colour and constitution

Seoul National Univ.

3.5 Colour and Constitution


3.5.1 Introduction(1) Early Theories(2) Modern Theories (3) Experimental Observation – Monoazo Dyes(4) Application of VB and MO Theories

– Rationalisation of colour

• VB Theory • MO Theory

1. Qualitative MO Theory - Dewar’s Rules2. Quantitave MO Theory – PPP Model

• Tinctorial Strength



3.5.2 Early Theories

1865 : Kekule, Benzene structure1867 : Graebe and Liebermann, Unsaturated theory 1876 : O. N. Witt's chromophore / auxochrome theory1879 : Nietzki, Nietzki's Rule 1887 : Armstrong, Quinonoid theory 1900 : Gomberg / Baeyer, Halochromism1904 : Baly, Isoropesis 1907 : Baeyer / Hantzsch, Conjugated theory (Hewitt’s Rule)1914 : Watson Tautomeric theory1914 : Adams, Rosenstein, Electron oscillation1916 : Lewis, selective absorption of light1928 : Dilthey and Wizinger1935 : Bury, Resonance Theory against Baeyer’s




• Chromphore Theory ( 발색단설 ) ※ 물질이 가시광선 스펙트럼의 일부를 선택적으로 흡수 할 경우 반사 혹은 투과 된 스펙트럼의 빛들은 우리 눈에 색으로써 느껴짐 .


Chromphore Theory

Quinonoid Theory

Resonance Theory

N

N N

O2N

C2H5

C2H4OH

Cl

Auxochrome

Chromophore

Dye

• Chromophore ; N=N-, -N=O, -NO2, -C=O, -C=C-

• Auxochrome ; OH, -NH2, -NHCH3, -COOH, -SO3H

• Chromogene



• Quinonoid Theory ( 퀴노이드설 ) ※ 분자 중에 o-quinone 혹은 p-quinone 의 형태를 직접 포함하거나 , Tautomerism 등으로 인한 구조적 변화로 quinone 형으로 되면 발색이 가능하다는 이론

O

O

OH

OH

O

O

OH

OH

p-Benzoquinone

(Yellow)

Hydroquinone

(Colorless)

o-Benzoquinone

(Red)

Catechol

(Colorless)


Chromphore Theory

Quinonoid Theory

Resonance Theory



• Resonance Theory ( 공명 구조설 ) ※ 분자 중에 존재하는 전자의 이동으로 인하여 나타나는 것으로 발색 설명 . 공명이 일어나지 않는 경우에는 색을 나타내지 않음 .


Doebner’s violetTautomeric form

By Baeyer

Electron oscillation

By Adams, Rosenstein

Ph

H2N NH2Cl Cl

Ph

H2N NH2

(94), (95A)

O O O O

CO2 CO2

(95), (95A)

Resonance TheoryBury-against Baeyer’s

Ph

H2N NH2

Ph

H2N NH2

(96),(96A)



3.5.3 Modern Theories

Two modern theories → VB (Valence bond) theory / MO (Molecule orbital) theory

※ VB Theory → Bonding valence electron pair is localized between specific atoms

* Schrodinger : Quantum Theory, (H, H2+), Heitler..

* Qualitative / Quantitative VB Theory , Bury Resonance Theory

※ MO Theory → Electron being distributed amongst a set of molecular orbitals of discrete energies

* LOAO-MO Theory, Hund..* Huckel : MO(HMO)* Kukn : Free Electron MO(FEMO)Model* Dewar : PMO Theory* Roothaan : Liner Combination Atomic Orbital(LCAO) MO Theory* PPP Model : High Occupied to Lowest Unoccupied (HO/LU)MO



• Electron Theory ( 전자론에 의한 발색설 )

※ 발색단의 유무와 상관없이 전자파의 흡수에 의하여 발색 . ※ 분자의 에너지 변화에 따라 기저상태의 에너지 준위 (Eg)와 여기 상태의 에너지 준위 (Ee) 의 차이 (ΔE) 가 흡수파장을 결정 .

