A fiber-optic spectroscopic setup for isomerization quantum
yield determination
Anouk Volker1,2, Jorn D. Steen*1 and Stefano Crespi*1

Full Research Paper
Address:
1Department of Chemistry - Ångström Laboratory, Uppsala University,
Box 523, 751 20 Uppsala, Sweden and 2Faculty of Science andEngineering, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands

Open Access
Beilstein J. Org. Chem. 2024, 20, 1684–1692.
https://doi.org/10.3762/bjoc.20.150
Received: 14 May 2024
Accepted: 10 July 2024
Published: 22 July 2024

Email:
Jorn D. Steen* - jorn.steen@kemi.uu.se; Stefano Crespi* stefano.crespi@kemi.uu.se

This article is part of the thematic issue "Harnessing light energy withmolecules".

* Corresponding author

Guest Editor: H. A. Wegner

Keywords:
isomerization; molecular photoswitches; photochemistry; photon flux;
UV–vis spectroscopy© 2024 Volker et al.; licensee Beilstein-Institut.
License and terms: see end of document.

Abstract
A spectroscopic setup for isomerization quantum yield determination is reported. The setup combines fiber-coupled LEDs, a commercially calibrated thermopile detector for measurement of the photon flux, and a fiber-coupled UV–vis spectrometer. By solving
the rate equations numerically, isomerization quantum yields can be obtained from the UV–vis absorption spectra. We show that
our results for the prototypical photoswitch azobenzene are in excellent agreement with the literature. The analysis of the errors
showed that the quantum yields determined using this method are in the same order of magnitude as when using actinometry, thus
demonstrating the reliability of our setup.

Introduction
Photoswitches are molecules that can undergo a light-driven
structural rearrangement to populate a metastable state of theinitial reactant. This isomerization can be reversed by light of adifferent wavelength or thermal stimuli. Depending on thenature of the reactant, different classes of photoreactions can beutilized: double bond isomerizations, electrocyclization, cycloadditions, and electron-, hydrogen-, or group transfer [1].

its trans-to-cis interconversion under direct sunlight dating backto the work of Hartley in 1937 [2]. Over the decades, azobenzene and its derivatives [3,4] have found numerous applicationsin the development of novel materials [5,6], photopharmacology [7-9] but also as actinometers [10,11], due to the large
geometry changes upon isomerization [12] and easily accessible derivatives [1,7,13,14].

Among the different switches known in the literature, azobenzene is a textbook example of a photoswitch operating via adouble bond isomerization mechanism, with the first reports of

The absorption spectra of the two isomers each show distinct
bands, as shown in Figure 1. For trans-azobenzene, there is aπ→π* band around 320 nm and a very weak (symmetry-

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Figure 1: a) Schematic overview of a photochemical isomerization and b) absorption spectra of the isomers of azobenzene.

forbidden) n→π* band around 440 nm. cis-Azobenzene shows
two bands at 240 and 280 nm, and the n→π* transition isslightly more allowed than for trans-azobenzene [15]. As the
bands of cis- and trans-azobenzene overlap, irradiation results
in a photostationary state (PSS) which is a mixture of theisomers. The distribution of the isomers at the PSS differs forirradiations at different wavelengths [16]. Irradiation at
340–350 nm, where the ratio εtrans/εcis is the highest, results inthe formation of >95% cis-azobenzene [17].
The quantum yields (Φ) of isomerization of this process havebeen determined using different irradiation wavelengths, solvents, and methods [18-24]. One of the most interesting properties of azobenzene is the dependence of the Φ on the irradiation
wavelength [25]. Clear differences are reported between irradiation of the different absorption bands, due to the so-called antiKasha behavior of this molecule, i.e., the wavelength dependency of the quantum yield of reaction [22,24].
Despite its textbook example status, the photoisomerization ofazobenzene is still under active investigation. Over the past two
decades, both experimental [23,26] and theoretical work [2732] has been performed to explain the differences in quantum
yield. Not only the exact details of the photoisomerization are
still under debate, but also the absorption spectrum of cisazobenzene has been redetermined as recently as 2017 [17].
With the re-evaluated values for the molar absorptivities, the Φof azobenzene were recalculated by the same group immediately afterwards [24].
As shown by Vetráková et al. [17], the spectrum of transazobenzene can be easily obtained by careful preparation of thesample in the dark. The absorption spectrum of cis-azobenzene,
however, is challenging to obtain via direct measurement, due

