Reduction of the Fluorescence Transition Dipole Moment by Excitation

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Reduction of the Fluorescence Transition Dipole Moment by Excitation Localization in a Vibronically Coupled Squaraine Dimer Henning Marciniak, Nina Auerhammer, Sophie Ricker, Alexander Schmiedel, Marco Holzapfel, and Christoph Lambert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11957 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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The Journal of Physical Chemistry

Reduction of the Fluorescence Transition Dipole Moment by Excitation Localization in a Vibronically Coupled Squaraine Dimer Henning

Marciniak,

Nina

Auerhammer,

Sophie

Ricker,

Alexander

Schmiedel, Marco Holzapfel, and Christoph Lambert* Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

Abstract A rigidly bridge squaraine dimer serves as a model compound to study exciton interactions between two chromophores without bothering conformational or other stereochemical isomers. We describe the synthesis as well as steady state and fs- and ps-time resolved optical spectroscopic data. The spectra are interpreted using a vibrionic coupling model which considers a single vibrational mode which produces a shallow excited state surface with two minima. These two minima cause symmetry breaking of the excited state which leads to a partial localization of excitation. The localisation of the wavefunction causes a reduced fluorescence transition moment although both the absorption and the emission spectra display exchange narrowing typical of excitonically coupled chromophores.

1. Introduction For using organic materials for optoelectronic applications such as OLEDs,1-2 photovoltaic cells3-4 or for two-photon absorption induced fluorescence,5-6 it is a basic necessity to gain control of the optical properties of the used compounds. To achieve this, a widely pursued approach is the design of supramolecular structures, i.e. oligomers, polymers or aggregates, starting from monomeric building blocks.7-9 A large 1 ACS Paragon Plus Environment

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variety of promising candidates with sharp and intense absorption bands have been identified in organic chemistry, among the most prominent being cyanine dyes, BODIPY dyes and squaraine dyes.7,

10-15

Especially the latter stand out for their

particularly intense absorption and fluorescence characteristics in the visible red to NIR region, where the number of alternatives is more restricted.11, 16-19 The supramolecular approach exploits the excitonic interactions between the monomers, leading to delocalized states with shifted transition energies and redistributed transition strengths.20 The basic effects can be explained by a relatively simple exciton theory, dating back to the works of Davydov21 and Kasha,22 where only intermolecular dipole-dipole coupling is taken into account. For a more exact prediction of the variation of optical properties, more thorough pictures, including e.g. transition densities and exciton-vibrational coupling, have also been developed.23-24 For a rational design of optical properties it is important to separate all impact factors that influence the excitonic interaction. Therefore, the smallest and thus most simple possible aggregate, the dimer, plays a prominent role in basic research on this subject. Among others, effects of relative orientation, distance, heterogeneity of the monomers, and also the influence of vibrational coupling have been studied.25-36 Quite recently, a series of squaraine dimers has been investigated, where transient localization depending on the excitonic coupling strength could be observed.37 However, due to the biaryl bridging between the monomers, rotation of the monomers relative to each other lead to a strongly inhomogeneous sample and complex absorption spectra. In this work, we investigate a squaraine dimer SQA2-r, which consists of the same monomers as one of the dimers, SQA2 in ref. 37, but with a rigid

bridge,

which

was

achieved

by

synthesis

starting

from

tetramethyldihydroanthracene and semisquaraine moieties (see SI for details). This 2 ACS Paragon Plus Environment

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dimer cannot form rotamers or other form of conformers or isomers. Due to its rigidity, it should allow to measure steady state and time resolved spectra, which can be quantitatively analyzed. In particular, the influence of vibrational coupling can be assessed, in addition to sheer excitonic effects. In the following, the model used for analysis will be briefly outlined, before the results are discussed. R N

