S1

Application of Small Molecule based on Dithienogermole core in Bulk

Heterojunction Organic Solar Cells and Perovskite Solar Cells

B. Yadagiri#a,b, K. Narayanaswamy#a, Towhid H. Chowdhuryc, Ashraful Islam*c, Vinay Guptad, Surya Prakash Singha,b*

[a]Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology (IICT), Uppal road, Tarnaka, Hyderabad, 500007, India

[b] Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, Uttar Pradesh, India

[c] Photovoltaic Materials Unit, Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, Tsukuba, 305-0047, Ibaraki, Japan

[d] Dr. Vinay Gupta

CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi, 110012, India

#Both author contributed equally

E-mail:spsingh@iict.res.in

Electronic Supplementary Material (ESI) for Materials Chemistry Frontiers.This journal is © the Partner Organisations 2020

S2

Contents

1. Experimental section………………………………………………………………………...S3

2. Theoretical calculations……………………………………………………...………………S6

3. Synthesis……………………………………………………………………………………...S8

4. Copies of NMR Spectra………………………………………………………………….....S12

5. References…………………………………………………………………………………..S21

S3

1. Experimental Section

1.1 Materials

10H-phenoxazine was purchased from TCI chemicals and ((4,4-bis(2-ethylhexyl)-6-

(trimethylstannyl)-4H-germolo[3,2-b:4,5-b']dithiophen-2-yl)dimethylstannyl)methylium (DTGe)

was purchased from 1-Material.The all other reagents and catalysts were purchased from

commercial suppliers and used without further purification. All reactions were carried out under

nitrogen atmosphere. All the solvents used in this work were distilled prior to use. The reagent

phosphorus oxychloride was freshly distilled under vacuum before use.

1.2 Measurement

1H and 13C NMR spectra were recorded in CDCl3on a Bruker 400-MHz or 500-MHz

spectrometer using TMS as standard and peak multiplicity was reported as follows: s, singlet; d,

doublet; t, triplet; m, multiplet; dd, doublet of doublet. Purifications were carried out through

silica gel using 100- 200 mesh and 230-400 mesh. Absorption spectra in solution and solid film

were recorded on Scimadzu UV-1800 model spectrophotometer. Cyclic voltammetry (CV) and

differential pulse voltammetry (DPV) were performed with a CH Instruments 620C

electrochemical analyzer with a scan rate of 100 mV/sin dichloromethane using 0.1 M TBAPF6

as the supporting electrolyte, a Ag/AgCl electrode as the reference electrode, a carbon-glass

electrode as the working electrode, a Pt wire as the counter electrode and

ferrocene/ferrocenium(Fc/Fc+) as an external reference.

1.3 Fabrication details of BHJ-OSCs Devices.

The prepared devices based on Ge-PO-2CN and their blends with PC71BM were

dissolved in 1-chlorobenzene and 1-chloronaphthalene (CN) (97:3 vol/vol). From this the

overall concentration was raised up to 10 mg ml−1 and also the solution was stirred up to

12 hours at 60 °C in an inert atmosphere. Indium tin oxide (ITO) substrates were cleaned

with different types of solvents such as water, acetone and isopropyl alcohol up to 60 min

under sonication method. The cleaned ITO substrates were exposed to ultraviolet ozone

irradiation for 15 min. Then MoOx (10 nm) was coated by the thermal evaporation

process in a glove box with chamber pressure of ~10−7 torr. The photo active layers were

S4

spin-coated using the previously prepared solutions with 1500 rpm in a glove box.

Finally, Al cathode (100 nm) was coated with thermally evaporation process. The made-

up devices were encapsulated with the epoxy and a cover glass also kept for 15 min under

ultraviolet irradiation. The current-voltage curves were measured by using Keithley 236

source meter unit. The light source was calibrated by using silicon reference cell (NREL)

with an AM 1.5 G solar simulator with an intensity of 100 mW/cm2. Throughout the

testing, an aperture with an area of 10mm2 was used to precisely measure the

performance of solar cells

1.4 Fabrication details of PSC Devices.

First, the cleaned FTO-substrate (14Ω sq-1) was coated with titanium

diisopropoxidebis(acetylacetonate) (Sigma-Aldrich) in ethanol solution by spray pyrolysis at

