RESEARCH PAPER https://doi.org/10.1071/CH21201

Using new solvatochromic parameters to investigate dye–solvent interactions Victor AkpeA,B,* , Timothy J. BiddleA , Christian MaduC , Tak H. KimA,B , Christopher L. BrownA,B

and Ian E. CockA,B,*

ABSTRACT

Solvatochromic behaviours of triazine substituted dyes were evaluated using a novel approach derived from the red shift index (RsI) and associated solvation energy (ASE). These parameters were used to describe the solvation trends of the dye–solvent interactions based on their polarity changes. The concept demonstrates the effect of substituent changes on the triazine scaffold and the induced solvent polarity changes as solvated dyes go through the HOMO–LUMO (highest occupied molecular orbital-lowest uncopied molecular orbital) phases. Primarily, these phases were characterised by evaluating the wavelength of the absorption and emission spectra in different solvents, which, in conjunction with the recently reported computational approaches, provides a well-adjusted model for predicting spectra polarity changes between the dye (solute) and the solvent. Based on the results from this study, predictive polarity changes on the triazine scaffold in different solvents can be empirically monitored both in ground and excited states. Moreover, the solvatochromic parameters can be extended to evaluate the predictive behaviours of different spectra dyes.

Keywords: implicit solvation model systems of dyes, positive and negative solvatochronism, solvatochromic behaviours of dyes, solvatochromic variation of dyes, solvent parameters, solvent polarity indicators, solvent polarity scale, solvent-effect.

Introduction

Solvatochromic effects are the changes that chemical species undergo during interactions with their local environments (solvents). These changes have been studied by several investigators to have a profound effect on the spectral shapes of organic dyes, and are not limited to the solution phase alone but have been extended to the gaseous state depend-ing on the solvent type.[1–3] Molecules express these spectral changes when dissolved in a variety of solvents, and these have been previously investigated using theoretical frame-works and experimental data.[1,3] For instance, the solvatochromic effect has been used to explain the charge-induced transfer between proxy molecules during intra-molecular transfer,[4,5] predict the direction of spectra shifts,[6] or track protein molecules in biological systems using fluorescent probes.[7,8]

To date, polarity changes in dye–solvent interactions have been predominantly explained using microscopic changes that occur during solvation. These examples have been reported for the dipole–dipole interactions of dye behaviour in different sol-vents.[9–22] Solvation dynamics of organic dye molecules have also been described macroscopically using spectral properties of the dyes in the ground and excited states.[23–25] The ground state is the lowest unoccupied molecular orbital (LUMO) of a molecule or an atom. It is also the most stable configuration as electrons are dia-magnetically positioned within the orbitals. The excited state is the highest occupied molecular orbital (HOMO) energy representation of a molecule. Because the conforma-tion at the excited state is short-lived due to single (unpaired) electron in the orbitals, the excited states may exhibit more drastic changes than in the ground state. Hence,

For full list of author affiliations and declarations see end of paper

*Correspondence to: Victor Akpe School of Environment and Science, Griffith University, Nathan Campus, Qld 4111, Australia Email: victor.akpe@griffithuni.edu.au

Ian E. Cock School of Environment and Science, Griffith University, Nathan Campus, Qld 4111, Australia Email: I.Cock@griffith.edu.au

Handling Editor: Manabu Abe

Received: 17 August 2021 Accepted: 26 November 2021 Published: 24 February 2022

Cite this: Akpe V et al. (2022) Australian Journal of Chemistry 75(3), 206–219. doi:10.1071/CH21201

© 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

OPEN ACCESS

the objective of this study was to evaluate the influence of substituent effect and solvent inclusion in solvated triazine dyes. Therefore, using the absorption and emission values of a class of triazine substituted dyes, 1–7 (Fig. 1) along with the new solvatochromic parameters developed, the dynamic changes in solute–solvent interactions in the ground and excited states were evaluated. These solvatochromic param-eters developed in this current study can be extended to evaluate the predictive behaviours of different spectra dyes.

Theoretical framework

The solvation process involves the interaction of solute with solvent species, resulting in charge stabilisation of the solute in solution. This process underpins all chemical equilibria in solutions. Fig. 2 illustrates the HOMO–LUMO energy of a solvated dye required to stabilise the ground and first excited states. According to the Franck–Condon principle, the first excited state (‘Franck–Condon state’) possesses the same shape as the equilibrium ground state, and the mole-cule remains in the excited state by reaching the equilibrium

solvation. Subsequently, the fate of electron relaxation from the excited state depends on the interaction between the dye and solvent polarities, which initially results in a non- equilibrium Franck–Condon ground state, and finally to a ground state with equilibrium solvation. The transition of an organic dye in solution and vapour states with frequencies v and vo, respectively, can be expressed as:

E hv hv=solv. o (1)

ΔEsolv. is the solvation energy of the HOMO–LUMO transi-tion state and h is the Planck’s constant.

