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Dyes and Pigments 105 (2014) 208e215

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Electronic properties of indan-1,3-dione-carbazole-based compoundsrevealed by time resolved spectroscopy

Renata Karpicz a,*, Maryt _e Da�skevi�cien _e b, Vytautas Getautis b, Alytis Gruodis a,c,Vidmantas Gulbinas a,c

aCenter for Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, LithuaniabDepartment of Organic Chemistry, Kaunas University of Technology, Radvilenu Plentas 19, LT-50270 Kaunas, LithuaniacDepartment of General Physics and Spectroscopy, Vilnius University, Saul _etekio 9-III, LT-10222 Vilnius, Lithuania

a r t i c l e i n f o

Article history:Received 26 September 2013Received in revised form24 January 2014Accepted 11 February 2014Available online 19 February 2014

Keywords:Indan-1,3-dione compoundCarbazoleFluorescenceTime resolved spectroscopyIsomerizationQuantum chemical calculations

* Corresponding author.E-mail addresses: renata.karpicz@ftmc.lt, renata@a

http://dx.doi.org/10.1016/j.dyepig.2014.02.0110143-7208/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Two new indan-1,3-dione and carbazole-based (IDC) compounds with single and four covalently con-nected chromophores were synthesized and their optical properties were investigated in solutions and insolid films with the emphasis on the interchromophore interactions. Because of molecule twisting,opening transitions to np* states, IDC chromophores experience ultrafast excited state relaxation withthe time constant of about 2 ps in solutions and several times slower solid films. Chromophoreconnection into quadruplicate molecules only weakly influence their spectroscopic properties in solu-tions, however prevents H-type aggregate formation and concomitant spectral changes in solid films.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Small molecules, forming glasses in solid state, are veryperspective for electronic applications, since formation of molec-ular layers may be often performed by vacuum evaporation and byprocessing from solutions, which not only gives technologicalflexibility, but also allows better manipulation of the layermorphology. Materials for organic electronics shall meet manyrequirements for mechanical, optical and electronic properties,which are often difficult to combine. Therefore, current state of theart molecular compounds are often composed of several buildingblocks responsible for one or another material functionality.

Carbazole, which is a strong electron-donating chromophore[1e13] is one of the most common building blocks, particularly indesign of molecular compounds where intramolecular or inter-molecular charge transfer is desirable. Indan-1,3-dione moiety is avery strong electron acceptor [10,14e19] and is also quite commonin materials for molecular electronics, in particular as an electron-accepting moiety of so-called pushepull molecules where intra-molecular charge transfer takes place under the molecule excita-tion. Electrooptically active materials containing such pushepull

r.fi.lt (R. Karpicz).

molecules are currently being investigated for their application inlight manipulation devices for optoelectronics and optical infor-mation processing [9,20]. Such donor-acceptor compounds enablecreation of materials possessing high bipolar, electron and holemobilities [21]. Because of the presence of absorption bands in thevisible spectral range, optical properties of indan-1,3-dione/carba-zole materials may be controlled by light of the visible spectralrange, convenient for laser sources, eg. these molecules may besubject to alleoptical orientation [22,23].

In this paper we report synthesis of new indan-1,3-dione andcarbazole-based (IDC) compounds with one and four covalentlyconnected chromophore units. We denominate the latter ones asquadruplicate compounds. Such connection of relatively smallmolecules into large compounds, prevents material crystallizationin solid films, but may also influence their optoelectronic proper-ties, and definitely should change their translational and rotationaldiffusion. The investigated compounds form well-defined molec-ular glasses. The existence of several diastereoisomers, the possi-bility of intermolecular hydrogen bonding and flexibility ofaliphatic linking chains ensure high morphological stability ofthese glasses. Another feature of these compounds is the presenceof two hydroxyl groups in the molecule. This improves adhesionand compatibility with various binders and they can be chemicallycrosslinked in the layer by reaction of the hydroxyl groups with

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215 209

polyisocyanates [24]. Very fast excited state relaxation, as will bediscussed in the current paper, suggests application of the inves-tigated compounds in high speed electronic devices, or in nonlinearoptics, for example, as fast saturable absorbers. We characterizeoptical properties and excited state dynamics of IDC compoundsand show that chromophore units weakly interact in the quadru-plicate compound, and branched structure of this compound pre-vents formation of crystallites and excitonicaly coupled aggregatespecies in solid films.

