Oligo p-Phenylenevinylene Derivatives as Electron Transfer Matrices for UV-MALDI

Article abstract of DOI:10.1007/s13361-017-1783-z

Phenylenevinylene oligomers (PVs) have outstanding photophysical characteristics for applications in the growing field of organic electronics. Yet, PVs are also versatile molecules, the optical and physicochemical properties of which can be tuned by manipulation of their structure. We report the synthesis, photophysical, and MS characterization of eight PV derivatives with potential value as electron transfer (ET) matrices for UV-MALDI. UV-vis analysis show the presence of strong characteristic absorption bands in the UV region and molar absorptivities at 355 nm similar or higher than those of traditional proton (CHCA) and ET (DCTB) MALDI matrices. Most of the PVs exhibit non-radiative quantum yields (φ) above 0.5, indicating favorable thermal decay. Ionization potential values (IP) for PVs, calculated by the Electron Propagator Theory (EPT), range from 6.88 to 7.96 eV, making these oligomers good candidates as matrices for ET ionization. LDI analysis of PVs shows only the presence of radical cations (M^+.) in positive ion mode and absence of clusters, adducts, or protonated species; in addition, M^+. threshold energies for PVs are lower than for DCTB. We also tested the performance of four selected PVs as ET MALDI matrices for analytes ranging from porphyrins and phthalocyanines to polyaromatic compounds. Two of the four PVs show S/N enhancement of 1961% to 304% in comparison to LDI, and laser energy thresholds from 0.17 μJ to 0.47 μJ compared to 0.58 μJ for DCTB. The use of PV matrices also results in lower LODs (low fmol range) whereas LDI LODs range from pmol to nmol. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13361-017-1783-z

B American Society for Mass Spectrometry, 2017
J. Am. Soc. Mass Spectrom. (2017)
DOI: 10.1007/s13361-017-1783-z
RESEARCH ARTICLE
Oligo p-Phenylenevinylene Derivatives as Electron Transfer
Matrices for UV-MALDI
Laura J. Castellanos-García,1 Brian Castro Agudelo,2 Hernando F. Rosales,1
Melissa Cely,2 Christian Ochoa-Puentes,2 Cristian Blanco-Tirado,1 Cesar A. Sierra,2
Marianny Y. Combariza1
1Escuela de Química, Universidad Industrial de Santander, Bucaramanga, Santander, Colombia
2Departamento de Química, Universidad Nacional de Colombia, Bogotá, Colombia
Abstract. Phenylenevinylene oligomers (PVs) have outstanding photophysical char-
acteristics for applications in the growing field of organic electronics. Yet, PVs are also
versatile molecules, the optical and physicochemical properties of which can be tuned
by manipulation of their structure. We report the synthesis, photophysical, and MS
characterization of eight PV derivatives with potential value as electron transfer (ET)
matrices for UV-MALDI. UV-vis analysis show the presence of strong characteristic
absorption bands in the UV region and molar absorptivities at 355 nm similar or higher
than those of traditional proton (CHCA) and ET (DCTB) MALDI matrices. Most of the
PVs exhibit non-radiative quantum yields (φ) above 0.5, indicating favorable thermal
decay. Ionization potential values (IP) for PVs, calculated by the Electron Propagator
Theory (EPT), range from 6.88 to 7.96 eV, making these oligomers good candidates as matrices for ET ionization.
LDI analysis of PVs shows only the presence of radical cations (M+.) in positive ion mode and absence of clusters,
adducts, or protonated species; in addition, M+. threshold energies for PVs are lower than for DCTB. We also
tested the performance of four selected PVs as ET MALDI matrices for analytes ranging from porphyrins and
phthalocyanines to polyaromatic compounds. Two of the four PVs show S/N enhancement of 1961% to 304% in
comparison to LDI, and laser energy thresholds from 0.17 μJ to 0.47 μJ compared to 0.58 μJ for DCTB. The use of
PV matrices also results in lower LODs (low fmol range) whereas LDI LODs range from pmol to nmol.
Keywords: Electron transfer, UV-MALDI, p-Phenylenevinylene matrix, Optoelectronic, Ionization potential, Quan-
tum yield, Porphyrins, Polyaromatics, Phthalocyanine
Received: 1 April 2017/Revised: 10 August 2017/Accepted: 10 August 2017
plexes [1–6]. In positive mode ET ionization, primary ions of
the matrix (radical cations) react with neutral analyte molecules
Introduction
as-phase protonation, deprotonation, and adduct forma-
tion reactions are very common to MALDI-MS; however,
in the gas phase. The electron transfer process, from the neutral
to the ionized matrix, occurs only if the ionization potential of
the matrix is higher than the ionization potential of the analyte
(IPM > IPA) as first reported by McCarley [7].