Fig. 2 also suggests that the solvation energies of both polar and non-polar interactions of organic dyes can also influence the spectral changes and can be empirically eval-uated. In this study, triazine dyes with varying substituent moieties have been investigated. It is expected that a change in the functional moiety within the triazine cyclic atom can result in wavelength changes of spectra because of substitu-ent effect and solvent-effect interactions. As a result, there can either be an increase or a decrease of aggregated dye molecules during solvation. Furthermore, substituent effects

NN

HN

O S

O

NNN

O S

OH2N

NN

HN

O S

ONN

HN

O

O

S

O

NN

N

N

[BPT] (1 )

[MPT] (2)[BDT] (3 )

[MOT] (4)

[AMT] (5 )

[BMT] (6 )

[EOT] (7)

5-(Benzylthio)-3-methyl-7-(2-(pyrrolidin-1-yl)acetyl)-3,6-dihydro-

4H-cyclopenta[d ][1,2,3]triazin-4-one

Ethyl 5-(methylthio)-4-oxo-4,6-dihydro-3H-cyclopenta[d ][1,2,3]triazine-7-carboxylate

3-Ethyl-5-(methylthio)-4-oxo-4,6-dihydro-3H-cyclopenta[d ][1,2,3]triazine-7-carboxamide

7-Acetyl-5-(methylthio)-3,6-dihydro-4H-cyclopenta[d ][1,2,3]triazine-4-one

7-Benzoyl-5-(methylthio)-3,6-dihydro-4H-cyclopenta[d ][1,2,3]triazin-4-one

5-(Benzylthio)-7-(dimethylglycyl)-3-methyl-3,6-dihydro-4H-

cyclopenta[d ][1,2,3]triazin-4-one

3-Methyl-5-(methylthio)-7-(2-(pyrrolidin-1-yl)acetyl)-3,6-dihydro-

4H-cyclopenta[d ][1,2,3]triazin-4-one

O S

O

NN

N

N

O S

O NN

N

N

O S

O

Fig. 1. The structure, IUPAC nomen-clature and abbreviated names of the triazine dyes 1 to 7 investigated.

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207

of this class of triazine dyes can also affect the cyclic atoms bearing the π-electrons, causing shifts in wavelength posi-tions of the absorption or emission spectra.

Computational details

Computational data for triazine dyes 1 to 7 were obtained from our recent article.[26] The calculations for 2- methylthio-3-methyluracil (2-MT-3MU) were completed under vacuum using an analogous method reported in this literature.[26] The structure of 2-methylthio-3-methyluracil was produced and visualised using Avogadro (software ver-sion 1.90.0).[27] Calculations were completed using Gaussian 16 software (Wallingford, Connecticut, USA) (full reference provided in the supplementary information, S1) for all methods utilised. The 2-methylthio-3-methyluracil structure was initially subjected to geometry optimisation through the Berny analytical gradient method[28] incorpo-rated into Gaussian 16 followed by vibrational frequency analysis. The resulting structures were confirmed to be sta-ble intermediates (minima on the potential energy surface) and it was ensured that no imaginary frequencies were present during simulation.

For the semi-empirical approaches, the structures were optimised using either the Austin Model 1 (AM1),[29] para-metric method 3 (PM3),[30,31] or parametric method 6 (PM6)[32] method followed by single-point energy (SPE) calculations at the ZINDO/S semi-empirical level,[33] speci-fying ten singlet excited states. For the TDDFT analysis, the ωB97X-D long-range corrected hybrid exchange–correlation

function[34] was selected in combination with the 6-31G(d) basis set[35,36] specifying six singlet excited states.

Polarisation efficiency of spectra dyes

According to Bayliss et al.[9,10], dye–solvent interactions have momentary transition dipoles in the ground state and in the excited state (Fig. 2). Herein, the polarisation red shift, Δλ has been defined as the wavelength of the spectra absorption difference of dye in solution and the vapour phase. However, in the vapour phase, there is no distortion of molecules caused by intermolecular interactions[37] and solvent-effect changes are expected to be negligible. Here, the wavelength of absorption ( )o of the experimentally reported value of 2-methylthio-3-methyluracil in the vapour phase[37] has been used as the reference compound. Also, it has been assumed that the wavelength of the absorption or emission (λ) of the investigated dye in the solution phase is greater than the wavelength of the absorption or emission of the reference compound in the vapour phase, o. To validate this assertion, 2-methylthio-3-methyluracil scaffold conformation was com-pared with triazine dyes investigated using a computational chemistry approach provided in the Supplementary material.