2. Materials and methods

2.1. Synthesis of materials

9-ethyl-3-carbazolecarboxaldehyde, indan-1,3-dione were pur-chased from Aldrich Chemical Co. and were used without purifi-cation. Bis{4-[6-(3-formylcarbazol-9-methyl)-7-(3-formylcarbazol-9-yl)-3-hydroxy-5-oxa-1-thiaheptyl]phenyl}sulphide was synthe-sized earlier according to the method described in [25].

2.1.1. 9-ethyl-3-(1,3-dioxoindan-2-ylmethylene)carbazole (IDC1,C24H17NO2)

5.58 g (0.025 mol) of 9-ethyl-3-carbazolecarboxaldehyde and3.65 g (0.025) of indan-1,3-dione were stirred in 70ml of ethanol atroom temperature overnight. The crystalline product that had beenformed was filtered off, washed with ethanol and recrystallizedfrom toluene. Yield 6.0 g (68.3%); IR (KBr): n ¼ 3069, 3049, 3000(CHarom), 2958, 2917, 2861 (CHaliph), 1715, 1675 (C]O) cm�1; 1HNMR (CDCl3, 300 MHz): d¼ 9.34 (1H, s, CH of methylene), 8.59 (1H,

NO

N

SR

S

NO

N

SHO

S

O

O

O

O

NO

N

SR

S

1, 2

aO

O

3,4

b,c,d

IDC4

Scheme 1. Synthesis of IDC4 possessing four indan-1,3-dione moieties. (a) acetic anhydride,dione, EtOH, r.t.

d, J ¼ 8.7 Hz, H-2 of carbazole), 8.18 (1H, d, J ¼ 7.7 Hz, H-5 ofcarbazole), 8.00e7.22 (9H, m, Ar), 4.31 (2H, q, J ¼ 7.2 Hz, CH2CH3),1.43 (3H, t, J ¼ 7.2 Hz, CH2CH3) ppm. Anal. Calcd for C24H17NO2: C,82.03; H, 4.88; N, 3.99. Found, %: C, 81.85; H, 4.60; N, 3.64.

2.1.2. Bis{4-[6-(3-(1,3-dioxoindan-2-ylmethylene)carbazol-9-methyl)-7-(3-(1,3-dioxoindan-2-yl-methylene)carbazol-9-yl)-3-hydroxy-5-oxa-1-thiaheptyl]phenyl}sulphide (IDC4, C112,H78,N4,S3,O12)

1.26 g (0.001 mol) of bis{4-[6-(3-formylcarbazol-9-methyl)-7-(3-formylcarbazol-9-yl)-3-hydroxy-5-oxa-1-thiaheptyl]phenyl}sulphide and 0.73 g (0.005) of indan-1,3-dione were stirred in themixture of 30 ml of ethanol and 15 ml of chloroform at roomtemperature overnight. After termination of the reaction solventswere removed and the residue was purified by column chroma-tography (eluent: acetone:n-hexane ¼ 1:4). The obtained com-pound was isolated as solid product. Yield 0.87 g (49.2%); IR (KBr):n¼ 3436 (OH), 3051 (CHarom), 2927, 2917 (CHaliph),1715,1679 (C]O)cm�1. 1H NMR (CDCl3, 300 MHz): d ¼ 9.45e9.14 (4H, m, CH ofmethylene), 8.67e6.67 (52 H, m, Ar), 4.64e3.83 (12H, m, NeCH2CHCH2eN, CHOH), 3.35e2.68 (4H, m, OCH2), 2.60e2.21 (4H, m,SeCH2),1.85e1.60 (2H,m, OH) ppm. Anal. Calcd for C112H78N4S3O12:C, 76.08; H, 4.45; N, 3.17. Found, %: C, 75.75; H, 4.12; N, 2.79.