to its intrinsic thermal instability and sensitivity to ambient
light. The authors showed that the cis-azobenzene previously
isolated by extensive irradiation [2], liquid chromatography[22], or preparative TLC [21,33] contained some of the transisomer. Their work highlights the care that needs to be taken to
obtain correct values for the molar absorptivities of cis-azobenzene. This conclusion holds for any photoswitch where only one
isomer can be isolated successfully. During the measurement
and at the PSS, the relative concentrations of the two isomers
are unknown. The total absorbance (Absobs) at a given total concentration (c) of the sample is a linear combination of the absorbance of each isomer, as illustrated by Equation 1.
(1)
The value for c at the PSS can be found using other spectroscopic methods, such as NMR spectroscopy [17] or the TEM(for Thulstrup, Eggers, and Michl) method [34,35]. With aknown value for c, the spectrum of the second component canbe found by a simple scaled subtraction [14]. Another powerful
method to obtain the spectrum of the second isomer is global
and target analysis, where time-resolved spectra can beanalyzed to find the number of spectral components and theevolution of their concentration over time [36,37]. Additional
information, such as a kinetic model describing the components
and the non-negativity constraint on the absorbance, can help to
guide the decomposition of the data into chemically meaningful concentration traces and species-associated spectra (SAS)[38]. Global and target analysis is one of the methods employed in recent literature to find the spectrum of cis-azobenzene [17].
To calculate the quantum yields, both the molar absorptivitiesof the isomers and the number of photons absorbed by the sam-

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ple needs to be known very accurately. Utilizing chemical actinometers remains the most used method for the determination ofphoton flux, i.e., the number of photons impinging a unit area ofthe sample in a unit of time [10]. Potassium ferrioxalate [39,40]
is one of the most used actinometers while also some azobenzene derivatives have been reported as reaction quantum yieldstandards [11,22]. Although new chemical actinometers arebeing proposed [41,42], actinometry is dependent on quantum
yields determined decades ago [10,43]. Using chemical actinometers is often labor-intensive, and measurements are
restricted to irradiation wavelengths where the actinometer
absorbs. With high-quality and reliable thermopile and solar
cell detectors now being commercially available at a reasonable
cost, instructions for building laboratory apparatus for quantum
yield determination using direct measurement of photon flux
using solar cell detectors have been reported by Riedle andco-workers [44,45]. With the design reported by Stadler et al.
[46], on the other hand, it is possible to follow the isomerization in operando, while the photon flux can be obtained bydisassembling the setup and reading the power of the excitation
source using an integrating sphere.

The first term gives the photochemical reaction from A to B,
which is governed by the number of photons absorbed at agiven wavelength, qA, as well as the quantum yield ΦA→B for
this reaction. NA is Avogadro’s number, V is the total volume ofthe sample. When the cis-isomer absorbs at the same wavelength, the reverse reaction will happen photochemically as
well, as described by the second term. The last term describes
the thermal reaction converting cis-azobenzene into the stabletrans-azobenzene. For the selected example, the rate constantfor the thermal back isomerization is 7.2∙10−7 s−1 at 20 °C [24].
This value is generally disregarded for routine experiments
where the isomerization happens in the minute timescale, however we explicitly included it in our quantum yield determination for better accuracy.
Using the Beer–Lambert law, the light absorbed by the sample,
Abstot, in a setup with a given path length l can be expressed as
the sum of the absorbances of the single species A and B andconsequently in terms of the concentrations of A and B andtheir respective molar absorption coefficients (εA and εB):(3)

In this paper, we introduce a relatively simple setup thatcombines the positive features of the aforementioned designs,
which can be used to determine the forward and backward
quantum yields of molecular switches without the need for
actinometry, by combining fiber-coupled LEDs with a commercially calibrated thermopile detector and following the isomerization of a molecular switch by UV–vis absorption spectroscopy in operando. We provide all the information necessary tobuild the setup as well as the scripts and examples to retrieve
the quantum yields of forward and backward isomerization of amolecular photoswitch following the evolution of the absorption in the low absorption regime [47] upon irradiation.

