Resonance-regime behaviour of a F€oorster-transferfluorescent dye couple dissolved in a chiral nematic

liquid crystal

Martin Chambers, Monika Voigt, Martin Grell *

Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK

Received 1 November 2001; in final form 10 January 2002

Abstract

We have studied the photophysics of a F€oorster-transfer fluorescent dye couple (Coumarin ‘sensitizer’/Pyrromethene

‘emitter’) dissolved in a chiral nematic liquid crystal (CNLC). Fluorescence spectra were dominated by the (lower

concentration) emitter although only the (higher concentration) sensitizer was excited. The reflection band of the CNLC

was chosen to overlap with the fluorescence band of the emitter. Emission was modulated by the photonic stopband

condition imposed by the CNLC in a similar manner as for single-dye systems dissolved in CNLCs. Using F€oorster-couples instead of single-dye systems may lead to lower threshold dye-doped CNLC lasers. � 2002 Published by

Elsevier Science B.V.

1. Introduction

Over the last few years, the study of chiralnematic liquid crystals (CNLCs) doped with flu-orescent dyes has seen a remarkable revival. Thekey conceptual development that sets apart therecent work from older investigations is in thechoice of the pitch length p of the CNLC withrespect to the emission wavelength of the dye, kE

typically kE is in the visible range kE � (400 to700) nm. The classic study by Pollmann et al. [1],dating from 1976, addressed dye-doped CNLCsin the so-called Mauguin limit [2] with p � kE.

This is also the regime in which conventional,twisted-nematic liquid crystal displays operate [3].Pollmann et al. observed that the dye fluorescencein CNLC solution may display a certain degree ofcircular polarisation (CP), and developed aquantitative theory of CP based on the guest–host alignment of the fluorescent dye in theCNLC.

The common feature of the more recent work inthe field [4–9] is that CNLCs with p close to thevisible regime were used. Short-pitch CNLCs dis-play a chiroselective optical reflection band cen-tred at kR ¼ –np, with the CNLCs’ averagerefractive index, and bandwidth Dk=kR � Dn=–n,with Dn the CNLCs’ birefringence. Within thatband, light with CP equal to the sense of chiralityof the supramolecular helix is reflected.

2 April 2002

Chemical Physics Letters 355 (2002) 214–218

www.elsevier.com/locate/cplett

* Corresponding author. Fax: +44-114-272-8079.

E-mail address: m.grell@sheffield.ac.uk (M. Grell).

0009-2614/02/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S0009 -2614 (02 )00119 -7

Work with dye-doped CNLCs in the ‘resonanceregime’ kR � kE has targeted two main applica-tions: Firstly, the manufacture of circularly polar-ised light sources [4–6], and secondly, themanufacture of low threshold lasers [7,8]. In par-ticular for lasing applications, the peculiar behav-iour of dye emission CP near the edges of the CNLCchiroselective reflection band is of outstanding in-terest. In the centre of the reflection band, dyeemission CP is of equal sense as it is for light froman external source transmitted through the CNLC.At the edges of the reflection band, however, thesense of CP is reversed [9]. In their groundbreakingpaper, Kopp et al. [7] have interpreted this behav-iour as the result of the enhanced density of statesfor the reflected sense of CP at the band edges. Theyhave recognised the potential of this phenomenonfor mirrorless lasing, and demonstrated lowthreshold band-edge lasing from a dye-dopedCNLC [7]. Recently, we have firmly established thelink between CP sign reversal and reflection bandedge: Sign reversal is ‘pegged’ to the reflection bandedge when the band blueshifts under non-normalviewing angle [9]. CNLCs are now treated withinthe wider framework of photonic crystal theory [10]as one-dimensional photonic crystals, and the re-flection band is identified as a ‘photonic stopband’for one (but not the other) sense of CP.