※ 가시광선의 파장에 상응하는 에너지 가져야 함 . → ΔE=167-297 kJ/mole→ 분자 내의 π 전자 에너지 차이 변화

※ 빛의 흡수에 의한 분자의 Ee↓; 장파장 / Ee↓; 단파장 측에서 흡수 .※ 가시광선 영역에서 흡수 → 이중결합의 π 전자 여기 ( 적은 Eg 필요 )

Electron Theory

∆E = Ee – Eg = hυ, υ = c / λ




Electron Theory

CH2=CH2 CH2=CHCH=CH2

excitation

π HOMO

π* LUMO

En

erg

y

π4*

π3* LUMO

π2 HOMO

π1

excitation

Antibonding Molecular orbitals

Bonding Molecular orbitals

• Excitation energy of ethylene vs. 1,3-butadiene

※ Conjugated double bond → Bathochromic shift




3.5.4 Experimental Observations

Monoazo Dyes

Monoazo Dye - Derivatives of 4-Aminoazobenzene

• simple Azobenzene (pale yellow)

⋅ introduction of EWG minor effect on the colour ➡

⋅ introduction of EDG bathochromic (Table 3.7)➡

→ 4-NN dimethylamino (deep yellow)

⋅ the greater the electron donating power, the more bathochromic




Table 3.7. Effect of electron-accepting and electron-donating groups of azo-benzene

R1 NN R

R

HHHNO2

HHHHHMeMeONH2

NH2

NMe2

HCO2Me

NO2

MeMeONH2

NMe2

NEt2

NH2

NMe2

R1 λmaxEtOH(nm)

320325332338333349385407415407407410399, 435s460

εmax

21,00021,90024,00022,90023,40026,00024,50030,90029,500---33,90033,100

NO2

NMe2

NMe2

NMe2



NN D

D

A

orthometa

para

C

D A maxEtOH(nm) max

OHNH2NEt2NEt2NEt2NEt2NEt2NEt2NEt2

p - NO2p - NO2p - NO2o - Clm - Clp - Clo - CNm - CNp - CN

386439486427423422462446466

29,50027,40034,00029,50029,80026,70030,00028,10032,700

Table 3.8. Combined effect of electron-donating and accepting groups in azo benzene


• EWG into the diazo ring & EDG in the coupling ring

• effect of conjugation



Electron-withdrawing groups in ring C hypsochromic ➡

➡ ε(tinctorial strength) ↓

NN NEt2

NN NH2

O2N DC

DC

X

Y

O2N

Y maxEtOH (nm) b max X max

EtOH (nm) b max

HClCNNO2CF3

486472472470467

-14-14-16-19

34,00033,80032,40031,80031,400

HClNO2

439427394

-12-45

27,40025,80024,900

a These dyes have been chosen to minimise steric effects (see 4.5.9) b Relative to X or Y = H

Table 3.9. Effect of electron-accepting groups in the coupling component ring C




A NN NEt2

A maxEtOH (nm) max

NC

NCCN

NC

NC

NCCN

CNCN

NC

NC

CN

CN

478

495

500

503

515

562

33,600

36,000

38,800

33,100

39,800

46,600

Table 3.10. Effect of several electron withdrawing groups in ring D




NN NEt2

O2N

Y

X Y maxEtOH (nm) a max

X

HHOMeOMeOMe

HOMeHOMeNHAc

486501488516530

+15+2+30+44

34,00032,80022,600b

29,600b

32,600b

a Relative to X = Y = H.b Low because of steric hindrance(see 3.5.9).