To synthesize the desired structure IDC4, possessing four indan-1,3-dione chromophores, we have selected a strategy in which themain step included the preparation of an intermediate 4 startingfrom readily available bis{4-[6-(3-formylcarbazol-9-methyl)-7-(3-formylcarbazol-9-yl)-3-hydroxy-5-oxa-1-thiaheptyl]phenyl}sul-phide (1) [26] (Scheme 1).

S OR

N

N

S OOH

N

N

O

O

O

O

S OR

N

N

O

O

1: R = OH2: R = COCH3

3: R = COCH34: R = OH

pyridine, 50 �C; (b) POCI3/DMF, 90e95 �C; (c) 85% KOH, acetone/H2O, b.t.; (d) indan-1,3-

Scheme 2. Synthesis route to IDC1.

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215210

At first, in order to protect hydroxyl groups, compound 1 wasconverted to acetyl derivative 2. The next step was a Vilsmeierformylation followed by deprotection of the hydroxyl groups of theresulting 3 to get the key intermediate 4, possessing four aldehydegroups in each carbazolyl chromophore. Finally, by condensation 4with indan-1,3-dione, the desired IDC structure IDC4 was isolated.

The parent derivative IDC1 comprising one covalently con-nected carbazolyl and indan-1,3-dione chromophore unit wassynthesized by interaction of 9-ethyl-3-carbazolecarboxaldehydewith indan-1,3-dione (Scheme 2).

The course of the reactions products were monitored by TLCALUGRAM� SIL/UV254 plates (eluent: acetone:n-hexane¼ 7:18) anddevelopment with I2 or UV light. Silica gel (grade 62, 60e200mesh,150 �A, Aldrich) was used for column chromatography. Elementalanalyses (C, H, and N) were conducted using the ElementalAnalyzer CE-44; their results agreed satisfactorily with the calcu-lated values. Melting points were determined in capillary tubes oncapillary melting point apparatus MEL-TEMP. The 1H NMR spectrawere taken on Varian Unity Inova (300 MHz) spectrometer inCDCl3. The IR spectra were taken for samples in KBr pellets on aTensor27 IR System spectrometer.

2.2. Characterization of materials

The 1H NMR spectra were taken on Varian Unity Inova(300 MHz) spectrometer in CDCl3. The IR spectra were taken forsamples in KBr pellets on a Tensor27 IR System spectrometer. Thecourse of the reactions products were monitored by TLCALUGRAM� SIL/UV254 plates (eluent: acetone:n-hexane¼ 7:18) anddevelopment with I2 or UV light. Silica gel (grade 62, 60e200mesh,150 �A, Aldrich) was used for column chromatography. Elementalanalyses (C, H, and N) were conducted using the ElementalAnalyzer CE-44; their results agreed satisfactorily with the calcu-lated values. Melting points were determined in capillary tubes oncapillary melting point apparatus MEL-TEMP. The differentialscanning calorimetry (DSC) measurements were carried out on a

50 100 150 200

a)Tg = 136ºC

Tg = 139ºC

2nd heating

1st heating

T/ºC

< en

do /

egzo

>

< en

do /

egzo

>

Fig. 1. DSC curves of IDC4 (a) and IDC1 (b).

Mettler DSC 30 calorimeter at a scan rate of 10 �C/min under ni-trogen atmosphere.

The formation of a glassy state for IDC4 was confirmed by DSCanalysis. These investigations revealed that IDC possessing fourindan-1,3-dione was found only in the amorphous phase (seeFig. 1(a)). No crystallization took place at first heating of IDC4, onlyglassing temperature (Tg) was revealed at 136 �C during the secondheating. In contrary, the endothermic melting peak was observedfor the synthesized IDC compound with one indan-1,3-dionemoiety in the first heating of DSC scan. The DSC curve of IDC1 ob-tained in the second heating run shows not only a glass transition at68 �C but also crystallization at 133 �C and then melting (Fig. 1(b))at 220 �C.