G
not all organic compounds are amenable to charge acquisition
through these processes. The electron transfer (ET) process, a
less travelled road in MALDI-MS, can expand the usefulness of
the technique because it allows ionization of compounds essen-
tial to material sciences such as polymers, fullerenes, polycyclic
aromatic hydrocarbons, organometallic, and coordination com-
Highly conjugated phenylenevinylene (PV) systems are
amongst the most studied architectures in organic polymers
for photophysical applications [8–10]. It is well known that
photoluminescence and electroluminescence phenomena in
PVs depend of many physicochemical factors, amongst which
molar absorption coefficients and maximum absorption wave-
lengths in the UV-VIS region play a major role [10, 11].
Although currently there are no reports of PVs as MALDI
matrices, basic research related to their solid-state behavior
and photophysical properties indicates potential usefulness
Electronic supplementary material The online version of this article (https://
doi.org/10.1007/s13361-017-1783-z) contains supplementary material, which
is available to authorized users.
Correspondence to: Marianny Combariza; e-mail: marianny@uis.edu.co

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
for this particular application. The structural versatility of PVs,
diphenylanthracene (9,10-DPA) was reported as MALDI ma-
trix for the ET ionization of model analytes like chlorophyll and
retinol [30]; this compound and its derivatives are commonly
used in OLEDS [31] and as reactants in electron transfer disso-
ciation ETD [32]. However, the photophysical characteristics of
9,10-DPA, e.g., IP (7.0 eV) and ε355nm (7220 L mol–1 cm–1)
[30], are not superior to established ET matrices like DCTB, IP
(8.5 eV) [33] and ε355nm (33980 L mol–1 cm–1) [25].
In an interesting review Wyatt [1] highlights the usefulness
of MALDI for the analysis of coordination and organometallic
complexes and points out the lack of research in this area
despite the great benefits of the technique. For instance, por-
phyrins and phthalocyanines are ionized through radical cation
formation and LDI is the technique of choice for their analysis;
however, LDI induces demetallation and increases fragmenta-
tion in labile analytes [5, 23]. Other analytes such as polycyclic
aromatic hydrocarbons (PAH), with low solubility in polar
solvents, are also susceptible to electron transfer ionization.
Solvent-free LDI analysis was put forward as an alternative
for the ionization of these non-polar analytes; however, molec-
ular cation abundances and survival yields in this technique are
very low [34].
Considering the wide applicability of ET ionization for the
analysis of polymers, fullerenes, polycyclic aromatic hydrocar-
bons, organometallic, and coordination complexes [1], in this
contribution we report the synthesis, photophysical, and mass
spectrometric characterization of eight PV derivatives with
potential application as ET MALDI matrices. We also test the
performance of four of these structures (PV-CN, PV-COOH-
CH3, PV-COOH-OCH3, and PV-Cl) as electron transfer
MALDI matrices for the analysis of labile compounds such
as phthalocyanines, porphyrins, and polycyclic aromatic hy-
drocarbons (PAH). In this regard, we compare the effectiveness
of the PV derivatives as ET MALDI matrices, in terms of
molecular ions energy threshold, S/N, and limit of detection
(LOD), with traditional LDI analysis and MALDI using DCTB
as ET standard matrix.
easily accessed by manipulation of the electronic conjugation
length or the chemical nature of the substituents in these
molecules [12–15], allows not only fine control of
photophysical behavior (e.g., absorption/emission wavelengths
and quantum efficiencies), but also of solubility and stability
properties, which are key factors in MALDI matrices. A series
of studies report interesting solid crystal properties for PVs
such as the enhancement in photophysical properties by aggre-
gation of PVs through π stacking by Amrutha and Jayakannan
[16], and the capability of PVs to undergo photo-induced intra-
and intermolecular energy and charge transfer processes like
those occurring in primary and secondary reactions in the
MALDI plume [17–20]. In addition, Samori et al. showed that
electronic conjugated systems such as stilbenes, basic units of
PVs, are able to produce radical cations during irradiation with
a 355 nm Nd:YAG laser [21]. Finally, Grozema et al. using
theoretical approximations described the formation of positive-
ly charged species (radical cations) from PVs with the ability to
maintain high absorption coefficients with minor changes in
molecular geometry [22].