Table 3.11. Effect of electron-donating groups in ring C

NN NEt2

O2N DC

X

(98) ( X = H )

NN NEt2

O2NNH

Ac

(99)




NN NEt2

O2N

AcHN

NO2

Typical violet dye

maxEtOH 543nm

(100)

X in (98) maxEtOH (nm) max v1/2cm-1)

HNHCOCH3

NEt

COCH3

486511

482

34,00047,000

36,000

5,0003,900

4,900

NN NEt2

O2N

AcHN

NO2

Br

OMe

Typical blue dye

maxEtOH 608nm

(101)


• Effect of acylamino group in ring C

(intramolecular H-bonding)



Increasing number of substituents (especially NO2) make dyes dull heterocyclic diazo components require fewer electron accepting groups to produce the bathochromic shift

N

SO2N NN N

Me

OHEt

Me

maxMeOH 593nm

thiazole

(102)

SN

NN

NEt C2H4CO2Me

O2N

maxMeOH 587nm

benzoisothiazole

(103)

SO2N NN N(C2H4OAc)2

Me

NO2

maxEtOH 614nm

dinitrothiophene dye

(104)

NN N(C2H4OAc)2

Me

NO2

O2N

maxEtOH 513nm

dinitroaniline dye

(105)




dyes with heterocycles as coupling components are more bathochromic, but not studied enough

O2N NN

YNMe2

azo dyes from furan and thiophene coupling components

YmaxEtOH max

OS

538546

¤Ñ50,000

(106) O2N NN NMe2

azo dyes with benzenoid counterparts

maxEtOH nmmax 33,000

(107)




Table 3.13. Substituent effect at the terminal amino group

R R' maxEtOH (nm) max

EtEtMea

C2H4CNH

EtC2H4CNCH2CNC2H4CNH

486452422432439

34,00030,90026,00030,00027,400

a Data for Et compound is not available.


Yellow dyes obtained when the diazo component is devoid of EWG photochromic commercial ones contain EWGs on both sides(109)

O2N N

N

Cl

NHC2H4CN



3.5.5 Application of VB and MO Theories

The first excited state of azobenzene is charge separated structure introduction of an EWG has a minor effect an EDG at ortho or para stabilizes the excited structure

N+ NN +

O¤Ñ

O¤Ñ

N+ NNO¤Ñ

O

N¤Ñ

N +

NN

N¤Ñ

N N+R2

NN NR2

Charge separated structure (Excited state)

Uncharged structure

E

(111)

(111A)

(110)

(110A)

(112)

(112A)



N¤Ñ

N N+Me2

D

NN NMe2

D

(113A)

(113) ortho- and para-cyano dyes sharing of charge markedly stabilises the excited state

∆➡ E is reduced and bathochromic shift

C NN N+Et2

N¤Ñ N¤Ñ

N N+Et2

NC

NN NEt2

NC

E

(114B) (114A)

(114)

unfavourable situation with EDG at both sides failure of VB theory since experimental results say otherwise(table 3.7)




Meta cyano dyes Not conjugated to the azo group No stabilisation of the excited state

N¤Ñ

N N+Et2NC

NN NEt2

NC

E

(115A)

(115)

NN NR2

N¤Ñ

N N+R2A

A'

A

A'

(116A)

(116)

substituents meta to the dialkylamino group (116A) inductive and mesomeric (resonance) effect (table 3,9)




substituents ortho to the amino group(117A) inductive effect only (table 3.9)

N¤Ñ

N N+R2A

A'

NN NR2A

A'

(117A)

(117)

bathochromic shift caused by introducing a methoxy group into ring D

N+ NN

O¤Ñ

O¤Ñ

OMe

N+Et

C2H4CN

N NN

O¤Ñ

O¤Ñ

O+Me

N+Et

C2H4CN

(97A) (97B)




the introduction of a methoxy group in the meta position to the diethylamino group in ring C ➡ stabilise the excited state by sharing the position charge on the terminal amino nitrogen atom ➡ bathochromic shift

N¤Ñ

N N+Et2

O2N

MeO

N¤Ñ

N NEt2

O2N

Me+O

(118) (118A)

the effective sharing of the negative charge results in a large

bathochromic shift

N¤Ñ

N N+Et2

NC

CN

NC

C

N

N¤Ñ

CN¤Ñ

CN

N

(119) (119A) (119B)




the RSE of a heterocyclic ring such as thiophene or thiazole is less than that of benzene ➡ more bathochromic

