 11.89% for Zn(Me) Pc-3-pic (5) and

Zn(Me) Pc-py

(4)

13.44% for octamethyl-substituted zinc phthalocyanine ligated by

3,4-lutidine (6) and 3,5-lutidine (7). Finally, the sample gives the

octamethyl-substituted zinc phthalocyanine Zn(Me) Pc that

undergoes sublimation above 400–450 °C.

Zn(Me) Pc-3-pic (5)

Zn(Me⁾8Pc-3,4-lut (7)

3,4-lut

Zn(Me) Pc-3,5-lut (6)

3,5-lut

3.3. Structural characterization

3-pic

Single crystals suitable for X-ray diffraction of all the Zn(Me) -

Pc-L derivatives (L = pyridine, 3-picoline, 3,4-lutidine and 3,5-luti-

dine) were obtained in a one-step method by the direct reaction of

Zn(Me) Pc

,5-dimethylphthalonitrile with Zn(CH

solvents, which play the role of not only as the axial ligation ligand

by also the crystallization medium. All the Zn(Me) Pc-L derivatives

3 2

COO) in the respective

crystallize in the centrosymmetric space group of the triclinic sys-

tem with two molecules per unit cell (Table 1). The asymmetric

100

150

200

250

300

350

400

450

Temperature [ C]

unit of the crystals consists of one Zn(Me) Pc-L molecule (Fig. 3).

The Zn(Me) Pc-L molecules exhibit a similar conformation, in

Fig. 2. Thermogram for Zn(Me)

6) and Zn(Me) Pc-3,5-lut (7).

Pc-py (4), Zn(Me)

Pc-3-pic (5), Zn(Me)

Pc-3,4-lut

which the divalent zinc ion is coordinated with the four isoindole

nitrogen atoms of the octamethyl-substituted phthalocyaninate

(

2-) macrocycle and, in addition, with the nitrogen atom of the

axial ligand (L). Thus, the coordination polyhedron of the zinc(II)

ion in all the complexes exhibits a slightly distorted square pyra-

which correlates with the axial ligand binding force, intermolecu-

lar interactions and the crystal packing. The weight losses are

mid. However, due to the interaction between the zinc(II) center

1.45 and 12.00% for 4 and 5, respectively, and the weight loss

of Zn(Me)

Pc and the lone electron pair on the sp orbital of the

for complexes 6 and 7 is almost the same and equal to 13.50%.

The weight losses are in agreement with the calculated values:

N atom of pyridine or its derivatives, with the formation of an axial

Zn–N bond, the divalent central zinc(II) ion does not lie in the cen-

J. Janczak

Polyhedron 197 (2021) 115024

(

(b)

(

(d)

Fig. 3. View of the asymmetric unit of 4 (a), 5 (b), 6 (c) and 7 (d).

tral hole of (Me)

deﬁned by the four isoindole N atoms of the (Me)

Pc(2-), but is deviated by 0.38(2) Å from the plane

Pc(2-) macrocy-

cle toward to the N atom of the axial ligand. The deviation of the Zn

Table 2

Selected geometrical parameters for Zn(Me)

Pc-py (4) (Å,^o).

X-ray

DFT

2.045

2.043

2.045

2.215

101.86

101.36

101.88

101.36

90.00

(

II) ion from the N

-isoindole plane is very similar in all the Zn

Zn—N2

Zn—N4

Zn—N6

Zn—N8

2.018 (2)

2.027 (2)

2.019 (2)

2.166 (2)

97.52 (9)

101.61 (9)

105.44 (9)

100.79 (9)

87.08 (3)

(

Me) Pc-L molecules (Table 2), however, a tendency to reduce

the Zn deviation from the ring plane going from the pyridine com-

plex (0.397 (3) Å) to 3-picoline (0.387(3) Å) and then to 3,4-luti-

dine (0.372 (3) Å) and 3,5-lutidine(0.365(4) Å) is noticed. This

tendency correlates well with the coordination properties and

Zn—N9

N2—Zn—N9

N4—Zn—N9

N6—Zn—N9

N8—Zn—N9

N2—Zn—N9—C45

Deviation of Zn from

the basicity of the ligands: pK

line, 7.85 for 3,5-lutidine and 7.54 for 3,4-lutidine [29]. The defor-

mation of the (Me) Pc(2-) ring form planarity can be determined

by the inclination of the four planar isoindole units to the N -plane

and is ranges from 0.91(3) to 6.00(3), 1.80(3) to 7.50(3), 2.011(3) to

.51(3) and 1.10(3) to 8.50(3) ^oin 4, 5, 6 and 7, respectively

Table 2). The four equatorial Zn–N bonds with the (Me) Pc(2-)

= 8.77 for pyridine, 8.32 for 3-pico-

the N

-isoindole plane

0.397 (3)

0.412

Dihedral angle

(

N -isoindole/N2,C1-C8

N -isoindole/N4,C9-C16

N -isoindole/N6,C17-C24

N -isoindole/N8,C25-C32

0.91 (3)

4.70 (3)

2.51 (3)

6.00 (3)

4.49

6.01

4.69

6.01

macrocycle are very similar in all the molecules, however, they

are still slightly shorter than the axial bond linking the central zinc

ion to the axial pyridine or its derivative ligands (Table 2). A search

of the Cambridge Structural Database (CSD, Version 5.41) shows a

similar correlation between the equatorial and axial Zn–N bonds

observed in several 4 + 1-coordinated ZnPc-derivatives that have

been structurally characterized [30].

