Exciton Coupling Enhancement in the Relaxed Excited State - The

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Exciton Coupling Enhancement in the Relaxed Excited State Nina Auerhammer, Alexander Schmiedel, Marco Holzapfel, and Christoph Lambert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03337 • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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The Journal of Physical Chemistry

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Exciton Coupling Enhancement in the Relaxed Excited State

Nina Auerhammer, Alexander Schmiedel, Marco Holzapfel, Christoph Lambert* Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany.

Abstract:

The steady-state and photoinduced dynamical optical properties of two

squaraine-bodipy dye conjugates are in focus of this work. While the squared absorption transition moments of the dye conjugates can be traced back in an additive way to the constituents of the conjugates, this is not possible for the squared fluorescence transition moments. We suggest an enhancement of electronic coupling in the relaxed excited state to be responsible for this observation. Transient absorption and fluorescence upconversion experiments with fs-time resolution give insight into the relaxation phenomena of the dye conjugates, in particular concerning the relaxation within the exciton manifold.

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INTRODUCTION Bringing a number of chromophores in close contact and thus providing electronic coupling between these chromophores is an attractive way to tune their optical and electronic properties.1-2 Nature makes abundant use of this concept in the light harvesting complexes of the photosynthetic machinery of some bacteria and higher plants.3-4 Excitonic coupling of chromophores leads to new or shifted bands in absorption and fluorescence spectra, either at higher or at lower energies compared to the undisturbed monomers, depending on the orientation of the chromophores.5 While in the so-called H-aggregates a face-to-face arrangement of chromophores leads to a blue-shift of absorption bands and to reduced fluorescence intensities, in the so-called Jaggregates a head-to-tail arrangement of chromophores induces a red-shift of absorption bands and the enhancement of the fluorescence (superradiance).6-7 Due to this interaction the absorption and emission may even be shifted down to the NIR region.8 Chromophores that show emission and absorption in the NIR region are highly attractive because of multiple applications in optoelectronics, NIR-OLEDs, bioimaging, biosensors and labelling purposes in microscopy.8-15 In order to achieve NIR emission, coupling of two or more squaraine dyes proved to be quite successful.16-23 Following this concept, we recently synthesized and investigated a number of heterochromophore triads consisting of one or two bodipy dyes (B) and one or two transindolenine-squaraine dyes (tS) connected by triple bonds (see Figure 1). These dyes were chosen because of their intense and narrow absorption bands in the red to near infrared (squaraine) or yellow (bodipy) spectral region and because they exhibit high fluorescence quantum yields.24-30 In these triads, BtSB and tSBtS, exciton coupling between their constituent dyes leads to the formation of NIR absorbing superchromophores31 with strongly allowed low energy transitions which show predominantly squaraine character in the absorption and fluorescence spectra. Compared to the parent squaraine tS the quantum yield increased from 57 % to 83 % for BtSB but decreased slightly to 53% for tSBtS. These observations could nicely be explained by exciton coupling theory which yields three states within the exciton manifold. While the increased oscillator strength of the lowest energy transition and the higher fluorescence quantum yield in BtSB can explained by intensity borrowing from higher energy, bodipy localized states, the lowering of the fluorescence intensity of tSBtS is caused by a mixture of transoid and cisoid conformers which distribute the oscillator strength among all three states in the exciton manifold. In this work we replaced the trans-indolenine-squaraine tS as the low energy chromophore by a cis-indolenine-squaraine, cS, which leads to two triads, BcSB and cSbcS, whose


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steady-state and dynamic optical spectroscopic properties are in focus of this paper. The replacement of tS by cS will supposedly change the exciton coupling in the following way: i) the lowest energy absorptions of the triads will be at even smaller energy because cS ( = 14300 cm-1) possesses an absorption maximum at significantly lower energy than tS ( = 15500 cm-1). ii) because of the lower absorption energy of cS the energy difference ∆E of the lowest energy states between the isolated (= noninteracting) parent chromophores (cS and B1 or B2) is larger than in case of BtSB and tSBtS. This will lead to a smaller splitting of exciton states although the formal exciton coupling energy may be the same as for the tS triads. iii) the relative orientation of transition moments localized at individual bodipy or squaraine chromophores is different in BcSB and BtSB because the central squaraine is bent in BcSB rather than s-shaped (see Figure 1). This will lead to a modified distribution of oscillator strength among all three exciton states (see Figure 4 in Ref. 31).