NN N+R2

X

S¤ÑA

NN NR2

X

SA

A = acceptorX = CH, N

(120A)

(120)

sulphur has available vacant 3d orbitals ➡ acceptable resonance structure with more than 8 electrons

SN

NN

NR2

O2N

S¤ÑN

NN

N+R2

O2N

(121) (121A)





MO Theory : Qualitative vs. Quantitative Qualitative MO Theory – Dewar’s Rules → Predicting qualitatively the effect of substituents on the colour of dyes.

VB Theory (qualitative)

Qualitative MO Theory (Dewar’s Rules)

Quantitative MO Theory (PPP Model)

Tinctorial Strength

(122)CH2

*

*

*

*

*

*

*

*

(i) No two starred atoms are adjacent, and(ii) That the number of starred atoms exceeds the number of

unstarred atoms.

Dewar’s Rules

1. Any E.W.sub. at a starred position, or any E. D.sub. At an unstarred position, should cause a large hypsochromic shift.

2. Any E.W.sub. At an unstarred position, or E.D.sub. At starred position, should cause a smaller bathochromic shift.

3. Replacing carbon by nitrogen has the same effect as an E.W.sub. At that position.4. Any neutral, unsaturated group such as vinyl or phenyl, anywhere, has a bathochromic

effect.

N

N NR2

●

●

●

● correct prediction

●

●

x

x

x

x

x

X incorrect prediction

Fig. 3.7 Dewar’s Rules prediction for 4-aminoazobenzene dyes




Quantitative MO Theory – PPP Model




Tinctorial Strength

- Only π-electron model effect, σ-electrons are not included

- Predicts colour (λmax) and tinctorial strength

- “electronic picture” of ground and excite states

Table 3.14. Experimental and PPP calculated λmax value for azobenzene and its o-, m-,p-amino derivatives




1. Comparison of the calculated electron densities in the origin of the absorption band by highlighting the donor and acceptor groups.

2. in first excited state (123A) the electron density

3. PPP model vs. VB theory α-azo N β-azo N

(123A)

(123)

(124A)

(124)

X

A

N

N

NH2

X = O, NH, S, CR2

1. Thiophene and thiazole more bathochromic

2. VB theory – sulphur atom plays a decisive role by acting as an efficient electron sink

3. PPP model – cis-diene structure and that the sulphur atom is not important




1. Tinctorial strength increases as the dyes become more bathochromic with many exceptions

2. Directly related to the economic viability of a dye.




Tinctorial Strength

f = 4.703 x 1029 x M2 x υM Eq. 3.1

f = 4.32 x 10-9 x Δυ1/2 x εmax Eq.3.2

Fig. 3.8. Relationship between tinctorial strength and the area

under the absorption curve




In MO term, the transition dipole moment M must increase sufficiently with increasing wavelength of absorption

NO2N

N

X

NEt2

(126)

Direction of the transition dipole moment lies along the molecular from the donor D the acceptor group A

The optimum orientation of the molecule for maximum absorption of radiation



protonation of 4-aminobenzene ammonium tautomer – colorless azonium tautomer – colored

The Azonium Tautomer

• azonium tautomer contains delocalized positive charge• more bathochromic, tinctorially stronger and brighter than neutral azo form

NN N+R2

N+

N NR2

DC

H HD

C

(127) (127A)

3.5.6 Protonated Azo Dyes



∆υ1/2 of azonium tautomer is narrower than that of neutral azo form

NN NEt2

O2N

XX

maxEtOH/HCl (nm) bmax

a In EtOH : conc. HCl 2 : 1 by volume.b Relative to corresponding neutral dye.