Thus, at the ground state, the polarisation red shift is defined by:

=go

go o (2)

where, go is the wavelength of the absorption of the inves-

tigated dye in the ground state. Also, at the excited state, the polarisation red shift is defined by, E

1:

=E1

E1 o (3)

E1 is the wavelength of the emission spectra of dye. The

polarisation efficiency of a dye sample transitioning from the ground state to the excited state has been defined as the ratio of the polarisation red-shift at the ground state to the polarisation red shift at the excited state:

Polarisation efficiency of a dye,

= go

E1 (4)

Associated solvation energy (ASE) and red shift index (RsI)

During the solvation process, energy is released when a solute associates with a solvent.[38] Therefore, when a dye transitions from the ground to the excited states, there is a measure of internal thermodynamic energy. Some investigators have been able to show the predictability of the solute–solvent behaviours using the spectral approaches.[1,24,25,39]

Solvent relaxation

Polar solvent

Non-polar or lesspolar solvent

Ground state

Exc

ited

sta

te

S1

hvA hvF

DEsolv. = hv 0–hv

hv

µ0

µG

h ¢vF

hv 0S2

Internal conversion andvibrational relaxation

Fig. 2. HOMO–LUMO of a solvated dye required to stabilise the ground and excited states. S1, S2 are the transitions from the ground and excited states. ΔEsolv. is the solvation energy. h is Planck’s constant. ν and νo are the frequency of the transition in solution and in vacuum, respectively. νA and νF are the absorption and emis-sion frequencies, respectively. µG and µO are the dipole moments from the transitions at the excited and ground states respectively.

V. Akpe et al. Australian Journal of Chemistry

208

The solvated energy is directly proportional to the fre-quency by a constant velocity expressed by c = vλ, and the energy in terms of wavelength expressed by E = hc . The ASE of spectra dyes in solution and vapour states with frequencies v and vo respectively can therefore be expressed as in Eqn 1.

E hv hv=solv. o

ΔEsolv. is defined as the ASE at the ground state, h is the Planck’s constant and v is the frequency. From Eqn 1,

Ehv

hv hvhv

=solv.o

o

o

Table 1. Spectral parameters were evaluated for the triazine derivative dyes studied. The reference compound (2-methylthio-3-methyl uracil) is in the vapour phase, λ° (290 nm).[ 37]

Dye Solvent (nm)go (nm)g

o RSI (nm)E1 (nm)E

1 RSI1 ξ (%)

BPT DMSO 385 95 13.3 481 191 16.6 50

MeCN 380 90 12.6 470 180 15.6 50

THF 381 91 12.7 457 167 14.5 54

2-MeTHF 381 91 12.7 463 173 15.0 53

Toluene 385 95 13.3 449 159 13.8 60

BDT DMSO 382 92 13.2 482 192 16.6 48

MeCN 378 88 12.6 466 176 15.2 50

THF 379 89 12.7 460 170 14.7 52

2-MeTHF 378 88 12.6 471 181 15.7 49

Toluene 384 94 13.5 459 169 14.6 56

MPT DMSO 387 97 13.3 491 201 16.9 48

MeCN 389 99 13.6 477 187 15.7 53

THF 384 94 12.9 471 181 15.2 52

2-MeTHF 384 94 12.9 473 183 15.4 51

Toluene 387 97 13.3 462 172 14.5 56

MOT DMSO 388 98 13.4 491 201 16.9 49

DMF 389 99 13.5 478 188 15.8 53

MeCN 381 91 12.4 478 188 17.2 48

THF 381 91 12.4 465 175 16.5 52

AMT DMSO 401 111 13.8 497 207 18.4 54

DMF 404 114 14.2 490 200 17.8 57

MeCN 393 103 12.8 483 193 17.2 53

THF 393 103 12.9 476 186 16.5 55

BMT DMSO 406 116 14.0 497 207 17.1 56

DMF 409 119 14.4 492 202 16.7 59

MeCN 396 106 12.8 485 195 16.1 54

THF 397 107 12.9 482 192 15.9 56

EOT DMSO 399 109 13.8 527 237 18.1 46

DMF 397 107 13.5 514 224 17.1 48

MeCN 395 105 13.3 505 215 16.5 49

THF 393 103 13.0 496 206 15.8 50

Wavelengths of the absorption and emission spectra were go and E

1 respectively; polarisation red shifts from absorption and emission were go and E

1, respectively; red shift index at the ground and excited states (RsI and RsI1 respectively); the scaling factor used at the ground and excited states of the dyes were Ndye and N1

dye, respectively; polarisation efficiency, .

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209

Eh

v v=solv. o (5)

Again, since the wavelength is inversely proportional to the frequency, Eqn 5 becomes:

Ehc

= 1 1solv.o

go

Ehc

=solv. go o

ogo

13.4 17.0

16.5

16.0

15.5

15.0

14.0

14.5

17.0

16.5

16.0

15.5

15.0

14.5

17.0

16.5

16.0

15.5

15.0

14.5

13.5

13.3

13.2

13.1

13.0

12.9

12.8

12.7

12.6

13.4

13.6

13.2

13.0

12.8

12.6

13.7

13.6

13.5

13.4

13.3

13.2

13.1

13.0

12.9

12.8

12.590 91 92 93 94 95

9088

94 95 96 97 98 99

89 91 92 93 94

155 160 165 170 175 180 185 190 195

SO

O

N

NN

N

SO

O

N

NN

N

SO

O

N

NN

N

165 170 175 180 185 190 195

170 175 180 185 190 195 200 205

Polarization red shift (nm)

