Fluorescence Spectroscopy

The distinction between P2 monomers and J-aggregates is also possible by fluorescence spectroscopy. In solutions/dispersions, containing monomers and aggregates, both species can be selectively excited within their specific absorption bands. As could be seen in the previous experiment in Fig. 20a and b, within the solvent mixture of anhydrous n-heptane and a small amount of anhydrous DCM, P2 is not only present as J-aggregates, but also partly resides dissolved as monomers. Such a solution/dispersion has been used to selectively excite monomers and J-aggregates at 420 nm and 480 nm, respectively. After recording the absorption and fluorescence spectra a few drops of methanol were added to this solution in order to disassemble the aggregates to monomers. Then the fluorescence measurements have been repeated. The results are shown in Fig. 22.

Fig. 21: Absorption spectra monitoring the dissolution of P2 J-aggregates in DCM

The same cuvette, with the P2 J-aggregates on the walls (dark) solvated in pure (anhydrous) n-heptane as shown in Fig. 20a, c, was filled with anhydrous DCM and shook gently. This initiated a dissociation process of the J-aggregates to monomers leading to a steady rise of the Soret band at 426 nm and of the monomeric Q band at 551 nm, whereas the red-shifted Soret and Q band around 470 nm and at 632 nm, respectively, decreased at the same time during 70 min (blue – red). The inset shows a magnification.

300 400 500 600 700 800

0.0 0.2 0.4 0.6 0.8 1.0

400 600 800

0.00 0.05 0.10

Absorbance

Wavelength / nm P2 + n-heptane added DCM + 9 min.

+ 11 min.

+ 24 min.

+ 26 min.

632 590 582 551 426

1.2.3 Optical Properties 36

The typical J-aggregate absorption spectrum changed almost to a pure monomeric one upon the addition of methanol (Fig. 22a). The slight red-shift of this monomeric absorption spectrum, which is even enhanced in the fluorescence spectrum, is caused by a complexation of the porphyrin's central zinc atom by methanol.

Upon the excitation at the J-aggregate's absorption band at 480 nm the fluorescence intensity was significantly lower, than that measured upon excitation of the monomers at 420 nm. Within the predominantly monomeric solution, containing methanol, there is no significant qualitative difference in the fluorescence spectra at both excitation wavelength (Fig. 22b, blue, green). Without the methanol instead, both fluorescence spectra differ also qualitatively from each other (dark, red). There the main fluorescence band of the monomers at 592 nm almost disappeared and the band around 713 nm, which was a small shoulder upon excitation of the monomers, developed to a distinct band upon excitation within the aggregate's absorption band. Thus, we assume, that these two fluorescence bands, at approx. 647 and 713 nm, origin from the J-aggregates and the bands at 592 and 647 nm origin from the monomers' decay.

The shoulder of the monomer's fluorescence at 713 nm may be caused by an energy transfer from the monomers to the J-aggregates. Another possibility would be a decay from the monomeric LUMO into the second excited vibrational level of the monomeric HOMO, whereas the band at 647 nm is supposed to be the decay into the first excited vibrational level and the band at 592 nm is the decay into the ground state of the monomeric HOMO. The almost disappearing of the monomeric band at 592 nm upon excitation of the aggregates at 480 nm and the development of the fluorescence band at approx. 713 nm can be explained by a red-shift of both (or the possible three) monomeric bands by 0.18 eV each. The possible third monomeric band can not be resolved in the red-shifted J-aggregate spectrum, due to the low intensity level. This obvious red-shift in the fluorescence spectrum asserts, that the J-aggregates have been indeed selectively excited and that they fluoresce in the solid-state, what is a good indication for a long-lived excited state.

Time resolved fluorescence measurements have been done by cooperation with Jędrzej Szmydkowski (group of Heinz Kalt, KIT) in order to investigate the lifetime of the excited states, which may allow to estimate the exciton diffusion time within the aggregates. A similar solution/dispersion of P2 as in the previous experiment has been prepared within n-heptane to selectively excite the monomers and J-aggregates by

Fig. 22: Fluorescence spectra of P2 within n-heptane with and without methanol

A dispersion of P2 within anhydrous DCM has been injected into a cuvette with anhydrous n-heptane to induce precipitation of P2 J-aggregates. After recording the absorption (a, dark) and fluorescence spectra (b, dark and red) 2-3 drops of methanol were added into the cuvette followed by a gently shake and repeat of the measurements (blue, green). The inset shows a magnification of the bottom part of the fluorescence spectrum. The tail of the absorption spectrum of the aggregate dispersion (a, dark) is an artifact caused by light scattering at the J-aggregates, because no integrating sphere was used here.

