Variation of Donor- and Acceptor Strength in Analogues of Brooker's

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Variation of Donor- and Acceptor Strength in Analogues of Brooker's Merocyanine and Generalization to Various Classes of Charge Transfer Compounds Marina L. Dekhtyar, Wolfgang Rettig, Annette Rothe, Vladimir V Kurdyukov, and Alexey I Tolmachev J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b10660 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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The Journal of Physical Chemistry

Variation of Donor- and Acceptor Strength in Analogues of Brooker's Merocyanine and Generalization to Various Classes of Charge Transfer Compounds M. Dekhtyara) *, W. Rettigb), A. Rotheb), V. Kurdyukov a), A.Tolmachev a) a) Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanskaya str. 5, Kiev, 02094, Ukraine b) Institute of Chemistry, Humboldt University of Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany

Abstract Derivatives of Brooker's Merocyanine (BM) have been investigated, which possess different donors and acceptors and therefore vary their donor-acceptor strength SDA. The 00-energies have been extracted from the spectra and compared. In basic conditions, where the neutral (merocyanine) form is present, the absorption energies for all compounds are similar, whereas there is a large difference for acidic conditions where the cationic (cyanine) form is present. This behaviour could be explained by a simple theoretical model involving the dependence of the excitation energy E01 on SDA. This model can be generalized to describe in a consistent way two different well-known classes of neutral chromophores with a certain degree of charge separation, namely merocyanine I (TICT) and merocyanine II (often betainic) compounds. Merocyanines I are characterized by a medium polar aromatic ground state and a zwitterionic quinoid excited state and hence positive solvatochromism, whereas merocyanines II are formally characterized by a zwitterionic aromatic ground state and a less polar quinoid excited state and, accordingly, by negative solvatochromism. On increasing the donoracceptor strength SDA sufficiently, merocyanines II can, however, move to the so-called overcritical region with the excited state dominated by the zwitterionic valence bond structure. For many of the merocyanine II molecules investigated here, a weakly positive solvatochromic behaviour is observed indicating that the ground state contains less of the zwitterionic valence bond wavefunction than the excited state and that these compounds belong to the overcritical region. The fluorescence spectra have been analyzed in terms of the Franck-Condon model and confirm these conclusions.

Introduction Merocyanines have a long history, starting with the early investigations of König in the 1920s,1 and continuing e.g. in the 1940s and 50s with the famous Brooker dyes.2,3 Apart from the "Brooker deviation" coined from these investigations and usable to describe quantitatively the unsymmetry of cyanine and merocyanine dyes, Brooker also described the so-called "cusping"

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behaviour for certain merocyanines, indicating that for a certain solvent polarity in mixtures of pyridine with water, the extinction coefficient shows a maximum, and the structure of the absorption spectrum has the smallest width.3 At the same point, the solvatochromic behaviour changes from positive to negative or vice versa.4,5,6 This was explained on the basis of a two-state model involving the resonance interaction of two different valence bond structures, a strongly polar "zwitterionic" one (Z), and a weakly or nonpolar one (N), see Scheme 1. In structure Z, the molecule can be considered as being of the type Donor-Bridge-Acceptor.

O

O X

X

Z

N

Scheme 1: Most important valence bond resonance structures for a generalized case of Brooker's merocyanine (BM, see also Scheme 2). The total wave function can be approximated by a mixture of the zwitterionic (Z) and the nonpolar (N) form. The structure shown for Z is only one of two equivalent resonance forms which differ only for the aromatic rings. Further less important structures with charges on different atoms also mix to the total wavefunction but can be neglected in a first approach. If the two valence bond structures have equal energy, their contribution in the system's wavefunction is equal for ground and excited state, i.e. 50%. In this case, which was later called the "cyanine limit" CL, the dipole moment of S0 and S1 must be identical, with a zero change upon excitation.7 It was also shown in this paper that for dyes which do not have such isoenergetic resonance structures, the energy difference ΔE01 between S0 and S1 increases, i.e. there is a minimum for the excitation energy at the CL. Förster7 and later Platt8 used this 2x2 description to set this model on a firm quantum chemical basis (see eq. 1), and Platt showed that the energy difference between Z and N structures is related to the Brooker deviation.3 In the usual notation, the ground state wavefunction of the donor-acceptor system DA(S0) depends on the square of a mixing coefficient c (eqs. 1 and 2). At the CL, c2 equals 0.5 and the valence bond wavefunctions Z and N therefore contribute equally to both S0 and S1. DA(S0) =

c Z

DA(S1) = (1-c2)1/2 Z

+

(1-c2)1/2 N

(1)

-

c N

(2)

For the general case of unsymmetric dyes, the value of the transition energy ΔE01 can be obtained by solving the 2x2 secular determinant (eq. 3) involving the interaction energy F as offdiagonal element and the energy difference b=bZ-bN regarding the two resonance structures, eq. 4.


2

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bZ  E

0.5 F

0.5 F

bN  E

0

(3)

E 01  F2  b 2

(4)

The energy difference b of N and Z VB-structures can be called the "donor-acceptor strength" SDA. Then eq. 4 predicts that the excitation energy E01 depends on SDA in a functional form which resembles a hyperbola (rotated by 45° degrees from the normal case), with a minimum F (the interaction energy) for SDA = 0. For SDA strongly differing from 0, E01 is proportional to SDA. We call this function the SDA-function here. Its dependence on F has been discussed previously.9 For zero coupling, F = 0 (e.g. 90° twist of one of the two formally single bonds of the structures in Scheme 2), the contribution of one VB-structure to S0 and the other to S1 is 100%. When nonlinear optics (NLO) became important, scientists quickly realized that the hyperpolarizability  is connected to the CL:  is at a maximum at a certain offset from the CL, where the dipole moment difference reaches a certain intermediate value.10 For this reason, the exact determination of c2 in eqs. 1 and 2 became an important task for developing efficient NLO dyes.11-16 From eqs. (3) and (4), the mixing coefficient can be derived as a function of b and F (eq. 5). It is readily seen that c2 = 0.5 for b = 0, i.e. when both resonance structures are of equal energy.

c 2 |

F 2  b2  b 2 F 2  b2

|

(5)

Another way of quantitatively deriving c2 is the use of eq. 6.17 For its application, the dipole moment change  between S0 and S1 and the transition dipole moment 01 have to be measured. This can be done by using absorption and solvatochromic techniques. The solvatochromic technique18 involves the solvatochromic slope in a series of differently polar solvents, i.e. it cannot give the dipole moment difference in one given solvent and cannot describe changes of c2 when the solvent polarity is varied. c2 = 0.5 [1 -  (4 012 + 2 )- 0.5


(6)

3


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a) N

O

O

O

O

N N

N CN

N CN

DMABN

N

DCB

ON-para

ON-ipso

Class I merocyanines (TICT-type)

B 30

BM

Class II merocyanines (Betaines)

b)

Bridge

Donor N

O

Bridge

CN

Acceptor N

Bridge

hp

dd

"nonpolar" N

"zwitterionic" Z

Bridge

hp "zwitterionic" Z

N

O

Bridge

C=N

N

class I

class II

dd "nonpolar" N

c)