In the present Letter, we study a dye-dopedCNLC system that we believe represents an im-provement over the system that Kopp et al. [7]have used in their original report. One advance isthat we place the long-wavelength, rather thanshort-wavelength, reflection band edge into closevicinity of the dye gain maximum. The CP reversalat the long-wavelength edge is generally morepronounced than at the short-wavelength edge [9].This can be understood as a result of the differentlocations of the standing-wave nodes and anti-nodes with respect to the high- and low-refractiveindex regions in the CNLC at the different stop-band edges. Another advance is that we are notusing a single dye, but two dyes that constitute aF€oorster-transfer couple. A F€oorster couple consistsof a sensitizer and an emitter dye that display goodspectral overlap between the sensitizer emissionand the emitter absorption. This allows for arapid, radiationless excitation transfer from the

sensitizer to the emitter, which is known asF€oorster-transfer [11]. Consequently, in F€oorstercouples, the emitter may dominate the fluorescencespectrum even if it is present at a lower concen-tration than the sensitizer. A F€oorster couple thusfacilitates a decoupling between excitation andemission processes, and redshifts the emissionwavelength relatively far away from the maximumabsorption. These properties of F€oorster couplesresult in distinct advantages over single-dye sys-tems for organic lasing applications, which wereconfirmed in a number of reports [12–15]. In par-ticular, we believe that the use of a F€oorster couplewith high sensitizer/low emitter concentration al-lows excitation ‘enrichment’, and thus populationinversion, on the emitter at lower pump intensities.

2. Experimental

As CNLC, we used the commercially availabletwo-bottle kit BL130/BL131 from Merck, whichallows for tuning of the reflection band by dilutinga CNLC (BL131) with an achiral nematic (BL130).As sensitizer and emitter dye, we used Coumarin153 and Pyrromethene 580, respectively, bothsupplied by Lambda Physik. Dyes were dissolvedin methanol and dye solution was added to theCNLC mixture with a microlitre syringe to allowfor precise control of concentration. Dye-dopedCNLCs were then gently heated and stirred forseveral hours to evaporate methanol. By capillaryaction, dye-doped CNLC was filled into commer-cially available LC test cells with aligning surfaces,supplied by E.H.C. of Tokyo. Sensitizer (Couma-rin 153) concentration was adjusted so that in a25 lm cell, the sensitizer absorbance at its maxi-mum (424 nm) was >1. This ensures that whenexcited at 424 nm, >90% of incoming photons areabsorbed by the sensitizer. Emitter (Pyrromethene580) was added up to a concentration that ensuresan almost complete dominance of emitter oversensitizer fluorescence, i.e. almost completeF€oorster-transfer. Absorption and transmissionmeasurements were carried out with a UNICAM 4UV/Vis spectrometer. Reflection spectra undernormal incidence were taken with the help ofa beamsplitter. For the characterisation of

M. Chambers et al. / Chemical Physics Letters 355 (2002) 214–218 215

fluorescence CP, an optical setup was used thatemployed a halogen lamp and programmablemonochromator for narrow bandwidth excitation.Cells were mounted so that they could be rotatedaround the axis vertical to light propagation forstudies of non-normal emission. A Fresnel rhombwas used to convert the emitted fluorescent light ofCP with opposite senses into light with mutuallyperpendicular linear polarisations independent ofwavelength. Light leaving the rhomb was collectedand focussed by a lens. A linear polariser that couldbe rotated around the axis of light propagation wasintroduced to discriminate between the differentpolarisations. One end of a liquid light guide wasplaced into the focus of the collection lens. Theother end of the light guide was fed into a diffrac-tion grating/ANDOR CCD camera spectrograph.

3. Results and discussion

Fig. 1 shows the absorption and fluorescenceemission spectra of sensitizer and emitter dyes,respectively, in methanol solution. All spectra arearbitrarily normalised to equal peak height. Notethat there is considerable spectral overlap betweenthe sensitizer emission spectrum and the emitterabsorption spectrum, as required for a goodF€oorster couple [11].