500 600 700 800

0 20 40 60 80 100 120

400 500 600 700 800

0.0 0.5 1.0 1.5 2.0 2.5

420

480

P2-DCM in ...

n-heptane

n-heptane + methanol

Absorbance

Wavelength / nm

600 700 800

0 4 8

665

400500 6007008000. 00. 51. 01. 52. 02. 560 3588420433638562480 n- heptane + DCM n- heptane + DCM + MeOHAbsorptionWavelen gth / nm

713 647 623

Intensity / a. u.

Wavelength / nm

Ex. at 420 nm Ex. at 480 nm

Ex. at 420 nm (+ methanol) Ex. at 480 nm (+ methanol) 592

b) a)

1.2.3 Optical Properties 37 different wavelengths. The precipitation of J-aggregates has been induced by injecting a small amount of an anhydrous P2-DCM solution into a major amount of n-heptane with a very low water content (≤ 0.005 %).

The monomers were excited by a pulsed laser at 430 nm and aside the monomeric absorption band the J-aggregates were excited at 447 nm. The time- and wavelength-resolved measurements are shown in Fig. 23.

The excitation within the Soret band of monomers at 430 nm gave a fluorescence spectrum with three bands, labeled as A, B and C (Fig. 23a: red). Band A and C are expected to be the two main fluorescence bands of P2 monomers, whereas the band B most probably originates from the complexation of P2 monomers with water, because a similar red-shift was caused by the monomer-methanol complex known from Fig. 22b. The excitation wavelength of 447 nm was intended to excite only the J-aggregates, but the biexponential fluorescence decay in the time-resolved measurement suggests the excitation of two different species ( Fig.

23b). Whereas the excitation of monomers at 430 nm led to a monoexponential decay of the fluorescence band A (Fig. 23b: red, black), the decay of the fluorescence at the wavelength D, after excitation at 447 nm, implies the existence of two excited state lifetimes, affirmed by the good fitting with a biexponential function (blue). As the fluorescence band A was only present after excitation within the monomer's absorption band, the fitted mean lifetime of 2.5 ns is expected to originate from the monomers. The fluorescence band D of the excited J-aggregates instead, has an overlap with the fluorescence band C, which occurred by exciting the monomers. Thus, the two decay times at the wavelength D of 2.0 nm and 40 ps are supposed to correspond to the fluorescence of monomers and J-aggregates, respectively. The question that arises here is how could the monomers got excited at 447 nm, too, when the experiment in Fig. 20a and b showed, that the absorbance of the residual monomers in solvent mixtures of n-heptane and DCM is almost zero at this wavelength. The answer may give the unexpected band B, which is at a similar wavelength as the monomer-methanol complex of the previous experiment and may be attributed to a similar complex with water, which was contained to a small amount (≤ 0.005 %) within the commercial n-heptane. The wavelength for exciting the aggregates could not have been set to higher values as 447 nm, because this was the limit of the used optical parametric oscillator (OPO), which tuned the laser wavelength. At this wavelength a red-shifted monomer complex with water, could be excited beside the J-aggregates, too. Thus, the longer excited state lifetime of 2.0 ns supposedly corresponds to the decay of the monomer-water complex, whereas the shorter lifetime of

Fig. 23: Wavelength- and time-resolved fluorescence of P2 monomers and J-aggregates

P2 was dissolved within anhydrous DCM and injected into commercial dry n-heptane (≤ 0.005 % water) to induce the precipitation of J-aggregates. After the pulsed excitation with a laser at 430 nm (red) and at 447 nm (green) the decay of the fluorescence signal was measured wavelength- (a) and time-resolved (b) by a CCD detector. The time-resolved fluorescence measurements of the fluorescence band denoted with “A”, which originated from an excitation at 430 nm (a, red), showed a monoexponential decay, where the mean lifetime could be fitted to 2.5 ns (b, dark). The decay of the fluorescence band “D” (green) could be fitted by a biexponential function, corresponding to two mean lifetimes of 2.0 ns and 40 ps (blue).