4

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Scheme 2: a) Examples for different classes of CT merocyanine compounds: Referring to the aromatic resonance structure depicted as the most stable one characterizing the ground state in the majority of cases, we can discriminate two different classes: Structures without charge separation (DMABN, DCB) and structures with charge separation (ON-para, ON-ipso, B30, BM). We define the first group as class I compounds (predominantly non-charge-separated aromatic N ground state with some admixture of Z in the planar conformation) and the second one as class II compounds (predominantly zwitterionic aromatic Z state with some admixture of N which can represent the ground or excited state). b) Examples DCB and ON-para and generalization to their bridge-related derivatives (merocyanines with still longer polymethinic chain) as comparable class I and class II Donor-Bridge-Acceptor merocyanines. The classes are characterized by a different connection of the aromatic character, the zwitterionic property Z and the electronic structure (hole-pair hp, i.e. closed shell system, and dot-dot i.e. open-shell system, see below). The transition between the two classes is performed by the replacement of two atoms being situated in adjacent columns of the periodic system (see text). If the bridge is completely insulating, for example by orthogonal twisting, the mesomeric arrow is broken. For other twist angles, the mesomeric resonance structures interact to a certain extent. Frequently investigated compounds involve -(C=C)n- bridges (biphenyls n = 0, the present compounds n = 1), but also polyphenyl or -(CC)n- bridges are often encountered. In the series of merocyanines (e.g. Brooker-type BM compounds) the donor or acceptor moieties can also be charged or uncharged as in 5- or 7-ring heteroaromatic systems. c) Two examples of related class I and class II merocyanine structures are depicted in their aromatic (upper part) and their quinoid VB forms (bottom). There are no unpaired electrons for the aromatic structures, and the frontier orbitals are doubly occupied (HOMO, pair) and devoid of electrons (LUMO, hole). These valence bond structures are therefore called "hole-pair" (hp). They are of Ntype for class I and of Z-type for class II compounds. In addition to the quinoid structure, a further structure belonging to this valence bond family, with two unpaired electrons (middle), is shown which is important for the theoretical model explaining the properties of class I and II compounds (see discussion section). This structure has an unpaired electron on each of the aromatic moieties and is formally derived from the aromatic structure by electron transfer of one electron from the donor (left moiety) to the acceptor. In terms of molecular orbitals, it is represented by single occupancy of HOMO and LUMO by one electron (represented here as a thick dot). This structure is called "dot-dot" (dd) structure in the literature.19 Recently, the electrooptical approach was used, which can determine dipole moments and their changes on excitation in one given solvent.17 Würthner and Kuball17 could show in this way, that the cusping behaviour is directly connected with changes of c2 passing through the value 0.5, and that for this condition, the dipole moment change  is zero. In an early work, Wizinger20 gave examples of compound series where the change of donor or acceptor can lead to blue- or red-shifted absorption. An anomalous case is observed, when the donor strength is strongly increased (or equivalently its ionization potential is decreased). Then a blue shift of the absorption spectrum is observed. Wizinger called this "the inversion of auxochromic behaviour". This approach of comparing the absorption spectra of different compounds to learn something about the electronic structure of a merocyanine will be used in the present work and we vary both


5


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donor and acceptor moieties of the derivatives of Brooker’s merocyanine (BM) in a systematic way. We explain the results on the basis of the 2x2 model and qualitatively derive the relative contribution of the Z-structure in the ground state, i.e. c2 of the compound with respect to the CL (c2 = 0.5). This allows a new approach based on a simple model to be developed here for tailor-making of absorption and NLO properties of merocyanine dyes. Some of the results have been presented in a previous publication21 and are analyzed with more refined methods here. Moreover, in this contribution, we want to go a step further and compare the merocyanines to other charge-transfer (CT) compounds. We define class I and class II compounds and develop their essential properties (Scheme 2). This formalism is applicable to well-known literature class I examples e.g., so-called twisted intramolecular charge transfer (TICT) compounds22-25 such as DMABN and DCB, as well as for betaines like Reichardt's dye B3026 and merocyanines like BM2,3,27 which belong to class II. Model compounds for B30 such as ON-ipso and ON-para can be discussed, which are directly comparable to the TICT compound DCB, and Schemes 2b and 2c show the relevant valence bond structures together with the definition of dd and hp states important for developing the theoretical model28,29,19 (see below in the discussion part).

Experimental Schemes 3 and 4 show the structures of the investigated BM analogues (class II merocyanines) in their neutral zwitterionic form (basic conditions) and in their ionic cyanine form (protonated, acidic conditions), respectively. For the sake of uniformity, the aromatic representation of merocyanines is used throughout the consideration even though for some of the compounds it does not correspond to the real ground state structure (which possesses more of the non-charge-separated than of the zwitterionic valence bond structure). Absorption spectra were measured on an ATI UNICAM Series UV–vis spectrometer UV4 and quantum corrected fluorescence spectra on an AMINCO Bowman series 2 Luminescence Spectrometer. The fluorescence spectra were converted from quanta per wavelength to quanta per wavenumber by multiplying with 2. Solvents were of Merck Uvasol quality and directly used without further purification. We chose aprotic polar solvents in order to minimize possible hydrogen bonding effects. Due to the very low emission intensity, concentrated solutions (OD ca 1-2) were used to reduce the solvent background, and quantum yields were not determined. The emission maxima did not depend on the excitation wavelength and on the concentration, and the fluorescence spectra were approximately the mirror image of the absorption spectra, however with differences in the relative importance of the subbands. This suggests the assumption that the observed emission is not from an impurity (from solvent or in the compound) nor from a dimeric species.


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The basic (neutral and formally zwitterionic) form was stabilized by addition of 3 to 5 drops of triethylamine. In most cases, the spectra were identical to the spectra without addition of acid or base, i.e. the dyes are in their unprotonated state in acetonitrile. The acidic (cationic) form of the merocyanines was stabilized by addition of one drop of 1 N HCl or 70% HClO4 to the cuvette, with identical results. Further addition of acid did not change the spectra, and we conclude that the merocyanines were fully protonated. Synthetic data can be found in.21

S

N

O

O

O

NO-DP

O

SO-DP S

OO-DP S

S

N

O

S

O

O

O

S

S

S

NO-DT

SO-DT

OO-DT

Scheme 3: Structures of the investigated class II merocyanines (basic form) together with their abbreviations.

N

S

HO

O

HO

NOH-DP

HO

SOH-DP

S

O

S

HO

HO

S

NOH-DT

S

S

N HO

OOH-DP

S

S

SOH-DT

OOH-DT

Scheme 4: Structures of the investigated protonated class II merocyanines (acidic, or cyanine, form) together with their abbreviations. ACS Paragon Plus Environment

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Results Absorption spectra of the 6 BM analogues were measured for basic and acidic conditions in the standard solvent acetonitrile as well as, for a few of the compounds, in a variety of differently polar solvents, in order to compare excitation energies for the compounds and to determine the solvatochromic properties. The fluorescence spectra are also reported and are estimated to contain some solvent impurity and second order inner filter effects in the short wavelength region where absorption and fluorescence overlap. The main part of the fluorescence spectra, however, is considered to be significant, although with a large experimental scatter due to the weak intensity, and we consider the maxima and the presence or absence of vibronic structure, and especially its solventpolarity induced changes, as significant. The results are contained in Figs. 1 and 2 for the standard solvent acetonitrile and Figs. 3 and 4 for the solvatochromic measurement. The analysis of the data is summarized in Tables 1-3 and S1, S2 (Supporting Information). These tables also contain the 00energies derived by two different methods as described further in the text and in the section “Determination of the 0-0 band”, Supporting Information. In some cases, clearly visible vibronic structure is seen, and the assignment of the 00-band is straightforward. In other cases with unstructured spectra, the maximum and halfwidth, as well as the energy at a third of the maximum intensity have been used for an estimation of the 00-energy. This approach has been verified in the case of the structured spectra by comparing the simplified analysis with the detailed peak-fitting analysis. With the peak-fitting method, also the relative intensities of the different vibronic bands could be quantified and compared for different solvent polarities and compounds with different donoracceptor strength. The results are also contained in the tables, as well as in Fig. 4. The following points should be highlighted: 1) In most cases, the spectra of the basic form possess vibronic structure, whereas the acidic form is unstructured. 2) The structure is less for the thiopyrylium compounds than for the pyrylium ones, or even absent. 3) The spectra of the basic form are of similar energy for all compounds. The spectra of the acidic form, on the other hand, are blue-shifted with respect to those of the basic form, but the blueshift is much stronger for the pyridinium than for the pyrylium and thiopyrylium compounds. 4) In acetonitrile, the 0-0-transition is the strongest one for the pyridinium compounds, whereas the pyrylium and thiopyrylium compounds have spectral maxima at higher vibronic transitions. 5) The absorption spectra of the dithienyl-substituted compounds exhibit somewhat broader bandwidths as compared to the diphenyl-substituted ones. In view of the fact that different vibronic transitions correspond to the maximum for different compounds, it is necessary to determine the 0-0 transition for the comparison of the absorption energy ACS Paragon Plus Environment

