Enhanced Intramolecular Charge Transfer in New Type Donor

Subscriber access provided by University Libraries, University of Memphis

Article

Enhanced Intramolecular Charge Transfer in New Type Donor-Acceptor Substituted Perylenes Aleksey Aleksandrov Vasilev, Kurt De Mey, Inge Asselberghs, Koen Clays, Benoît R. Champagne, Silvia E. Angelova, Milena I. Spassova, Chen Li, and Klaus Mullen J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Oct 2012 Downloaded from http://pubs.acs.org on October 6, 2012

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

The Journal of Physical Chemistry

1 2 3 4 5 6

Enhanced Intramolecular Charge Transfer in New 7 8 9 10 1

Type Donor-Acceptor Substituted Perylenes

12 13 14 15 16

Aleksey A. Vasilev,†±* Kurt De Mey,§ Inge Asselberghs,§ Koen Clays,§ Benoît Champagne,# 17 18

Silvia E. Angelova,□ Milena I. Spassova,□ Chen Li,† Klaus Müllen†* 19 20 21 22 23 24 25 26 27 28 29

†

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

±

University of Sofia, Faculty of Chemistry, Department of Applied Organic Chemistry, James

Bourchier 1, 1164 Sofia, Bulgaria

30 31

§

32

Department of Chemistry, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven,

3

Belgium 34 35 36 37

#

38 39

Laboratoire de Chimie Théorique, Facultés Universitaires Notre-Dame de la Paix, rue de

Bruxelles, 61, B-5000 Namur, Belgium 40 41 42 □

43 4 45

Institute of Organic Chemistry with Centre of Phytochemisty,Bulgarian Academy of Sciences,

Sofia 1113, Bulgaria 46 47 48

KEYWORDS 49

perylene

dyes,

solvatochromism,

transition

energy,

nonlinear

optics,

50 51 52

intramolecular charge transfer (ICT)

53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

1


Page 2 of 31

1 2 3

ABSTRACT 4 5 6 7

Here we report the facile synthesis and physical characterization of new type N-(2,68 9

diisopropylphenyl)-3,4-perylenedicarboximides (PMI) with alkyl-substituted quinoline-4(1H)10 1 12

ylidenemethyl or acridine-9(10H)-ylidenemethyl units as strong donors in the 9-position. When

13 14

compared to parent PMI, these perylene dyes, 9-((1-methylquinoline-4(1H)-ylidene)methyl)15 16

PMI, 17

9-((1-benzylquinoline-4(1H)ylidene)methyl)-PMI,

9-((1-heptylquinoline-4(1H)-

18 19 20 21 22 23 24 25 26 27 28 29

ylidene)methyl)-PMI

and

9-((10-methylacridine-9(10H)-ylidene)methyl)-PMI,

show

a

pronounced bathochromic shift of their electronic absorption with solvatochromism because of their intramolecular charge transfer. The solvatochromic behavior of these dyes is further confirmed by second-order nonlinear optical experiments. Remarkably high second-order nonlinear optical values (βHRS up to 1300±50×10-30 esu at 880 nm in dichloromethane) are

30

obtained by femtosecond hyper-Rayleigh scattering. The one-step synthesis to-gether with the 31 32 3

spectroscopic, solvatochromic and nonlinear optical characteristics qualify these new type 34 35

perylene dyes as promising candidates for solvent polarity probes, photovoltaics or nonlinear 36 37 38

optical applications. 39 40 41 42

Introduction

43 4

Perylene dyes have been intensively investigated as functional materials of optoelectronics 45 46

owing to their versatile chemical functionalization and favorable physical properties.1 Donor and 47 48 49

acceptor substituted perylenes are particularly important compounds in terms of intramolecular 50 51 53

52

charge transfer (ICT) and photovoltaics.2 Recently, a series of novel push-pull type perylene sensitizers consisting of amino-substituted perylene anhydrides has been introduced by our

54 5

group.3 These dyes possess an excellent incident monochromatic photon-to-current conversion 57