O O

N R

R N

O

N R

O

R=

SQA2-r

2. Theoretical Framework An approximate, yet quantitative model for an excitonic dimer of two identical molecules 𝐴 and 𝐵, linearly coupled to a single vibrational mode, was introduced by Fulton and Gouterman38-39 and has since been well established.31, 33, 40-43 It is used in this work to interpret the experimental results and will therefore be outlined here. The vibrational mode, which is assumed to be totally symmetric in the monomer point group, is included in the form of one harmonic oscillator mode per molecule. Upon excitation, the harmonic potential energy surface (PES) of the dimer is then linearly shifted from its equilibrium position either along the oscillator coordinate of molecule 𝐴 or molecule 𝐵. The PES of the dimer ground state (𝐸0) and the excited states (𝐸1± ) can be written as: 𝐸0 =

𝐸𝑣𝑖𝑏 2

(𝑞2A + 𝑞2B)

𝐸1± = 𝐸exc +

𝐸𝑣𝑖𝑏 2

(1a)

𝜆 (𝑞2A + 𝑞2B) + 2(𝑞A + 𝑞B) ±

(

𝜆

2

)

(𝑞A ― 𝑞B) + 𝐽2

2

3 ACS Paragon Plus Environment

(1b)


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Here, the oscillator coordinates 𝑞 have been transformed to dimensionless quantities. The energy 𝐸𝑒𝑥𝑐 comprises the monomeric excitation energy as well as the change in interaction energy between the two monomers upon excitation.21-22 𝐸exc, the vibrational mode 𝐸vib, the vibrational coupling constant 𝜆, and the electronic coupling term 𝐽 are given in units of cm-1. In Figure 1, these PES are exemplarily depicted for 𝐸0 and the lower of the excited states, 𝐸1― . The coordinate transformation 𝑞 ± =

1

(𝑞A ± 𝑞B) leads

2

to the more convenient description: 𝐸0 =

𝐸vib 2

(𝑞2+ + 𝑞2― )

𝐸1± = 𝐸exc +

(2a)

𝐸𝑣𝑖𝑏 2

(𝑞2+ + 𝑞2― ) +𝜆𝑞 + ± (𝜆𝑞 ― )2 + 𝐽2

(2b)

Figure 1. Left: Potential energy surfaces (PES) for 𝐸 0 (red) and 𝐸 1― (blue). The dashed and the solid black line indicate the 𝑞

― -axis

Right: Cut through the PES along the 𝑞

at 𝑞

― -axis

+

= 0 and at 𝑞

at the 𝑞

2

+

+ -values

= ― 𝜆 𝐸 𝑣𝑖𝑏 , respectively.

indicated in the left panel.

The corresponding adiabatic dimer wave functions 𝜓 are expressed through linear combinations of products of the monomer wave functions 𝜙: 𝜓0 = 𝜙A0𝜙B0

(3a) 4 ACS Paragon Plus Environment

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𝜓1― = cos 𝛼 ∙ 𝜙A1𝜙B0 + sin 𝛼 ∙ 𝜙A0𝜙B1

(3b)

𝜓1+ = sin 𝛼 ∙ 𝜙A1𝜙B0 ― cos 𝛼 ∙ 𝜙A0𝜙B1

(3c)

where 𝛼 is defined through tan 2𝛼 = |𝐽| |𝜆𝑞 ― |. 𝜓1― is implicitly assumed to be the “brighter” state, i.e. to have a stronger transition dipole moment to the ground state, as it would be the case for a “J”-like dimer, where the monomers are in a head-to-tail arrangement.7, 22-23 It turns out, that for |𝜆 𝐸vib| > |𝐽|, 𝐸1― exhibits a structure with a double minimum 2

at: 𝜆

and

𝑞 + = ― 𝐸vib

𝑞― = ±

2

(𝜆 𝐸vib)

2

― (𝐽 𝜆)

(4a and 4b)

(marked with 𝒇 and 𝒇′ in Figure 1), from where fluorescent transition to ground state takes place. The point where vertical absorption takes place is marked with 𝒂. The energetic shift between absorption and fluorescence of the dimer can thus be determined as: 𝜆2

(𝐸1― (𝒂) ― 𝐸0(𝒂)) ― (𝐸1― (𝒇) ― 𝐸0(𝒇)) = ― |𝐽| +2𝐸vib

(5)

The wave function at point 𝒂 has the form: 𝜓1― (𝒂) =

1

(𝜙A1𝜙B0 + 𝜙A0𝜙B1)