METHODS The synthetic protocols of the triads can be found in the Supporting Information. Steady-state absorption spectroscopy. All experiments were carried out in 1 cm quartz cuvettes from Hellma in toluene (Uvasol® from Merck) and cyclohexane (Acros Organics) at r.t. using a Jasco V670 spectrometer. The solvents were used as received. The pure solvent was used as a reference. Emission Spectroscopy. All experiments were carried out in 1 cm quartz cuvettes in Uvasol® solvents from Merck. Before the measurement the dissolved samples were purged with argon for 15 min. Steady-state fluorescence measurements and excitation spectra were performed with an Edinburgh Instruments FLS980 spectrometer. The fluorescence quantum yields were determined with the FLS980 spectrometer using optically dense samples in an integrating sphere following the method of Bardeen et al.32 to correct for self-absorption. Fluorescence lifetimes were measured by time correlated single-photon counting (TCSPC) at the FLS980 spectrometer using pulsed laser diodes at 15200 cm -1 (656 nm) or 19400 cm-1 (515 nm) as excitation source. The instrument response was determined by using a scatterer solution consisting of colloidal silicon in deionized water. All spectra were recorded under magic angle conditions using a fast PMT detector (H10720). Lifetimes were determined by deconvolution of the experimental decay (4096 channels) with the instrument response function and by fitting the decay curves with an exponential decay function using the FAST software (version 3.4.2). ACS Paragon Plus Environment


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Femtosecond transient absorption spectroscopy. All experiments were performed in quartz cuvettes from Spectrocell (Oreland, PA) with an optical path length of 2 mm equipped with a micro-stirrer to allow stirring during the measurement at r.t. All samples were dissolved in the solvent indicated, filtered and degassed for 30 min by purging with nitrogen gas. The optical density was adjusted to ca. 0.2 at the corresponding excitation wavenumber. The transient absorption spectra were performed with a Newport-Spectra-Physics Solstice onebox amplified ultrafast Ti:Sapphire laser system with a fundamental wavenumber of 12500 cm–1 (800 nm), a pulse length of 100 fs and a repetition rate of 1 kHz. The output beam from the Solstice amplifier was split into two parts. One part was focused onto a vertically oscillating CaF2 crystal to produce a white light continuum between 11800 cm–1 (850 nm) and 28600 cm–1 (350 nm). The resulting beam, which was polarized horizontally, was used as the probe pulse. The second pulse was used to pump an optical parametric amplifier (TOPAS-C) from Light Conversion to generate the pump pulse with a pulse length of 140 fs at the appropriate excitation wavelength. By using a wire grid (Moxtek) the polarization axis of the pump pulse was set to magic angel relative to the probe pulse. The pump pulse (50 nJ, Ø ca. 0.4 mm) and the probe pulse (Ø ca. 0.1 mm) met at an angle of 6° vertically in the cuvette. The attenuated probe pulse was measured by means of an CMOS sensor (Ultrafast Systems, Helios) in the range of 11900 cm–1 (840 nm) to 25000 cm–1 (400 nm) with an intrinsic resolution of 1.5 nm. To compensate fluctuations of the intensity of the white light continuum, a reference beam was split off and detected with an identical spectrograph. Every second probe pulse was blocked by a mechanical Chopper (500 Hz) to measure the ratio of I und I0. The computer-controlled stage (retro reflector in double pass setup) sets the time difference between pump and probe pulse in 20 fs intervals from 0 fs to 4 ps and 4 ps to 8 ns in logarithmic steps with a maximum length of 200 ps. Before global data analysis with GLOTARAN33-34 to obtain the decay associated difference spectra (DADS) the raw data were corrected for stray light. The white light dispersion (chirp) was corrected by fitting a polynomial to the cross phase modulation signal of the pure solvent under otherwise identical experimental conditions. Femtosecond fluorescence upconversion spectroscopy.