56 58 59 60

ACS Paragon Plus Environment

2

Page 3 of 31


1 2 3

efficiency (IPCE) of up to 87 %, with an overall power conversion efficiency of up to 6.8 % 4 5

under standard AM 1.5 solar conditions.3 The stronger are the donor acceptor pairs in perylenes, 6 7 8

the more efficient is the intramolecular charge transfer, the higher is the orbital partitioning, and 9 10

consequently the lower are the charge recombination rates between electrons on the TiO2 and 1 12 13

holes on the dye cations.4,5 The interfacial electron transfer from the dye to TiO2 is known to be

14

strongly influenced by solvents,6 suggesting a further investigation of dyes with strong 15 16 17

solvatochromic behavior. According to the Franck-Condon principle (atoms do not change 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

position during light absorption) the excited-state solvent shell is not in equilibrium with the excited state of the dye molecule.7 Thus, the sign of the solvatochromism depends on the variation of the energy difference between the ground and excited states of the dyes. Therefore, the HOMO (the highest occupied molecular orbital) and the LUMO (the lowest unoccupied molecular orbital) energies of dyes, and thus the driving force for the light induced interfacial electron transfer, can be tuned not only by the substitution type, but also by the choice of

3 34

solvents. 35 36 37 38 39 40 41 42 43 4 45 46 47 48 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

3


Page 4 of 31

1 2 3

Scheme 1. Reagents, conditions and yields: (i) lepidine 1a or 9-methyl-acridine 1b and methyl 4 5

iodide or benzyl bromide or heptyl bromide, 145 °C, 2 h, yields: 1c 97 %, 1d 95 %, 1e 83 %, 1f 6 7 8

88 %. (ii) 9-bromo-N-(2,6-diisopropylphenyl)-3,4-perylenedicarboximide 2a (1 equiv.), 1c-1f (3 9 10

equiv.), N-ethyldiisopropylamine (6 equiv.), pyridine (12 equiv.), N- methylpyrrolidone (NMP), 1 12

130 °C, 2h, yields: 2b 46 %, 2c 52 %, 2d 41 %, 2e 57 %. 13 14 15 16

It is well known that the centrosymmetrical perylenebis(dicarboximide)s are not 17 18

solvatochromic or possess negligible solvatochromism. Zoon and Brower demonstrated that 919 20 21 22 23 24 25 26 27 28 29

aminosubstituted perylenemonoimides show moderate positive solvatochromism, resemble to that in merocyanine dyes.8 Recent studies reveal that the presence of a quinoidal fragment in the molecules of the NIR absorbing dyes leads to significant bathochromic shifts of their absorption.9,10 To achieve NIR absorbing perylene dyes with strong intramolecular charge transfer similar to that of the known donor-acceptor substituted perylene chromophores,4,10,11

30 31 32 3

we herein report

on quinoidal push-pull perylene monoimide dyes 2b-2e, which exhibit

34

pronounced ICT behavior and solvatochromism. In addition to the linear optical properties, 35 36 37

the second-order nonlinear 38 39

optical

(NLO)

responses

are

determined

by

the

hyper-

Rayleigh scattering (HRS) technique.12 HRS measures the second harmonic generation (SHG) 40 41 42 43 4

first hyperpolarizability [β(-2ω;ω,ω)], of which the amplitude can be correlated to the ICT.13 These results are analyzed in the light of Density Functional Theory (DFT) and ab initio

45 46

Molecular Orbital calculations.14 47 48 49

Experimental part 50 51 52

The solvents and chemicals used are commercial products and were used without further

54

53

purification. Column chromatography was performed on silica gel (Silicagel 60A, 0.06-0.2 mm, 56

5

Acros). Melting points were performed on a Büchi MP B-545 melting point apparatus and are 57 58 59 60 ACS Paragon Plus Environment

4

Page 5 of 31


1 2 3

not corrected. Infrared spectra were obtained on a Nicolet FT-IR 320. 1H and 13C NMR spectra 4 5

were recorded on Bruker DPX250 and Bruker DRX500 spectrometers. FD mass spectra were 6 7 8

recorded with a VG Instruments ZAB 2-SE-FPD apparatus. MALDI-TOF mass spectra were 9 10

recorded on a Bruker Reflex II-TOF spectrometer. UV/vis spectra were recorded on a Perkin1 12 13

Elmer Lambda 900 spectrophotometer in different solvents within concentrations of 2x10−5 M.