The absorption spectrum of a cell filled with thesensitizer/emitter/CNLC mixture is shown in

Fig. 2. We see the superposition of sensitizer ab-sorption (peak at 424 nm), emitter absorption(peak at 528 nm), and CNLC reflection band(long-wavelength edge at 588 nm). For clarity, Fig.2 also shows the reflection spectrum from the samecell. The reflection band has no contribution of thedye absorption, and we clearly see the short-wavelength stopband edge at � 537 nm as well asthe long-wavelength edge at � 590 nm. The oscil-lations outside the reflection band are Fabry–Perotinterferences of light reflected from the front andrear faces of the cell. When a droplet of sensitizer/emitter/CNLC mixture is diluted in methanol andabsorption spectra are taken in a cuvette, theCNLC phase is destroyed and we can measure theabsorption spectra of the dyes without contribu-tion from the reflection band. This complementsthe information contained in Fig. 2. From the in-tensity ratio of sensitizer/emitter absorbancemaxima found in such a measurement (not shownhere), and the known molar extinction coefficientsof both dyes [16], a ratio of � 8 : 1 (mol/mol) ofsensitizer to emitter can be calculated. In absoluteterms, the concentrations of sensitizer and emitterare approximately 15 and 2 mmol/L, respectively.The emitter concentration is thus rather low, andfor possible lasing applications, we may expect lowgain. However, Kopp et al. [7] have observed lowthreshold lasing with the same emissive dye-dopedinto a CNLC at a similar cell thickness and con-centration. This is a result of the excellent feed-

Fig. 1. The normalised absorption and emission spectra of the

Coumarin 153 sensitizer (x: absorption, o: emission), and the

Pyrromethene 580 emitter (+: absorption, ): emission). Note

the overlap between sensitizer emission and emitter absorption,

which is essential for efficient F€oorster-transfer.

Fig. 2. The absorption spectrum of a 25 lm cell filled with our

CNLC/sensitizer/emitter mixture, showing sensitizer and emit-

ter absorption as well as the reflection band. The broken line

shows a (non-calibrated) reflection spectrum of the same cell.

216 M. Chambers et al. / Chemical Physics Letters 355 (2002) 214–218

back provided at the CNLC band edge, and sincewe expect even better feedback at the long-ratherthan short-wavelength edge, we expect to find evenlower threshold lasing despite low gain.

Fig. 3 shows the circularly polarised fluorescencespectra from a cell filled with sensitizer/emitter/CNLC mixture under excitation at 424 nm. At 424nm, Pyrromethene absorbance is negligible andlight will almost exclusively be absorbed by theCoumarin sensitizer. However, the left-handed CPspectrum is very similar to the Pyrrometheneemission spectrum found in methanol solution,which is shown as the doted curve in Fig. 3. Thisindicates that F€oorster-transfer is highly efficient intransferring almost all excitons from the sensitizerto the emitter. Since the CNLC displays right-han-ded sense of chirality, there is no effect of the CNLCmatrix on emission into the left-handed sense of CP.

The emission into the right-handed sense of CPagain is clearly dominated by the Pyrrometheneemitter, however, it is strongly affected by thephotonic stopband condition established by theCNLC. As in our previous study on a CNLC/single dye system [9], the right-handed fluorescenceintensity in the stopband centre is strongly sup-pressed. In fact, we find a remarkably high po-larisation ratio r of left-handed/right-handedintensity of 8.85:1 at 545 nm. At the long-wave-length stopband edge, CP is reversed within anarrow wavelength band due to the enhanceddensity of states for right-handed emission at the

stopband edge. This enhancement is more pro-nounced at the long-wavelength stopband edgedue to the favourable node/antinode locations ofthe standing wave. The pronounced maximum ofright-handed emission is found at k ¼ 595 nm,slightly redshifted from the long-wavelengthstopband edge as seen in Fig. 2b. This is in ac-cordance with our earlier observations on singledye/CNLC systems [9]. The right-handed CP peakat the long-wavelength stopband edge has a widthof � 17 nm FWHM.