The combination of a wide array of molecular properties
makes an efficient ET matrix, among others relatively high IPs,
high molar absorptivity in the solid state, good solubility prop-
erties, appropriate crystallization, vacuum stability, and low
tendency to form clusters. Many structures for ET purposes
have been tested, mostly through trial and error; however
currently ET matrices are mostly based on highly conjugated
aromatic compounds such as DCTB and 9-nitroanthracene. As
examples, Chait et al. successfully obtained radical cations of
phthalocyanines, porphyrins, and multiporphyrin arrays using
1,4-benzoquinone [23], whereas Boltalina et al. studied the
ionization of synthetic fluorofullerenes in negative mode using
9-nitroanthracene, the latter, also hindered defluorination of the
substituted fullerenes, a reaction observed when using LDI [3,
24]. However, Drewello et al., performing the same experiment
on positive ion mode, found two major problems with the use
of 9-nitroanthracene. The first related to an oxygen transfer
reaction from the matrix to the analyte, and the second associ-
ated with the presence of abundant clusters in the low mass Experimental
region of the spectrum (from m/z 220 to 800) interfering with
Chemicals
the analysis of low molecular weight analytes [5].
In 2000, Luftmann et al. [25] introduced the most successful
to-date electron transfer matrix, commonly known as DCTB
(trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]
malononitrile). DCTB is useful for the ionization of fullerenes
derivatives [3, 25], polymers [25, 26], and porphyrins [27],
among others [28]. The advantages of DCTB, over other ET
matrices, lie in the relatively low laser fluence needed for
ionization and its ability to prevent fragmentation of labile
analytes [5, 23, 25]. However, there are still some issues re-
garding the performance of DCTB such as formation of adduct
ions with analytes containing amine-groups. The carbon adja-
cent to the dicyanomethylene functionality in DCTB can act as
an electrophile for primary and secondary aliphatic amines,
leading to the formation of imines [29]. More recently, 9,10-
Unless otherwise stated, all compounds are commercially avail-
able and were used without further purification. DMSO, DMF,
CHCl3, THF, acetonitrile, p-xylene, methanol, n-hexane, ethyl
acetate, terephthalaldehyde, dimethoxybenzene, potassium tert-
butoxyde, and methanol were purchased form Merck (Darm-
stadt, Germany). Methyl triphenylphosphonium bromide, meth-
yl 4-Iodo benzoate, and sodium periodate were purchased from
Panreac (Barcelona, Spain). Pd(dba)2, triphenyl phosphite, bo-
ron tribromide, 2-chloroethanol, 4-vinyl benzoic acid, anthra-
cene, 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine
cobalt (II) (cobalt porphyrin), zinc 29H,31H-phthalocyanine
(zinc phthalocyanine), coronene, 2,3-naphthalocyanine
(naphthalocyanine), rubrene, 29H,31H-phthalocyanine (phtha-
locyanine), MALDI grade α-CHCA, and MALDI grade trans-2-

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene (DCTB) were
purchased form Sigma-Aldrich (St. Louis MO, USA).
PV derivatives, anthracene and quinine sulphate concentra-
tions in solution were adjusted to yield a UV-vis absorbance
of approximately 0.05 for all fluorescence experiments; this
translates in solutions ranging from 2 to 20 μM in DMF.
Calibration curves were prepared in ethanol for anthracene
and in a 0.1 M solution of H2SO4 in deionized water for
quinine sulphate [36, 37]. The calculated error for the
intercrossing calibration was less than 5%.
PVs Derivatives synthesis
Scheme 1 depicts the general procedure for PV synthesis and
the structure of the eight molecules tested as MALDI matrices.
PV synthesis starts by preparation of a series of styrene and
iodobenzene precursors, followed by coupling reactions. The
iodobenzene precursors are obtained through a SEAr mecha-
nism by refluxing appropriate reactants with KIO3 and I2 in
acid media [19]. On the other hand, following the Wittig
olefination the corresponding aromatic aldehydes are subse-
quently converted (in high yield) to their styrene derivatives
using methyl triphenyl phosphonium bromide and the appro-
priate base [15]. Finally, the iodobenzene and styrene deriva-
tives are connected by means of the Mizoroki-Heck coupling
reaction using Pd(dba)2 and triphenylphosphite, according to
our previously reported methodology [35]. In the case of com-
pounds with carboxylic functional groups, an extra hydrolysis
step was needed after the final coupling reaction. Full synthetic
details and spectroscopic characterization of all precursors and
target compounds are provided in section S1 of the Supple-
mental information.