The orientation of the axial ligand in 4, 5, 6 and 7 ligated to the

central Zn(II) ion in relation to the octamethyl-substituted

phthalocyaninate(2-) macrocycle is well described by the N2—

Zn—N9—C45 torsion angle. Rotation of the ligands around the axial

Zn—N9 bond would reduce the steric hindrance effect, i.e. the non-

bonding distances between the hydrogen atoms in the ortho posi-

tion to the N-pyridine or its derivatives atom of the axial molecules

and the atoms of the octamethyl-substituted phthalocyaninate(2-)

ring. In particular, if the rotation angle is equal 0 or 90°, the planar

axial molecule is parallel to the Zn—Niso bond. The second orienta-

tion, with a rotation angle of 45° (or the equivalent 135°) makes

. .

the axial ligand parallel to the Zn

Naza bond. Both orientations

of the axial ligands are preferred by the electrostatic interaction

between the positively polarized hydrogen atoms from the ortho

position of the ligand and the indole or azamethine nitrogen atoms

J. Janczak

Polyhedron 197 (2021) 115024

of the macrocycle with opposite polarization (see Fig. 1). However,

in the crystals, due to intermolecular interactions and crystal pack-

ing forces, the rotation of the axial ligands around the axial Zn–N9

bond is slightly deviated from the ideal torsion angle mentioned

above (see Tables 2–5). The non-bonding distances between the

ortho-H atom of the axial pyridine derivatives and the N-isoindole

atoms of the macrocycle are 2.763 and 2.882 Å in 4, 2.660 and

Table 3

Selected geometrical parameters for Zn(Me)

Pc-3-pic (5) (Å,^o).

X-ray

DFT

2.046

2.044

2.046

2.043

2.210

102.02

101.36

102.03

101.75

90.00

Zn—N2

Zn—N4

Zn—N6

Zn—N8

2.0172 (17)

2.0252 (17)

2.0186 (17)

2.0137 (17)

2.1761 (19)

98.59 (7)

Zn—N9

.864 Å in 5, 2.669 and 2.775 Å in 6 and 2.711 and 2.734 Å in 7.

N2—Zn—N9

N4—Zn—N9

N6—Zn—N9

N8—Zn—N9

N2—Zn—N9—C45

Deviation of Zn from

In the crystal structure, the molecules interact with each other

mainly through H. . .H dispersion and van der Waals forces. The

99.63 (7)

104.99 (7)

100.96 (7)

89.23 (3)

inversion related molecules in each crystal are arranged in face-

. .

to-face dimeric structures with weak p p interactions between

partially overlapped macrocycles with a distance of 3.65

to ~ 4.22 Å (Cg—Cg (Cg = centrum gravity) of the peripheral six

the N

Dihedral angle

-isoindole plane

0.387 (3)

0.440

. .

. . .

membered rings of the macrocycle) and CAH

with H Cg dis-

N -isoindole/N2,C1-C8

N -isoindole/N4,C9-C16

N -isoindole/N6,C17-C24

N -isoindole/N8,C25-C32

7.50 (3)

2.61 (3)

2.41 (3)

1.80 (3)

4.29

4.69

4.60

tances of ~ 3.25 to ~ 3.55 Å (Fig. S4), whereas the face-to-face

dimeric structures are weakly interacting since the distance

between the back-to-back and partially overlapped macrocycles

is ~ 3.35 Å (Fig. S5). The

overlapping saucer-shaped macrocycles in crystals 4–7 are less

effective compared to the . . . interactions in the normal ZnPc

p. . .p interactions between the partially

Table 4

_P_c_-₃_,₄_-_l_u_t₍₆₎₍_Å_,o₎_.

Selected geometrical parameters for Zn(Me)

crystal. The interaction between the octamethyl-substituted

phthalocyaninate(2-) macrocycles in the studied crystals 4–7, the

crystal stability, as well as the relationships between the inter-

molecular interactions in the crystals and their solubility are dis-

cussed in detail in the next paragraph, based on Hirshfeld surface

analysis.