2.3. Sample preparation

Dilute solutions of the investigated IDC compounds were pre-pared by dissolving them in different organic solvents at 10�5 Mconcentration. Neat films of the studied compounds were depos-ited on glass substrates from chloroform solutions by spin-coatingtechnique. The glass substrates were cleaned in chloroform andtreatment with oxygen plasma before the deposition of organiclayers. The solutions of 8 mg/ml concentrationwere spin-coated onsubstrates for 5 s at 600 rpm and for 30 s at 1000 rpm. The samplethickness was of about 300 nm. The obtained IDC4 films had quite agood optical quality. However, optical quality of IDC1 films wasmuch worse, squire and needle shape crystallites of up to severalmm in size were observed with fluorescence microscope (Fig. 2).The low-molecular-weight materials possessing indan-1,3-dionemoieties are prone to crystallization [16]. Similar crystallites wereobserved and for another indan-1,3-dione compound [27].

2.4. Instrumentation

Absorption spectra of investigated IDC materials waremeasured by using the Jasco V670 spectrophotometer. Fluores-cence spectra and fluorescence transients were measured usingEdinburg Instruments Fluorescence Spectrometer F900. Semi-conductor diode lasers EPLe375 and EPL-470 emitting 50 psduration pulses at 375 and 470 nm, respectively, with an averagepower of 0.15 mW/mm2 was used in the transient measure-ments. The pulse repetition rate was 5 MHz and the time reso-lution of the setup was of about 100 ps taking into accounttemporal deconvolution procedure. All fluorescence spectra werecorrected for the instrument sensitivity.

Transient absorption spectra and decay kinetics of the studiedIDC compounds have been investigated by using the pump-probe

50 100 150 200 250

2nd heating Tc = 133ºC

Tg = 68ºC

T/ºC

1st heating

Tm = 220ºC

b)

Heating rate 10 �C/min, N2 atmosphere.

Fig. 2. Fluorescence microscope images of IDC1 (a) and IDC4 (b) neat films in 605e685 nm spectral region.

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215 211

technique with a femtosecond time resolution. The optical densityat the maximum of the lowest energy absorption band wasapproximately 0.5 for solutions in 1-mm thick cuvette and about0.3 for neat films.

Transient absorption measurements were performed by meansof conventional femtosecond pump-probe absorption spectrom-eter built on the basis of the Quantronix Ti:Sapphire laser Integra-Crunning at 1 kHz repetition. Duration of the laser pulses was of130 fs. Excitation pulses at 400 nm were obtained by frequencydoubling of the fundamental laser radiation, while parametricamplifier TOPAS-C pumped by the femtosecond laser has been usedfor the sample excitation (200 mW average optical power at420 nm). Sample probing was performed by a white light contin-uum generated in a LiF plate. The probe light transmitted throughthe sample was recorded by Avantes AvaSpec-1650F spectrograph.Transient differential absorption spectra have been determinedfrom the probe light spectra recorded with and without excitation.Transient absorption spectra at different delay times have beenrecorded simultaneously within the entire investigated spectralrange. Later the detected spectrawere numerically corrected for thegroup velocity dispersion of the probe light. Transient absorptionkinetics at specified probe wavelength has been extracted from thearray of the transient absorption spectra measured at multipledelay times.

0.5

1.0

0.5

1.0

N

a

Flu

ores

cenc

e, a

rb.u

.

Abs

orpt

ion,

arb

.u.

IDC1 in toluene neat film

2.5. Quantum chemical calculations

Quantum chemical simulations have been provided usingGaussian09 package [28]. Optimization of the ground state mo-lecular geometry was done by means of density functional B3LYPmethod in 6/311G basis set without polarization functions. Elec-tronic excitation energies for “freezed” structures were calculatedusing ZINDO functional (singlets only).

0.0 0.0

300 400 500 600 700 8000.0

0.5

1.0

0.0

0.5

1.0b IDC4 in toluene neat film

Flu

ores

cenc

e, a

rb.u

.

Abs

orpt

ion,

arb

.u.

Wavelength, nm

Fig. 3. Absorption and fluorescence spectra of IDC1 (a) and IDC4 (b) compounds indilute toluene solution (solid lines) and neat films (dashed lines).