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of the different compounds. Even further, a method has to be evaluated which allows to judge the 0-0 transition even for the unstructured spectra, such as the dithienyl compounds and the acidic forms. A detailed procedure used by us for determination of the 0-0 band is described in Supporting Information. It was preceded by a full nonlinear least-squares band fitting for the structured spectra, involving Gaussian shaped vibronic subbands.30

abs / fluorescence

1

a

NO-DP

0 1

OO-DP

0 1

SO-DP

0 30000

25000

20000

15000

-1

wavenumber  [cm ]

1

b

abs / fluorescence

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NOH-DP

0 1

OOH-DP

0 1 0 30000

SOH-DP

25000

20000

15000 -1


Figure 1: Absorption and fluorescence spectra of the dyes with diphenyl (DP) substituents (NO-DP; OO-DP; SO-DP) in acetonitrile at room temperature; a) merocyanine (basic) and b) cyanine (acidic) form. Due to the very weak fluorescence for the basic form, the Rayleigh and Raman contributions of the solvent are visible and have not been deleted. ACS Paragon Plus Environment

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abs./ fluorescence

1

a

NO-DT

0 1

OO-DT

0

SO-DT

1

0 30000

25000

20000

15000

-1


1

b

abs / fluorescence

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NOH-DT

0 1

OOH-DT

0 1

SOH-DT

0 30000

25000

20000

15000 -1


Figure 2: Absorption and fluorescence spectra of the dyes with dithienyl (DT) substituents (NO-DT; OO-DT; SO-DT) in acetonitrile at room temperature; a) merocyanine (basic) and b)cyanine (acidic) form. Due to the very weak fluorescence for the basic form, the Rayleigh and Raman contributions of the solvent are visible and have not been deleted.


10

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Table 1: Absorption and fluorescence characteristics of the dyes in the basic (a) and acidic forms (b) in acetonitrile at room temperature. Maximum max(abs), halfwidth (FWHM) ; 00-band 00(abs); vibrational quantum at the absorption maximum nabs; similar values for the fluorescence. Table 1a) absorption a) fluorescence a), b) compound

max(abs)

NO-DP

cm-1 15800

2350

OO-DP

18750

SO-DP



00(abs)

nabs

max(fluo)

00(fluo)

nfluo

15800

0

cm-1 14200

14200

0

4900

16130

2

13300

14600

1

18600

4800

15980

2

13200

14500

1

NO-DT

15350

2900

15350

0

13200

13200

0

OO-DT

18500

5100

15880

2

12800

14100

1

SO-DT

18400

5100

15780

2

13000

14300

1

a) vib = 1310 cm-1 is used for the determination of nabs and nfluo. b) Due to the very weak fluorescence of the merocyanines (f < 10-4 ), the spectra were measured at optical densities between 1.2 and 1.7, and the quantum yield could not be determined quantitatively for these conditions, and the spectral maxima are somewhat uncertain (ca 200 cm-1). The absorption energies are precise to ca 50 cm-1. Table 1b) absorption a) compound

max(abs)

NOH-DP

cm-1 24650

4300

OOH-DP

19950

SOH-DP



00(abs)

fluorescence a) nabs

max(fluo)

00(fluo)

nfluo

22050

2

cm-1 18150

20750

2

3150

18650

1

16150

17450

1

18900

3250

17600

1

15050

16350

1

NOH-DT

24150

6000

21550

2

17400

20000

2

OOH-DT

19700

4200

18370

1

15800

17100

1

SOH-DT

18400

4450

17100

1

14450

15750

1

a) For the spectral analysis, vib = 1310 cm-1 is used. The absorption and fluorescence energies are precise to ca. 50 and 200 cm-1. ACS Paragon Plus Environment

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Table 2: Relation of base to acid shifts and influence of heteroatoms on the spectra of DP and DT compounds in acetonitrile (from Table 1) Number of comparison

Comparison of compounds

1) change of heteroatoms

NO-DP to OO-DP increasing the acceptor strength basic B to acidic form A: decreasing the donor strength basic B to acidic form A: decreasing the donor strength

2) effect of protonation for NO-DP 3) effect of protonation for OO-DP Number of comparison

Comparison of compounds

1) change of heteroatoms

NO-DT to OO-DT increasing the acceptor strength basic B to acidic form A: decreasing the donor strength basic B to acidic form A: decreasing the donor strength

2) effect of protonation for NO-DT 3) effect of protonation for OO-DT

Basic form max(abs) [cm-1] 15800 to 18750: apparent strong blue shift

Basic form 00(abs,) [cm-1] 15800 to 16130 no significant shift

Acidic form max(abs) [cm-1] 24650 to 19950: strong red shift

Acidic form 00(abs,) [cm-1] 22050 to 18650: intermediate red shift

15800 to: 

15800 to: 

24650 very strong blue shift


18750 to: 

16130 to: 

19950 weak blue shift

18650 intermediate blue shift

Basic form max(abs) [cm-1] 15350 to 18500 apparent strong blue shift

Basic form 00(abs,) [cm-1] 15350 to 15880 no significant shift

Acidic form max(abs) [cm-1] 24150 to 19700 strong red shift

Acidic form 00(abs,) [cm-1] 21550 to 18370 intermediate red shift

15350 to: 

15350 to: 



18500 to: 

15880 to: 

19700 weak blue shift

18370 intermediate blue shift


12

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a

absorption / fluorescence

1

EOE

0 1

EtAc

0 1

THF

0 1

ACN

0 30000

25000

20000

15000

wavenumber  [cm-1]

1

b


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EOE

0 1

EtAc

0 1

THF

0 1

ACN

0 30000

25000

20000

15000

-1



13


EOE

c

abs / fluorescence

1 0

THF

1 0

ACN

1

0 30000

25000

20000

15000


1

d


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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EOE

0 1

EtAc

0 1

THF

0 1

ACN

0 30000

25000

20000

15000


Figure 3: Solvatochromic behaviour of NO-DP (a), NO-DT (b), OO-DP (c), and SO-DP (d) in the merocyanine (basic - black) and cyanine forms (acidic - red) in solvents of different polarity at room temperature (diethyl ether, EOE:  = 4.3; ethyl acetate, EtOAc:  = 6.02; tetrahydrofuran, THF:  = 7.58; and acetonitrile, ACN:  = 37.5). For a: Noteworthy is the jump of the maximum of the spectra of the basic dye with solvent polarity: In acetonitrile, the maximum is 15800 cm-1 for absorption and 14200 cm-1 for fluorescence and corresponds to the 00-band. In less polar solvents, especially in diethyl ether, the absorption maximum jumps to the higher vibronic 01-band reaching 16900 cm-1 and the fluorescence maximum still corresponds to the 00-band reaching 13800 cm-1. For a, b, c: Due to the very weak fluorescence for the basic form, the Rayleigh and Raman contributions of the solvent are visible and have not been deleted. For d: The fluorescence under basic conditions is so weak, that it could not be measured with confidence in solvents except ACN – the latter spectrum is contained in Fig 1.


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2.0

16000

ACN EOE

EtOAc CH Cl 2 2

-1

1.5

THF

rel I(01)

wavenumber   cm

a

15000

1.0

14000 0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.5 0.32

f

3.5

17000

EOE THF ACN

16000

3.0

rel I(01)

b

wavenumber  , cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5

15000

14000 0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

2.0 0.32

f

Figure 4: Plot of the experimental 00-energy (squares, left y-axis) and relative importance of the 01band, rel I (01) = I (01) / I (00) (circles, right y-axis), for NO-DP (a) and OO-DP (b) basic forms.