X-ray

DFT

Zn—N2

Zn—N4

Zn—N6

Zn—N8

2.016 (3)

2.017 (3)

2.009 (3)

2.012 (3)

2.148 (4)

97.52 (13)

100.83 (14)

105.04 (13)

99.17 (14)

85.29 (3)

2.048

2.046

2.048

2.045

2.197

102.31

101.76

102.31

101.86

90.00

Zn—N9

N2—Zn—N9

N4—Zn—N9

N6—Zn—N9

N8—Zn—N9

N2—Zn—N9—C45

Deviation of Zn from

3.4. Hirshfeld surface analysis

The Hirshfeld surface (HS) [31] and the analysis of 2D ﬁnger-

print plots [32] are good tools to illustrate the interactions

between crystal-forming components and can also be useful in

explaining the solubility of the compounds studied. The HS not

only allows for qualitative analysis of the intermolecular interac-

tions in the crystal, but also allows for quantitative analysis, i.e.

it allows the calculation of the contributions of individual types

of interactions between the molecules constituting the crystal-

building units. To get more detail in the interactions between the

the N

Dihedral angle

-isoindole plane

0.372 (3)

0.428

-isoindole/N2,C1-C8

-isoindole/N4,C9-C16

4.51 (3)

2.51 (3)

2.01 (3)

3.70 (3)

4.71

5.99

4.71

5.99

N -isoindole/N6,C17-C24

N -isoindole/N8,C25-C32

octamethyl-substituted zinc phthalocyanine derivative Zn(Me)

Pc-L molecules building the crystals, the HS mapped with dnorm

33] and the 2D ﬁngerprint plots were calculated for the whole

Zn(Me) Pc-L molecules of 4–7 (Fig. 4). In the HS, the d value cor-

responds to the distance between the external atom and the sur-

face, and the d value corresponds to the distance between the

Table 5

Selected geometrical parameters for Zn(Me)

Pc-3,5-lut (7) (Å,^o).

[

X-ray

DFT

Zn—N2

Zn—N4

Zn—N6

Zn—N8

2.020 (3)

2.013 (3)

2.020 (3)

2.157 (3)

99.68 (10)

99.37 (10)

104.44 (10)

100.14 (11)

84.08 (4)

2.047

2.045

2.047

2.045

2.203

102.22

101.77

102.20

101.77

90.00

internal atom and the surface. The different interactions between

the molecules are related to different colours in the HS. The red

areas in the HS correspond to contact distances between atoms

inside and outside the surface, and they are smaller than the

sum of the respective van der Waals radii. Blue and white areas

represent distances that are longer than and equal to the sum of

the respective van der Waals radii, respectively.

Zn—N9

N2—Zn—N9

N4—Zn—N9

N6—Zn—N9

N8—Zn—N9

N2—Zn—N9—C45

Deviation of Zn from

the N

-isoindole plane

0.365 (4)

0.425

On the Hirshfeld surface mapped with dnorm there are slightly

visible red spots originating from the intermolecular CAHÁÁÁN and

CAHÁÁÁC interactions (Fig. 4, left), whereas on the HS mapped with

Dihedral angle

N -isoindole/N2,C1-C8

N -isoindole/N4,C9-C16

N -isoindole/N6,C17-C24

N -isoindole/N8,C25-C32

8.50 (3)

3.21 (3)

1.31 (3)

1.10 (3)

4.40

5.70

4.40

5.70

d broad orange-yellow depressions are visible (Fig. 4, middle). The

D ﬁngerprint plots for complexes 4–7 (Fig. 4, right) provide a con-

cise summary, illustrating the intermolecular interactions occur-

ring in these crystals.

the main contribution in the Hirshfeld surface of these complexes

represents HÁÁÁH dispersive forces. The contributions of the HÁÁÁH

dispersive forces in the HS of these complexes are 53.1% in 4,

The intermolecular CAHÁÁÁN /CAHÁÁÁC or NÁÁÁHAC/CÁÁÁHAC inter-

actions are clearly visible as donor and acceptor spikes on the 2D

ﬁngerprint charts (Fig. 4, right). The contributions of the CAHÁÁÁN/

NÁÁÁHAC and CAHÁÁÁC/CÁÁÁHAC interactions in the Hirshfeld surface

5.1% in 5, 56.2% in 6 and 57.0% in 7. The two-dimensional ﬁnger-

print plot for these complexes (Fig. 4, right) shows an exceptionally

short HÁÁÁH contact with d = d ꢀ 1.02 Å (i.e. an HÁÁÁH separation of

.04 Å) in the crystal of 4, whereas for the crystals 5–7 the HÁÁÁH

for these complexes are 10.9 and 26.7% in Zn(Me)

Pc-Py (4), 10.3

Pc-3-Pic (5), 9.9 and 24.9% in Zn(Me) Pc-

,4-Lut (6) and 10.0 and 24.7% in Zn(Me) Pc-3,5-Lut (7). However,

and 25.0% in Zn(Me)

J. Janczak

Polyhedron 197 (2021) 115024

(

(b)

(c)

(d)