3. Results

Optical properties of these IDC compounds were investigated invarious solvents and in neat films. Absorption and fluorescencespectra of IDC compounds in dilute toluene solutions and in neatfilms are shown in Fig. 3. Spectroscopic data of IDC compounds inother solvents are summarized in Table 1. Both compounds havestrong absorption bands in the 400e500 nm spectral range. Theirabsorption spectra in toluene solutions are very similar to those ofother donor-acceptor indan-1,3-dione compounds in weakly polarsolvents [10,17,18,29,30]. Similar spectra have been reported forother carbazole-containing donor-acceptor-type conjugated mate-rials [1,9e11,31]. Thus, the long wavelength absorption band shouldbe attributed to the intramolecular charge transfer states formed byelectron-accepting indan-1,3-dione and electron-donating carba-zole moieties. The long wavelength absorption band of IDC1 solid

film is splitted into two components separated by about 30 nm. Thisband of the IDC4 film is broadened and red shifted, however showsno clear splitting.

Fluorescence spectra of both studied IDC compounds showrelatively small Stokes shift in toluene, however it significantlyincreases in more polar solvents (see Table 1). This is in agreementwith the suggested charge transfer character of the lowest energyelectronic transition. Fluorescence spectra of solid films arestrongly shifted to the long wavelength side. The dramatic shift ofthe fluorescence spectra suggests that some strongly interactingaggregate or crystalline species are formed in films. The particularlylarge Stokes shift of about 140 nm of the IDC1 film fluorescenceshould be attributed to the strongly fluorescing crystallitesobserved in microscopic images (Fig. 2). The long wavelength tail ofthe fluorescence band of IDC4 solution in toluene, which closelyresembles the longwavelength part of the film fluorescence, shouldbe also attributed to the residual of nondissolved solid particles, orto large aggregates, remaining because of the bad solubility of thiscompound.

We also found that the fluorescence Stokes shift of both com-pounds in solvents with similar polarity slightly decreases with thesolvent viscosity (from 83 nm in ethanol to 68 nm in octhanol). It

Table 1Absorption and fluorescence peaks and the fluorescence lifetimes of IDC1 and IDC4compounds in dilute (10e5 M) solutions and neat film.

labs (nm) lfl (nm) s1 (ns) s2 (ns) s3 (ns) save (ns)

IDC1 in toluene 451 494 �0.05[89%]

1.3 [11%] 0.18

inchloroform

458 510 �0.05[87%]

0.24 [13%] 0.07

in ethanol 454 537 �0.05[79%]

1.2 [21%] 0.29

in octanol 458 527 �0.05[54%]

0.68 [46%] 0.34

neat film 446, 469 607 2.9[28%]

12.8 [72%] 10.03

IDC4 in toluene 443 500 �0.1[38%]

0.62 [58%] 3.0 [4%] 0.52

inchloroform

448 505 �0.05[68%]

0.46 [25%] 2.1 [7%] 0.3

in ethanol 454 545 �0.14[34%]

0.77 [38%] 4.4 [28%] 1.57

neat film 464 573 �0.15[31%]

0.88 [42%] 4.2 [27%] 1.55

450 500 550 600 650 700

-20

-15

-10

-5

0

5

0 5 10 15 20

-20

-15

-10

-5

0

5

a

ΔΑ

,10-3

Wavelength, nm

IDC1 in toluene 0 ps 0.5 ps 3 ps 10 ps

b

ΔΑ

,10-3

Time, ps

IDC1 in toluene460 nm475 nm530 nm640 nm

Fig. 4. Transient differential absorption spectra at various times after excitation at420 nm (a) and transient absorption kinetics (b) of IDC1 in toluene.

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215212

implies that the Stokes shift is related to the conformational mo-tions of molecules.

The fluorescence decays of both investigated compounds arenon-exponential, with two relaxation time constants in case ofIDC1 molecules and with three in case of IDC4 molecules (Table 1).Fluorescence of IDC1 in solid film decaysmore than 50 times slowerthan in toluene solution suggesting that the fast relaxation in so-lutions is related to the conformational motions of molecules.Fluorescence of IDC4 in ethanol and in neat film have been char-acterized by similar decay time constants of about 140 ps, 800 psand 4.3 ns. It supports the conclusion that fluorescence of the IDC4solutions is strongly affected by the residual nondissolved particles.Sonication of the solutions in ultrasonic bath temporary reducesthe fluorescence decay time.