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Discussion 1) Determination of the 0-0 Band and the Difference in the 00-Energies In order to compare shifts induced by changes of heteroatoms or by protonation, we cannot simply compare the maxima of broad absorption spectra because the 0-0 band may be situated at different places in the absorption spectrum. This is vividly shown by the spectra of NO-DP and OODP where some structure is visible: A change of the absorption maximum from the 0-0 to the 0-1 band can e.g. be observed for the merocyanine form of NO-DP when the solvent is changed from acetonitrile to THF or diethylether (see Fig. 3a). Similarly, when comparing NO-DP and OO-DP in acetonitrile, the maxima correspond to the 00 and the 02 band (see Fig. 1), respectively. Although OO-DP is only weakly structured, the derivative spectra (not shown) are clearly indicative of the structuring, and band fitting (see below) is possible. The spectra for SO-DP (see Figs. 1 and 4d) do not show structure but possess similar band halfwidths as compared to OO-DP. This holds also for the acidic form (see Tables 1a and 1b). This conclusion is corroborated by the comparison of a second pair of compounds, the thienyl derivatives SO-DT and OO-DT (see Fig. 2 and Tables 1a and 1b). As the excitation energy E01 is identical to the 00-energy, 00(abs), we have to compare the latter if we want to compare spectra of different compounds (see Figs. 1 and 2) or spectra in different environments (see Figs. 3 and 4). We can conclude that a change of the halfwidth is a good indicator for a change of the vibronic quantum nabs of the absorption maximum: For the basic form of the DP compounds with a given nabs, there is some variation in  but the values are close to 2400 (nabs = 0), 3800 (nabs = 1) and 5000 cm-1 (nabs = 2) (see Tables 1a, S1a, c, d). Thus, the comparison of halfwidths directly determined with the simplified analysis in conjunction with the fitting analysis of the structured spectra of similar halfwidths allows to conclude directly from 1/2 to nabs. The frequency 3L at one third of the maximum intensity is also a way of determining n as described above in the experimental section and in Fig. S1. For higher nabs, it can directly be taken as a good measure for the 00-energy 00(abs). For small nabs (0 or 1), 3L underestimates the 00-energy, but in this case, the spectra are mostly structured (see Figs. 1a, 3a and 3b), and the energy discrepancy can be corrected by comparison to the fitted 00energy of the structured spectra reported here. The acidic spectra (see Fig. 1b and Tables S2a-d) of the DP compounds are completely unstructured, but we can again use the above criteria for evaluation if we assume that the vibrational spacings are similar and the band maximum corresponds to the maximum of a very broad vibronic subband. In fact, if the vibronic bands are very broad, this assumption may not be rigorously fulfilled and translates into a stronger variation of the band halfwidth (3200-3800 cm-1 for nabs = 1, 4300 - 4500 cm-1 for nabs = 2, for DP compounds, see Tables 1b and S2a, c, d). Thus, the values group around the halfwidths 3500 and 4400 cm-1close to those of the basic forms. This supports our approach used to ACS Paragon Plus Environment

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evaluate nabs and 00(abs) on a similar basis for both basic and acidic forms, taking vib around 1300 cm-1, with due allowance to a much larger error margin in the case of the acidic spectra. On the basis of the 00-energies, we can compare the compounds with different donor-acceptor strength, and also the effect of protonation – see Table 2. This table shows in exemplary form, how misleading the comparison of only band maxima can be: In the basic form of NO-DP and OO-DP, the donor part (phenolate) is the same, but the acceptor strength is increased. There is a strong blue shift observed, if only the maximum is considered, but the spectral analysis shows that the 00-energy is hardly affected (see also Table 1a). This is a consequence of the fact that the vibronic quantum nabs changes from 0 to 2. For the acidic form, on the other hand, both max(abs) and 00(abs) indicate a considerable redshift for OOH-DP, the compound with the stronger donor-acceptor strength, but again, this redshift is weaker for 00(abs) than for max(abs), leading to nabs = 2 for NOH-DP and nabs = 1 for OOH-DP consistent with the difference in the halfwidths (see Table 1b). The same effects are observed for the family of DT compounds (see Tables 1a, 1b, and 2). In general, one can conclude that spectral comparisons on the basis of the band maximum can be done as long as the band halfwidths are similar, but a safer way of comparison is the evaluation of 00(abs) as described, especially if the halfwidths differ. We can ask why an increase of the acceptor strength with constant donor part has a negligible effect for neutral (merocyanine) compounds but leads to a considerable redshift for the protonated (cyanine) species. If we refer to the aromatic valence bond structures in Schemes 3 and 4, the donor of the protonated species is a substituted phenol, i.e., a weaker donor as compared to the phenolate moiety of the merocyanines. Consequently, the DA-strength for the acidic species is substantially reduced for both NO and OO species as compared to the neutral merocyanine form. We are confronted with the fact that a change of the DA-strength SDA leads to large effects on the excitation energy E01 for the acidic forms (which are situated in the region of weak SDA) but to small effects for the basic forms (in the region of intermediate to large SDA). This will be answered in the following section, which establishes the dependence of E01 on SDA. If we compare E01 for the protonated to the unprotonated compounds, we observe a blue shift in all cases (see Table 2), but it is stronger for NO/NOH-DP than for OO/OOH-DP. This difference can also be explained by the model developed below.

2) The Connection of Class I and Class II Merocyanine Compounds In Schemes 1 and 2, the merocyanines are depicted in their zwitterionic form, the rings possessing aromatic structure. In this form, the negative end is a strong electron donor, the positive end an acceptor. The change to the non-charge-separated valence bond form is formally effected by transferring an electron from the donor to the acceptor and hence involves an electron transfer. As ACS Paragon Plus Environment

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outlined in the introduction section, the closed-shell valence bond structures depicted in Schemes 1 and 2 can be regarded as true electronic configurations of the ground state if the frontier orbitals are completely decoupled between donor and acceptor. This is the case e.g. if these systems are orthogonally twisted around one of the flexible noncentral (formally) single bonds connecting the donor and acceptor moieties. Although these twisted conformations are energetically unfavourable in the ground state, the orthogonal conformations and, more importantly, the aromatic VB-structures connected are an important keystone for discriminating two classes of DA-chromophores: There is a related class of systems with uncharged donor and acceptor moieties in the aromatic VB-structure, the so-called TICT compounds.22-24 In this case, the orthogonal geometry, the state corresponding to the zwitterionic valence bond structure, is mainly represented (in MO language) by the configuration (Slater determinant) involving excitation of an electron from the HOMO to the LUMO orbital. This state corresponds to a zwitterionic biradical, with donor radical cation and acceptor radical anion. In usual TICT molecules, this is the excited CT state showing the characteristic "anomalous" emission. Indicating the open-shell nature of this state, it was called a "dot-dot" (dd) state in the theoretical model.23,24,28,29,19 The corresponding state after electron transfer from the radical anion to the radical cation is the "hole-pair" (hp) state indicating the doubly occupied HOMO and the empty LUMO orbital: the ground state in this case. If donors and acceptors become stronger, the dd state will be lowered energetically with respect to the hp-ground state because their energy difference is related to IP(D) - EA(A) where IP is the ionization potential of the donor, EA the electron affinity of the acceptor. For sufficiently strong donors and acceptors, the situation can in principle arise that both states become equal in energy. This corresponds to a conical intersection of S0 and S1, i.e. to a very strong coupling of these states and ultrafast S1 deactivation. From the theoretical point of view, an example for a relatively large system has recently been described using high-level ab initio calculations.31 A very small model compound for such type of TICT molecules is aminoborane and the formaldiminium cation.29,19 As we can see by inspecting the valence bond structures of the compounds investigated here (Schemes 3 and 4), the zwitterionic structure depicted does not correspond to an open-shell dd configuration but to a closed-shell hp one. In this case, the nonpolar form has the dd structure for twisting around the noncentral bonds. For planar systems, this corresponds to the quinoid resonance form. Again, in both cases, the hp and dd structures are connected by an electron transfer process. For the quinoid VB-structure, the twisted bonds considered here are formally double bonds. To avoid ambiguities in the comparison of the two classes of systems, we prefer to use the stable closed-shell aromatic VB-structures as reference. Then the twist considered occurs around a formal single bond for both cases.