A commercially available

fluorescence upconversion setup (Halcyone from Ultrafast Systems) was used for the measurements. The laser system was the same as for the femtosecond transient absorption spectroscopy. All lenses in the setup had a focal length of 100 mm and a thickness of 1.85 mm. The output beam from the Solstice amplifier was split into two beams, of which one part was used to pump an optical parametric amplifier (TOPAS-C) from Light Conversion to


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generate the pump pulse. The other part of the output beam was used as the gate pulse (12500 cm–1, 800 nm) which was delayed over a maximum of 3 ns in 20 fs steps from 0 fs to 4 ps and in logarithmic steps from 4 ps to 3 ns with a maximum step size of 80 ps with a computer-controlled linear stage. The pump pulse was focused on the cuvette and the resulting fluorescence and the gate beam were focused on a 0.5 mm BBO type I (for BcSB) or type II (for (cSBcS) crystal for frequency upconversion. The upconverted light was focused on the entrance slit of a double monochromator and measured by a PMT detector.

RESULTS AND DISCUSSION

Figure1. Squaraine-bodipy triads and their parent compounds. The yellow and blue arrows sketch localized transition moments.

Exciton Coupling. Extensive descriptions of exciton coupling theory can be found in literature.3, 35 Thus, we give an only qualitative pictorial insight into the situation found for the triads here. A quantitative evaluation for heterotriads can be found in the SI of ref.

31

. As

mentioned above, coupling of the lowest energy excited state of n chromophores generally leads to n different exciton states forming the exciton manifold. In case of three coupled ACS Paragon Plus Environment


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chromophores where two of those are chemically identical and can be transformed into each other by a symmetry operation, three different exciton states result which are given in Figure 2. The upper and the lowest state are separated by δE: 2 ∆ 2

(1)

where ∆E = (EB - EcS)/2 with EB and EcS being the energies of the noninteracting states of B and cS and where J is the exciton coupling energy. The phase relationship and orientation of transition moments localized at the individual chromophores result in more or less allowed or forbidden transitions from the ground state into the exciton states as depicted in Figure 2. For BcSB (and also for BtSB) there are several conformers corresponding to different rotational isomers of the bodipy chromophore relative to the squaraine dye. However, because of the parallel alignment of the bodipy transition moment and the triple bond as the axis of rotation, this will hardly affect the relative orientation of transition moments and therefore can be disregarded in context of exciton coupling theory. Much in contrast, the cisoid and transoid conformers of cSBcS (and likewise of tSBtS) show totally different relative orientation of transition moments.

Figure 2. Energy diagram of exciton states of the two triads BcSB and cSbcS. The diagram neglects a possible total stabilisation of all exciton states relative to those of the parent chromophores. The transition moment vector diagrams refer to the sequence of exciton state. The localized transition moments are given in yellow for cS and in blue for B with the orientation of the arrow indicating the phase relationship. The estimated resulting transition moment for each exciton state is given in black. The orientation and length of transition moments are estimates using the equations for the coefficients of eigenvectors as given in the SI of Ref.

31

.

UV-VIS-NIR Steady-state spectroscopy. For better comparison with literature data of BtSB and tSBtS and the parent compounds cS and B1 or B2 the absorption and fluorescence spectra of both triads BcSB and cSBcS were measured in toluene at room temperature (see Figures 3 and 4). ACS Paragon Plus Environment