500°C in 30 min to make a thickness of 60 nm compact TiO2 layer(cl-TiO2). Prior to deposition

of compact layer the cl-TiO2 substrates were annealed at 500°C for 30 min and cooled down to

room temperature. Then a mesoporous TiO2 film was deposited on the cl-TiO2 coated substrate

by spin coating (3000 rpm, 30s) from an ethanol solution of diluted TiO2 paste(30 nm particle

Dyesol-30NRD, Dyesol) with a mass ratio of 1:4 followed by drying at 70 °C for 10 min and

sintered for 500 °C for 1.5 h in air. After annealing, the mesoporous TiO2(mp-TiO2) substrates

were cooled down to room temperature and transferred to a N2 glove box. Subsequently, a 1.2M

CH3NH3PbI3 perovskite precursor was prepared in dimethylformamide and dimethyl sulfoxide

solution with a ratio of 4:1.The CH3NH3PbI3 precursor solution was spin coated by two

consecutive spin coating steps at 1000 rpm for 12 s followed by 4000rpm for 30 s. After 10 sec

of starting the second spin-coating stage, 800 μL of anhydrous chlorobenzene (99.8%) was

dropped onto the spinning substrate. These perovskite deposited substrates were then heat-treated

at 100°C for 10 min on a hotplate. The Ge-PO-2CN solutions were deposited on the individual

perovskite films by spin-coating and solutions were prepared by the following details. Ge-PO-

2CNwas dissolved in 1 mL anhydrous chlorobenzene (7 mM). The spiro-OMeTAD (60 mM)

solution was prepared with 4-tert-butylpyridine (10μL) and a0.043 mM Li-TFSI acetonitrile

solution (72μL) was used. The spin-coating process was carried out at 3000 rpm for 30 s for Ge-

PO-2CN.The spiro-OMeTAD solutions were spin coated with 4000 rpm for 30 s. All the hole

conducting materials were spin-coated on CH3NH3PbI3/mp-TiO2/cl-TiO2/FTO. Finally, 80 nm of

S5

elemental gold was deposited by thermal evaporation under vacuum (4.1x10-4 Pa) to complete

the fabrication of the solar cells. The active areas of all the cells were 1.02 cm2.

Characterization

The current-voltage characteristics were measured using a solar simulator with standard air mass

1.5 sunlight (100mWcm-2, WXS-155S-10: Wacom Denso) under ambient conditions. The J-V

curves were measured by forward (-0.2V to 1.2V forward bias) or reverse (1.2V to -0.2V) scans.

The step voltage was fixed at 5-10mV and the delay time was set at 50 ms. J-V curves for all

devices were measured by masking the cells with a metal mask 0.1 cm2 area. Monochromatic

incident photon-to-current conversion efficiency (IPCE) spectra were measured with a mono

chromatic incident light of 1 x 1016 photons cm2 in director current mode (CEP-2000BX, Bunko-

Keiki). The light intensity of the solar simulator was calibrated by a standard silicon solar cell

provided by PV Measurements.

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2. Theoretical calculations

Table S1: Molecular orbital of Ge-PO-2CNcalculated at B3LYP/6-311g (d,p)1 level of theory in

DCM solvent.

Table S2. Absorption spectra calculated atB3LYP/6-311g(d,p)* in chloroform

Ge-PO-2CN

HOMO LUMO

HOMO-1 LUMO+1

HOMO-2 LUMO+2

HOMO-3 LUMO+3

S7

3. Synthesis

Excited state λ (nm) Osc. Strength

(f)

Major contributions Minor contributions

S1 630.4495 1.2054 HOMO->LUMO (95%)

H-1->L+1 (4%)

S2 602.86 0.0458 HOMO->L+1 (97%) H-1->LUMO (2%)

S3 513.031 0.0755 H-1->LUMO (92%) H-2->L+1 (3%), HOMO->L+1 (2%)

S4 503.7551 0.4209 H-1->L+1 (83%) H-2->LUMO (4%), HOMO->LUMO (5%),

HOMO->L+2 (7%)S5 456.2772 0.5716 H-1->L+1 (11%),

HOMO->L+2 (81%)H-2->LUMO (6%)