600 650 700 750

0.0 0.5 1.0

C D B

A

Fluorescence / arb. u.

Wavelength / nm λ_exc= 430 nm λ_exc= 447 nm

0 500 1000 1500

0.0 0.5 1.0

Time / ps

Fluorescence / arb. u.

λ_exc= 430 nm, λ_emi= band A fit with τ₁=2.5 ns

λ_exc= 447 nm, λ_emi= band D fit with τ₁=2.0 ns; τ₂=40 ps

a) b)

1.2.3 Optical Properties 38 40 ps is attributed to the lifetime of the J-aggregates.

The fluorescence of a similar solution/dispersion of P2, aggregated from an anhydrous DCM solution/dispersion in commercial n-heptane (≤ 0.005 %), has been been measured wavelength-resolved freshly after preparation and after 4 days storage at room temperature (Fig. 24).

At the excitation wavelength of 440 nm all four bands (A-D) are present within the fluorescence spectrum (Fig. 24a, dark). At the excitation wavelength of 450 nm the monomeric band A, as well as the band B of the supposed monomer-water complex, decreased significantly and didn't decrease further at the excitation wavelength of J-aggregates at 480 nm (red, green). After storage of the sealed cuvette for four days at room temperature the fluorescence band of the aggregate (D) vanished almost completely and only the monomeric bands (A, B, C) remained at the excitation wavelength of 440 nm (blue). The additional vanishing of the monomeric fluorescence bands at the excitation wavelength of 480 nm proves that at this wavelength no monomers can be excited, just exclusively the J-aggregates.

Although the absorption spectrum of this solution/dispersion shows the presence of monomers and J-aggregates (not shown here) the latter lost their ability to fluoresce almost completely within this liquid. As the chemical stability of P2 within J-aggregates is very high, what will be shown later, this absence of fluorescence may be due to a change in aggregate morphology. A possible change at this conditions and within this period of time may be the growth of small dispersed J-aggregates to bigger ones or the disordered agglomeration of small aggregates to larger clusters which settled onto the cuvette walls. This can mean, that the trap density, at which excitons quench, increases with the growth of the aggregates or the agglomerates.

Fig. 24b shows the three dimensional fluorescence spectrum of the four days old solution in its original sealed cuvette. The decrease of the band A and the evolution of the band B can be seen in the smaller topographic view (top right corner) at higher excitation wavelengths. This measurement is an excerpt from a three dimensional (3D) fluorescence spectrum of this solution, which is shown in Fig. 25.

Fig. 24: Fluorescence spectra of P2 within commercial n-heptane as prepared and after 4 days

An anhydrous P2-DCM solution was injected into a cuvette with a major amount of commercial n-heptane to induce the precipitation of J-aggregates. The fluorescence of this solution/dispersion was measured immediately after preparation (a: dark, red, green) and after four days storage within the sealed cuvette (blue, magenta). The two last spectra (blue, magenta) are also shown in b) together with a series of fluorescence spectra measured at different excitation wavelengths of this four days old solution. The curves in a) are very noisy, due to the low intensity and a very fast scan rate of 10 nm/s of the emission wavelengths. In addition the resolution was limited by the slit bandwidth to 5 nm for the excitation and the emission wavelength.

550 600 650 700 750

0 5 10 15 20 25

30 After 4 days

Ex.: 428 nm Ex.: 430 nm Ex.: 432 nm Ex.: 434 nm Ex.: 436 nm Ex.: 438 nm Ex.: 440 nm Ex.: 480 nm

Intensity / a.u.

Wavelength / nm

600 650 700 750 800

0.0 0.5 1.0 1.5

Intensity / a.u.

Wavelength / nm Ex.: 440 nm Ex.: 450 nm Ex.: 480 nm

Ex.: 440 nm after 4 days Ex.: 480 nm after 4 days

a) b)

A B

C

D

A

B C

D

1.2.3 Optical Properties 39

Im Dokument Biomimetic Dye Aggregate Solar Cells (Seite 35-39)