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The lowest energy absorption band of triad BcSB at 13400 cm-1 is redshifted by 900 cm-1 compared to that of cS but shape and intensity are similar. This band is assigned to the lowest exciton state (see Figure 2). The squared transition moments (, which is proportional to the oscillator strength, see Table 1) of this lowest exciton band and of the lowest energy absorption band of cS are indeed very similar (101 vs. 92.7 D2). At ca. 18900 cm-1 there is a broad band covering the highest and middle exciton state. From the maxima of the lowest and the highest exciton state we evaluate δE = 18900 cm -1 -13400 cm -1 = 5500 cm-1, and with the lowest energy absorptions of B1 and cS we obtain ∆E = (18900 cm-1 – 14300 cm-1)/2 = 2300 cm-1 which yields the absolute value of the exciton coupling IJI = 1070 cm-1 by eq. 1. This value is almost identical to that of BtSB (IJI = 1100 cm-1). From Figure 2 and eq. 1 it is apparent that the lowering of the lowest exciton state, ∆ − ∆ 2 , compared to the site energy of the squaraine parent chromophore becomes smaller the larger ∆E is. This explains why the redshift of BcSB vs. cS (900 cm -1) is smaller than that of BtSB vs. tS (1100 cm-1). The lowest energy peak in the absorption spectrum of cSBcS also is shifted by 700 cm-1 to lower energy compared to cS but not as much as in BcSB. The intensity of the band has doubled ( = 210 D2) but the shape is significantly broader than that of cS. This band comprises the lowest two exciton states as can be seen in Figure 2. In contrast to BcSB there is no distinct maximum which could unequivocally be assigned to the highest exciton state. Thus, the evaluation of the exciton coupling energy is impossible in this case. Taking the squared transition moments of the lowest energy band of cS (92.7 D2), BcSB (101 D2), and cSBcS (210 D2) shows that the behavior is practically additive and no intensity borrowing from higher excited exciton states occurs. Integrating the upper and middle exciton band of BcSB from 16500 to 22200 cm-1 yields 82.3 D2 which is approximately twice that of B1. This contrasts the behavior of BtSB where we found a pronounced enhancement of the lowest energy absorption on the expense of the higher energy exciton states. This might be a consequence of the chromophore orientation where the angle between the bodipy transition moment and the squaraine transition moment is smaller in BcSB than in BtSB because the wider the angle and the more linear the chromophore arrangement is, the larger the transition moment of the lowest exciton state should be, see Figure 2 and Figure 4 in Ref. 31.



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8 λ / nm 800 700

600

500

400

400000

ε / M–1 cm–1

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

cSBcS BcSB cS B2 B1

300000

200000

100000

0 15000

20000

25000

ν / cm

30000

–1

Figure 3. Absorption spectra of triads and reference compounds in toluene at r.t.

Table 1. Optical Spectroscopic Data of Squaraine and Bodipy Dyes at r.t. / cm–1 a

ε / M–1 cm–1 b

2c

2e

/D

/ cm–1 d

/D

kfl=φfl/τfl

–1 f

( / cm )

8 -1 g

/10 s

B1

toluene

18900

70600

36.5

18300

33.1

0.78 (20000)

1.66

B2

toluene

18000

86900

44.9

17500

47.9

0.92 (18870)

2.04

cS

toluene

14300

202000

92.7

14000

92.2

0.75 (15400)

2.17

13000

135

0.67 (14700)

2.58

BcSB

cSBcS

toluene

13400

193000

cyclohexane

13300

195000

104

13000

127

0.65 (14700)

2.41

toluene

13600

387000

210

13400

149

0.77 (14700)

3.08

cyclohexane

13500

388000

207

13400

147

0.81 (14900)

3.0

a

Absorption energy.

b

Extinction coefficient.

c

101 (82.3)

h

Squared transition moment of the lowest energy band

ε (ν~ ) ~ . 9n determined by integration via µ 2 = 3hc ε 0 ln 10 dν 2 2 2 ∫ ν~ 2000 π N (n + 2)

d

Fluorescence energy.

e

Squared

f

fluorescence transition moment determined via the Strickler-Berg eq. 2. Fluorescence quantum yield g

h

(excitation energy). Radiative rate constant; for the lifetimes see Table 2. Middle and upper exciton band.

Emission spectroscopy. Both triads show an intense fluorescence in toluene solution when excited at the lowest energy exciton band. The fluorescence spectra display the typical narrow band shape of squaraine fluorescence with a small Stokes shift (200-400 cm-1, see Table 1 and Figure 4).