S6 433.2082 0.0723 H-2->LUMO (87%) HOMO->L+2 (9%)

S7 429.0704 0.0333 H-2->L+1 (93%) H-1->LUMO (4%)

S8 391.4137 0.0454 H-1->L+2 (95%)

S9 350.0895 0.1621 H-2->L+2 (96%)

S10 345.7355 0.0101 H-4->L+1 (23%), H-3->LUMO (29%),

HOMO->L+3 (24%)

H-5->LUMO (7%), H-1->L+4 (4%), HOMO->L+5

(7%)

S8

S

Ge

SO

N N

O

N

N

N

N

S

Ge

SSn SnO

N

Br

N

N

Pd2(dba)3

(o-tol)3P,Toluene85oC,4 days, 71%

O

N

DCE, OoC-80oC12hrs, 86%

ODry DMF, POCl3

1 2O

HN

O

N

Phenoxazine

KOH, Acetone, TEAB60oC,14hrs, 97%

NBS,Acetone0-5oC-RT,12hrs83%

O

N

Br

N

N4

2-Ethylhexylbromide

Malononitrile

THF, Ethanol (1:1)RT, 3hrs. 75%

O

NOBr

3

4DTGe Ge-PO-2CN

Scheme-1: Synthetic route to Ge-PO-2CN molecule

10-(2-ethylhexyl)-10H-phenoxazine (1):

In a two neck round bottom flask Phenoxazine (3.6 g, 19.64 mmol), tetraethyl ammonium

bromide (420mg, 2 mmol) and potassium hydroxide (3.3g, 58.9 mmol) were taken and

dissolved in 100 ml of acetone. After heating the reaction mixture for 1.5 hrs at 50 oC, 2-

ethylhexylbrimide (4.53ml, 25.53 mmol) was added drop wise, and then the mixture was

refluxed for 24 hrs. After completion of the reaction, the solvent was removed under

reduced pressure, and then organic layer was extracted with hexane workup with water

and NaCl aqueous solution. Collect the organic layer, dried over Na2SO4 and then solvent

was removed by rotary evaporation. The residue was purified by column chromatography

using silica gel as stationary phase and petroleum ether as mobile phase afforded the

product 1 as colorless viscous oil (5.7 g, 97%).1H NMR (400 MHz, CDCl3): δ(ppm) 6.76

(t, J= 8.1 Hz, 2H), 6.62 (d, J = 5.6 Hz, 4H), 6.52 (s, 2H), 3.40 (s, 2H), 1.87 (m,1H), 1.49

– 1.28 (m, 8H), 0.93 - 0.85 (m, 7.4 Hz, 6H).13C NMR (101 MHz, CDCl3): δ (ppm)

S9

145.20, 134.23, 123.44, 120.78, 115.47, 112.08, 47.87, 36.67, 30.79, 28.83, 24.18, 23.15,

14.08, 11.01.ESI-MS calcd for C20H25NO [M+H]+ m/z 296; found 296.

10-(2-ethylhexyl)-10H-phenoxazine-3-carbaldehyde (2):

In a 100 mL two neck round bottom flask dry phosphorus oxychloride (7.13 mL, 76.36mmol)

was added drop wise very slowly to the dry N, N- dimethylformamide (DMF) (4.41 mL, 57.27

mmol) at 0 oC in nitrogen atmosphere with stirring. After formation of white crystals, 10-(2-

ethylhexyl)-10H-phenoxazine (5.64g, 19.09mmol) in 30 mL of dry 1, 2-dichloroethane (DCE)

was added drop wise to the reaction mixture over 1 hrs. Then reaction mixture was heated to

reflux for 24 h. After cool down to room temperature, the reaction mixture was added drop wise

into dilute sodium hydroxide aqueous solution and extracted with ethyl acetate. The organic

layer was dried over anhydrous Na2SO4, the solvent were removed under reduced pressure. The

residue was purified by silica gel column chromatography using and ethyl acetate and hexane as

the eluent gave 2 as yellow liquid, then slowly became yellowish solid. Yield (5.3g, 85.8%).1H

NMR (500.1 MHz, CDCl3):δ(ppm) 9.66 (s, 1H), 7.29 (dd, J = 8.3, 1.7 Hz, 1H), 7.08 (d, J = 1.7