Table 3.16. Spectral data for neutral and protonated azo dyes

HOMeNHCONH2NH2

486501518514

maxEtOH (nm) max v1/2 (cm-1)

Neutral Dye

v1/2 (cm-1)

Azonium Tautomera

34,00032,80041,00045,000

5,0005,0004,3004,200

515486500482

43,50070,00047,30063,800

3,1003,4003,7003,500

+29-15-18-32





NN NMe2

NN NMe2

O2N

NN NMe2

MeO

NN NEt2

O2N

MeO

Neutral Dye Azonium tautomer

¥ëEtOHmax (nm) ¥ëEtOH/HCl

max (nm) ¥ëa

408

405, 440s

475

501

516

548

508

486

+32

-8

-30

Table 3.17. Spectral effects in neutral and protonated azo dyes




NN NMe2

O2N

F3CO2S

λEtOHmax 500 nm ;

λEtOH/HCl max 500 nm ;

εmax 29,800

εmax 56,300

ring D of azonium tautomer – electron donating group → bathochromic

– electron withdrawing group → hypsochromic ring C of azonium tautomer

– electron donating group → hypsochromic

– electron withdrawing group → bathochromic

it is possible to obtain a dye in which the neutral and protonated form exhibit the same colour




NN NR2

X

N N

NCC C

NC CN

R X

Me

Et

Neutral Dye Azonium form

¥ëEtOHmax (nm) ¥ëEtOH/HCl

max (nm)

¥ë

596

592

-90

-6

506

586

Table 3.18. Negative halochromism by substituents in ring D

negative halochromism - hypsochromic shift caused by protonation - difficult to achieve by substitution of ring D - achieved with electron-donating group meta to the terminal amino group in ring C (table 3.16)




O2N NN

SNMe2

SO2N N

N

CO2Et

NEt2

¥ëEtOHmax 594 nm ; 531 nm(EtOH/HCl) ¥ëEtOH

max 554 nm ; 464 nm(EtOH/HCl)

Azo dyes containing heterocyclic rings – also display negative halochromism




E

(132)

E e.w.

(132)

(132)

E e.d.

First excitedstate

Groundstate

X=Electron-donating group

X=Electron-withdrawing group

batochromicity

Fig. 3.10. Effect of electron-withdrawing and electron-donating groups on the colour of azonium tautomers

ground state of azonium tautomer (132) → quinoid structure first excited state of azonium tautomer (132A) → benzenoid structure electron donating group on ring D stabilize (132A), but destabilize (132) which causes bathochromicity

D

D

X NN NR2

H

X NN NR2

H

C

C

(132)

(132A)



3.5.7 Azo-Hydrazone Tautomers

NN OH

X

NN OH

XD

Eazo

Ehyd.

NN O

X H

D

D

NN O

XD

HE

(134A)

(134)

(133A)

(133)

hydrazone tautomer – generally more batochromic than the azo tautomer



λ max (nm)

Azo (134) in EtOH

para X λ max (nm)

Hydrazone (133) in HOAc

MeOHMeHCNNO2

404405407424432

λa

-3-2

+17+25

485489478462465

+7+11

-16-13

λa

Table 3.19. Visible absorption maxima of the azo(134) and hydrazone (133) tautomers

ring D of azo tautomer - electron donating group – hypsochromic → localize negative change of the first excited state (134A) - electron withdrawing group – bathochromic ring D of hydrazone tautomer - electron donating group – bathochromic

→ reduce positive charge on hydrazo nitrogen atom (133A) - electron withdrawing group – hypsochromic





NN

X

XD

NN

X

D

D

(135)

(137)

(136)

(138)

OH

OH

NN

HO N

N

R

Ar

NN

HO N

XD

R

R

R

O

dyes in hydrazone tautomer: electron-donating groups on D ring cause bathochromic shift



3.5.8 Polyazo Dyes

CH2

HNCONH

N N

N

X

S

O

NH

CO

methylene

ureido

triazinyl

important insulationg groups

non-conjugated polyazo dye spectra is the sum of individual monoazo dyes individual azo subunit is insulated by more than two single bonds



NN CH2 N

N NMe2

Me2N

NN NMe2

Me

disazo dye has similar colour, is 2 times stronger, but has no economical advantage over the monoazo dye