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9

1.0 0.8

BcSB cS cSBcS

0.6 I / a.u.

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

620 640

0.4

840

0.2 0.0 10000

11000

12000 13000 ν / cm–1

14000

15000

Figure 4. Fluorescence spectra of triads and reference compounds in toluene at r.t.

The fluorescence excitation spectra (see Figure S1 in the SI) nicely match the absorption spectra which shows quantitative energy transfer/internal conversion to the lowest exciton state irrespective of excitation energy. Most notably is the difference of full width at half maximum (fwhm) of the fluorescence bands which are much narrower in the triads (620-640 cm-1) compared to cS (840 cm-1). Such an exchange narrowing is generally believed to be a hint to exciton delocalization.36-39 In contrast, the fwhm of the absorption bands are ca. 680 cm-1 for both cS and BcSB. This indicates that exchange narrowing and thus pronounced exciton delocalization is present in the relaxed excited state only but not in the FranckCondon state. The fluorescence quantum yields were measured using an integrating sphere and appear to be slightly smaller for BcSB than for cS but slightly larger for cSBcS (see Table 1). In order to gain more insight into the dynamic processes of the triads we measured the fluorescence lifetime by TCSPC in toluene and in cyclohexane, see Table 2. Within time resolution, the fluorescence decays of the triads turned out to be monoexponential with lifetimes in the lower ns-time region. Using the relation kfl=φfl/τfl we determined the radiative rate constant kfl which increases on going from cS to BcSB and cSBcS (see Table 1). This rate constant can be used to estimate the fluorescence transition moment by the Strickler-Berg equation (2).

∙ !" '(' ) * + #$%& ,

)

〈.#〉. 01

(2)

where 〈.# 〉. ⁄2 3 .#4 is the average cubic fluorescence energy.40 The thereby 01 2 3 4 evaluated squared transition moments are also collected in Table 1. Ideally, in the absence of any major structural reorganization they should be the same as those of the absorption. This is fulfilled for the parent compounds B1, B2, and cS to a very good approximation. In ACS Paragon Plus Environment


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contrast, while BcSB shows a significantly higher by about 30%, cSBcS has distinctly smaller value by about 20%. However, when we take into account that the lowest energy band of cSBcS covers the two transitions into the lowest two exciton states of both the cisoid and transoid conformer, we considerably overestimate the squared absorption transition moment of the lowest exciton state from which fluorescence is emitted. Thus, the true squared transition moment of absorption into the lowest exciton state is unknown but most likely much smaller than 210 D2. For comparison, if we integrate the absorption band down to 20000 cm-1, which most likely covers the whole exciton manifold, we obtain a squared transition moment of 254 D2. The sum of squared transition moments of B2 and two times cS yields 233 D2 in reasonable agreement, which proved the validity of the Thomas-ReicheKuhn sum rule41 in the present case. Nevertheless, for BcSB the enhancement of vs is still surprising. This difference may

be explained by a structural reorganization of the lowest energy exciton state after excitation, that is, the exciton coupling is different for the excitation process where the chromophores are in the ground state configuration from that of the emission process where the excited state is in the relaxed excited state geometry.


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Table 2. Time-Resolved Optical Data of Triads and Reference Chromophores in Cyclohexane (CH) and Toluene at r.t.

τ/ns (TCSPC) a toluene

B1

4.7

B2

4.5

cS

3.45

BcSB

2.6

τ/ps (TA) b

τ/ps (TA) b

τ/ps (FLUC) c

(@lowest exciton state) CH

CH

CH

(@upper exciton state) CH

2.7

0.12 29 270 2600

2.7 2.9 75 280 2600

0.20 (-0.65) 14 (-0.17) 920 (-0.11) 2200 (1.00)

0.28 0.12 (-0.17) 100 79 (-0.10) cSBcS 2.5 2.7 980 2700 (1.00) 2600 a –1 Fluorescence lifetime measured by TCSPC, excitation for B1 and B2 at 23920 cm , cS at 15240 cm 0.05 480 2500

1

, and for BcSB and cSBcS at 19420 cm–1. Emission was measured for BcSB at 13100 cm-1 and for

cSBcS at 13300 cm–1.

b

TA measurements. The lifetimes refer to a global deconvolution. Excitation -1

-1

upper/lower exciton state of BcSB: 18800/13300 cm ; cSBcS: 16800/13500 cm .

c

Fluorescence

upconversion measurement (amplitudes). Excitation/probe wavenumber: 18800/12700 cm-1 (BcSB); 18200/12800 cm-1 (cSBcS).