(6)

2

and the squared transition dipole moment to the ground state is: 𝜇2(𝒂) = 2𝜇2M

(7)

where 𝜇2M is the squared transition dipole moment of the monomer. At the points 𝒇 and 𝒇′, the wave function experiences stronger localization on one of the two monomers:



𝜓1― (𝒇)

=

1± 1―

2

( 𝐽) ∙ 𝜙A1𝜙B0 + 𝐸vib 𝜆2

2

1∓ 1―

( 𝐽) 𝐸vib 𝜆2

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2

∙ 𝜙A0𝜙B1

(8)

2

Thereby, the transition dipole strength to the ground state is reduced according to:

( | 𝐽|)

𝜇2(𝒇) = 𝜇2M 1 +

𝐸vib

(9)

𝜆2

3. Experimental Absorption spectra were measured with a Jasco V-570 or V-670 UV/vis/NIR spectrometer in 1 mm or 10 mm quartz cuvettes and referenced against the pure solvent. An Edinburgh Instruments FLS980 fluorescence lifetime spectrometer was used for steady state fluorescence measurements, determination of fluorescence quantum yields and fluorescence lifetime measurements. The observed fluorescence quantum yields were determined with an integrating sphere and afterwards corrected for self-absorption applying the method of Bardeen et al..44 Fluorescence lifetimes were determined by time-correlated single-photon counting (TCSPC). The samples were excited by a pulsed laser diode at 15 200 cm-1 (656 nm) under magic angle conditions and the fluorescence was detected with a high-speed PMT detector (H10720). The IRF was 270 ps. Transient absorption measurements were performed using a Helios transient absorption spectrometer from Ultrafast Systems with white light generation45 in a 3 mm sapphire plate. A Solstice amplified Ti:sapphire laser from Newport Spectra-Physics served as pulse source for the whole setup (see SI for further details). The pump pulses were generated with a noncollinear optical parametric amplifier (NOPA)46 and compressed with a fused silica prism compressor to durations between 20 and 25 fs. The IRF was ca. 66 fs. The sample solutions were pumped through a flow cell (Starna)


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with 0,2 mm thick quartz windows and 0,2 mm path length during the measurements, using a micro annular gear pump (HNP Mikrosysteme). All measurements were done in toluene.

4. Results and Discussion The steady state absorption and fluorescence spectra of SQA and SQA2-r are depicted in Figure 1 in the transition dipole representation, for better comparison47 (for numbers see Table 1).

Figure 2. Transition dipole representation of the normalized absorption 𝜀 /𝜈 and fluorescence 𝐹/𝜈 3 of SQA 2 -r in toluene. The arrow in the dimer absorption spectrum indicates a weak shoulder.



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Figure 3. Time-correlated single-photon counting (TCSPC) measurements at 15000 cm 1,

excitation at 15200 cm - 1 . The time constants from a biexponential fit are given in blue

for the monomer SQA and in red for the dimer SQA 2 -r, with the relative amplitudes in round brackets.

Table 1. Absorption and fluorescence maxima ( 𝝂 𝒎𝒂𝒙 ), 1 s t moments ( 〈𝝂 〉) and half widths at half maximum ( 𝒉𝒘𝒉𝒎 𝒓𝒆𝒅 and 𝒉𝒘𝒉𝒎 𝒃𝒍𝒖𝒆 , measured on the red and blue side of the spectrum, respectively), maximum extinction coefficients ( 𝜺 𝒎𝒂𝒙 ), fluorescence quantum yields ( 𝝓 𝒇𝒍 ) and lifetimes ( 𝝉 𝒇𝒍 ), and squared absorption and fluorescence transition dipole moments ( 𝝁 𝟐𝒂𝒃𝒔 and 𝝁 𝟐𝒇𝒍 ). Absorption ~𝜺(𝝂) 𝝂