Fig. 4. Hirshfeld surface mapped with dnorm over the range À0.10 to 1.50 (left), with d

for the molecules of 4 (a), 5 (b), 6 (c) and 7 (d).

over the range 1.00 to 2.00 (middle) and the two-dimensional ﬁngerprint plots (right)

J. Janczak

Polyhedron 197 (2021) 115024

contacts are slightly longer, de = di ꢀ 1.07 Å (i.e. an HÁÁÁH separation

of c.a. 2.14 Å). The contribution of the HÁÁÁH dispersive forces in the

HS of these complexes in crystals 4–7 is about 1/3 greater than that

in the normal ZnPc crystal (see Fig. 5). The CÁÁÁC contact resulting

ative to unsubstituted normal zinc phthalocyanine (over 4 times),

it only improves their solubility relative to normal ZnPc by about

30%. Analysis of the Hirshfeld surface shows that the reduction of

the

ÁÁÁ

interactions with the simultaneous increase of the

interactions in the investi-

from

ÁÁÁ

stacking interactions appears in normal unsubstituted

CAHÁÁÁN, CAHÁÁÁN, HÁÁÁH and CAHÁÁÁ

ZnPc [34] on the diagonal of the ﬁngerprint plots (i.e. where d

= d

gated complexes 4–7 is the main reason for the relatively small

beginning near the C-atom van der Waals radius of 1.7 Å [35]. In

the investigated complexes 4–7, due to the steric hindrance of

the axial ligands as well as due to the octamethyl-substituted Pc-

macrocycle and the shift of the macrocycles, each other CÁÁÁC con-

increase in solubility relative to the normal unsubstituted ZnPc.

3.5. DFT studies

tact resulting from

ÁÁÁ

interactions is signiﬁcantly longer and

= d above

.8–1.9 Å (red circle in the Fig. 4, right). The contribution in the

interac-

DFT calculations were carried out for the all the octamethyl

substituted zinc phthalocyanines axially ligated by pyridine, 3-

picoline, 3,4-lutidine and 3,5-lutidine derivatives (4–7). Fully opti-

mized molecules of 4–7 exhibit conformations similar to that in

the crystal structures. In general, the DFT optimized parameters

are in good agreement with those observed in the crystal struc-

tures. Selected DFT parameters for the investigated zinc phthalo-

cyanine derivatives are collected together with their X-ray values

in Tables 2–5, whereas the whole detailed geometrical parameters

are listed in Tables S1–S4 (in the SI). It should be noted that the X-

ray experimental results refer to the solid phase and the DFT calcu-

lated results refer to the conformation of the molecules in the gas

phase. Thus, as seen in Tables 2–5, some differences between these

values can be understood. The calculated four equatorial Zn–N

bond lengths are slightly longer (~0.02 – 0.03 Å), whereas the axial

Zn–N bonds in these zinc phthalocyanine derivatives are longer

by ~ 0.05 Å than that in the crystal structures. The deviation of

appears on the diagonal of the ﬁngerprint plots at d

Hirshfeld surface of the CÁÁÁC contact resulting from

p p

ÁÁÁ

tions in the investigated complexes 4–7 of 3–4% is signiﬁcantly

lower in comparison to that in the normal unsubstituted ZnPc

complex of ~ 22%, with strong

p p interactions resulting from

ÁÁÁ

the sum of CÁÁÁC and C N/N C intermolecular interactions [34].

interactions in the investigated complexes manifest

. .

. . .

The CAHÁÁÁ

themselves on the Hirshfeld surface. McKinnon et al. previously

have noted that the shape of the surface clearly reﬂects this contact

[

36]. The map of the shape index shows these depressions as

orange regions with a concave curvature on both sides of the

macrocycle, while the CAH donor regions have exactly the oppo-

site curvature and have the opposite shape index and are blue

(Fig. 4, middle). The subtle differences of the complementary

regions with different colours on the shape index surface are char-

acteristic of any regions of the surface that come into contact with

each other and these are observed for the complexes, slightly vis-

ible as the ’wings’ on the two-dimensional ﬁngerprint plots

4 8

the Zn(II) ion from the N -isoindole plane in all Zn(Me) Pc-L mole-

cules in the gas phase, as obtained by the DFT calculations, is

slightly greater than that found in the crystal geometries (Table 2).

However, a tendency to reduce the Zn deviation from the ring

plane going from the pyridine complex (0.412 Å) to 3-picoline

(0.440 Å) can be noticed. The intermediate value for the Zn devia-

. .

(

Fig. 4, right), and their location depends on the CAH

distances

. .

see Fig. S4, H Cg of ~ 3.25 to ~ 3.55 Å). The contribution of the

. .

CAH

p interactions to the Hirshfeld surface for the complexes

4–7 is relatively low and does not exceed 5–6%. The contribution

4 8

tion from the N -plane was found in the Zn(Me) Pc-L molecules

of the main intermolecular interactions in the crystals 4–7 is illus-

trated in Fig. 5, whereas the deconvolution of the 2D-ﬁngerprint

plots for individual types of interactions is given in Figs. S5–S8 (SI).