3.5.8 Polyazo Dyes

Fig. 3.13. The absorption spectra of an insulated diazo dye and the parent monoazo dye


linking of two different dyes: produce tertiary shades but no economical advantage

brown

NN

Me

NaO3S

OH

NaO3SOMe

NN

HNN

NN N

N N

CO2Na

OHNaO3S

HO

NH

PhH

SO3Na

N

N

N N

NN

N

NHCNH2

NN

SO3H

SO3HHO3S

O

O

Cu

HO3S

O

HH

ClSO3H

L

purple yellow

blue yellow

(143)

(144)

green

3.5.8 Polyazo Dyes



AM

NN N

N DE

increasing the electron-donating strength of the terminal group D incorporation of electron-donating groups in M or E changing M or E from phenyl to naphthyl

→ all cause a bathochromic shift

3.5.8 Polyazo Dyes



NN N

N D

NN N

N D

(146)

(147)

VB explanation - bathochromic due to stabilized first excited state

3.5.8 Polyazo Dyes



Y X

N

N

OH

NH

N

NH

HO3S SO3H

hydrazo

imino

para C

ortho C

disazo dye

one of azo subunit exists in hydrazone form

three peaks (Fig. 3.14)

dull blues, greens or blacks

3.5.8 Polyazo Dyes



3.5.9 Steric Effects

steric hindrance in azo molecule - cause hypsochromic or bathochromic shift always lower tinctorial strength

3 types of steric hindrance in azo dyes - at the azo group at the dialkyl amino group of coupling ring at the nitro group of diazo ring




Steric Hindrance at Azo Group

Two planar conformations are possible.

conformation (a) is sterically more crowded than conformation (b)

crowding between ortho substituent and electrons of nitrogen

NN

DC

NN

DC

¥â¥â

(a) (b)

Fig. 3.15. Alternative conformations for an ortho-substituted azobenzene




X

HClClNO2

NO2

NO2

CNHCNCN

Y

HHClHH NO2

HHHCN

NO2

NO2

NO2

HNO2

NO2

HCNNO2

NO2

Z ¥ëmax (nm)

453475417425491520434433504549

¥åmax

44,00040,00031,00036,00038,00048,00042,00045,00045,00038,000

¥ëa

+22-36-28+38+67 - -+51+96

Table 3.21. Absorption band of some derivatives of dye(150) in methanol

Z NN N

Et

C2H4CN

X

Y

(150)




SO2N

NN

N(C2H4CO2Me)2

X

N

H

Ac

126

120

for 5-membered heterocyclic system used as diazo compounds - steric hindrance between the ortho substituent and lone pair orbital of the α-azo nitrogen atom is diminished because of larger bond angle




HHMe

R1

HMeMe

R2 ¥ëEtOHmax (nm)

480495468

¥åmax

30,40031,00026,800

¥ë

+15-12

Table 3.22. Absorption bands of some ortho-substituted azo dyes

NN

O2NNMe2

R1

R2

one ortho substituent caused a bathochromic shift two ortho substituents caused a hypsochromic shift




HHHHNO2NO2

HHMeHHMe

X

HMeMeClMeMe

Z ¥ëEtOHmax

(nm)

408375380377422423

¥åmax

28,25018,20010,30019,90019,50011,200

¥ë

-33a

-28a

-31a

-57a

-56a

Y

a relative to (153 : X=Y=Z=H)b relative to (153: X=Y=H, Z=NO2)

Table 3.23 Absorption bands of some dyes with a sterically hindered terminal group

NN

ZNMe2

X

Y

Steric Hindrance at the Terminal Dialkylamino Group




NN

O2NNMe2

X

Y

(157)

X

HMeMe

Y

HHMe

¥ëmax (nm)

479460443

¥åmax

31,30032,00031,000

¥ëa

-19-36

Table 3.24 Absorption bands of dye

Steric Hindrance at the Nitro Group

colour and constitution

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