Hz, 1H), 6.84 – 6.77 (m, 1H), 6.73 (t, J = 7.4 Hz, 1H), 6.66 (dd, J = 7.8, 1.2 Hz, 1H), 6.58 (t, J =

7.4 Hz, 2H), 3.48 (d, J = 7.1 Hz, 2H), 1.92 – 1.84 (m, 1H), 1.41 – 1.25 (m, 8H), 0.94 (t, J = 7.4

Hz, 3H), 0.88 (d, J = 6.9 Hz, 3H).13C (75.4 MHz, CDCl3): δ (ppm)189.5, 145.1, 144.7, 140.1,

132.0, 129.7, 128.3, 123.6, 122.5, 115.8, 114.3, 112.7, 111.2, 47.7, 36.6, 30.6, 28.7, 24.0, 23.0,

13.9, 10.9.ESI-MS calcd for C20H25NO [M+H]+ m/z 296; found 296.

7-bromo-10-(2-ethylhexyl)-10H-phenoxazine-3-carbaldehyde (3):

The two neck flask was charged with magnetic stir bar, compound 2 (5.7g, 17.62 mmol)

was dissolved in 100 ml of dry acetone under nitrogen atmosphere. The reaction contents

were cooled in ice salt bath, maintain reaction temperature at about 0-5oC, then N-

bromosuccinimide (NBS, 4.36g, 24.67 mmol) was added portion wise (300 mg X 14) into

the solution and the mixture was stirred for 16 h. After completion of the reaction, 100

mL of water are added and the acetone was completely removed. Then organic layer was

extracted with dichloromethane washed several times with water. Collect the organic

layer and the solvent was removed by rotary evaporation. The residue was purified by

column chromatography using chloroform and hexane as mobile phase and 100-200 mesh

S10

silica gel as stationary phase, yields intermediate product 3 as yellowish solid. Yield (5.9g,

83.1%).1H NMR (400 MHz, CDCl3): δ (ppm)9.68 (s, 1H), 7.31 (dd, J = 8.3, 1.9 Hz, 1H), 7.09

(d, J = 1.9 Hz, 1H), 6.91 (dd, J = 8.6, 2.3 Hz, 1H), 6.80 (d, J = 2.3 Hz, 1H), 6.58 (d, J = 8.4 Hz,

1H), 6.43 (d, J = 8.7 Hz, 1H), 3.44 (d, J = 7.7 Hz, 2H), 1.91 – 1.80 (m, 1H), 1.49 – 1.27 (m, 8H),

0.96 – 0.86 (m, 6H).13C NMR (126 MHz, CDCl3):δ(ppm)189.55, 145.42, 144.78, 139.48,

131.52, 130.05, 128.60, 126.36, 118.94, 114.66, 113.93, 113.85, 111.49, 47.95, 36.59, 30.68,

28.76, 24.11, 23.07, 14.04, 10.98.ESI-MS calcd for C21H24BrNO2 [M+2]+m/z 404; found 404.

2-((7-bromo-10-(2-ethylhexyl)-10H-phenoxazin-3-yl)methylene)malononitrile(4):

Under nitrogen atmosphere, Compound 3 (800 mg, 1.98 mmol) and malanonitrile (196.1 mg,

2.97 mmol) were dissolved in 40 mL of 1:1 ratio of THF and Ethanol mixture in a two neck 100

ml round bottom flask. Then the reaction content was stirred for 15 minutes, NH4OAc (335 mg,

4.3 mmol) was added portion wise, then the reaction turns dark red colour. The reaction was

stirred at room temperature until total starting material disappears on TLC. The solvents were

removed under reduced pressure; the crude product was extracted with DCM and washed with

water and NaCl aqueous solution. Organic layer was collected, dried over anhydrous Na2SO4.