Femtosecond time resolved spectroscopy. In order to gain insight into the photoinduced dynamics we measured transient absorption (TA) spectra of BcSB and cSBcS for excitation of the upper exciton state and the lowest exciton state, respectively. We performed these measurements in cyclohexane solution because toluene shows strong coherent artefacts and tends to broaden the instrument response considerably by its strong group velocity dispersion. From the steady state and ns time resolved fluorescence spectra (see Table 1 and 2), no major differences between toluene and cyclohexane solution is visible which justifies the use of the latter solvent for the fs-time resolved experiments. In these experiments, the samples were pumped by a ca. 140 fs pump pulse at the appropriate wavenumber and probed by a white light continuum (ca. 140 fs) between 12500 cm-1 and 24000 cm-1. For BcSB the transient spectra for both excitation wavenumbers are given in Figure 5 along with time traces at selected wavenumbers. A global analysis of the transient map yields decay associated difference spectra (DADS) which represent amplitude spectra of a ACS Paragon Plus Environment


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multiexponential deconvolution. These are also given in Figure 5; the evolution associated difference spectra (EADS) which pertain to a unidirectional sequential model are given in the SI. For excitation at the upper exciton state at 18800 cm-1, the deconvolution gave four components. The DADS associated with the longest time constant (2.6 ns) shows a strong ground state bleaching (GSB) at ca. 13200 cm-1 which is caused by a depletion of ground state population and the strong absorption of the squaraine absorption at this wavenumber. The low energy side of the GSB merges with stimulated emission (SE). There is also significant excited state absorption (ESA) around 15000-20000 cm-1. The time constant of this DADS agrees very well with the fluorescence lifetime in cyclohexane (2.7 ns, see Table 2) and represents ground state recovery. The two weak DADS with lifetimes in the ps time regime (29 and 270 ps) show a wave-like signature around 13000 cm-1. This is typical of band shifts associated with vibrational relaxation. The shifts of the combined GSB/SE signal may either be due to hot ground state population which affects an overlaying absorption signal to the low energy side of the GSB42 and/or to dynamic shifts of the SE caused by vibrational relaxation in the excited state.43-46 Solvent reorganization contributions (dynamic Stokes shift) can be excluded as the source of these shifts because of the low polarity of the cyclohexane solvent. However, the viscoelastic response of the solvent to the change of solute volume upon excitation may play a role here instead of the dielectric response.47-48 In addition, there is a very pronounced DADS which has almost mirror symmetry of the DADS with 2.6 ns. This DADS represents a rise of the transient absorption signal with τ = 0.12 ps and is caused by the population transfer from the highest to the lowest exciton state and thus corresponds to the internal conversion within the exciton manifold. Somewhat shorter times (0.03-0.09 ps) were observed recently for the internal conversion between the exciton states of squaraine dimers.23 Excitation of BcSB at the maximum absorption of the lowest exciton band at 13300 cm-1 gives very similar transient spectra which rise with the instrument response function without the additional rise time as in the experiment before (see e.g. time traces in Figure 5g and h around t = 0). The global analysis shows five contributions, one DADS with a 2.6 ns lifetime and again two DADS with wave-like shape and 75 and 280 ps lifetime, all in good agreement with the other experiment. At this point we stress that the DADS with wave-like shape represents shifts of the SE + ESA whose dynamics cannot necessarily be represented by exponential functions. The lifetimes of these shift components are therefore difficult to compare directly. The biggest difference to the former pump-probe experiment are two DADS components with lifetimes between 2-3 ps which show almost mirror image behavior. These two components turned out to be necessary to fit the transient map around 15000-20000 cm-1 accurately. In this wavenumber region, the TA amplitude decreases and rises slightly on the lower ps time domain which can


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13 also nicely be seen in the time trace in Figure 5f. The physical origin of the behavior is unclear but an explanation may be given later.