Fluorescence ~𝑭(𝝂) 𝝂𝟑 𝜺𝐦𝐚𝐱

𝝁𝟐𝐚𝐛𝐬

𝝂𝐦𝐚𝐱

〈𝝂〉fl

𝒉𝒘𝒉𝒎𝐛𝐥𝐮𝐞

𝝉𝐟𝐥

/cm-1

/M-1 cm-1

/D2

/cm-1

/cm-1

/cm-1

/ns

15840

250

365000

127

15340

15040

260

1.71

0.62

114

15430

200

858000

245

15020

14750

180

1.33

0.71

177

𝝂𝐦𝐚𝐱

〈𝝂〉abs

/cm-1

/cm-1

SQA

15530

SQA2-r

15120

𝒉𝒘𝒉𝒎𝐫𝐞𝐝

𝝓𝐟𝐥

𝝁𝟐𝐟𝐥 /D2

SQA Monomer. The SQA monomers exhibit typical characteristics of squaraine compounds.11, 16, 48 The main absorption peak is intense and sharp with a maximum extinction coefficient of 365000 M-1cm-1 and 250 cm-1 half width at half maximum (hwhm, measured on the red flank of the spectrum) and can be assigned to the HOMOLUMO transition. A weak shoulder of a vibronic progression can be observed from 8 ACS Paragon Plus Environment

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which a vibrational mode of 𝐸𝑣𝑖𝑏 = 1340 cm-1 is estimated (see Figure 2). The squared dipole moment of the transition is 127 D2, calculated from the absorption spectrum according to:49-50 𝜇2abs =

3𝜀0ℎ𝑐ln 10 2

20𝜋 𝑁A

∙

9𝑛

(𝑛 + 2) 2

𝜀

2

(10)

∙ ∫𝜈𝑑𝜈

where 𝜀0 is the vacuum permittivity, ℎ is the Planck constant, 𝑐 ist the speed of light, 𝑁A is the Avogadro constant, 𝑛 is the index of refraction of the solvent, 𝜖 the extinction coefficient in M-1cm-1, and 𝜈 the wavenumber in cm-1. The fluorescence spectrum largely exhibits mirror symmetry to the absorption spectrum. To determine the spectral shift between absorption and emission, the first moments or centers of gravity of the absorption and the fluorescence spectrum are calculated as:39, 51-52

〈𝜈〉abs =

∫𝜀𝑑𝜈 𝜀

∫𝜈𝑑𝜈

and

〈𝜈〉fl =

∫

𝐹(𝜈) 𝑑𝜈 𝜈2

∫

𝐹(𝜈) 𝑑𝜈 𝜈3

(11a and 11b)

respectively. For the SQA monomer, 〈𝜈〉abs is located at 15840 cm-1 and 〈𝜈〉fl at 15040 cm-1, yielding a shift of 800 cm-1 (the shift between the most probable absorption and fluorescence transitions, i.e. the maxima of the spectra, is 190 cm-1). The fluorescence lifetime has been measured by time correlated single photon counting (TCSPC, see Figure 3) and could be fitted with a bi-exponential function, having a small contribution of a shorter decay time constant of 𝜏1 = 242 ps (amplitude 𝐴1 = 1.4%) and a dominant term with a time constant of 𝜏2 = 1.709 ns (𝐴2 = 98.6%). The resulting expectation value is 1. 71 ns according to: ∑𝐴𝑖𝜏2𝑖

(12)

𝜏 = ∑𝐴 𝜏

𝑖 𝑖



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with 𝑖 = 1,2. The quantum yield was determined to 𝜙 = 0.62. Using this lifetime, the squared transition dipole moment for the fluorescence 𝐹: 𝐹(𝜈)

𝜇2fl

=

∫ 3 𝑑𝜈 3𝜀0ℎ 9 𝜙 𝜈 ∙ ∙ ∙ 𝜏 16 ∙ 106𝜋3 𝑛(𝑛2 + 2)2 ∫𝐹(𝜈)𝑑𝜈

(13)