Polarization red shift (nm)

Polarization red shift (nm) Polarization red shift (nm)

Polarization red shift (nm)

[BPT]

[BDT]

[MPT]

Polarization red shift (nm)

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

R2 = 0.99Slope = 0.14s.e. = 0.4%

R2 = 0.99Slope = 0.15s.e. = 0.47%

R2 = 1Slope = 0.14s.e. = 0.3%

R2 = 1Slope = 0.08s.e. = 0.12%

R2 = 1Slope = 0.09s.e. = 0.16%

R2 = 1Slope = 0.09s.e. = 0.08%

THF,

THF

THF,

2-MeTHF

2-MeTHF

MeCN

MeCN,

MeCN

THF

2-MeTHF

MeCN

Linear fit of ground state

Linear fit of ground state

Linear fit of ground state Linear fit of excited state

Linear fit of excited state

Linear fit of excited state

Toluene

Toluene

Toluene

DMSO,

DMSO

2-MeTHF

Toluene,DMSO

THF

THF

2-MeTHF

2-MeTHF

MeCN

MeCN

Toluene

Toluene

DMSO

DMSO

DMSO

Fig. 3. Polarity changes measured by spectra indices (red shift index (RsI) versus polarisation red shift).

NN

NH

O S

ON

N

NH

O S

O

Scheme 1. π-electron delocalisation between the electron donor and the electron acceptor groups of the triazine substituted scaf-fold (AMT).

V. Akpe et al. Australian Journal of Chemistry

210

10212.6

12.8

13.0

13.2

13.4

13.6

13.8

14.0

14.2

14.4

13.6 17.4

17.2

17.0

16.8

16.6

16.4

16.2

16.0

15.8

15.6175 180 185 190 195 200 205

13.4

13.2

13.0

12.8

12.6

12.4

90 92 94 96 98 100

18.5

17.5

16.5

18.0

17.0

12.6

12.8

13.0

13.0

13.2

13.4

13.6

13.8

13.2

13.4

13.6

13.8

14.0

14.2

14.4

14.6 17.2

16.8

16.6

16.4

16.2

16.0

15.8

18.5

17.5

16.5

15.5

18.0

17.0

16.0

17.0

104 106 108 110 112 114 185 190 195 200 205 210

190

205 210 215 220 225 230 235 240

192 196 198194 200 202 204 206 208

O

O

[AMT]

[MOT]

HN

NN

S

O

OO

HN

NN

S

O

O

[BMT]

HN

NN

S

O

O

[EOT]

N

NN

H2N

S

104

103 105 106 107 108 109104

106 108 110 112 114 116 118 120

Polarization red shift (nm)

Polarization red shift (nm)

Polarization red shift (nm)

Polarization red shift (nm)Polarization red shift (nm)

Polarization red shift (nm)

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

Polarization red shift (nm)

Polarization red shift (nm)

Red

shi

ft in

dex,

RsI

Red

shi

ft in

dex,

RsI

R2 = 0.99Slope = 0.12s.e. = 0.5%

R2 = 1Slope = 0.14s.e. = 0.26%

No linear fit at theexcited state

R2 = 1Slope = 0.09s.e. = 0.25%

R2 = 1Slope = 0.12s.e. = 0.17%

R2 = 0.99Slope = 0.13s.e. = 0.7%

R2 = 1Slope = 0.08s.e. = 0.19%

R2 = 1Slope = 0.07s.e. = 0.1%

THF

THF

THF

THF

THF

THF

THF

MeCN

MeCN,

MeCN

MeCN

MeCN

MeCN

MeCN

Linear fit of ground state

Linear fit of ground state

Linear fit of excited state

Linear fit of excited state

Linear fit of excited state

Linear fit of ground state

Linear fit of ground state

DMF

DMF

DMF

DMF

DMF

DMF

DMF

DMSO

DMSO

DMSO

DMSO

DMSO

THF

MeCN

DMF

DMSO

DMSO

DMSO

Fig. 4. Polarity changes measured by spectra indices (red shift index (RsI) versus polarisation red shift).

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211

The difference between the wavelengths of the investi-gated dye ( )g

o and the reference standard (λo) can be expressed as g

o:

=go o

go

Therefore:

Ehc

=solv. go

ogo (6)

Multiplying Eqn 6 through by the ratio of the scaling factor of the dye, Ndye and g

o:

E Nhc

N· ·=

·solv. go

dye go

dyeo (7)

Therefore, considering the right hand side of Eqn 7, the red shift index of an organic dye, RsI, is defined as the fraction of the polarisation red-shift of the dye (sample) to the wavelength of the reference compound in the vapour phase[37] multiplied by a scaling factor, Ndye

NRsI =

·go

dyeo (8)

Ndye is a scaling factor applied to a dye dissolved in a particular solvent. For the present study, Ndye was deter-mined for each dye 1 to 7. Here, Ndye has been calculated by dividing the polarisation red-shift of the dye when dissolved in the solvent with the highest polarity index for which the dye is soluble ( )g

o by the difference between go and the

wavelength of the reference compound (2-methylthio-3- methyluracil) at the Q-band in the vapour phase ( )o , multi-plied by 10.[24,25,39]

Therefore, Ndye was calculated as:

N = 10 = 10dyego

go o

go

go (9)

DMSO possessed the highest polarity index of all the sol-vents in which dyes 1 to 7 were dissolved and therefore was utilised for obtaining g

o. The scaling number (in this study, 10) is an arbitrary number and was also used by Freed et al.[24] in their investigation for simplicity.