Excited state relaxation dynamics of studied IDC compounds insolutions and of solid films has been investigated by means of thefemtosecond transient absorption spectroscopy. Transient absorp-tion spectra of IDC1 in toluene at different delay times after exci-tation at 420 nm are presented in Fig. 4(a). Fig. 4(b) shows thetransient absorption kinetics at various probe wavelengths. Thetransient absorption spectra and kinetics of IDC4 in toluene wereobtained within experimental errors being identical to those ofIDC1, therefore are not presented. The negative signal in the 450e480 nm region should be attributed to the absorption bleaching,while the negative signal at longer wavelengths (480e570 nm),where the steady state absorption is absent, should be attributed tothe stimulated emission. During initial 3 ps, the stimulated emis-sion band shifts to the long wavelength side by about 20 nm andbroadens. A weak, broad induced absorption band was observed inthe red spectral region. A weak induced absorption band in the600e700 nm spectral region is characteristic of carbazole cationradical [13,32,33], thus it apparently originates from the electron-ically depleted carbazole group in the intramolecular CT state. Anew induced absorption band forms during 3e6 ps at about480 nm. This band appears at the long wavelength side of the ab-sorption band, thus shows a longwavelength shift of the absorptionband. Similar shift has been observed in a number of moleculesexperiencing ultrafast excited state twisting and was attributed tothe conformationaly distorted ground state [29,34].

Transient absorption relaxation kinetics in the bleaching, stim-ulated emission and induced absorption regions are presented inFig. 4(b). The absorption bleaching band decays and turns into theinduced absorption with the time constant of about 2 ps. The

stimulated emission at 530 nm and excited state absorption at640 nm, decay with very similar time constants, which apparentlycorrespond to the electronically excited state lifetimes. Conse-quently relaxation of the stimulated emission is much faster thanthe time resolution of the fluorescence measurements and it re-veals the real relaxation time of the fastest fluorescence relaxationcomponent. On the other hand, transient absorption reveals noslower relaxation processes evident in fluorescence, apparentlybecause minor strongly fluorescent species are responsible for thesubnanosecond and nanosecond fluorescence. The slowest processobserved in the transient absorption is the decay of the inducedabsorption band at about 480 nmwith about 20 ps time constant. Itshould be attributed to the ground state stabilization.

Figs. 5a and 6a present the transient absorption spectra of IDC1and IDC4 solid films respectively under excitation at 420 nm. Incontrast with the solutions, the transient absorption spectra of thetwo films are significantly different. The transient absorptionspectra of IDC1 have several evident differences in comparisonwiththe spectra of this compound in toluene: (i) only slight trace of thestimulated emission band is observed in the 540e580 nm region,(ii) induced absorption band in the 500e550 nm region is evidentin the zero delay time spectrum, and (iii) the negative signal in theshort wavelength region (at about 450e510 nm) is shifted to thelong wavelength side by about 10 nm during several tens of pico-seconds. In contrast with the IDC1 film, the transient absorptionspectra of IDC4 film show a strong stimulated emission band in the520e600 nm spectral region.

The transient absorption kinetics of the IDC1 film is presented inFig. 4(b). The strong negative signal in the short wavelength regionreveals some very fast relaxation process with about 1 ps and 10 ps

450 500 550 600 650 700-15

-10

-5

0

5

0 5 10 15 20-15

-10

-5

0

5

aΔΑ

, 10-3

Wavelength, nm

IDC1 neat film 0 ps 1 ps 10 ps 100 ps

b

ΔΑ

, 10-3

Time, ps

IDC1 neat film 480 nm 530 nm 650 nm

Fig. 5. Transient differential absorption spectra at various times after excitation at420 nm (a) and transient absorption kinetics (b) of IDC1 neat film.