λ / nm 800

700

λ / nm

600

a

500

800

700

excitation

600

500

b

excitation

∆ OD

0.01

0.00

0.00

-0.01 -0.01 -0.02

-0.02 0.02

-0.03

c

d

DADS

0.01

0.00

2.7 ps

*

-0.01 -0.02

14000

∆ OD

16000

18000 ν / cm–1

e

-0.01

0.12 ps

2.9 ps

29 ps

75 ps

270 ps

280 ps

2600 ps

2600 ps

-0.03

0.003

0.01

*

0.00

20000

22000

14000

16000

18000 ν / cm–1

20000

-0.02

-0.03 22000

f

16700 cm–1

0.004

0.002 0.002 0.001

16100 cm–1

0.000

0.000

0.00

0.00

∆ OD

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

-0.01

g

h

-0.01 -0.02

13200 cm–1

-0.02 0

1

2

10

100

1000

13200 cm 0

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–1

-0.03

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Figure 5. Chirp corrected transient absorption spectra of BcSB in cyclohexane at r.t. after excitation at (a) 18800 cm–1 (533 nm) and (b) 13300 cm–1 (753 nm). Early spectra are given in blue, later spectra in red. Decay associated difference spectra (DADS) resulting from the TA spectra (c and d) and time scans at selected wavenumbers (e-h). The features marked with * are caused by Raman scattering.



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For cSBcS similar results as for BcSB were obtained at pumping the sample at the upper exciton level (see Figure 6). Again, the lifetime of the longest lived DADS (τ = 2.5 ns) agrees very well with that of the fluorescence lifetime (2.7 ns). The much weaker DADS with a wavelike shape around 13500 cm-1 with τ = 480 ps may again be ascribed to a shift possibly caused by a vibrational relaxation phenomenon. The DADS with τ = 0.05 ps represents the population transfer from the upper to the lower exciton state. Because this component approaches our time resolution, its exact time constant is rather uncertain. Furthermore, there are spurious contributions from the coherent artefact visible around 14000 and 20000 cm-1. Excitation of cSBcS at the lowest exciton state at 13500 cm-1 gives rise to a prominent DADS with τ = 2.6 ns and a series of much weaker DADS with time constants 0.28 ps, 100 ps and 980 ps again indicating a dynamic shift of the SE/ESA around 13500 cm-1. The shape of the DADS with τ = 2.6 ns is remarkable in regard to the unusually steep flank at the high energy side of the strong GSB signal at 13500 cm-1. Therefore, we modelled the sum of GSB and SE contribution by using the steady state fluorescence spectrum plotted vs. wavenumbers (divided by )49 and the steady state absorption spectrum which are given in Figure 7b and subtracted this sum from the transient absorption spectrum with maximum amplitude at 13500 cm-1. This difference spectrum represents the ESA contribution which shows a prominent positive absorption at 13800 cm-1. Such an ESA at slightly higher energy of the GSB is typical of an excitation into a two-exciton state where both squaraine chromophores are excited.4, 21, 37 A similar observation has been made for tSBtS.31 Likewise, the ESA of BcSB estimated in the same way (see Figure 7a) shows a peak at 18600 cm-1 which also is caused by a two-exciton state. However, because this two-exciton state is formed by two excited bodipy chromophores it is at higher energy than that of cSBcS.


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Figure 6. Chirp corrected transient absorption spectra of cSBcS in cyclohexane at r.t. after excitation at (a) 16800 cm

–1

(594 nm) and (b) 13500 cm

–1

(741 nm). Early spectra are given in blue, later spectra

in red. Decay associated difference spectra (DADS) resulting from the TA spectra (c and d) and time scans at selected wavenumbers (e-h). The features marked with * are caused by Raman scattering.