14 15

The photochemical stability was measured in dry toluene with irradiation at 254 nm in equal 16 17

intervals of 10 minutes. Fluorescence spectra were recorded on a Spex Fluorolog 3 spectrometer. 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Fluorescence quantum yields were determined by the relative method using Rhodamine 101 as a reference. Elemental analyses were performed on a Foss Heraeus Vario EL at the Institute for Organic Chemistry at the University of Mainz. Synthesis of quaternary lepidinium and 9-methylacrydinium salts (1c-1f): Lepidine (0.001 mol) or 9-methyl-acrydine (0.001 mol) and 0.002 mol of alkylhalide were added in a sealed tube and flushed with argon for 30 minutes. The reaction mixture was heated at 145 oC for two hours,

3 34

and then poured into 200 ml of acetone after cooling to room temperature. The resultant 35 36

precipitate was suction filtered, washed with acetone and dried in a desiccator. Synthesis of 37 38

perylene-merocyanine 39

dyes

(2b-2e):N-(2,6-Diisopropylphenyl)-9-bromoperylene-3,4-

40 41 42 43

dicarboximide 2a (0.001 mol) and 0.003 mol of the quaternary intermediates 1c-1f were added in a Schlenk flask and were dissolved in 20 ml NMP and flushed with argon. The reaction mixture

4 45

was heated to 130 oC and a mixture of 0.006 mol N-ethyldiisopropylamine and 0.012 mol 46 47 48

pyridine was added rapidly. After two hours at this temperature the reaction mixture was cooled 49 50

to room temperature and poured into 200 ml of a 3:1 mixture of distilled water:acetone. The 51 53

52

formed precipitate was suction filtered, washed with water, and air dried. The analytical samples 54 5 56 57 58 59 60 ACS Paragon Plus Environment

5


Page 6 of 31

1 2 3

were obtained after column chromatography on silica gel (methylene chloride/ethyl acetate = 10: 4 5

1). 6 7 8

Computational details 9 10

The molecular ground-state geometries were optimized using Becke's three parameter hybrid 1 12 13

exchange-correlation (XC) functional B3LYP15 and the 6-31G(d) basis. Local minima were

14 15

verified by establishing that the Hessians have zero negative eigenvalues. The vertical excitation 16 17

energies [ ∆E 18

0n

19 20 21 22 23 24 25 26 27 28 29 30 31 32

= hω0 n = h(ωn − ω0 ) ]

and oscillator strengths (f0n) were determined using the time-

dependent DFT (TDDFT) scheme with the 6-31G(d,p) basis set16 considering the first 10 lowest energy states. Additionally to B3LYP, the PBE017 functional was also employed. This choice is based on proofs of the validity of these functionals to determine the excitation energies and oscillator strengths of a broad range of organic compounds.18 Spectra were simulated by associating to each transition a 50/50 Gaussian/Lorentzian line shape having a height proportional to the oscillator strength and a full width at half-maximum (fwhm) of 0.2eV. In

3 34

order to take into account the solvent effect, the integral equation formalism (IEF) of the 35 36

polarizable continuum model (PCM)19 was employed for toluene, DCM, and DMSO solvents. 37 38

The time-dependent Hartree-Fock (TDHF) method with the 6-31G(d), 6-311G(d), and 639 40 41 42 43

31G+(d,p) basis sets was used to calculate the HRS first hyperpolarizability (βHRS) following the computational procedure described in Ref.20. To account for correlation effects, the second-

4 45

order Møller-Plesset (MP2) method was employed in combination with the finite field (FF) 46 47 48

procedure,21 implying a Romberg scheme to improve the accuracy of the numerical derivatives.22 49 50

FF/MP2 calculations have been shown to provide reliable first hyperpolarizabilities for organic 51 52

chromophores since they recover the largest part of the electron correlation effects.23 To account 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

6

Page 7 of 31


1 2 3

for frequency dispersion at the MP2 level, the multiplicative correction scheme was applied.24 It 4 5

consists in multiplying the static MP2 value by the TDHF/CPHF ratio: 6 7 8

(

9

)

(

)

β MP 2 −2ω ; ω , ω ≈ β MP 2 0; 0, 0 ×

10

(

βTDHF −2ω ; ω , ω

(

β CPHF 0; 0, 0

)

) (1)