Also, from Eqn 7:

E Nhc

N· ··

=·solv. dye g

o

ogo

dyeo

the ASE of a dye sample at the ground state, ΔEsolv. defined above can be simplified to Eqn 10, so that,

Ehc

=·

solv.go

go (10)

Therefore, Eqn 10 represents the empirical ASE at the ground state of the dye while Eqn 9 is the empirical RsI at

Table 2. The dye–solvent polarity scale for triazine substituted positions.

Theoretical scale Graphical scale

Dye Solvent Ndyeo Ndye

1 Ndyeo Ndye

1

BPT DMSO 40.5 25.2 55.0 42.0

MeCN

THF

2-MeTHF

Toluene

BDT DMSO 41.5 25.1 57.0 42.0

MeCN

THF

2-MeTHF

Toluene

MPT DMSO 39.9 24.4 54.0 38.0

MeCN

THF

2-MeTHF

Toluene

MOT DMSO 39.6 24.4 54.0 0

DMF

MeCN

THF

AMT DMSO 36.1 25.8 48.0 44.0

DMF

MeCN

THF

BMT DMSO 35.0 24.0 48.0 39.0

DMF

MeCN

THF

EOT DMSO 36.6 22.2 51.0 36.0

DMF

MeCN

THF

Note: Ndyeo and Ndye

1 are the corresponding scaling factors at the ground

electronic state and excited state. The theoretical Ndye was calculated from

Eqn 9; N = 10 = 10dyego

go o

go

go . The graphical Ndye was calculated from

the slope ( Fig. 3, 4). The slope of the graph was calculated from Eqn 8,

RsI =N.g

odye

o .

V. Akpe et al. Australian Journal of Chemistry

212

the ground state. The speed of light is c. Similarly, the RsI and ASE were used to deduce the excited states of the dye, following the stepwise equations previously derived elsewhere.[25,39]

Results and discussion

The bond polarity of a dye can be defined as the electro-negative difference between two or more atoms. The polar-ity becomes more apparent in the solvent–dye interactions in the solvated state. Herein, the polarity of triazine substi-tuted dyes has been evaluated using new solvatochromic concepts described by the red shift index (RsI) and associ-ated solvation energy (ASE). The derived formula was used to empirically evaluate the ground and excited states of these dyes as the substituent changes.

The computational chemistry approach was used to vali-date the choice of the reference compound, 2-methylthio-3- methyluracil. The computational data calculations validate lower absorption and emission maxima of the reference

compound to the triazine dyes 1 to 7 investigated. This study was confirmed using the semi-empirical optimisation methods of AM1, PM3, and PM6 utilised for vertical excita-tion energies combined with the ZINDO/S semi-empirical method. Each of these calculations indicated that the absorption maximum of 2-methylthio-3-methyluracil was lower than that of the triazine dyes 1 to 7[26] (supplemen-tary information, S2, Supplementary Table S1).

Additionally, the absorption and emission data were eval-uated using time-dependent density functional theory (TDDFT) at the ωB97X-D/6-31G(d) level. The calculations showed that 2-methylthio-3-methyluracil provided consid-erably lower values than 3-Ethyl-5-(methylthio)-4-oxo-4,6- dihydro-3H-cyclopenta [d][1,2,3] triazine-7-carboxamide EOT (supplementary information, S3, Supplementary Table S2). Hence, the results confirmed the choice of 2- methylthio-3-methyluracil as a suitable reference compound for the investigated dyes.

Solvatochromic behaviour of triazine substituted dyes (Fig. 1) in different polar and non-polar solvents was studied using spectra properties derived from Eqn 1 to 10. The

140

Toluene

Toluene

Toluene

Toluene

Ground state of BPTExcited state of BPT

DEsolv. ASE ratio

DEsolv.(a)

(b)

(c )

(d )

Ground state of BDT

BPTBDTMPT

Excited state of BDT

2-MeTHF

2-MeTHF

2-MeTHF

2-MeTHF

KK K

THF

THF,

THF

THF

MeCN

MeCN

MeCN

MeCN

DMSO

DMSO

DMSO

DMSO

130

120

0 10 20 30 5040

0 10 20 30 40 50 0 10 20 30 40 50

100

110

90

80

140

Toluene

Toluene

Ground state of MPTExcited state of MPT

DEsolv.