Looking at these results in more detail, it should be stated that

although the axial ligand, as a steric hindrance barrier, signiﬁcantly

with 3,4- and 3,5-lutidine as axial ligands (0.428 and 0.425 Å).

The calculations expose two equivalent global minima and two

equivalent local minima on the potential energy surface (PES). The

values of the rotational barriers were derived from the variation of

the total energy as a function of the rotation angle (the N2—Zn—

N9—C45 torsion angle) and the results are illustrated in Fig. 6.

The rotational barriers of the axial ligands (py, 3-pic, 3,4-lut and

reduces the stacking effect resulting from the

pÁÁÁ

interactions rel-

,5-lut) were calculated by DFT using the same basis set as for

the optimization.

As can be seen from Fig. 6, during the rotation of the axial ligand

around the Zn—N9 bond from 0 to 180°, two pairs of minimum of

energy global (at 0 and 90°) and local (at 45 and 135°) are

observed; thus the total energy of the molecule is equal for the four

symmetrically equivalent conformations (the same value for the

rotation from 0 to 180° as for 180 to 360°). The conformation of

Zn(Me) Pc-py

(4)

Zn(Me)8Pc-3-pic (5)

Zn(Me) Pc-3,4-lut (6)

Zn(Me) Pc-3,5-lut (7)

the Zn(Me) Pc-L molecules with the global energy minimum is at

normal ZnPc

the rotation angle of the axial ligand when its plane coincides with

the Zn-Niso bond, while the local energy minima appears when the

plane of the axial ligands coincides with the axis of the virtual Zn-

Naza bond. During rotation of the axial ligand around the Zn—N9

bond from 0 to 180°, two pairs of different rotational barriers are

observed. The ﬁrst energy barrier at the rotation angle of 22.5°

(

and equivalent at 112.5°) equals ~4.5 kJ/mol in the complexes

with pyridine (4) and 3-picoline (5) and ~4.8 kJ/mol in the com-

plexes with 3,4-lutidine (6) and 3,5-lutidine (7), whereas the sec-

ond energy barrier at the rotation angle of 67.5° (and equivalent

at 157.5°) is comparable with the kT energy (~2.5 kJ/mol). This con-

formational relationship is understandable by looking at the maps

of the electrostatic potential as well as the interactions between H

atoms in the ortho position to the nitrogen atom of the ligand with

H...H

H...C+C...H H...N+N...H

C...C

N...C+C...N

Types of interactions

a slight positive EP and the macrocycle (Me) Pc ring with a variable

Fig. 5. Percentage contributions of the intermolecular interactions in crystals 4–7

and for comparison in the crystal of the normal ZnPc.

EP (See Fig. 1).

J. Janczak

Polyhedron 197 (2021) 115024

Fig. 6. Rotational barriers during rotation of the axial ligand (py, 3-pic, 3,4-lut and

,5-lut) around the axial Zn—N9 bond (change of the torsion angle N2—Zn—N9—

C45 from 0 to 180°).

3.6. UV–Vis spectroscopic characterization

To further characterize the octamethyl substituted zinc

phthalocyanine derivatives 4–7, the electronic absorption spectra

were recorded. The spectra were recorded in pyridine and in

toluene solutions (Fig. 7). The spectra of the investigated Zn(Me) -

Pc-L derivatives (4–7) in both solvents are very similar and show

two bands (Q and B) characteristic for the phthalocyaninate(2-)

macrocycle [37]. The Q and B bands are observed at ~ 680

and ~ 350 nm in both solvents (Fig. 7). The Q band corresponds

to excitation between the HOMO and LUMO levels and the B band

corresponds to the HOMO-1 to LUMO level transition. In addition,

vibrionic splitting of the Q band with a splitting value of ~ 60 nm is

observed, which has been mentioned in the literature [38]. The

UV–Vis spectra of the octamethyl substituted zinc phthalocyanine

Fig. 7. UV–Vis spectra of Zn(Me)

solutions.

Pc-L derivatives in pyridine (a) and toluene (b)

Zn(Me)

Pc were also recorded in pyridine and toluene solutions.

Pc are

The maximum absorption of the Q and B bands of Zn(Me)

slightly hypsochromically shifted (blue shifted), by about 6–8 nm

for the Q band and by ~ 20 nm for the B-band (Fig. 7) in relation

increase in the Pc concentration, the intensity of absorption of

the Q-band increased linearly, with no new bands and wavelengths

shifts observed, which clearly indicate that there were no aggre-

gated species [40]. This behaviour results from the effect of periph-

eral substitution and the axial ligation of a large ligand, leading to

the deformation of the Pc-macrocycle from planarity, limiting the

to 4–7. So, the bathochromic shift of the axially ligated Zn(Me)

Pc-L derivatives 4–7 in relation to Zn(Me) Pc may be associated

with a change in the energy gap between the electronic levels as

a result of axial ligation. Time-dependent DFT calculations conﬁrm

the differences in energy gaps between the electronic HOMO/

interaction and preventing association.