1 12 13 14

Moreover, the solvent effects on the first hyperpolarizabilities are determined by inserting in Eq. 1 β values evaluated within the IEFPCM scheme.19 Full expressions of βHRS, without

15 16 17

assuming Kleinman conditions, were employed within the B convention. More details about the 18 19 20 21 22 23 24 25 26 27 28 29

computational procedure can be found in Refs.14 and therein. All calculations were performed using Gaussian0325 as well as homemade codes to carry out the FF Romberg differentiations. Results and Discussion Synthesis. The synthesis of the target perylene dyes requires two building blocks: the known

30 31 32 3

N-(2,6-diisopropylphenyl)-9-bromo-3,4-perylenedicarboximide (2a) and the cyanine precursors (1c-1e). Using a modified reaction procedure,26 lepidine (1a) or 9-methylacridine (1b) were

34 35 36

quaternized with methyl iodide, benzyl bromide or heptyl iodide to afford the known quaternary 37 38

salts (1c-1f)27-29 (Scheme 1) and the new compound 1e in high yields. 39 40

Many condensation methods have been described for the synthesis of cyanine dyes.30-35 Some 41 42

have significant drawbacks such as the evolution of methyl thiol,30 while other methods were not 43 4 45

appropriate for our purposes due to the absence of suitable functional groups on the N-(2,646 47

diisopropylphenyl)-3,4-perylenedicarboximide (PMI) moiety.31-35 48

This has prompted us to

49 50

search for new reaction conditions leading to the condensation of 2a and the quaternary salts 1c51 53

52

1f (Scheme 1). Our attempts to perform the condensation reaction with only N-

54

ethyldiisopropylamine (Huenig’s base) without pyridine were unsuccessful. In this case only by5 57

56

products (via self-condensation) were detected.36 The addition of a sterically less hindered base 58 59 60 ACS Paragon Plus Environment

7


Page 8 of 31

1 2 3

(e.g. pyridine) successfully led to the desired condensation. The dyes 2b-2e were finally obtained 4 5

by the reaction of 2a with the appropriate quaternary salts 1c-1f in N- methylpyrrolidone (NMP) 6 7 8

in the presence of N-ethyldiisopropylamine with pyridine as additional base (Scheme 1). An 9 10

excess of the quaternary salts was necessary due to the formation of the self-condensation by1 12

products (blue cyanine dyes) which could easily be removed by column chromatography and 13 14 15

subsequent recrystallization from acetone/water. Performing the reaction at temperatures below 16 17

120 ºC gave rise only to the cyanine by-products. Increasing both the reaction temperature 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

(above 140 ºC) and the reaction time (more than one hour) led to the partial decomposition of the target dyes and lower reaction yields. The chemical structures of the novel dyes 2b-2e were proven by NMR, UV-VIS and IR spectroscopy, by FD-MS and MALDI-TOF spectrometry, as well as by elemental analysis. The singlet between 6.73 and 6.89 ppm in all 1H-NMR spectra arises from the proton of the monomethine group.28

3 34

Photophysical properties. The dyes 2b, 2c and 2e are insoluble in alkanes, poorly soluble in 35 36

acetonitrile (except dye 2d), moderately soluble in alcohols, and well soluble in toluene, 37 38

dichloromethane (DCM), DMF and DMSO. Dye 2d shows very good solubility in all solvents 39 40 41 42 43

used in the present study. The photostability of dye 2d in dry DMF is much higher than that of 1methyl-4-[(4-oxocyclohexadienylidene)-ethylidene]-1,4-dihydropyridine

4

(the

Brooker’s

45

merocyanine - BM (Scheme 2, Figure 1), which finding could be assigned to the conjugative 46 47 48

effect of the large aromatic perylene core. 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

8

Page 9 of 31


1 2 3

Scheme 2. Structures of dyes BM and MeQMBr2.37 4 5 6

The photostability of dye 2e appears to be more enhanced in dry toluene and with irradiation at 7 8

254 nm than that of 2b-2d. The absorption intensity is only decreased by 20 % after 24 hours. 9 10 1 12 13

The absorption behavior of dye 2e in a toluene solution remained unchanged in sunlight after three weeks.