2-MeTHF

2-MeTHF

K

THF

THF,

MeCN

MeCN

DMSO

DMSO

130

120

0 10 20 30 5040

100

110

90

80

130

120

100

110

1.65

1.60

1.55

1.45

1.40

1.50

90

80

Fig. 5. Scattered central plot. Net solvated state of dye measured by spectra index (solvation energy (ΔEsolv.) versus dielectric constant (K)).

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213

solvatochromic shifts in the studied solutions focused on the effect of substituent changes and the dye–solvent interac-tions in the ground and excited states (Fig. 2). Particularly, substituent change within a dye molecule can influence the spectra changes but can also cause solvent-induced changes during solvation. Table 1 are the parameters used to evalu-ate the dye–solvent behaviours. From Scheme 1, the delo-calisation of electrons within an aromatic cyclic structure is expected to improve charge distribution and separation between electron donor and acceptor of the triazine scaf-fold. Subsequently, the effect of these dynamic changes was investigated in polar and non-polar solvents.

Firstly, we compared triazine scaffolds that differed at the 2-pyrrolidine position, BPT; 5-methylthio position, MPT; 7-dimethylglycl derivative position, BDT. An example of the intramolecular charge transfer of the triazine substi-tuted dye has been shown in Scheme 1. From equation 8, the RsI was plotted against Δλ to obtain the solvatochromic shift

of the dye in different solvents. For BPT triazine substituted dye, the 2-pyrrolidine position interactions with DMSO and toluene had greater charge stabilisation in the ground state than MeCN for the same substituted dye. Nonetheless, tran-sition of BPT at the excited state increased the charge stabilisation effect of MeCN and placed the solvent position (at n = 2) after DMSO (at n = 1), and toluene (at n = 5) for the dye–solvent interactions. A similar solvatochromic shift was evaluated for the graphs of BDT and MPT. Secondly, we compared triazine scaffolds that differed at the 7-carboxylate position, MOT; 7-acetyl position, AMT; 7-benzoyl position, BMT; 3-ethyl and 7-carboxamide posi-tions, EOT. The RsI was again plotted against Δλ to obtain graphs delineated in Fig. 3, 4 The result of the triazine substituted MOT at the excited state suggests that the dye–solvent interactions is largely destabilising (Fig. 4).

Another parameter considered in this study is the dielectric constant, which, despite being used as a guide to

140

130

THF

THF

DMF

MeCN

DMSO

DMF

MeCN

DMSO

120

1100 010 20 30 40 50

100

90

140

150

130

THF

THF

DMF

DMF

MeCN

DMSO

MeCN

DMSO

120

110

10 20 30 40 50

100

90

Ground state of AMT Ground state of EOTExcited state of EOTExcited state of AMT

Ground state of BMT

Excited state of BMT

DEsolv. DEsolv.

140

130

THF

THF

DMF

MeCN

DMSO

DMF

MeCN

DMSO

120

110

0 10 20 30 40 50

100

90

DEsolv.

AMTBMTEOTMOT

0 10 20 30 40 50

1.65

1.60

1.55

1.45

1.40

1.50

(a)

(b)

(c )

(d )

KK K

K

ASE ratio

Fig. 6. Scattered central plot. Net solvated state of dye measured by spectra index (solvation energy (ΔEsolv.) versus dielectric constant (K)).

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categorise solvents into polar and non-polar, does not implicitly correlate to the dye–solvent trends, as indicated by this order: DMSO (46.7), MeCN (37.5), DMF (36.7), THF (7.58), 2-Me-THF (6.97) and toluene (2.38). Based on the relative polarity of the triazine substituted spectra values, there was no clear correlation to justify that the solvent

polarity only can be used to evaluate the dye–solvent inter-actions. Furthermore, in Fig. 4 triazine substituted MOT graph had solvent positions occupied at n = 1 for DMSO, toluene and the least position for MeCN at n = 3 in the ground state. Unequivocally, the solvatochromic shift observed in the absorption and emission spectra is caused by the net-charge changes during solvation. The influence of these pseudo-polarity changes of dye–solvent interactions has also been demonstrated from the following stud-ies.[1,25,39] It was therefore imperative to establish a polarity scale that reflects the dynamic changes of the dye–solvent interactions and not the solvent polarity only. From Eqn 9, the parameter ‘Ndye’, which represents the dynamic polarity scale of the dye–solvent interactions was introduced into the equation. Ndye is defined as the relative ratio of the absorp-tion or emission spectrum of the dye to the polarisation red in the ground or excited state. Table 2 provides the theoreti-cal and actual or graphical Ndye scale for each of the dyes. Theoretical value was calculated from Eqn 9 while the graphical Ndye was calculated from the solvatochromic shifts at the ground and excited states.