Results and Discussion
Azobenzene was chosen as the model compound for testing, as
the recent work by Vetráková et al. [17] and Ladányi et al. [24]has provided us with very reliable molar absorptivities andquantum yields to use for validating our methods.
A molecular photoswitch generally has a stable state A and ametastable state B, which would be the trans- and cis-isomers,
respectively, for azobenzene. The rate equations of the isomerization can be described in terms of the quantum yields and themolar absorption coefficients. It is possible to show that thechange in the concentration of A is given by Equation 2 [47]:(2)

For absorbances below 2, not all photons are absorbed when
passing through the sample. In this case, the number of photons
absorbed by a species (qA for species A in the following example) can be given by Equation 4 [47]:(4)

Where q0 is the total number of photons absorbed by the sample. Combining Equations 2 and 4, the rate equations for thesystem are given by Equation 5 and Equation 6:

(5)

(6)

So far, the spectral shape of the excitation light source (an LED
in our case, vide infra) has not been explicitly considered andhas been assumed to be monochromatic. Although this approximation is effective for many light sources with narrow bandwidths, the emission spectrum can be included in the rate equa-

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tions. This correction is done by multiplying Equation 5 by thespectral distribution f(λ) of the light source and integrating over
all wavelengths [45]:

(7)

where
(8)

Setup
The UV–vis irradiation experiments were performed using a
setup assembled in-house, see Figure 2 and Supporting Information File 1. A Quantum Northwest Luma 40 Peltier-based
temperature-controlled cuvette holder with an insulator jacket
with four optical windows was used to allow irradiation perpendicular to the spectrometer light path. The light source is an
Avantes AvaLight-DH-S-BAL light source coupled with a
400 µm fiber (Avantes FC-UVIR400-1-BX) to an SMA-to-SM1
fiber adapter (Thorlabs CVH100-COL) containing a planoconvex lens (20.1 mm focal length, Thorlabs LA4647) in a
1 inch diameter lens mount (Thorlabs LMR1S/M). The transmitted light is collected by a plano-convex lens (20.1 mm focal
length, Thorlabs LA4647) in a 1 inch diameter lens mount(Thorlabs LMR1S/M) contained in an SMA-to-SM1 fiber
adapter (Thorlabs CVH100-COL) connected to a 400 µm fiber(Avantes FC-UVIR400-1-BX). The spectrometer we used is an
Avantes AvaSpec-ULS2048CL-EVO-RS. The light path for theirradiation consists of a fiber-coupled LED (Thorlabs M340F4,

M395FP1, M455F3) coupled to a 600 µm fiber (Thorlabs
M114L01) to an adjustable fiber collimator (Thorlabs CFCS5A) mounted on an adapter (Thorlabs AD15F2) in a 1 inch diameter lens mount (Thorlabs LMR1S/M). On the opposite side of
the cuvette holder, a power meter (Thorlabs PM16-401) ismounted. The LED is powered by an LED driver with trigger
mode (Thorlabs LEDD1B T-cube) controlled using an Arduino
UNO R3 connected to a PC.
The AvaLight-DH-S-BAL light source is connected to the spectrometer via an interface cable (Avantes IC-DB26-RM), whichallows the proprietary software AvaSoft from Avantes tocontrol the internal shutter of the light source via a PC. With
this configuration, starting a single measurement in AvaSoft
sends a TTL signal to the light source that opens the shutter,
which is followed by the acquisition of a spectrum, and finally a
TTL signal is sent to close the shutter again. By utilizing scripts
we developed (see Supporting Information File 1), it is possible
to precisely control the timing of irradiation of the sample with
the LED and the perpendicular illumination with the UV–visprobe. In this way, when a UV–vis spectrum is recorded, theshutter of the D2-tungsten lamp is open and the excitation LED
is switched off for the entirety of the measurement (average of
500 spectra acquired for 1.0 ms). When the measurement isfinished, the script closes the shutter and the LED is switchedon for a given amount of time. This sequence can be easily
looped to obtain a desired illumination sequence (see Supporting Information File 1 for more detailed information on theloops and exact timings). The shutter prevents undesired illumination from the UV–vis probe that could influence the measurement altering the kinetics of isomerization, a known problem
for similar setups [24].
The power readings from the thermal power sensor were recorded by using Thorlabs’ Optical Power Monitor (OPM) software. The resulting data showed some fluctuations depending

Figure 2: a) Scheme of the setup and b) picture of the setup.