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We can define the following two classes of dd-hp molecules: Class I possesses closed-shell uncharged and open-shell charged donors and acceptors (the usual TICT compounds), class II closedshell charged and open-shell uncharged donors and acceptors (the merocyanine compounds investigated here). In addition to the derivatives with an ethylene bridge studied here, a further wellknown family of class II compounds are biphenyl-type betaines, such as the solvent-polarity indicator dye ET30 of Dimroth and Reichardt (for a review see Ref. 26), or related zwitterionic pyridinium or pyrylium biphenyls.32 All these betaines are also relevant for tailor-making of efficient NLO dyes.33 For class II compounds, the situation where the S0 and S1 states switch between the dd and hp nature (i.e. the "cyanine limit") can in general more easily be reached than for class I compounds, and a number of BM analogues are known which demonstrate "solvatochromic inversion".2-6,34 For orthogonally twisted derivatives of these molecules, S0 and S1 are nearly degenerate (conical intersection) and are predicted to exhibit infrared absorption.33 Class I and class II CT compounds differ by the number of electrons on the aromatic moieties in the Z VB-structure: an odd number for compounds of class I (the TICT state has biradicaloid properties), an even number for compounds of class II. As shown by Michl and Koutecky,28,29,19 the transition from class I to class II compounds can formally be done in a general way by exchanging one atom in the donor and the acceptor moiety by an atom which is situated in an adjacent column of the periodic system. A simple example with the donor and acceptor being a single -atom is the pair ethylene/aminoborane: Ethylene is a class II compound (zwitterionic state of 90° twisted ethylene is of hp type), and aminoborane, the prototype of TICT molecules, is a class I compound (zwitterionic state of dd type).29,19 In this case, the transition between these two molecules is performed by the change of C to B and of C to N. The distinction of neutral molecules into two classes I and II is rather general and can be applied to the most families of merocyanine compounds. Important examples are given in Scheme 2 starting with the most well-known TICT molecule DMABN.24 The corresponding biphenyl-type class I TICT compound DCB can directly be compared to the corresponding class II betaine-type systems, e.g. pyridinium-phenolate ON-para and ON-ipso, which can be viewed as model compounds for B30 but also for BM and for NO-DP and NO-DT investigated here. Considering DCB and ON-para, the transition between class I and class II is performed by exchanging the amino nitrogen in DCB by a charged oxygen, and an uncharged carbon atom (connected to a nitrile substituent) by a charged nitrogen, similar as the exchange of two carbon atoms in ethylene by B and N in aminoborane.19


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3) Different Behaviour for Unprotonated and Protonated Class II Merocyanine Dyes and the Relation to the TICT Model The model discussed in section 2 corresponds to a limiting case without resonance, i.e. with decoupled -molecular orbitals on donor and acceptor. In TICT compounds, this is achieved by twisting to 90° in the excited state. One can also envisage an insulating bridge between D and A to have the same effect. If resonance is present, for example for planar molecules, the two limiting hp and dd valence bond wavefunctions of the localized orbital model interact and are both included as a linear combination into the wave function of both ground and excited state (eqs. 1, 2), as described in the basic treatment of Förster, Platt and others.7,8 The resonance leads to c  1 or 0 in eqs. 1 and 2. The interaction "repels" the two states (Fig. 5a). In a treatment using localized orbitals, this can be quantitatively described by linear combinations of dd and hp electronic configurations.29,19 class I (TICT-type) State energy a)

State energy

dd Z (Q)

S1

dd

Z(ar)

SDA CL

Excitation energy

overcritical

normal

N (ar)

N (Q)

N (ar) CL

Z (Q)

Z (ar)

negative

solvatochromism solvat, c2

dd

overcritical

Z (ar) CL

negative

N(Q)

CL

Z (Q)

positive

hp

S0

dd

CL normal

Z(ar)

hp

SDA Z (Q)

N(Q)

S1

hp

S0 CL

b)

N (ar)

hp N (ar)

Excitation energy

class II (betainic-type)

N (Q)

positive

solvatochromism

SDA

solvat, c2

SDA

Figure 5: Energies of hp and dd type states (a) and the excitation energy (b) as a function of the DAstrength for both classes of merocyanines. The hp (aromatic) state is taken as reference and is drawn as a horizontal line. The dd state corresponds to HOMO-LUMO excitation. In the normal region (l.h.s. of all 4 figures), LUMO is of sufficiently higher energy than HOMO such that dd corresponds to S1. Upon increasing the donor strength, HOMO is raised, upon increasing the acceptor strength, LUMO is lowered. Therefore, an increase of SDA narrows the HOMO-LUMO energy gap in the normal region (l.h.s.), and the energy difference between dd and hp in decoupled systems diminishes (straight lines), and correspondingly also the S1 and S0 energy gap in merocyanines with coupled chromophores (curved lines). At the bottom, the arrows labelled "solvat, c2" indicate the direction of the movement along the SDA -axis and the direction of increasing weight of the Z-wavefunction in the ground state when the solvent polarity is increased. ACS Paragon Plus Environment

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For class I compounds, dd is more polar than hp and energetically above it and corresponds to the S1 state (i.e. these compounds are situated to the left of CL in the "normal region" for TICT-type molecules and the aromatic N resonance structure prevails in the ground state) – see Fig. 5. Due to the increase of the dipole moment by the excitation process, they show positive solvatochromic behaviour. In contrast, for class II molecules in the "normal region" (where the charge-separated resonance structure Z is aromatic and prevails in the ground state), S0 is more polar than S1, and negative solvatochromic behaviour is usually observed because the ground state is stabilized more efficiently by polar solvents than the excited state. If the CL is crossed from the left to the right by class I compounds so that they enter the "overcritical" region (this term refers to the "critical situation" of the theoretical model29,19 with degenerate S0 and S1 states for noninteracting moieties), negative solvatochromism is expected, because their ground state becomes zwitterionic quinoid Z(Q), i.e., more polar than the excited N(ar) state. Vice versa, class II compounds show positive solvatochromism in the overcritical region since in this region, their ground state is predominatly of non-charge-separated quinoid N(Q) nature, i.e., less polar than the excited Z(ar) state. Due to this different nature of the prevailing resonance structure, increasing solvent polarity corresponds to an increase of SDA values for class I (TICT) merocyanines and a decrease for class II merocyanines independently of the region (normal or overcritical). This is indicated on the axis in Fig. 5b. As a consequence, class II compounds which experimentally show positive solvatochromism are situated in the overcritical region (Fig. 5b, r.h.s.): The excitation energy is lowered on going to the left along the SDA scale within the overcritical region, up to the CL point. Vice versa, the negative solvatochromism in the normal region is understandable as a rise of the excitation energy on a leftward shift along the SDA scale. Thus, for class II compounds which cross the CL upon solvation and show an inversion behaviour for absorption solvatochromism, the overcritical situation corresponds to the low-polarity solvents and the normal region to the strongly polar solvents. The opposite applies to class I compounds, because in this case increasing solvent polarity corresponds to a movement to the right on the SDA scale. It is also seen from Fig. 5b that solvatochromism vanishes for both classes at the CL point. In addition to solvatochromism, i.e. the solvent polarity influence, comparison of absorption spectra (0-0 bands) of merocyanines with different DA-strength will follow the same pattern, i.e. a movement along the SDA scale. Increasing the donor strength of D and/or the acceptor strength of A shifts the molecule of both classes to the right (increasing SDA). For class I (TICT molecules – see l.h.s. of Fig. 5), this implies an increase in c2 (the ground-state weight of the polar zwitterionic structure): c2 equals to 0.5 at the CL as defined by eqs. 1 and 2, is smaller than 0.5 in the normal region and larger than 0.5 in the overcritical region. As a result, increasing SDA for TICT compounds causes a movement towards the CL and, accordingly, a red-shift if c2 < 0.5 (normal region left to the ACS Paragon Plus Environment