The ASE parameter was used to predict the net-charge changes of dye–solvent interactions delineated in Fig. 5, 6. Fig. 5 shows graphs from the scattered central plots of triazine substituted scaffolds at the 2-pyrrolidine, BPT; 5- methylthio, MPT; 7-dimethylglycl derivative positions, sum-marised in Table 3. The first and second graph plots (ΔEsolv. versus k) were used to allocate solvent positions within the different solvents considered. These graphs were then used to evaluate the resultant net charge solvation effect of the dye–solvent interactions. The same evaluation was consid-ered for triazine substituted scaffolds at the 7-carboxylate position, MOT; 7-acetyl position, AMT; 7-benzoyl position, BMT; 3-ethyl and 7-carboxamide positions, EOT, sum-marised in Table 4.

Therefore, in comparison to previous models for solvatochromic behaviours of dyes using spectral approaches,[1,24,25,39] the newly developed concept pro-vides more insight into the dynamic behaviours of dye–solvent interactions. In addition to the polarity changes measured by spectra indices using the RsI versus polarisation red shift in Fig. 3, 4 for the HOMO–LUMO transitions, the scattered central plot in Fig. 5, 6 was used to describe semi-quantitative behaviours of the tria-zine substituted dyes. A summary of the semi-quantitative transitions of the dyes has been provided in Table 4. Particularly, Table 4 provides a detailed description of the dye–solvent interactions.

Conclusions

Overall, this study demonstrates a novel approach to evalu-ate the solvatochromic behaviours of triazine substituted dyes in different solvents using the RsI, polarisation red

Table 3. Solvation energy parameters and dielectric constant values for triazine substituted positions.

Dye Solvent k ASE ratio

E (kJ)go E (kJ)E

1

BPT DMSO 46.7 1.61 85 137

MeCN 37.5 1.61 82 132

THF 7.58 1.54 82 126

2-MeTHF 6.97 1.57 82 129

Toluene 2.38 1.44 85 122

BDT DMSO 46.7 1.65 83 137

MeCN 37.5 1.63 80 130

THF 7.58 1.57 81 127

2-MeTHF 6.97 1.66 80 133

Toluene 2.38 1.51 84 127

MPT DMSO 46.7 1.64 86 141

MeCN 37.5 1.53 88 135

THF 7.58 1.57 84 132

2-MeTHF 6.97 1.58 84 133

Toluene 2.38 1.50 86 129

MOT DMSO 46.7 1.62 87 141

DMF 36.7 1.53 88 135

MeCN 37.5 1.65 82 135

THF 7.58 1.59 82 130

AMT DMSO 46.7 1.51 95 143

DMF 36.7 1.45 97 141

MeCN 37.5 1.53 90 138

THF 7.58 1.49 90 134

BMT DMSO 46.7 1.44 99 143

DMF 36.7 1.40 101 141

MeCN 37.5 1.50 92 138

THF 7.58 1.47 93 137

EOT DMSO 46.7 1.65 94 155

DMF 36.7 1.61 93 150

MeCN 37.5 1.60 92 147

THF 7.58 1.59 90 143

Note: k, dielectric constant; ASE, associated solvation energy (ΔEsolv.); Ego

and EE1 are the ASE values in the ground and excited states respectively; ASE

ratio, is E E/E1

go.

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Table 4. Qualitative descriptions of the dye–solvent interactions at the ground and excited states.

Polarity index Dye solvation Net polarityA during solvation

Dye Solvent Non-polar Polar ΔEsolv. vs k ΣEffect of polarity index, ΣASE ratio vs k

BPT DMSO Neutral Neutral

MeCN + Neutral

THF + +

2-MeTHF + +

Toluene −

**** BPT with these solvents tend to favour non-polar interactions (THF, 2- MeTHF) but most effective as a fluorescent dye in polar solvent and partially in non-polar solvent (see Fig. 5d).

BDT DMSO + Neutral

MeCN + Neutral

THF Neutral +

2-MeTHF + +

Toluene −

*** ** BDT with these solvents tend to favour non-polar interactions (THF, 2- MeTHF) but most effective as a fluorescent dye in polar solvent and partially in non-polar solvent (see Fig. 5d).

MPT DMSO + +

MeCN −

THF + +

2-MeTHF + +

Toluene −

**** ** MPT with these solvents tend to favour non-polar interactions (THF, 2- MeTHF) but most effective as a fluorescent dye in non-polar solvents and partially in polar solvent (see Fig. 5d).

MOT DMSO − Neutral

DMF − Neutral

MeCN + Neutral

THF + Neutral

* * MOT with these solvents tend to favour either polar or non-polar interactions (MeCN or THF). Nonetheless, the dye is most effective as a fluorescent dye in polar solvent (see Fig. 6d).

AMT DMSO + +

DMF −

MeCN + Neutral

THF Neutral Neutral

*** AMT with these solvents tend to favour polar interactions (DMSO, MeCN). Additionally, the dye has high absorption in polar solvents (see Fig. 6d).

BMT DMSO + +

DMF + −

MeCN Neutral Neutral

THF − Neutral

*** BMT with these solvents tend to favour polar interactions (DMSO, DMF). Additionally, the dye has high absorption in polar solvents (see Fig. 6d).