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on environment temperature and air movement. The background power was therefore monitored to get an accurate power
reading before turning on the LED. The raw data was then fittedwith a simple polynomial baseline using a Python script (seeSupporting Information File 1). After baseline subtraction, theaverage power and standard deviation for the power were determined, as shown in Figure 3. Since transmission through quartz
cuvettes and windows is not complete [48], we also corrected
for the power loss.
The power of the LED was first determined without any quartz
elements in the path between the collimator and the sensor.
Then the LED power was determined with the insulated jacket
for the cuvette holder and a cuvette containing solvent in place.
The power loss at the sample was approximated to be half of thetotal power loss recorded for the system. Table S1 in Supporting Information File 1 gives an overview of the measured
powers and averages. The loss was between 6–8%, with nomarked dependence on the wavelength of the LED. The final
power was taken to be the weighted mean of 3–4 measurements, using the reciprocal of the variance as the weights. The
average error in the measured power is 10−6 W. These results
grant a precision which is high enough to calculate the quantum yields, as our total error on the determined Φ is of the sameorder of magnitude as the ones earlier reported (vide infra)[22,24].
The current output of the LED driver is set by a manual dial,
which adds another variable to the setup, inevitably causing
some variation in the output power between different days. To
avoid reproducibility errors, we measured the output current of
the LED associated with a given power readout utilizing a
multimeter connected to the LED driver. Nevertheless, we also

checked the reproducibility of the measurements with different
currents to test the stability of the setup (see Supporting Information File 1).

Quantum yield determination
A concentrated solution of azobenzene (Sigma-Aldrich) inmethanol (Supelco LiChrosolv) was prepared by dissolving an
aliquot of crystalline azobenzene in 1.00 mL of solvent. Samples of 3.00 mL were prepared in a quartz cuvette (Hellma 117100F QS) by diluting an aliquot of the concentrated solution inmethanol, to obtain an absorbance of the π→π* band between
0.8 and 1.0. The samples of azobenzene were kept at 20 °C andstirred throughout the measurement (1200 rpm). The samples
were irradiated for 1 h, while a UV–vis absorption spectrum
was recorded every 30 s. The results for one such determination are shown in Figure 4A.
We utilized the powers obtained following the procedure described in the previous section to retrieve the quantum yields ofisomerization, assuming strictly monochromatic light at first[24,49]. We chose the peak of the emission spectrum of the
LED as the approximated monochromatic wavelength, since
this value can substantially differ from the nominal wavelengthof the LED, e.g., the 455 nm LED from ThorLabs has a peak inthe emission spectrum at 445 nm. We furthermore determined
the quantum yields again, taking the emission spectrum of the
LED into account. The emission spectra were measured using
our spectrometer by connecting the LEDs to the detector(Figure S1 in Supporting Information File 1). The following
input parameters were used in both methods: the photon flux
was calculated from the weighted mean of the power measured;molar absorptivities, εA and εB, as well as the thermal rate constant, kB→A, were taken from the most recent study reporting

Figure 3: A visual example of the power determination. a) Power without any elements (left) and with insulated jacket and cuvette (right side, thechange in baseline is noticeable), b) power with insulated jacket and cuvette after baseline subtraction.

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Figure 4: UV–vis absorption spectra of azobenzene upon irradiation at 340 nm (methanol solution, 20 °C). a) Evolution of spectra over time under irradiation and b) absorption at 340 nm over time, with fitted total absorbance curve obtained from Equation 5. The curve was obtained from entry 5 ofTable S2 (Supporting Information File 1).

these values [17]. The initial spectrum was confirmed to contain
100% trans-azobenzene by comparison with the known absorption spectrum of the pure switch [17].

obtained quantum yields using Equation 9 and Equation 10 toderive Φmin and Φmax:(9)

To retrieve the quantum yield values for forward and backward
isomerization, we solved rate equations Equations 5 and 6 (orEquation 6 and Equation 7, when the monochromatic excitation
light approximation was not considered) numerically using two
different python scripts (provided in Supporting Information
File 1). The rate equations are ordinary differential equations(ODEs) and can thus be solved numerically using existing
methods in the SciPy package [50]. An absorbance function wasconstructed, which takes the solution of the ODEs to model theabsorbance over time. This absorbance function was then fitted
to the measured absorbance at the wavelength of irradiation (orwavelengths, for a non-monochromatic excitation light). The
parameters Φ A→B and Φ B→A were evaluated to minimize
the residual sum of squares (RSS) between the measured andfitted absorbance values. This minimization is done by a Levenberg–Marquardt algorithm [51] as implemented in the lmfit
package [52]. Using this package, the standard error of the fitted
parameters is also obtained.
By evaluating the magnitude of the errors, we noticed that thelargest source of uncertainty comes, not surprisingly, from the
LED power measurement. This error is not included in the standard error of the fit, making the latter a poor reflection of theuncertainty in the retrieved quantum yields.
To overcome this limitation, we decided to evaluate the quantum yields using the powers corresponding to the lower andupper limits recorded with the thermopile. This procedure leads
to two different values of quantum yields, Φ min power andΦmax power, respectively, for both ΦA→B and ΦB→A. In order toinclude the uncertainty of the fit (ΔΦfit), we corrected the thus