450 500 550 600 650 700

-4

-2

0

2

0

2

a

ΔΑ, 1

0-3

Wavelength, nm

IDC4 neat film 0 ps 1 ps 10 ps 100 ps

b

-3

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215 213

time constants and a slow decay with the time constant of about170 ps. Kinetics in the stimulated emission region shows an initialnegative signal, which decays on a subpicosecond time scale andturns into the induced absorption. Thus, it reveals very fast stim-ulated emission decay. The induced absorption decay in the longwavelength region at 650 nm, which we attributed to the elec-tronically depleted carbazole group absorption, evidently corre-sponds to the decay of the electronically excited state. It revealsexcited state relaxation time of about 15 ps, and slightly fasterrelaxation at short times. The transient absorption kinetics of theIDC4 film at 480 and 650 nm are very similar to those of the IDC1film, while kinetics at 530 nm shows that the stimulated emissionin this film decays only slightly faster than the population of theexcited state. Slow component of the absorption bleaching relax-ation should be attributed to the molecule stabilization in theground state.

0 5 10 15 20

-4

-2ΔΑ, 1

0

Time, ps

IDC4 neat film 480 nm 540 nm 650 nm

Fig. 6. Transient differential absorption spectra at various times after excitation at420 nm (a) and transient absorption kinetics (b) of IDC4 neat film.

4. Discussion

Very fast excited state relaxation revealed by the transientabsorption investigations of both IDC compounds in solutionsand much slower in solid films imply that the excited state dy-namics is at least partly determined by the large amplitudeconformational changes of chromophore moieties. Such confor-mational changes leading to the excited state relaxation aretypical of donor-acceptor molecules connected by conjugatedbridge. Bridge twisting takes place in the excited state and causesthe fast nonradiative decay of the excited state population. Fig. 7

shows calculated total energies of chromophore group in aground state and three lowest excited states as functions ofmolecule twisting around double central bond. The total energyin all electronic states has minimumwhen the molecule is almostflat and monotonicaly increases with twisting angle. Only in thefirst excited state the total energy has a second minimum atabout 90 twisting angle. Electronic transitions to the first andsecond excited states have negligible oscillator strengths, whenthe molecule is flat. Therefore the lowest energy absorption bandat about 460 nm is apparently caused by transitions to the S3excited state, having large oscillator strength. The fact that thestimulated emission in solutions (530 nm) decays simultaneouslywith the absorption bleaching (460 nm) indicates that eitherrelaxation to the S1 excited state does not take place, or relaxa-tion of S1 to the ground state is very fast. Very similar excitedstate dynamics and similar properties of excited states have beenobserved for N,N-dimethylaminobenzylidene-1,3-indandionemolecules [17]. The fast excited state relaxation for these mole-cules has been attributed to the molecule twisting on relativelyflat excited state potential surface formed by several excitedstates and ultrafast subsequent relaxation to the S0 state. Such arelaxation scenario is apparently valid and for IDC chromophores.Fig. 7 shows the anticipated relaxation pathway. Twisting of theindan-1,3-dione moiety to about 50�. takes place on the S3 statepotential surface, then ultrafast transition to the S1 potentialsurface, double bond twisting to 90� and finally relaxation to theS0 potential surface via cornel intersection take place. It shouldbe noted, that the quantum chemical calculations of the excitedstate potential surfaces was not completely accurate because

0

1

2

3

4

5

6

0 30 60 900.0

0.2

0.4

0.6

Tota

l ene

rgy,

eV

S0

S3(ππ∗)

S1(nπ*)

S2(nπ*)

S0

Osc

illat

or s

treng

th

Dihedral angle, deg

S1

S0 S3

S0S2

Fig. 7. Calculated potential surfaces of indan-1,3-dione-carbazole-basedchromophores.

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215214

other conformational parameters except of dihedral angle werenot optimized. Such optimization would lead to the additionalreduction of the total energy at large twisting angles. Thereforethe excited state energies at large twisting angles are evidentlyoverestimated. Thus energy barrier, which accordingly to Fig. 7exist for the molecule twisting, is most probably lower or ab-sent. Consequently the twisting reaction is barrierless andtherefore very fast. On the other hand, fast excited state relaxa-tion in solid films, where conformational motions of moleculesshould be strongly suppressed, suggests that molecule relaxationfrom S3(pp*) state to S1(np*) state is also quite fast even withoutlarge amplitude molecule twisting.