LUMO levels of the unsubstituted Zn(Me)

Pc and the axially ligated

Besides, the UV–Vis spectroscopic characterization of the inves-

Zn(Me) Pc-L derivatives 4–7 (see Fig. 9 and S12, S13 and S14 in the

SI).

tigated Zn(Me) Pc-L derivatives (4–7) in different solutions, diffuse

reﬂectance spectroscopic (DSR) characterization on the solid Zn

The aggregation behaviour of the investigated complexes was

examined by varying their concentration in different solvents

within the limits of the Beer-Lambert’s law. Usually, the aggrega-

tion of phthalocyanines occurs in any system through a coplanar

association that makes the conversion of a monomer to higher

order complexes, leading to insolubility in many organic solvents.

This behaviour depends on the nature of the substitution, metal

ions, concentration, solvent polarity and temperature of the stud-

ied system [39]. In this study, the aggregation behaviour of the

zinc(II) phthalocyanine complexes peripherally octa-substituted

by methyl groups and with axially ligated N-donor ligands was

examined in pyridine and toluene solvents at different concentra-

tions. These metallophthalocyanines have minimum aggregation

in both solvents, as illustrated in Figs. S10 and S11 (SI). With the

(Me) Pc-L derivatives (4–7) was performed. Due to the high diffuse

reﬂectance of the 4–7 derivatives and to minimize the effects of

regular reﬂection and particle size, the samples were diluted with

non– or weak absorbing colorless standard of Al O . The concen-

tration of the studied Zn(Me) Pc-L derivatives (4–7) in the used

Al O diluent was 5% by weight. As shown in Fig. 8, the DSR spectra

2 3

of Zn(Me) Pc-L derivatives (4–7) exhibit similar bands of similar

intensities. The k_m_a_xpositions of the Q and B bands of the Zn(Me)₈-

Pc-L derivatives (4–7) are observed at 680 and 355 nm, which are

bathochromically shifted in relation to the Zn(Me) Pc complex

(Fig. 8). Quite a similar correlation was also found in the UV–Vis

spectra in solution. However, due to the interactions between the

molecules in the solid state, the DSR bands are signiﬁcantly much

wider in relation to the UV–Vis bands of the complexes in solution.

J. Janczak

Polyhedron 197 (2021) 115024

Table 6

TD-DFT results for the low-energy

-p

states for Zn(Me)

Pc-py (4), Zn(Me)

Pc-3-pic

(5), Zn(Me)

8 8

Pc-3,4-lut (6) Zn(Me) Pc-3,5-lut (7) and for Zn(Me)

Pc and ZnPc for a

comparison.

(

a) Zn(Me)

Pc-py (4)

k, nm E, eV

Contribution (weight, %)^*⁾

14.66 2.0171 0.4498 200 ? 201 (94.6), 195 ? 202 (3.4), 196 ? 202

2.9)

08.37 2.0380 0.4660 200 ? 202 (95.5), 195 ? 201 (3.4), 196 ? 201

2.8)

(

12.87 2.4174 0.0025 200 ? 203 (99.8)

20.24 2.9503 0.0004 200 ? 204 (99.9)

93.04 3.1545 0.0013 199 ? 202 (87.7), 195 ? 202 (7.3), 196 ? 202

(

2.4)

91.36 3.1680 0.0105 199 ? 201 (33.4), 194 ? 201 (32.9), 195 ? 201

28.6), 196 ? 201 (3.9)

88.77 3.1891 0.0049 199 ? 201 (50.4), 194 ? 201 (45.0), 196 ? 201

2.1)

86.82 3.2052 0.0003 194 ? 202 (85.3), 195 ? 202(10.9), 199 ? 202

3.2)

(

Fig. 8. UV–Vis diffuse reﬂectance spectra of Zn(Me) Pc-L derivatives (4–7) and Zn

(

b) Zn(Me)

Pc-3-pic (5)

(

8 2 3

Me) Pc diluted with Al O (5%).

k, nm

E, eV

Contribution (weight, %)^*⁾

614.66 2.0171 0.4470 204?205 (94.6), 199?206 (3.3), 200?206 (3.0)

The broadening of the absorption bands in the solid thin ﬁlms is a

common feature of metallophthalocyanines, which has been

widely discussed in the literature [41].

608.17 2.0386 0.4644 204?206 (94.4), 199?205 (3.3), 200?205 (3.0)

04.14 2.4593 0.0019 204?207 (99.8)

98.56 3.1108 0.0005 204?208 (99.7)

92.84 3.1545 0.0139 203?206 (86.4), 199?206 (8.1), 200?206 (2.1)

To further explore the optical properties of the Zn(Me)

complexes, time-dependent TD DFT calculations were performed.