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minimum in Fig. 5b, l.h.s.) but a movement away from the CL and a blue shift if c2 > 0.5 (overcritical region right to the minimum). Vice versa, for class II compounds (see r.h.s. of Fig. 5), c2 is reduced with rising SDA so that c2 > 0.5 in the normal region and c2 < 0.5 in the overcritical region. However, the resulting behaviour is similar to class I compounds: an increase in SDA leads to a red shift of absorption spectra in the normal region and to a blue shift in the overcritical region. The model can be generalized for ionic species (e.g. the protonated compounds, NOH, SOH and OOH, investigated here or, more generally, unsymmetric cyanine systems) where in addition to the dipole moment, a monopole contribution is present35 which does, however, not change upon excitation. We can therefore neglect its influence on the excitation process and use the same equations 1-5 as for neutral TICT and class II merocyanine systems. Because the positive overall charge does not play a decisive role, we can also view these ionic cyanine-type chromophores as related to the corresponding deprotonated merocyanines and conclude that, again, the weight of the polar Z wavefunction and therefore c2 is smaller for OOH- as compared to NOH-compounds, because OOHcompounds possess the stronger acceptor and the larger SDA value. The difference to the neutral merocyanines, however, is that the donor is now a phenol moiety with a smaller donor strength instead of the phenolate donor. Consequently, the SDA of NOH and OOH is much smaller than that of NO- and OO-merocyanines, and we can group the compounds in the order of increasing SDA as NOH < OOH < NO < OO. If the OO-compounds are situated in the neighbourhood of the CL, then the excitation energy E01 is in the order OO  NO < OOH < NOH, which corresponds to the observation (see Table 1). The situation is graphically depicted in Fig. 6 and explains why the protonation of the OO- and SO-compounds, though leading to noticeable blue shifts, results in a much weaker effect than for NO-compounds. It also explains why the difference in excitation energy is small for the NO- and OO-merocyanines but large for the NOH- and OOHcyanines. Moreover, the placement of the merocyanines close to the CL signifies a nearly horizontal part of the SDA-function, and the solvatochromic effect is expected to be weak (c2  0.5 for both ground and excited state). The cyanines of the NOH-, OOH-, and SOH-family are placed far to the left of the CL, hence c2 > 0.5, and the ground state is significantly more polar than the excited state. A negative solvatochromism would be expected but is not observed (see Table S2). We can explain that by the overwhelming effect of the monopole moment (the overall positive charge of the cyanines) which also polarizes the solvent molecules. The polarizing effect is nearly identical for ground and excited state, if the dipolar contribution is much weaker than the monopolar one. This seems to be generally the case for unsymmetric cyanine type molecules.36


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NOH

 E01

OOH SOH normal region

SO

NO

OO

overcritical region

SDA

c² = 0.5 c²

Figure 6: Schematic placement of the investigated class II merocyanine compounds on the SDA-scale in acetonitrile. Filled circles indicate the diphenyl DP-compounds, open squares the dithienyl DTcompounds. The general redshift for both acidic and neutral DT-dyes is indicated by a second hyperbola in red, and the DT-points are placed at slightly smaller SDA values consistent with the reduction of the strength of the acceptor moiety by the donor thienyl. Due to the dominating role of the mesomeric effect, the SO-points are placed closer than the OO-points to the hyperbola bottom though the former correspond to larger SDA values (see text).

4) Solvatochromic Behaviour As discussed above, we can understand the negligible solvatochromic effect of the NO to OO merocyanines by their placement close to the bottom of the SDA-function where c2 is around 0.5 and ground and excited state dipole moments are similar. Nevertheless, we observe a distinct effect on the weight of the different vibronic peaks as the solvent polarity is changed (see Figs. 3 and 4, Table S1): With increasing polarity, the weight of the 00-vibronic peak increases in all cases. Moreover, we see a change of the halfwidths to smaller values as the quantum number of the vibration which constitutes the maximum, decreases, a fact which we used in the assignment of the 00-frequencies (see results section and the method for determination of the 0-0 band, Supporting Information). In terms of the Franck-Condon model, the reorganization energy, i.e. the extent of solvent and solute relaxation, is smallest for the case where the minima of the S1 and S0 potential energy curves are exactly vertically placed. Then, from the Franck-Condon factors, the 00-band is expected to be the most prominent one. Such a case is predicted if the wavefunctions of ground and excited state differ as little as possible. Considering eqs. 1-2, these wavefunctions are identical for c2 = 0.5, except for the phase of the two contributions, which, however, is not important for the electronic distribution and the shape of the potential energy curve. All cases with c2 = 0.5 are therefore expected to possess a prominent 00-band. This is well-known e.g. for symmetric cyanines which are all characterized by the same value 0.5 for the S0 and S1 weights of two identical charged resonance structures (see the narrow ACS Paragon Plus Environment

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spectra of long-chain cyanines, and of triphenylmethane and rhodamine dyes.37,38 As far as merocyanines are concerned, going beyond the simple 2x2 model (Z and N only) and taking into account further resonance structures (e.g. those defining the aromatic character) will lead to c2 < 0.5 but identical at the CL for S0 and S1. Thus the polar character does not change upon excitation and the FC-condition as discussed above is similar with the strongest structuring for the CL. For more clarity, however, in the discussion within this paper we prefer to use the 2x2 model where CL corresponds to c2 = 0.5. The prominent 00-band in the case of NO-DP and NO-DT in acetonitrile therefore signifies a situation close to c2 = 0.5. In ether and THF, on the other hand, the 01-band is the most prominent one, i.e. c2 is unequal to 0.5. Considering that c2 increases as the solvent polarity is raised, corresponding to a decrease of SDA (Figs. 5 and 6), less polar solvents correspond to a placement at higher SDA values, and the FC-bandshape tells us that this weakly polar surrounding induces a situation which is further away from the CL than the situation in acetonitrile. Hence, we can conclude, that the OO-, SO- and even the NO-merocyanines in weakly polar solvent are placed in the overcritical region with c2 < 0.5 but still close to it because the solvatochromic shift is not significant. NO-DP and NO-DT are derivatives of BM (Scheme 2), which is known3,27 to exhibit a reversal in its solvatochromic behaviour, i.e. crosses the CL with solvent polarity change. This reversal effect is, however, small for BM and even smaller in our case (see Figs. 3a,b and 4a and Tables S1a,b). A possible reason for this very small effect is the larger size of NO-DP as compared to the unsubstituted BM. The solvatochromic effect as measured by the Stokes shift18,39 can be described by eq. 7, and for a given change of the ground to excited state dipole moment  = e - g, the effect will be strongly weakened for an increased Onsager radius a consistent with the increased molecular size of NO-DP as compared to BM. F1(,n) is the solvent polarity function.