Fig. 4 shows that again, as for a single dye/CNLC system [9], the location of the CP sign re-versal is pegged to the CNLC stopband edge. Thereflection band location kr of a CNLC blueshiftswith viewing angle according to Eq. (1) [17],

kRðhÞ ¼ kRð0Þ cos h; ð1Þwherein h stands for the internal angle of lightpropagation with respect to the cell normal, whichis related but not identical to the external viewingangle. Fig. 4 shows the degree of CP gðkÞ for thesame cell as used for Fig. 3 when it is viewed underincreasing angle. gðkÞ is defined in Eq. (2):

gðkÞ ¼ 2ILðkÞ � IRðkÞILðkÞ þ IRðkÞ

¼ 2rðkÞ � 1

rðkÞ þ 1; ð2Þ

with IL=R the intensity of left-/right-handed lightand r ¼ IL=IR the CP ratio. CP reversal translatesinto a change of sign in g. In Fig. 4, we see that thesign reversal in g blueshifts as it tracks the shift of

Fig. 3. The left-handed (+) and right-handed ()) circularly

polarised emission spectrum from the sensitizer/emitter/CNLC

cell. Excitation was at 424 nm. For comparison, the dotted line

shows the emitter fluorescence when recorded in methanol so-

lution when excited at 365 nm.

Fig. 4. The degree of CP gðkÞ for a sensitizer/emitter/CNLC

cell viewed under different angles to the normal: 0� (�), 10� (}),

20� (D), and 30� ().

M. Chambers et al. / Chemical Physics Letters 355 (2002) 214–218 217

the stopband: Sign reversal is pegged to the stop-band edge [9]. Note that the gðkÞ spectrum for zeroviewing angle is an alternative representation ofthe data in Fig. 3. Maximum g and intensity ofsign reversal are reduced under off-axis viewing, asthe optical quality of a CNLC suffers from in-creasing viewing angle.

The Pyrromethene fluorescence maximum islocated at 570 nm, while its tuning range for lasingis centred at 590 nm [16]. Such a redshift of thegain spectrum with respect to the fluorescencespectrum is common for laser dyes, resulting fromresidual dye absorbance in the gain region. In aF€oorster couple, residual dye absorbance is reduceddue to the lower emitter concentration, and thegain maximum may be closer to the fluorescencemaximum. From Fig. 4, we see that off-axisviewing allows us to scan the stopband edgethrough the gain region to match maximum gain.

4. Conclusions and future work

We have demonstrated that in the resonanceregime of a CNLC doped with a suitable F€oorster-transfer dye couple, the CP of the resulting fluo-rescence displays a peculiar behaviour which is verysimilar to that of single dye/CNLC systems studiedpreviously [4,7,9]. We find a high degree of CPðgMAX � 1:6Þ and a pronounced reversal in thesense of CP at the long-wavelength stopband edge.We believe that a F€oorster couple may offer signifi-cant advantages over single dye systems for the useof dye-doped CNLCs as low threshold lasers. Thisis because of reduced self-absorption, and the de-coupling of the excitation and emission process inF€oorster couples. The logical next step for our workis the study of the system introduced here underhigh excitation densities, in the hope to find lowthreshold lasing.We briefly wish to comment on thechoice of emitter concentration. We have chosen anas low as possible concentration that is still suffi-cient to ensure (almost) complete F€oorster-transfer.This will lead to reduced self-absorption and en-hanced excited state population ratio due to exciton‘enrichment’, which we believe may outweigh thedrawback of low gain due to low concentration.However, we have carried out spectroscopy at very

low excitation density. As we approach the lasingthreshold, we may find that F€oorster-transfer maybecome incomplete (i.e., there will be a relative riseof sensitizer emission) due to emitter saturation. Inthat case (and we believe, only in that case), it maybe helpful to somewhat increase emitter concen-tration.

Acknowledgements

MV wishes to acknowledge support from theEuropean Community under the ‘EUROLED’research and training network. We wish to ac-knowledge the EPSRC for support of our workunder grant No GR/R41200, and MERCKDarmstadt for the provision of CNLC mixtures.

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