The ultrafast excited state relaxation dynamics within theexperimental accuracy is identical for both IDC1 and IDC4 com-pounds. It shows that a) interchromophore interactions inside IDC4compounds are negligible and b) linking of carbazole group to thelarge compound body makes no influence on the chromophoretwisting dynamics. This is not very surprising because indan-1,3-dione moiety is slightly smaller than the carbazole moiety there-fore its motion determines the twisting dynamics and the restrictedfreedom of cabazole unit motion in IDC4 compound has no sig-nificant influence.

According to the above presented scenario, the long-livingfluorescence should be attributed to some molecular speciesexperiencing no fast excited state relaxation because of hamperedexcited state twisting, or different energy positions of excitedstates. It is reasonable to attribute the long lasting fluorescence tomolecular aggregates, most probably to dimers. Molecule aggre-gation apparently prevents excited state twisting of molecules and/or changes energetical positions of excited states. Large IDC4compounds possess higher amplitude of the slow relaxation

component in solutions. Apparently large molecules are moreprone to formation of aggregates. This is also consistent with lowersolubility of IDC4 molecules.

Aggregate states apparently play even larger role in the excitedstate relaxation of solid films. The absence of the stimulatedemission of the IDC1 film should be related to the formation ofaggregate states with weakly allowed low energy transitions.Splitting of the absorption band of this film supports this conclu-sion. IDC1 films tend to crystallization, therefore formation of ag-gregates or microcrystalline structures is very likely. Very faststimulated emission decay in IDC1 film should be apparentlyrelated to the fast localization of excitations on the low energy H-type aggregate states with forbidden transitions to the groundstate. It should be also noted, that even a relatively low concen-tration of aggregates in solid films may play decisive role inrelaxation process. This is because of the fast exciton migration inmolecular solids. Typical intermolecular exciton transfer times inmolecular solids are of the order of tens or hundreds of femtosec-onds. Therefore exciton localization on molecular species possess-ing low energy states may occur on a subpisocesond time scaleobserved as a decay of the stimulated emission. Disordered struc-ture of molecular films causes formation of variety of aggregatestates with different excited state lifetimes responsible for non-exponential fluorescence decay. Films of large IDC4 molecules, incontrary, do not show absorption band splitting, have narrower andless red shifted fluorescence band and shorter fluorescencelifetime, thus apparently form more amorphous films and thisdisordered material structure hampers aggregate formation.Consequently connection of IDC compounds into quadruplicatemolecules plays controversial role in their aggregation and thus, inexcited state properties. On the one hand, large molecules are morelinked to aggregation in solutions, on the other hand, large mole-cules form more amorphous molecular films.

5. Conclusions

Compounds with single and covalently connected four IDCchromophores have been synthesized. IDC chromophores inquadruplicate compounds retain spectroscopic properties of indi-vidual IDC molecules indicating that interchromophore in-teractions in quadruplicate compounds are weak. IDCchromophores experience barrierless excited state twisting in so-lutions, which causes ultrafast excited state relaxation, which alsooccurs identically in single and quadruplicate compounds.

Only fraction of molecules experience excited state twisting andfast relaxation to the ground state in solid molecular films. Anotherfraction relax to the low energy aggregate states and experiencerelaxation to the ground state on a nanosecond time scale. Bothsingle and quadruplicate compounds form molecular glasses;however single molecule films are prompt to crystallization, and toformation of weakly emissive H-aggregate-like species. Fast exci-tation localization on the nonemissive aggregate states causesdecay of the stimulated emission on a subpicosecond time scale.Large quadruplicate molecules form more amorphous filmswithout aggregate states.

Acknowledgments

R.K., A.G. and V.G. acknowledge the European Social Fund underthe Global Grant measure (Grant No. VP1-3.1-�SMM-07-K-01-006).The public access supercomputer from the High PerformanceComputing Center (HPCC) of the Lithuanian National Center ofPhysical and Technology Sciences (NCPTS) at Physics Faculty ofVilnius University was used (HPC Sauletekis).

R. Karpicz et al. / Dyes and Pigments 105 (2014) 208e215 215

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