 St   St (0) 

( e   g ) 2 2 0 hca 3

F1 ( , n)

(7)

The OO- and SO-compounds are placed at still larger SDA values than the NO-compounds and therefore are expected to be situated somewhat further in the overcritical region. In fact, the solvatochromic spectra of OO-DT in Fig. 3c and the solvatochromic plot of OO-DP derived from the 00-energies in Fig. 4b show a very slight positive solvatochromic tendency consistent with c2 < 0.5. Also consistent is the increase of the relative importance of the 00-band if solvent polarity increases (see Fig. 4b, Table S1c). With increasing solvent polarity, the OO- and SO-compounds move leftwards to the c2 = 0.5 (to the CL), so that their 00-band becomes more intense. The placement of the OO-compounds further in the overcritical region is also corroborated by inspecting the most prominent vibronic band (nabs) for the basic and the acidic form. For NO-DP and ACS Paragon Plus Environment

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the closely related NO-DT, it changes from nabs = 0 (basic) to nabs = 2 (acidic) in acetonitrile, while OO-DP and the closely related SO-DP, OO-DT and SO-DT change from nabs = 2 (basic) to nabs = 1 (acidic) (see Table 1). Therefrom we conclude that in the merocyanine form, the OO-compounds are further away from the CL than the NO-compounds and are situated to the right of it (overcritical region). For acidic conditions, SDA is decreased similarly in both cases, and the OO-compounds are now closer to the CL than the NO-compounds, and are to the left of the CL, in the normal region. We can use the absorption energies for acidic conditions to gain information on the relative DA-strength of the different compounds, because the SDA-function is steep in the normal region far away from the CL. Sometimes, however, DA effects are obscured by other factors. For example, we observe the following order for the excitation energies E01: NOH-DP > NOH-DT, OOH-DP > OOHDT, and SOH-DP > SOH-DT (see Table 1b), with the shifts of ca. 500 cm-1, as if SDA were slightly increased for DT compounds. But this is not the case, since thienyl substituents being stronger donors than phenyl groups decrease the strength of the acceptor moiety and, accordingly, the overall SDA value. On the other hand, a compound with smaller SDA would be blue-shifted in the normal region, contrary to what is observed for acidic DT vs. DP absorption. At the same time, a slight red shift of 200-450 cm-1 is also observed for basic DT dyes relative to their DP counterparts (see Table 1a). All basic dyes are close to the CL and respond little to the changes of SDA induced by the acceptor within the series of DP and DT dyes (just as with NO-DP vs OO-DP, see Table 1 and Fig. 6). As seen, the red shift is observed for the DT-series relative to the DP-series throughout the SDA scale, both for basic and acidic forms and it must therefore be due to a source different from SDA. As a possible explanation for the DT vs. DP bathochromic effects both in the normal and overcritical regions, we suggest that it is the effectively larger overall chromophoric system of DT compounds (increased size of the "mesomeric box", e.g. through conformational effects such as twisted phenyl substituents but planar thienyl substituents) that adds to the red shift for basic DT dyes and solely causes the red shift for acidic DT dyes completely overcompensating a possible SDA-induced blue shift. Though a “mesomeric box” argument is purely qualitative and by far not universal and indisputable, it can, nevertheless, be justified for closely related structures, which is just the case of the molecules under consideration. A further comparison can be done for SO vs. OO and SOH vs. OOH compounds: The E01 values change in the order OOH-DP > SOH-DP and OOH-DT > SOH-DT, with a difference of around 1000-1300 cm-1 (see Table 1b). This large red shift of the SOH compounds in acidic media is consistent with larger SDA values for the SOH than for the OOH compounds. Contrary to simple intuitive expectation based on the smaller electronegativity of the S than of the O atom and to the evidently stronger mesomeric donor ability of the SH than of the OH group,40 a thiopyrylium acceptor moiety acts as a stronger acceptor than a pyrylium moiety, in accordance with experimentally ACS Paragon Plus Environment

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measured spectral deviations of unsymmetric polymethine dyes41-43 as well as polarographic data.44 (For comparison, the respective half-wave reduction potentials for pyrido-, pyrylo-, and thiopyrylocarbocyanines were measured in Ref. 44 to be -1.6, -0.65, and -0.46 eV). This difference of SDA is strongly reflected only on a steep portion of the SDA-function and its effect is expectedly much smaller at the flat bottom of the hyperbola (see Fig. 6). Accordingly, the shift in transition energies reduces to ca. 100 cm-1 for basic dyes (see Table 1a). However, it is not inverted in the overcritical region, contrary to what would be expected based on the larger SDA for the SO than for the OO compounds. This situation again suggests the interference of conjugation effects, though less pronounced than in the case of DT vs. DP (compare the corresponding conjugation red shifts: 100 cm1 for

SO vs. OO and 200-450 cm-1 for DT vs. DP). Indeed, it appears that sulfur-containing molecules

have a larger "mesomeric box" which increases the red shift of SOH as compared to OOH acidic dyes and completely overcompensates a possible SDA-induced blue shift of SO as compared to OO basic dyes. Though the resulting red shift of SO vs. OO compounds in the overcritical region is very small, ca. 20 times less than the red shift of NO vs. SO or OO dyes (see Table 1a), it is of importance being the only indication of the interference of non-SDA factors in transition energies. Interestingly, the more pronounced acceptor property and the larger "mesomeric box" of the thiopyrylium moiety as compared to its O-counterpart have a common origin, namely weak C-S bonds, which can be rationalized even at the topological level. In terms of topological -methods,45 the DA ability of a conjugated heterocycle is governed both by the electronegativity and connectivity of the heteroatom(s) X, i.e. by the Coulomb integral X and the resonance integral CX, respectively. It is clear that the larger the electronegativity of X, the less it is able to carry a positive charge. But it is also intuitively evident that the weaker the bonds of atom X with other atoms are, the more the heteroatom splits off from the conjugation thus likewise losing the ability to share the positive charge on the ring, if any. In other words, the electron-acceptor strength of a six-membered heteroaryl ring increases with increasing X and decreasing CX. This is consistent with the topological estimate of the electrondonor ability of the 2-X or 4-X heterosubstituted phenyl group in the framework of the long-chain approximation46-48 (see Appendix, Supporting Information). On the other hand, the weaker the conjugation between the heteroatom and the rest of the ring, the closer is the system to an acyclic branched conjugated chromophore containing five carbon atoms. A chain is known to have a larger effective chromophoric length than a cycle, other conditions being equal.49 Moreover, going from the cyclic 6-atomic to linear 5-atomic chromophore yields so large chromophore lengthening that it overcompensates even the loss of an atom (cf. entries for the 6-membered (het)aryl and 3-penta-1,3dienylium residues in Table 1SA in Appendix, Supporting Information).


26

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The relevant topological parameters of the O and S atoms in the six-membered heterocyclic residue indicate a weaker bond strength for S50 (see Appendix, Supporting Information), with the consequence that thiopyrylium proves to be less capable of bearing the positive charge of the aromatic ring: the effect of the smaller S electronegativity is overcompensated by the effect of weaker C-S bonds. Also, a more efficient removal of the S as compared to the O atom from conjugation gives rise to a larger effective chromophoric length of the S-containing heterocycle (see Table 1SA in Appendix, Supporting Information). Accordingly, the thiopyrylium nucleus should have more pronounced electron-acceptor properties and provide a smaller first transition energy than its pyrylium analogue, as is the case in experiments.41,44 Comparing the effects of changes of acceptor strength and mesomeric power for DT vs. DP and thia vs. oxa analogues (see Table 3), it is notable that in the former case (DT/DP), the SDA and mesomeric effects act in opposite directions in the normal region but in the same direction in the overcritical region. Indeed, smaller SDA values of DT vs. DP compounds (due to a less efficient acceptor moiety) should reduce the DT vs. DP red shift (due to a larger "mesomeric box") in the normal region and increase it in the overcritical region. This is consistent with our experimental result that the red shifts of DT vs. DP even at the flat bottom of the hyperbola (200-450 cm-1 in basic media) are quite comparable to those at its steep descending slope (ca. 500 cm-1 in acidic media) – see also Tables 1a, 1b, and Fig. 6. Vice versa, comparison of thiopyrylium and pyrylium moieties reveals the opposite trend: the SDA and mesomeric effects are added in the normal region and counteract each other in the overcritical region. Accordingly, the red shifts of SO vs. OO compounds at the valley bottom of the curve (ca. 100 cm-1) are an order of magnitude smaller than those of SOH vs. OOH at the steep slope portion (1000-1300 cm-1) – see the same sources as above. At the same time, it is understood from Fig. 6 that, anyway, the SDA-induced contributions to absorption shifts are largest at the steep portions and smallest at the flat portions of the curve. Hyperbola wings are, therefore, informative about the role of the SDA factor, whereas the near-bottom sites demonstrate the interference of mesomeric effects, if any. Importantly, however small the energy differences may be at the flat bottom of the hyperbola, their sign should not be ignored, because it is only due to this sign that the possible interference of the factors other than SDA can be noticed at all.