(10)
The total uncertainty on the quantum yield of a single measurement obtained by employing the weighted mean of the power(ΦΑ→Β in the following example), is:(11)

The values listed in Table 1 are the weighted means and sample
standard deviations of multiple measurements, using the reciprocal of the variance of the quantum yields as the weights. For
completeness, we report all the primary data that were utilized
to compile Table 1 in Supporting Information File 1.
The measured quantum yields show good agreement with thevalues reported in the literature [22,24], especially for the
340 nm irradiation. It should be noted that an exact match of thequantum yields should not be expected, as the irradiation wavelengths are slightly different from the ones in the previous
studies, because different excitation sources were used (nonmonochromatic LED in our case, versus monochromatic light).
In line with the findings of Ladányi et al., the precision of
Φtrans→cis is generally better than the precision for the reverse
photoreaction. This difference in precision could be attributed
to the trans→cis reaction being more dominant, especially forirradiation in the π→π* band, where the absorption of transazobenzene is considerably higher than the cis (see Figure S4 in
Supporting Information File 1). The use of Equation 7 to takethe emission spectrum of the LED into account increased the1689

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Table 1: Calculated quantum yields and related quantum yields from the literature.

Wavelength of irradiation (nm)This work;
single wavelength

This work;
integrated emission

Ladányi et al. (2017)[24]

Gauglitz and Hubig(1985) [22]340a/334btrans → cis

0.160 ± 0.003

0.166 ± 0.003

0.155 ± 0.006

0.15 ± 0.02

cis → trans0.376 ± 0.070

0.384 ± 0.069

0.388 ± 0.053

0.30 ± 0.03

trans → cis

0.314 ± 0.050

0.264 ± 0.030

0.288 ± 0.007

0.20 ± 0.02

cis → trans0.578 ± 0.076

0.513 ± 0.040

0.452 ± 0.018

0.57 ± 0.03

trans → cis––

0.315 ± 0.002

0.22 ± 0.03

cis → trans––0.469 ± 0.003

0.63 ± 0.05

trans → cis

0.352 ± 0.003

0.354 ± 0.025

cis → trans0.542 ± 0.034

0.569 ± 0.009

395a/405b
436
455

aIrradiation used in this work; birradiation used in literature.

quantum yield values by a few percent. The difference whenusing the integration is larger for Φcis→trans. Since the emissionof the LED has a larger bandwidth than the emission using a
monochromator and filter, the integration over the bandwidth of
the LED should be performed for the most accurate results. We
would recommend measuring the emission spectra of the LEDs
if possible. Although we did not find any significant changes
between the quantum yields found using the measured emission spectra and the ones provided by the vendor, it is known
that small variations in the LED spectral distribution can occur
per lot [53].

Conclusion
In this work, we report a simple setup for the determination ofquantum yields using LEDs for irradiation and a thermopile
detector for the direct determination of the power. The setup isbuilt from commercially available optical elements and(together with the scripts provided in Supporting Information
File 1) can be replicated at moderate financial costs. The inclusion of a temperature-controlled cuvette holder and the possibility for simultaneous UV–vis absorption spectroscopy makes
this setup ideally suitable for a wide range of photoswitches, including those with fast thermal back isomerization. Our setup
was tested by determining the quantum yields of azobenzene fordifferent wavelengths of irradiation. The measured quantum
yields showed good agreement with the most recently reported
values [24], making this method and setup a viable alternative
to actinometry.

Acknowledgements
A.V. thanks J. J. van der Wal for the synthesis of azobenzene.

Funding
S.C. thanks the Swedish Vetenskapsrådet for a Starting Grant(2021-05414), the Stiftelsen Lars Hiertas Minne (FO20220139) and the Göran Gustafsson Foundation. J.D.S. thanks theWenner-Gren Stiftelserna for a postdoctoral stipend (UPD20220079). A.V. thanks the Erasmus+ Programme for mobility
allowance.

ORCID® iDs
Anouk Volker - https://orcid.org/0009-0006-1561-2458
Jorn D. Steen - https://orcid.org/0000-0002-7568-9370
Stefano Crespi - https://orcid.org/0000-0002-0279-4903

Data Availability Statement
The data generated and analyzed during this study is openly available in afigshare repository at https://doi.org/10.6084/m9.figshare.25818016.

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