14 15 16

BM 2d

17 18 100

19

80

Abs. (%)

20 21 22 23 24 25 26 27 28 29

60

40

20

30 31 32

0 0

3

10

20

30

40

50

Time (min)

34 35 36

Figure 1. Photostabilty of Brooker’s merocyanine (BM) and 2d. 37 38 39

The longest absorption wavelengths of the dyes 2b-2e occur in the range of 572-748 nm with 40 41 42

molar absorptivities between 15000 and 20000 L mol-1 cm-1 (Table 1). The broadened absorption

43 4

bands already suggest an ICT effect.36-42 Additionally, the dyes 2b-2d exhibit strong absorptions 45 46

in the NIR region between 711 and 748 nm in DMSO (bathochromic shift of 200 nm as 47 48 49

compared to 2a, Figure 2).. 50 51 52

Table 1. Absorption maxima and molar absorptivities of dyes 2b-2e, BM and its analog

53

MeQMBr2 in solvents with varying polarities. 54 5 56 57 58 59 60 ACS Paragon Plus Environment

9


Page 10 of 31

1 2 3 4 5 6

Solvent (polarity index)7c

λmax (nm) (ε/L•mol-1•cm-1) 2b

2c

Toluene (2.4)

660 (19720)

650 (19860) 690 (19030) 574 (18380) no data

699

DCM (3.1)

689 (15800)

677 (20104) 709 (20106) 572 (17880) no data

671

i-PrOH (3.9)

not soluble

not soluble

CHCl3 (4.1)

bad solubility 669 (17050) 686 (12278) not soluble

2d

2e

MeQMBr237

BM

7 8 9 10 1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

711 (11332) not soluble

54639

619

62040; 61841

687

61440

691

Dioxane (4.8) not soluble

648 (22060) 663 (18570) not soluble

MeOH (5.1)

697 (10600)

698 (14480) 717 (13809) 572 (14540) 48341; 49040

560

EtOH (5.2)

not soluble

695 (17450) 716 (18344) 573 (15780) 51440

591

DMF (6.4)

715 (15000)

692 (16020) 718 (16200) 579 (15720) 58342

630

DMSO (7.2)

731 (15200)

711 (20200) 748 (18140) 579 (17800) 57240

626

The UV-Vis absorption spectra of dyes 2b-2d show a pronounced positive solvatochromism

30 31 32 3

(Table 1, Figure 2). The difference between the absorption maxima (∆λmax) in toluene and DMSO is +71 nm ( ∆ υ~ = 1472 cm-1) for the dye 2b, +61 nm ( ∆ υ~ = 1320 cm-1) for 2c, +58 nm (

34 35

∆ υ~ = 1124 cm-1) for 2d, and only +5 nm ( ∆ υ~ = 150 cm-1) for 2e (Table 1, Figure 2). The ∆λmax 36 37 38

values from dioxane to DMSO are +63 nm ( ∆ υ~ = 1367 cm-1) for 2c, +85 nm ( ∆ υ~ = 1714 cm-1) 39 40

for 2d. (Table 1, Figure 2). Dye 2c exhibits a blue color in toluene (Figure 2) and a green one in 41 42 43

dichloromethane, DMF and DMSO solutions. In protic solvents (methanol and ethanol) dyes 2b4 45

2d slowly undergo a color change from green (absorption at 697-717 nm) to red (absorption at 46 47

485 nm), presumably due to the protonation of the carbon atom of the monomethine group. The 48 49 50

color is fully changed after five hours in methanol and after four days in ethanol. Comparison 51 53

52

between the long wavelength absorption maxima of dyes 2b-2d and those of dyes BM and its

5

54

derivative MeQMBr2 (Table 1) in various polar solvents shows that the

solvatochromic

57

56

behavior of the perylene dyes is more pronounced for aprotic than for protic solvents, while 58 59 60 ACS Paragon Plus Environment

10

Page 11 of 31


1 2 3

conversely the merocyanine derivatives are more solvatochromic in alcohols than in aprotic 4 5

solvents. 6 7 8 9 10

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

(650) Toluene (677) DCM (692) DMF (711) DMSO (695) EtOH

1.0 0.8 0.6 0.4 0.2 0.0 400

Normalized absorption

12

Normalized absorption

1 1.0

(485) MeOH 5h (507) DCM acid (668) DCM base (698, 485) MeOH 1h

0.8 0.6 0.4 0.2 0.0

500

600

700

800

900

400

500

Wavelength (nm)