can be calculated from the spectrum,49-50, 53 yielding 114 D2. This is somewhat smaller than 𝜇2abs, yet comparable within the experimental uncertainty. SQA2-r Dimer. The main absorption peak (as well as the first moment) of the SQA2-r dimer is 410 cm-1 redshifted compared to the monomer and the hwhm is reduced to 200 cm-1 (see Figure 2). At the same time, the absorption is much more intense than that of the monomer with an extinction coefficient of 858000 M-1cm-1 in maximum, leading to a squared transition dipole moment of 245 D2 according to eq 10. This is almost twice the monomer transition strength. All these features hint at an excitonically coupled dimer in a “J”-like configuration.7, 23 Within the framework of exciton theory, the spectroscopic properties of the dimer are assumed to be emerging directly from the properties of the constituting monomers.21-22 In particular, the vectorial addition of the monomer transition dipole moments leads to the dimer transition dipole moments for delocalized exciton states, which are energetically separated due to the coulombic coupling between the monomers. As mentioned before, in a “J”-dimer, the monomer transition dipole moments are arranged parallel and in head-to-tail configuration. Therefore, the two possibilities 𝜇 ± to add the monomer transition dipole moments 𝜇𝑀 result in one forbidden transition (to the energetically higher exciton state) and one strongly allowed transition (to the energetically lower exciton state), where the full oscillator strength is concentrated:


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𝜇± =

1

(𝜇M ± 𝜇M)

2

→

|𝜇 ― |2 = 0 , |𝜇 + |2 = 2|𝜇M|2

(14)

The reduction of the hwhm can be interpreted in this context as resulting from exchange narrowing, where the inhomogeneous broadening is reduced due to the delocalization of the states over both monomers.54-55 The fluorescence spectrum of the dimer exhibits the same mirror symmetry as the monomer, including the spectral narrowing compared to the monomer. However, a closer inspection shows a significant difference in the absorption spectrum at ca. 15700 cm-1 where a small shoulder is visible which is missing in the fluorescence spectrum. This shoulder may be caused by the higher exciton state which is partially allowed because of vibronic coupling to an asymmetric mode which reduces the molecular symmetry. In comparison to the monomer, the shift between absorption and fluorescence (determined from the first moments of the spectra) is reduced to 680 cm-1. Also reduced is the lifetime with 𝜏 = 1.33 ns (see eq 12), determined from a biexponential fit of a TSCPC measurement (see Figure 3) with 𝜏1 = 367 ps (𝐴1 = 1.7%) and 𝜏2 = 1.335 ns (𝐴2 = 98.3%). This is in agreement with an expectedly higher squared transition dipole moment for fluorescence, analogous to the squared absorption transition dipole moment. With a fluorescence quantum yield of 0.71, the former can be calculated according to eq 13, resulting in 177 D2. This is indeed higher than for the monomer, but significantly smaller than the squared absorption transition dipole moment of the dimer. To rationalize this reduction of the transition dipole moment, vibronic coupling has to be taken into account. With the framework described in the section above and identification of 〈𝜈〉abs = 2

𝐸1― (𝒂) ― 𝐸0(𝒂) and 〈𝜈〉fl = 𝐸1― (𝒇) ― 𝐸0(𝒇),52, 56 it is clear, that the term ― |𝐽| +2𝜆 𝐸vib corresponds to the dimer shift of 680 cm-1 between the first moments of absorption and 11 ACS Paragon Plus Environment


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fluorescence (see eq 5). The coupling strength 𝐽 can be estimated to -180 cm-1 from the transition dipole interaction of the monomers:22-23, 25 1

𝜇2M

1

𝐽 = 4𝜋𝜀0 ∙ 𝑅3 (1 ― 3cos2 𝜗) ∙ 100ℎ𝑐

(15)

where |𝜇M| has been set as the average of |𝜇abs| and |𝜇fl|. 𝑅 = 1.87 nm is the centerto-center distance of the monomers, and 𝜗 = 7° is the angle between the monomer transition dipole moments and the vector 𝑅 (both from DFT calculations). Therefore 𝜆2 𝐸vib is 430 cm-1, which leads to a reduced squared transition dipole moment for transition to the ground state of 𝜇2(𝒇) ≈ 162 D2 (see eq 9), in reasonable agreement with the measured squared fluorescence dipole moment of 177 D2. The origin for the, compared to the absorption, reduced squared transition moment of fluorescence is to be seen in a significantly stronger localized wave function (see eq 8): 𝜓1― (𝒇) ≈ 0.95 ∙ 𝜙A1𝜙B0 + 0.05 ∙ 𝜙A0𝜙B1