Then organic solvents were removed through rotary evaporator, purified finally by column

chromatography using chloroform and hexane as mobile phase and 100-200 mesh silica gel as

stationary phase yields intermediate product 4 as red solid. Yield (674 mg, 75.2%).1H NMR

(500 MHz, CDCl3): δ (ppm)7.37 (s, 1H), 7.31 (dd, J = 8.6, 2.1 Hz, 1H), 7.24 (d, J = 2.1 Hz,

1H), 6.94 (dd, J = 8.6, 2.2 Hz, 1H), 6.81 (d, J = 3.6 Hz, 1H), 6.54 (d, J = 8.6 Hz, 1H), 6.46 (d, J

= 8.6 Hz, 1H), 3.47 (d, J = 7.1 Hz, 2H), 1.89 – 1.78 (m, 1H), 1.49 – 1.27 (m, 8H), 0.94 (t, J = 7.4

Hz, 3H), 0.89 (t, J = 7.0 Hz, 3H). 13C NMR (126 MHz, CDCl3): δ (ppm)156.80, 145.19,

144.35, 139.87, 130.83, 130.48, 126.65, 124.09, 119.13, 115.57, 114.98, 114.95, 114.28, 113.80,

111.79, 76.32, 48.01, 36.76, 30.67, 28.76, 24.11, 23.06, 14.04, 10.98.MALDI TOF-MS [M+1]+

calcd for C24H24BrN3Om/z 451.10; found 451.01.

2,2'-(((4,4-bis(2-ethylhexyl)-4H-germolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(10-(2-

ethylhexyl)-10H-phenoxazine-7,3-diyl))bis(methanylylidene))dimalononitrile(Ge-PO-2CN):

In a 15 mL pressure vial, solution of compound DTGe (200 mg, 0.25 mmol) and compound 4

(261 mg, 0.58 mmol) in dry toluene (10 mL) was purges with nitrogen gas for 15 minutes

S11

followed by the addition of tri(o-tolyl)phosphine (5 mg, 1.6 µmol) and

tris(dibenzylideneacetone)dipalladium(0)-chloroform (5mg, 0.005 mmol) then sealed with a

Teflon® cap. The reaction mixture was reacted at 100oC for 3 days. After removal of solvent, the

reaction mixture was extracted with dichloromethane, and then poured into water. The organic

layer was washed with sodium chloride solution, and then dried over Na2SO4. The solvents were

removed by rotary evaporator, the crude product was precipitated in methanol, washed several

times with methanol (100x3), acetonitrile (100x3) followed by hexane (100x5). Finally purified

by column chromatography on silica gel (230 – 400 mesh) with hexane and chloroform as eluent

to afford compound Ge-PO-2CN as dark pink solid (218mg, 71.3%).1H NMR (400 MHz,

CDCl3): δ (ppm)7.37 (s, 2H), 7.30 (d, J = 9.9 Hz, 4H), 7.16 (s, 2H), 7.05 (d, J = 8.2 Hz, 2H),

6.93 (s, 2H), 6.58 (dd, J = 20.3, 8.4 Hz, 4H), 3.53 (d, J = 7.4 Hz, 4H), 1.89 (s, 2H), 1.50 – 1.26

(m, 24H), 1.22 - 1.15 (m, 10H), 0.96 (t, J = 7.3 Hz, 6H), 0.90 (t, J = 6.7 Hz, 6H), 0.82 (t, J = 6.7

Hz, 12H. 13C NMR (101 MHz, CDCl3): δ (ppm)156.73, 145.38, 145.31, 144.83, 144.51,

139.99, 130.65, 130.45, 129.83, 125.57, 124.01, 120.81, 115.56, 115.14, 113.97, 113.54, 112.82,

111.62, 75.77, 47.95, 37.02, 36.92, 35.50, 30.72, 29.74, 28.96, 28.78, 24.15, 23.07, 23.05, 20.76,

14.18, 14.04, 11.01, 10.94.MALDI TOF-MS [M]+ calcd for C72H84N6O2S2Ge m/z1202.53;

found 1202.43.

3. Copies of NMR Spectras

S12

1H NMR of 1:

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5ppm

6.28

8.39

1.11

2.00

1.96

3.88

2.23

13C NMR of 1:

102030405060708090100110120130140150160ppm

11.0

114

.08

23.1

524

.18

28.8

330

.79

36.6

7

47.8

7

112.

08

115.

47

120.

7812

3.44

134.

23

145.