600

700

800

900

Wavelength (nm)

Figure 2. Normalized absorption spectra of dye 2c in solvents with different polarities. A change in the electron donor strength affects the HOMO and the LUMO levels of the dye and, consequently, the absorption properties.4,38 This effect is illustrated in Table 1, as the

31 32 3

absorption maxima of the benzyl derivative 2c are blue- shifted in comparison with those of the 34 35

alkyl derivatives 2b and 2d. This means that even an inductive effect of the N-alkyl-substituents 36 37

has a significant influence on the electron-donating properties of the nitrogen atom in the 1,438 39 40

dihydroquinoline end group and this in turn exerts an influence on the longest wavelength 41 42 43

absorption maxima. Additionally, we observed an effect of the alkyl substituent in the quinoline

4

end group on the chemical stability of these dyes. Dyes 2c-2e are more stable than dye 2b in the 45 46 47

presence of oxygen and acid or basic reagents. Consequently, the substituents at the quinoline 48 49

part of the chromophore protect the nitrogen atom from oxidation and provide a higher chemical 50 51 52

stability. This seems to be due both to the increased push-pull effect as well as to the steric

54

53

hindrance.10 5 56 57 58 59 60 ACS Paragon Plus Environment

11


Page 12 of 31

1 2 3

Table 2. ET(probe) (kcal mol-1) from experimental data (calculated for each dye in different 4 5

solvents according to Equation 2), theoretical results (in parentheses) and literature data35,37-40 for 6 7 8

dyes BM and MeQMBr2. 9 10 1 12

Solvent (εr)

13

2b

2c

2d

2e

BM

MeQMBr2

Dioxane (2.21)

Not soluble

44.1

43.1

not soluble

46.6

41.4

Toluene (2.4)

43.3 (41.0)

44.0 (41.2)

41.4 (40.7) 49.8 (44.2)

no data

40.9

CHCl3 (4.8)

Bad solubility

42.7

41.7

not soluble

46.1

41.6

DCM (8.9)

41.5

42.2

40.3

50.0

no data

42.6

i-PrOH (19.9)

Not soluble

not soluble

40.2

not soluble

52.4

46.2

EtOH (24.6)

Not soluble

41.1

39.9

49.9

55.6

48.4

MeOH (32.7)

41.0

41.0

39.9

50.0

59.0

51.1

DMF (37.8)

40.0

41.3

39.8

49.4

49.0

45.4

DMSO (46.5)

39.1 (39.5)

40.2 (39.7)

38.2 (39.1) 49.4 (42.9)

50.0

45.7

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

ET(probe) (kcal mol-1)

30 31 32 3 34

An addition of small amounts of hydrochloric acid to the dichloromethane solution of 2c-2e leads to the protonation of the monomethine carbon atom in 9-position, which suppresses the

35 36

electron-donating effect and thus only the Soret band is observed (Figure 2). Subsequent addition 37 38 39

of triethylamine to the same solutions results in a full reversal to the quinoidal structure (Figure 40 41 42

2). The same phenomenon occurs with fluorescence: upon protonation, dye 2d (with a

43 4

fluorescence quantum yield of 4 %) undergoes an enhancement of fluorescence quantum yield to 45 46

76 % (Figure 3) suggesting a suppression of the electron-donating effect of the quinoline group 47 48

upon acidification. 49 50 51 52 53 54 5 56 57 58 59 60 ACS Paragon Plus Environment

12

Page 13 of 31


1 2 3

6 7 8 9 10 1 12 13 14 15 16

ex 500 ex 690 abs 714 abs 506 (HCl) ex 490 (HCl)

1.0 0.8

10 8 6

0.6

4

0.4

2

0.2 0.0 400

0 500

600

700

800

900

Wavelength (nm)

17

Normalized fluorescence (a.u.)

5

Normalized absorption (a.u.)