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0.00 -0.01

cSBcS -0.02 -0.03

TAmax abs + em ESA=TAmax-(abs + em)

-0.04 14000 16000 18000 20000 22000 24000 ν / cm–1

Figure 7. Difference spectra of TA spectrum (Fig. 5b and 6b) at maximum negative intensity (TAmax) and the sum of steady state absorption (abs) and emission (em) spectra to model the excited state (ESA) contribution in a) BcSB and b) cSBcS.

Fluorescence Upconversion Measurements. Both triads show very similar dynamics in the fluorescence upconversion measurements (FLUC) when excited in the upper exciton state and probed at the low energy flank of the fluorescence band (see Figure 8). This was necessary to circumvent reabsorption problems. In both cases there is a short rise component on the 100-200 fs time range and one or two components on the order of 101000 ps. The ns-time components (2.2 and 2.7 ns) agree reasonably well with those of the TCSPC measurements given the fact that the complete decay could not be covered because of the limited length of the delay stage. The rising components, which are on the ps time scale, are caused by vibrational relaxation phenomena which lead to a shift of the emission band towards lower energy which causes a signal rise because the probe wavenumber is at the low energy flank of the emission band. However, the rising component with τ < 1 ps could either also be caused by a vibrational relaxation or by the population transfer from the excited upper exciton state to the emitting lowest exciton state. A clear separation of these ACS Paragon Plus Environment

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17 two processes would require the analysis of time resolved fluorescence spectra which is impossible because of the small Stokes shift in the present case.

a

BcSB

12700 cm–1

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100

τ1 = 0.12 ps τ2 = 79 ps

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Figure 8. Fluorescence upconversion measurements of a) BcSB excited at 18800 cm–1 (IRF: 190 fs) and b) cSBcS excited at 18200 cm

–1

(IRF: 150 fs) in cyclohexane at r.t.

CONCLUSIONS While excitation of the squaraine-bodipy triads in the upper exciton level is followed by ultrafast (~ 0.1 ps) internal conversion to the lowest exciton level (see Figure 9), the most remarkable features of the photophysical behavior of BcSB and of cSBcS are the significant exchange narrowing of fluorescence spectra and the enhancement of squared fluorescence transition moments. Both can be understood if one assumes that the ground state potential energy surface is shallow but the lowest excited state potential curve rather steep, as has been observe for e.g. oligo(p-phenyleneethynylene).50 Fluctuations in the ground state potential will then reduce the electronic coupling between the chromophores which leads to additivity of their transition moments and to a “normal” band width. However, an excited state potential energy curve with steep flanks gives rise to narrow emission band widths. Due to



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the steep excited state potential energy curve, fluctuations are small and, thus, coupling is larger than in the ground state configuration. This will then enhance the transition moment and narrow the band width of fluorescence from the relaxed excited state relative to that of the absorption into the Franck-Condon state, see Figure 9. In this context it is noticeable that in the TA spectra of BcSB we see a weak decrease and increase of ESA around 16000 cm-1 which cannot be caused by population/depopulation processes and must consequently be ascribed to a relaxation phenomenon in the excited state potential energy surface. In conclusion, our study shows that exciton coupling can be different for the Franck-Condon and the relaxed excited geometry with significant consequences for the coupling which ultimately leads to sizable fluorescence transition moments and fluorescence quantum yields of these bodipy-squaraine triad NIR chromophores.

Figure 9. Potential energy scheme for ground and three exciton states of squaraine-bodipy triad BcSB. Two processes are sketched: a) absorption into S3 and internal conversion (IC) to S1, followed by vibrational relaxation (VR) and b) spectrally broad absorption into S1 (blue arrow) and narrow emission (red arrow) caused by the different shape of potentials.


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ASSOCIATED CONTENT Supporting Information Comparison of absorption and fluorescence excitation spectra. Evolution associated difference spectra. Syntheses of the triads. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx AUTHOR INFORMATION Corresponding author *E-mail: [email protected] ORCID 0000-0002-9652-9165 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge support from the German Science Foundation (FOR 1809) and the Solar Technology Goes Hybrid program of the Bavarian State. We thank Dr. H. Marciniak for helpful comments.

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Exciton Coupling Enhancement in the Relaxed Excited State - The

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