The optical absorption spectra were calculated for the all Zn(Me)

Pc-L derivatives (4–7) as well as for Zn(Me) Pc and normal ZnPc for

Pc-L

391.32 3.1683 0.0085 198?205 (33.4), 199?205 (28.9), 203?205

(19.9), 200?205 (28.6)

89.03 3.1870 0.0065 203?205 (56.0), 198?205 (29.6), 201?205

10.7), 200?205 (2.2)

87.72 3.1978 0.0013 201?205 (86.6), 203?205(6.9), 198?205 (3.1)

(

a comparison. The results are summarized in Table 6 and full

details are listed in Table S6 (SI). Partial molecular energy dia-

grams, HOMO and LUMO frontier orbitals and the calculated elec-

(

c) Zn(Me)

Pc-3,4-lut (6)

k, nm

E, eV

Contribution (weight, %)^*⁾

14.09 2.0190 0.4469 208?209 (94.5), 203?210 (3.1), 204?210 (3.3)

07.97 2.0393 0.4635 208?210 (94.4), 203?209 (3.1), 204?209 (3.3)

72.96 2.6214 0.0011 208?211 (98.3)

tronic absorption spectra for Zn(Me)

shown in Fig. 9. The calculated energy gap between the HOMO

and LUMO levels is 2.0171 eV (614.66 nm) for the Zn(Me) Pc-py

complex, whereas for the Zn(Me) Pc complex it is 2.0912 eV

592.89 nm) and a similar value is obtained for the normal ZnPc

complex (2.0857 eV, 594.46 nm). The calculated energy gaps

between the HOMO and LUMO levels for Zn(Me) Pc-3-pic (5), Zn

Pc-3,5-lut (7) are 2.0171 eV

614.66 nm), 2.0190 eV (614.09 nm) and 2.0172 eV (614.59 nm),

8 8

Pc-py (4) and Zn(Me) Pc are

92.80 3.1565 0.0007 208?212 (98.3)

392.49 3.1589 0.0151 207?210 (84.9), 203?210 (8.3)

391.02 3.1708 0.0083 202?209 (50.3), 203?209 (28.7), 207?209

(

17.3)

88.54 3.1910 0.0071 207?209 (59.0), 202?209 (29.7), 205?209

7.4), 204?209 (2.2)

87.21 3.2020 0.0015 205?209 (89.1), 207?209(5.9)

(

8 8

Me) Pc-3,4-lut (6) and Zn(Me)

(

d) Zn(Me)

Pc-3,5-lut (7)

respectively. Partial molecular energy diagrams, HOMO and LUMO

frontier orbitals and the calculated electronic absorption spectra

_C_o_n_t_r_i_b_u_t_i_o_n₍_w_e_i_g_h_t_,_%₎*)

k, nm

E, eV

14.59 2.0172 0.4440 208?209 (94.6), 203?210 (3.1), 204?210 (3.3)

07.96 2.0393 0.4629 208?210 (94.4), 203?209 (3.1), 204?209 (3.3)

97.16 2.4939 0.0016 208?211 (99.8)

for Zn(Me)

,5-lut (7) are shown in Figs. S11, S12 and S13 (SI) and they are

very similar to that of the complex Zn(Me) Pc-py (4), whereas

8 8 8

Pc-3-pic (5), Zn(Me) Pc-3,4-lut (6) and Zn(Me) Pc-

392.62 3.1579 0.0147 207?210 (85.8), 203?210 (8.5)

391.23 3.1691 0.0073 202?209 (51.9), 203?209 (28.8), 207?209

the partial molecular energy diagrams, HOMO and LUMO frontier

orbitals and the calculated electronic absorption spectra for Zn

(

12.9)

88.70 3.1897 0.0100 207?209 (69.4), 202?209 (24.9), 204?209

3.4)

(

Me)

A small bathochromic shift of the maximum absorption wave-

length (kmax) in the spectra of the Zn(Me) Pc-L complexes 4–7 in

relation to Zn(Me) Pc dye was observed. The red-shift of about

2 nm in the spectra of 4–7 is related to the slightly decreasing

energy gap in relation to Zn(Me) Pc that should be assigned to

Pc and the normal ZnPc dye are shown in Fig. S14 (SI).