4

18 19 20 21 22 23 24 25 26 27 28 29 30

Figure 3. Absorption and emission spectra of 1×10-6 M ethanol solution of dye 2d. The reason for the observed acido(fluoro)chromism was specified by addition of hydrochloric acid to the DMSO-d6 solution of dye 2e. The appearance of a new signal in the 1H-NMR spectra with integral intensity for two protons at 5.96 ppm corresponding to the newly formed methylene group and decrease of the intensity of the signal for the monomethine proton fully confirms the

31 32

above mentioned suggestion (Figure S6). The absorption maxima of 2b-2d in different solutions

3 34

exhibit a positive solvatochromic behavior. Therefore, the dipole moments of the solute increase 35 36

during the electronic transition, which indicates that the Franck-Condon excited state of the 37 38

solute is formed in a solvent cage of already partly oriented solvent dipoles. Herein, the 39 40 41 42 43

empirical solvent polarity parameter, ET was calculated for each dye in different solvents, according to Equation (2).7

4 45 46

ET/(kcal mol-1)=hc υ~ NA =2.8591 υ~ /(cm-1)= 47 48

28591

λmax / nm

(2)

49 50

The ET values (Table 2) quantify the above analysis and reveal a significant change of the 51 53

52

molar transition energies of dyes 2b-2d in solvents of different polarity. With increasing solvent

54

permittivity (i.e. dielectric constant εr), a subsequent decrease of the ET was detected for dyes 2b5 56 57 58 59 60 ACS Paragon Plus Environment

13


Page 14 of 31

1 2 3

2d. More importantly, in the case of dye 2c, the decrease of the transition energies has been 4 5

observed even in protic solvents (i.e. methanol and ethanol).6 6 7 8

As described in Table 1 and Table 2, the acridinium derivative 2e is not solvatochromic. A 9 10

plausible explanation is the strong twist of the acridinium moiety with respect to the perylene 1 12

core as supported by the DFT geometry optimization (vide supra, Figure 4). This finding is also 13 14 15

related to the absorption maximum of 2e, which occurs at a shorter wavelength than those of 2b16 17

2d. 18

30 31 32 3

1.0

B3LYP PBE0 experiment

2c in toluene

0.8

0.6

0.4

0.2

0.0 300

400

500

600

700

800

900

Normilized absorbance, arb.units

20 21 22 23 24 25 26 27 28 29

Normilized absorbance, arb.units

19

1.0

PBE0 B3LYP experiment

2c in DMSO

0.8

0.6

0.4

0.2

0.0 300

400

Wavelength, nm

500

600

700

800

900

1000

Wavelength, nm

34 35 36 37

Figure 4. Experimental and TDDFT/6-31G(d,p) absorption spectra of 2c in toluene and DMSO. 38 39 40

It is known that high ET(30) values correspond to high solvent polarity.7c Additionally, 41 42 43

ET(probe) could be defined as the molar electronic transition energy of the dissolved dye.7c The

4 45

most prominent feature of 2b-2d is solvatochromism. Table 2 exhibits the correlation of 46 47

ET(probe) of dyes 2b-2e, BM and MeQMBr237,39-42 and εr of the solvents investigated. The 48 49

ET(probe) values for the new dyes 2b-2d decrease with increasing dielectric constant (Table 2). 50 51

54

53

52

This suggests that an enhanced stability of the excited states of dyes 2b-2d in polar solvents as compared to nonpolar ones is achieved.

5 56 57 58 59 60

ACS Paragon Plus Environment

14

Page 15 of 31


1 2 3

Additionally, the better stabilization of the excited states relative to the ground states with 4 5

increasing solvent polarity results in a bathochromic shift. 6 7 8

Furthermore, the ∆λmax values in solvents with different polarities (Table 1 and Table 2) are 9 10

quite similar to those of 1-methyl-4-[(4-oxocyclohexadienylidene)ethylidene]-1,4-dihyd1 12 13

ropyridine BM,7 which is one of the strongest solvatochromic dyes. The ICT can be additionally

14

observed in the 1H-NMR spectra of dye 2c in solvents with different polarities (Table 3). The 15 16 17

signal of the NCH2Ph protons is shifted to lower fields in more polar solvents, which is in 18 19 20 21 22 23 24 25 26 27 28 29

agreement with the results described above.

30

Toluene-d8

CDCl3

CD2Cl2

DMF-d7

DMSO-d6

32

Parameter εr

2.4

4.8

8.9

37.8

46.5

3

AN

-

20.4

23.1

16.0

19.3

Enhanced Intramolecular Charge Transfer in New Type Donor

Recommend Documents