Theoretical Calculations
PVs and analytes vertical ionization potentials (IPs) were de-
termined by the Electron Propagator Theory (EPT) using the
Gaussian 09 software suite [38]. Closed shell HF/6-311G(d,p)-
optimized geometries were used to start EPT calculations using
the same basis set [39, 40]. EPT simulations of all PVs exhib-
ited pole strengths above 0.85 giving credence to their ability to
reproduce the experimental data.
LDI and UV-MALDI Sample Preparation
For PVs LDI analysis, 2.5 mM stock solutions of the PVs
derivatives in THF were prepared. Because of the low solubil-
ity of some of the PVs, specifically PV-COOH and PV-
COOH-CH3, stock solutions were placed on a Bransonic ultra-
sonic bath (Danbury CT, USA) at 40 KHz and 130 W during
times ranging from 1 to 20 min, depending on the PV solubil-
ity; 1.0 μL of each solution was placed on a stainless steel
target and let dry at room temperature. For MALDI analysis,
2.5 mM stock solutions of the PV derivatives (PV-CN, PV-
COOH-CH3, PV-COOH-OCH3, and PV-Cl) in THF were
prepared. For all analytes, 2.5 mM stock solutions were pre-
pared and diluted to 25, 5, and 2.5 μM. Equal volumes of the
PVs and analyte solutions were mixed (1500 rpm for 15 min) to
reach analyte-to-matrix molar ratios of 1:100, 1:500, and
1:1000; 1 μL of each mixture was applied to the sample target
using the dried droplet method.
UV-vis Spectroscopy Measurements
UV-vis spectra for the PVs derivatives and the standard matri-
ces were acquired at 22 °C using 12.5 μM solutions of the
compounds in DMSO and a double channel Shimadzu UV-
2401PC spectrophotometer (Kyoto, Japan) with a scanning
range of 200 to 700 nm. Molar absorptivity (ε) at λ355nm was
calculated using the Beer-Lambert law for serial dilutions, from
2 to 25 μM, of PV derivatives in DMSO correlation coeffi-
cients for the measurements are higher than 0.97 (See section
S2 of the Supplemental Information).
Quantum Yield Measurements
Mass Spectrometry
Quantum yields were measured with a PTI/QM 40 Spectro-
fluorometer (NJ, USA) with an excitation wavelength of
355 nm and fluorescence emission monitoring in the range
of 360 to 700 nm. Anthracene and quinine sulphate were
used as reference fluorophores for intercrossing calibration.
A Bruker Ultraflextreme MALDI TOF-TOF instrument
(Bruker Daltonics, Billerica, MA, USA) was used to perform
MALDI and LDI analyses. The instrument is equipped with a
1 kHz Smart Beam Nd:YAG laser (355 nm), with a maximum
Scheme 1. General procedure for PV synthesis using the Mizoroki-Heck cross coupling reaction

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
energy output of approximately 8.5 μJ per shot; 6 ns pulse and
and maximum absorption wavelengths (calculated by deri-
vation of the spectral functions). In accordance with previ-
ous reports [12, 43], PV derivatives exhibit two character-
istic strong absorption bands ranging from 300 to 390 nm
and 320 to 520 nm. As expected the presence and position
of various functional groups on the phenylenevinylene core
causes displacements in the experimental absorption wave-
lengths, starting from the blue shifted absorption bands in
PV-COOH-CH3 (at 367 nm) and ending with the red shifted
bands of PV-NO2 (at 447 nm). We calculated the effective
wavelength shift (fourth column in Figure 1) for each com-
pound as the difference between the experimental maximum
absorption wavelengths for the PV derivative and the max-
imum absorption wavelength reported in literature for the
PV core (381 nm). Previously, Heller et al. [43] published
the physicochemical and photophysical characterization of
the PV core; however due to poor solubility this compound
was never used in electroluminescence applications. We can
establish a trend when examining PV derivatives UV-vis
spectra: clearly the presence of electron withdrawing
groups, -NO2 and –Cl, in the terminal phenylenes (R2
positions, Scheme 1) produces a bathochromic (red) shift
of 66 and 18 nm respectively with respect to the maximum
absorption wavelength of the PV core (381 nm). This shift
is due probably to an increase in effective π conjugation
length in the excited state, as has been previously reported
[42, 43]. The presence of alkoxy groups, with moderate
electron donating capabilities, still induces a red shift on
the maximum absorption band corresponding to 27, 17 and
14 nm for the PV-COOH-OCH3, PV-OCH3 and PV-OH
structures, respectively. On the other hand, the presence of
–COOH and -CN groups in the side rings causes a
hypsochromic (blue) shift in absorption maxima of 6, 14
and 9 nm for PV-COOH, PV-COOH-CH3 and PV-CN,
respectively. Blue shifts are attributed to substituents that
prevent planarization by reducing effective π conjugation
length in the excited state as has been previously reported
[42, 43].