(

387.09 3.2030 0.0001 202?210 (87.4), 203?210 (7.1), 207?210 (4.9)

379.73 3.2651 0.0004 208?212 (70.2), 208?213(26.1), 206?209

(

2.9)

(

e) Zn(Me)

k, nm E, eV

Contribution (weight, %)^*⁾

the axial ligation effect of the pyridine or its derivative molecules,

as well as to the saucer-shaped distortion of the octamethyl substi-

tuted phthalocyaninate macrocycle. The red-shift of the maximum

92.89 2.0912 0.4489 179?180 (92.6), 174?181 (4.5), 169?181 (2.9)

593.89 2.0912 0.4489 179?181 (92.6), 174?180 (4.5), 169?180 (2.9)

94.89 3.1397 0.0061 178?180 (87.9), 178?181 (4.1), 169?181 (3.3),

74?181 (2.7)

94.89 3.1397 0.0061 178?211 (87.9), 178?180 (4.1), 169?180 (3.3),

74?180 (2.7)

absorption wavelength in the spectra of these Zn(Me)

plexes (4–7) in relation to Zn(Me) Pc in solution was also observed

Fig. 7). The energy gap of the HOMO-LUMO level in the octa-

methyl substituted zinc phthalocyanine (Zn(Me) Pc) is only

Pc-L com-

(

(f) ZnPc

k, nm

E, eV

Contribution (weight, %)^*⁾

slightly red-shifted (~2 nm) in relation to that in the normal ZnPc

dye (see Table 6 and Fig. S15). Thus, the methyl substitution effect

of all eight peripheral positions of the phthalocyanine ring only

slightly modiﬁes the energy gap of the HOMO-LUMO levels and

594.46

2.0857

0.4084

147?148 (93.8), 145?149 (6.6)

(

continued on next page)

practically it does not affect the dye color of Zn(Me) Pc in relation

J. Janczak

Polyhedron 197 (2021) 115024

Table 6 (continued)

Aromatic macrocycles, including metallophthtalocyanines and

especially zinc phthalocyanines and their derivatives due to their

non-toxicity, have been extensively studied as photosensitizers in

a non-invasive treatment called photodynamic therapy due to

their strong absorption in the red region of the optical spectrum

(

f) ZnPc

_C_o_n_t_r_i_b_u_t_i_o_n₍_w_e_i_g_h_t_,_%₎_*)

k, nm

E, eV

594.43

373.77

2.0858

3.3171

0.4085

0.0137

147?149 (93.8), 145?148 (6.6)

146?148 (90.6), 137?148 (6.7)

146?149 (90.5), 137?149 (6.7)

[43]. In PDT, a photosensitizer in its ground state (S

absorbs energy and is excited to its short-lived ﬁrst excited state

(S ), then it undergoes conversion to the ﬁrst excited triplet state

by intersystem crossing (S ? T ). The triplet state (T ) of the pho-

) ﬁrstly

) HOMO (200), LUMO (201).

) HOMO (204), LUMO (205).

) HOMO (208), LUMO (209).

) HOMO (179), LUMO (180).

tosensitizer transfers its energy to the surrounding biological tis-

sue containing oxygen in its triplet ground state to the highly

active singlet state, which will eventually kill targeted cells. Some

criteria must be met for a photosensitizer to generate satisfactory

results. First, the photosensitizer should have a strong absorption

band in the therapeutic window (600–900 nm) and preferably it

should be red-shifted because a higher wavelength stimulates dee-

) HOMO (147) and LUMO (148).

to ZnPc. The TD DFT calculated HOMO-LUMO transition for the all

Zn(Me) Pc-L derivatives (4–7) as well as for the Zn(Me) Pc com-

plex is shifted by ~ 70 nm in relation to that of the experimental

values. A similar discrepancy between experimental and theoreti-

cal results is also found for other metallophthalocyanines [42].

per penetration. Moreover, the energy gap between T

ground singlet state (S ) must be greater than or equal to 22.5 kcal/-

mol [44]. A partial molecular energy diagram, HOMO and LUMO

and the

8 8

Fig. 9. Partial molecular energy diagrams, HOMO and LUMO frontier orbitals and the calculated absorption spectra for Zn(Me) Pc-py (4) and Zn(Me) Pc as a comparison.

J. Janczak

Polyhedron 197 (2021) 115024

frontiers orbitals and the calculated electronic absorption spec-

Acknowledgment

trum for Zn(Me) Pc-py (4) are shown in Fig. 9 and the spectra for

the other complexes (5–7) are shown in Figs. S8-S10 (SI). The

investigated complexes (4–7) have a strong absorption band in

the therapeutic window and the HOMO-LUMO energy gap is sufﬁ-

cient to excite the ground state of oxygen; therefore they can be

tested as potential photosensitizers for PDT.

DFT and TD DFT calculations have been carried out using

resources provided by Wrocław Centre of Networking and Super-

computing (http://www.wcss.wroc.pl).

Appendix A. Supplementary data

Supplementary data to this article can be found online at

https://doi.org/10.1016/j.poly.2021.115024 .

. Conclusion

The molecular properties of an aromatic phthalocyanine macro-

cycle can be signiﬁcantly affected by axial binding and by methyl

substituents at peripheral positions. In this work, the crystalline

form of a series of methyl group octa-substituted zinc phthalocya-

nine complexes ligated axially by pyridine or its derivatives, Zn

Me )Pc-L, were synthesized, and studied to see how the properties

of the macrocycle were affected. The methyl substituents on the

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