(16)

compared to 𝜓1― (𝒂) (see eq 6). Insight into the dynamics of the localization process can be obtained by transient absorption measurements. The transient spectra of the SQA2-r dimers mainly show two signatures (see Figure 4): a negative one around 15100 cm-1 and a positive one around 15600 cm-1. The former depicts a combination of ground state bleaching and, red shifted to it, stimulated emission from the lowest exciton state, both leading to a negative change in optical density. The latter, which has no equivalent in the transient absorption of the monomers (see SI, Figure S4), can be assigned to absorption from the lowest exciton state to the two exciton state,57 thus generating a positive change in optical density. A global analysis of the transient spectra using a parallel model with the minimum number of exponential functions yielded decay associated amplitude spectra (DADS) with four components (see Figure 4 bottom). First of all, these 12 ACS Paragon Plus Environment

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measurements support the numbers gained from TSCPC, which reoccur within experimental uncertainty. Close inspection of the spectra reveals, that the 300 – 400 ps time constant is not accompanied by significant changes of the spectral shape despite a very slight narrowing of the bands (see also Figure S1 in SI), which might be due to small inhomogeneities in the sample, that is, a distribution of slightly different geometries/solvation of the chromophores. The influence of rotational diffusion could be excluded by thorough analysis of the anisotropy in the transient measurement, whose time constant could be determined to ~930 ps (see SI). The high time resolution of the measurement also brings out two short time constants of 0.08 and 2.74 ps, which can be associated with initial spectral changes, marking the shift of the stimulated emission to shorter wavenumbers, i.e. what is commonly denoted the Stokes shift.52 Especially the sub-100 fs time constant, which is again not present in the monomer (see SI, Figure S4), manifests itself in the time traces as decay of the absolute signal on the blue side and rise on the red side of the main transient band. The rapidness of the process is in good agreement with theoretical predictions for the localization of excitons on a vibronic dimer33 and can thus be assigned to that.



Figure 4. Top: Transient absorption spectra of SQA 2 -r in toluene after excitation at 15150 cm - 1 with magic angle orientation of the pump pulse relative to the probe pulse orientation. Middle: Time traces probed at 15220 cm - 1 (open circles) and 14950 cm - 1 (full circles) and the corresponding fit curves from a global exponential fit. Bottom: Decay associated difference spectra from the global exponential fit.

5. Conclusions We investigated the optical properties of a rigidly bridged squaraine dimer by means of steady state and absorption and fluorescence spectroscopy as well as transient absorption spectroscopy. Comparison with the monomeric building block showed that the dimer behaves as two excitonically coupled monomers as predicted by exciton coupling theory. The reduction of the fluorescence transition dipole moment 14 ACS Paragon Plus Environment

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relative to the absorption transition dipole moment can be explained quantitatively by a vibronic dimer model, incorporating one single vibrational mode per monomer. The coupling to the vibrational mode leads to a double minimum in the PES of the excited state, that causes a symmetry breaking effect58-60 on the wave function along with a localization of the excitation on one of the monomers. The time constant of the localization process could be shown to be on the sub-100 fs timescale. Despite this localization, the transient absorption shows evidence of one-to-two-exciton absorption and therefore the existence of collective dimer states. Also, the fluorescence band exhibits exchange narrowing compared to the monomer fluorescence. This shows that dynamic localization and excitonic effects do not exclude each other.

Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Evolution of transient absorption spectra of SQA2-r; Analysis of the anisotropy in the transient absorption measurements; Transient absorption of SQA monomers; Synthesis

Author Information Corresponding Author *E-mail: [email protected] ORCID Christoph Lambert: 0000-0002-9652-9165 15 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

Acknowledgements We thank the Deutsche Forschungsgemeinschaft for funding this work within the Research Group FOR 1809 and the Bavarian Ministry of Education, Culture, Research, and the Fine Arts for support within the consortium “Solar Technologies Go Hybrid”.

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Reduction of the Fluorescence Transition Dipole Moment by Excitation

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