(Continued on next page)

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shift and ASE. This concept can be used for any type of dye that displays solvatochromic behaviours in a wide range of solvents, therefore the theoretical framework discussed should prove useful to researchers investigating the solva-tochromic properties of other classes of dyes for a range of applications. For the first class of triazine substituted dyes evaluated, BPT tend to favour non-polar interactions (THF, 2-MeTHF), but it is the most effective as a fluorescent dye in polar solvent and partially in non-polar solvent. BDT with these solvents tend to favour non-polar interactions (THF, 2-MeTHF), but is most effective as a fluorescent dye in polar solvent and partially in non-polar solvents. MPT with these solvents tend to favour non-polar interactions (THF, 2-MeTHF), but is most effective as a fluorescent dye in non-polar solvents and partially in polar solvent. For the other class of triazine substituted dyes evaluated, MOT with these solvents either favour polar or non-polar inter-actions (MeCN or THF). Nonetheless, the dye is most effec-tive as a fluorescent dye in polar solvent. AMT with these solvents tends to favour polar interactions (DMSO, MeCN). Additionally, the dye has a high absorption in polar solvents. BMT with these solvents tends to favour polar interactions (DMSO, DMF). Additionally, the dye has a high absorption in polar solvents. EOT with these solvents neither favour polar nor non-polar interactions (effect is neutral). Nonetheless, the dye is most effective as a fluo-rescent dye in polar solvent.

NOTE: The triazine substituted dyes have either been previously synthesised or modified and characterised.[40,41]

The wavelengths of the absorption and emission of the dyes were obtained using a Cary 50 Bio UV/Vis and Fluorolog 22 (Jobin Yvon Horiba) spectrophotometer, respectively. The

work reported in this study is original and is an extension of author’s published previous works.[25,39]

Abbreviations

Ego Associated solvation energy in the ground

state EE

1 Associated solvation energy in the excited state

ΔEsolv. Associated solvation energy ξ Polarization efficiency κ Dielectric constant

go Polarization red shift for absorption E1 Polarization red shift for emission

o Absorbance maxima of 2-methylthio-3- methyluracil in the vapour phase

go Wavelength of the absorption spectra for

solvated dye E1 Wavelength of the emission spectra for solv-

ated dye Ndye

1 Scaling factor used for the excited state of dyes

Ndye Scaling factor used for the ground state of dyes

ν Frequency of electronic transition in solution νo Frequency of electronic transition in vacuum νA Frequency of absorption νF Frequency of emission µO Dipole moment of excited state transitions µG Dipole moment of ground state transitions 2-MeTHF 2-Methyltetrahydrofuran 2-MT-3MU 2-Methylthio-3-methyluracil AM1 Austin Model 1

Table 4. (Continued)

Polarity index Dye solvation Net polarityA during solvation

Dye Solvent Non-polar Polar ΔEsolv. vs k ΣEffect of polarity index, ΣASE ratio vs k

EOT DMSO Neutral Neutral

DMF Neutral Neutral

MeCN Neutral Neutral

THF Neutral Neutral

EOT with these solvents neither favour polar or non-polar interactions (effect is neutral). Nonetheless, the dye is most effective as a fluorescent dye in polar solvent (see Fig. 6d).

ANet polarity was evaluated from the total number of positive transitions from HOMO to LUMO. It is the sum of the effects of (i) polarity changes of dye–solvent interactions and (ii) ASE ratio vs k. + represents the positive effect of (i) dye–solvent interactions under the polarity index columns evaluated from Fig. 3, 4; (ii) dye solvation under the net polarity solvation column evaluated from Fig. 5, 6. − represents the regions with negative polarity changes for the dye. **** represents the regions of highest polarity changes for the dye. *** represents the regions with the next high polarity changes for the dye. ** represents the regions with the lower polarity changes for the dye. * represents the regions with the least polarity changes for the dye. Neutral represents no polarity change.

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ASE Associated solvation energy DMF Dimethylformamide DMSO Dimethyl sulphoxide h Planck’s constant HOMO Highest occupied molecular orbital LUMO Lowest unoccupied molecular orbital MeCN Acetonitrile PM3 Parametric Model 3 PM6 Parametric Model 6 RsI Red shift index at the ground state RsI1 Red shift index at the excited state TDDFT Time-dependent density functional theory THF Tetrahydrofuran ZINDO Zerner’s Intermediate Neglect of Differential

Overlap

Supplementary material

Supplementary material is available online.

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Data availability. All data is either included in the manuscript or is available from the corresponding author on request.

Conflicts of interest. All authors declare no conflicts of interest.

Declaration of funding. The authors declare no funding.

Acknowledgements. Dr Kristine Kilså Jensen is thanked for the gift of the triazine derivative dyes and some of the absorption and emission data obtained from her previous work.[41]

Author affiliations ASchool of Environment and Science, Griffith University, Nathan Campus, Qld 4111, Australia. BEnvironmental Futures Research Institute, Griffith University, Nathan Campus, Qld 4111, Australia. CDepartment of Chemistry, Collin College, Preston Ridge Campus, Frisco, TX 75035, USA.

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