20

ESI-MS of 1:

O

N

S13

1H NMR of 2:

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5ppm

2.59

3.12

8.61

1.16

1.01

1.90

1.99

0.96

1.00

1.01

0.84

1.06

1.00

6.66.87.07.27.47.67.88.08.28.48.68.89.09.29.49.69.8ppm

13C NMR of 2:

O

N

O

S14

ESI-MS of 2:

1H NMR of 3:

S15

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.510.0ppm

6.19

8.26

1.03

1.99

1.02

1.02

0.87

0.96

0.92

1.05

1.00

6.67.07.47.88.28.69.09.49.8ppm

1.02

1.02

0.87

0.96

0.92

1.05

1.00

13C NMR of 3:

102030405060708090100110120130140150160170180190200ppm

10.9

814

.04

23.0

724

.11

28.7

630

.68

36.5

9

47.9

5

111.

4911

3.85

113.

9311

4.66

118.

9412

6.36

128.

6013

0.05

131.

52

139.

4814

4.78

145.

42

189.

55

ESI-MS of 3:

O

NOBr

S16

1H NMR of 4:

13C NMR of 4:

O

N

Br

N

N

S17

0102030405060708090100110120130140150160ppm

10.9

814

.04

23.0

624

.11

28.7

630

.67

36.7

6

48.0

1

76.3

2

111.

7911

3.80

114.

2811

4.95

114.

9811

5.57

119.

1312

4.09

126.

6513

0.48

130.

83

139.

8714

4.35

145.

19

156.

80

MALDI-TOF MS of 4:

1H NMR of Ge-PO-2CN:

S18

0.00.51.01.52.02.53.03.54.04.55.05.56.06.57.0ppm

11.9

66.

056.

1310

.07

24.7

1

2.01

3.88

4.00

1.89

2.01

1.93

3.89

2.00

6.56.66.76.86.97.07.17.27.3ppm

4.00

1.89

2.01

1.93

3.89

2.00

13C NMR of Ge-PO-2CN:

102030405060708090100110120130140150160ppm

10.9

411

.01

14.0

414

.18

20.7

623

.05

23.0

724

.15

28.7

828

.96

35.5

036

.92

37.0

2

47.9

5

75.7

7

111.

6211

2.82

113.

5411

3.97

115.

1411

5.56

120.

8112

4.01

125.

5712

9.83

130.

4513

0.65

139.

9914

4.51

144.

8314

5.31

145.

38

156.

73

MALDI-TOF MS of Ge-PO-2CN:

S

Ge

SO

N N

O

N

N

N

N

S19

0 1 2 3 4 51E-5

1E-4

1E-3

0.01

0.1

Ge-PO-2CN(0) Ex Ge-PO-2CN(0)

h= 1.7 x 10-4 cm2 V-1 s-1

Ge-PO-2CN(0.3) Ex Ge-PO-2CN(0.3) Th

h= 4.6 x 10-4 cm2 V-1 s-1

Ge-PO-2CN(0.6) Ex Ge-PO-2CN(0.6) Th

h= 3.2 x 10-4 cm2 V-1 s-1

Hole Mobility in blend

Curre

nt d

ensit

y / A

cm-2

Voltage / V0 2 4

1E-4

1E-3

0.01

0.1

PCBM Current PCBM HCLC

e= 2.4 x 10-4 cm2 V-1 s-1

PCBM Current PCBM HCLC

e= 4.8 x 10-4 cm2 V-1 s-1

PCBM (HT) Current PCBM (HT) HCLC

e= 3.7 x 10-4 cm2 V-1 s-1

Curre

nt d

ensit

y / A

cm-2

Voltage / V

Electron mobility in blend

(a) (b)

Figure S1. Hole mobility (a) and electron mobility (b) of Ge-PO-2CN and PC71BM molecules

S20

(a) (b) (c)

Figure S2. AFM height images of Ge-PO-2CN:PC71BM active layer with (a) no CN additive,

(b) 0.3% CN additive and (c) 0.6% CN additive.

Figure S3. (a) Surface morphology of the MAPbI3 layer (b) Ge-PO-2CN on top of the MAPbI3

layer

(a) (b)

S21

4. References

1. J.S. Binkley, J.A. Pople, and W.J.Hehre, J. Am .Chem. Soc., 1980, 102, 939-947