27

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 28 of 36

Table 3: Effects of acceptor strength and mesomeric power on absorption spectra for DT vs. DP and thia vs. oxa analogues

Interplay of SDA and mesomeric effects

DT vs. DP

DT vs. DP

SOH vs. OOH (normal

SO vs. OO

(normal region)

(overcritical region)

region)

(overcritical region)

Counteract: mesomeric

Act

concordantly

in Act

concordantly

in

box is larger for DT but favour of smaller DT favour of smaller SOH SDA works against

absorption energy

absorption energy

Counteract: mesomeric box is larger for SO but SDA

smaller DT absorption

works against smaller

energy

SO absorption energy

Observed red shifts,

500

200-450

1000-1300

100

cm-1

(moderate values –

(significant values –

(expectedly large

(expectedly small

smaller than expected

larger than expected at

values)

values – an order of

at the steep wing of the

the flat bottom, much

magnitude smaller than

hyperbola)

the same as at the steep

at the steep portion in

portion in the normal

the normal region)

region)


28

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5) Relation between Absorption and Fluorescence Although fluorescence is very weak for the merocyanines, we observe a clear structuring of the spectra also for NO-DP in different solvents. Fig. 3a shows that the fluorescence spectra are the approximate mirror image of the absorption spectra. In all cases, however, the relative contribution of the 00-band is clearly increased with respect to the absorption spectrum. This behaviour of the fluorescence can also be explained on the basis of our model and the placement of the compounds on the SDA-axis. For NO-DP in acetonitrile with a most prominent 00-band in the absorption spectrum, we concluded to a placement close to the CL, and ground- and excited-state dipole moments are nearly equal. Hence, the solvent relaxation which occurs after the FC-excitation process is small and the movement along the SDA axis, induced by this relaxation, is also small. Hence, the relative importance of the 00-band changes very little from the FC to the relaxed situation. For weakly polar solvents, e.g. diethyl ether, on the other hand, NO-DP is placed in the overcritical region, with (S0) < (S1). After the FC excitation process, the solvent relaxes to a situation which stabilizes the excited state and therefore brings S1 and S0 closer to each other. This corresponds to a movement to the left, to smaller values on the SDA axes, i.e. to an approach towards the CL and a concomitant increase of the relative contribution of the 00-band. In fact, for diethyl ether, the 01-band is the most prominent one in absorption (FC-situation), but the 00-band in fluorescence (relaxed situation), see Fig. 3a and Table S1. The behaviour of the fluorescence vibronic structure as compared to the absorption one thus leads to conclusions which corroborate the above analysis, namely that NO-DP is situated in the overcritical region in weakly polar solvents and moves to the CL in acetonitrile.

Summary and Conclusions The results of the spectral investigations of a set of derivatives of BM which differ in the strength of donor and acceptor moieties can be understood on the basis of a simplified 2x2 model involving valence bond resonance structures of aromatic/zwitterionic (Z) and quinoid/non-chargeseparated (N) character. A relevant parameter in this model is the donor-acceptor strength SDA, which determines both the excitation energy E01 and the contribution c2 of the Z VB-structure to the ground state wavefunction. The shape of the E01 dependence on SDA resembles a hyperbola, with a minimum for the excitation energy at the so-called cyanine-limit CL, where c2 = 0.5. For class II merocyanines, the region with c2 > 0.5 is defined as the normal region, that with c2 < 0.5 as the overcritical region. The model developed also incorporates CT compounds of a different type, class I merocyanines, or the so-called TICT-family. In this case, the normal region corresponds to c2 < 0.5.


29


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In order to compare the spectra, a method for determining the 00-energies has been developed, which is useful especially for unstructured spectra and has been tested by comparison to the spectra of some of the compounds where structuring was observed. The experimentally found 00-energies are very similar for the basic merocyanine forms of the dyes in all cases but differ significantly for the acidic forms. On the other hand, the shift of the 00energy by protonation is much stronger for the NO-compounds with a weaker acceptor than for the OO-compounds. This can be interpreted within the SDA model by the changes of donor- and acceptor strength. The results indicate that the acceptor strength increases in the order pyridinium < pyrylium < thiopyrylium. For the comparison of acidic to basic dyes, the donor strength increases in the order phenol to phenolate and leads to spectral changes which are also predicted by this model. Solvatochromic measurements additionally allow important conclusions. The absence of a significant solvatochromic shift for the basic forms of merocyanines indicates a placement in the vicinity of the cyanine limit CL with c2  0.5. The relative contribution of the 00-band, however, is solvent polarity dependent and increases for more polar solvents. If we place the spectrum with the most prominent 00-band at the CL, as for instance NO-DP in acetonitrile, then we can conclude from our results that this compound is situated in the overcritical region in the less polar diethyl ether. The OO and SO type compounds with a still stronger acceptor than in NO-DP are situated in the overcritical region in all solvents investigated. This assignment is corroborated by the analysis of the vibronic structure of the fluorescence spectra. There is an approximate mirror-image behaviour for NO-DP absorption and fluorescence in acetonitrile, and the prominent 00-band contribution is similarly indicating a placement close to the CL. In less polar solvents, especially diethyl ether, the fluorescence contains an increased 00contribution as compared to the absorption, and the maximum jumps from the 01-band in absorption to the 00-band in fluorescence, confirming the assignment of the overcritical region to NO-DP in weakly polar solvents and indicating a movement to the left, towards the CL at smaller SDA values, by excited-state relaxation. The hyperbolic model applied here makes it possible to estimate, though very roughly, the differences not only in SDA values but in the mesomeric box sizes, as well. If SDA effects clearly prevail over other factors and essentially govern the order of transition energies in the series of related compounds, this order should necessarily be opposite in the normal and overcritical regions (as in the case of aza vs. oxa compounds). In contrast, if SDA-induced trends are significantly interfered by “other than SDA” (as a rule, mesomeric) factors, it is indicated by the constancy of the transition energy order in both regions of the hyperbola (just as with DT vs. DP and thio vs. oxa compounds). That is why, the sign of energy differences is of significance, however small their absolute magnitude ACS Paragon Plus Environment

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may be at the flat bottom of the hyperbola: it is solely by the constancy of their sign throughout the hyperbola that the interference of non-SDA factors can be revealed. Steep hyperbola wings illustrate well the SDA contribution to first transition energies, whereas flat near-bottom sites manifest the interference of other possible effects.

Associated Content Supporting Information A method for determination of the 0-0 band, tables presenting absorption and fluorescence characteristics of the basic and acidic forms of the dyes obtained from experimental spectra and by fitting, and Appendix describing the relevant aspects of the long-chain approximation. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement The support by the Bundesministerium für Forschung und Technologie within project UKR 02/004 is gratefully acknowledged.


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For Table of Contents Only

Class I (TICT) merocyanines Bridge

N

CN

Bridge

N

CN

dd "zwitterionic" Z

hp "nonpolar" N

Class II (betainic) merocyanines O

Bridge

O

N

Bridge

N

dd "nonpolar" N

hp "zwitterionic" Z

Class I (TICT) merocyanines Bridge

N

CN

N

Bridge

CN

dd "zwitterionic" Z

hp "nonpolar" N

Class II (betainic) merocyanines O

Bridge

N

O

hp "zwitterionic" Z


Bridge

N

dd "nonpolar" N

36

Variation of Donor- and Acceptor Strength in Analogues of Brooker's

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