average spot size of 100 μm, according to the manufacturer’s
specifications. For matrices threshold energies laser power was
varied from 0.14 to 2.5 μJ per pulse using the instrument’s
attenuator. The energy output of the laser was measured using a
PowerMax-USB UV/vis Wand (Coherent, Santa Clara, CA,
USA); all laser power measurements were performed by trip-
licate. Positive ion mass spectra, from m/z 180 to 2000, were
acquired in reflectron mode with delayed extraction set at 100
ns, and an accelerating voltage of 25 kV. Instrument calibration
was performed using CHCA and a mixture of standard pep-
tides: leu-enkephaline, bradykinin, bombesin, and renin sub-
strate purchased from Sigma Aldrich (St. Louis, MO, USA).
To ensure unbiased data acquisition, we used an
autoexecution method within the FlexControl software (Bruker
Daltonics, Billerica, MA, USA) of the instrument. The
autoexecution method uses a random walk algorithm, which
takes 10 laser shots in the same coordinates, then the target
moves to another random position until it completes the re-
quested total shots, in this case 500. If the summed spectra fit
the evaluation parameters, they are added to the total spectrum,
if they do not then another acquisition takes place; each report-
ed analysis corresponds to the sum of 2000 spectra. Data
analysis was performed using the FlexAnalysis software
(Bruker Daltonics, Billerica, MA, USA). The software auto-
matically reports ion abundances, S/N ratios, signal resolution,
peak area, and monoisotopic masses. Experimental and theo-
retical isotope patterns, calculated with ChemCalc [41], were
compared to verify compound identification.
Results and Discussion
UV-vis Characterization
Many reports regarding photophysical properties of oligo PVs,
particularly optical absorption and photoluminescence, are
available in literature [12, 19, 42–44]. There is general agree-
ment that substituent types and their placement in the PV core
strongly influence PV derivatives optical performance [12, 13,
42, 43]. Typically, the presence of alkyl or alkoxy groups on
the central ring improves compound solubility [12, 45]; while
substituents in the peripheral rings induce shifts in UV-vis
absorption and fluorescence emission wavelengths. By using
PV (1,4-distyrylbenzene) as core structure, we introduced weak
and medium electron-donating groups (-CH3, -OCH3, and –
OC2H4OH) on the central phenylene ring (Position R1, Scheme
1), and both electron-donating (-OCH3) and electron-
withdrawing groups (-CN, -Cl, -NO2, -COOH) on the periph-
eral rings (Positions R2 and R3, Scheme 1). For some of the
selected structures (PV-CN, PV-COOH, PV-Cl, PV-NO2, PV-
OH and PV-OCH3) basic photophysical, spectroscopic and
physicochemical characteristics were available from previous
works; [18, 19, 46–48] while the molecules PV-COOH-CH3
and PV-COOH-OCH3 have not been previously reported.
Figure 1 shows UV-vis absorption spectra for the tested
PV series, molar absorptivities (ε) at λ355nm, solubility data
MALDI-relevant UV-vis absorption bands located between
300 and 400 nm, corresponding to π-π* transitions of the phenyl
and vinyl groups, are common to all of the tested PV derivatives.
According to Figure 1 all absorption maxima for PV derivatives
lie to the right of the 355 nm value. PV-COOH, PV-CN and PV-
COOH-CH3 exhibit the closest absorption wavelengths to the
355 nm line, with λmax of 367, 372 and 375 nm respectively;
these molecules have a slight red shift of 12, 17 and 20 nm with
respect to the laser wavelength. Although PV-Cl, PV-OH, PV-
OCH3 and PV-COOH-OCH3 are further red-shifted than both
acid and cyano PV derivatives, they still exhibit significant
absorption at 355 nm. Finally, PV-NO2 has the most dramatic
shift to red wavelengths: 92 nm from the 355 nm value, due to
effect of the electron withdrawing functional group.
Commercial MALDI instruments operate at fixed wave-
length values of 337 and 355 nm for N2 and Nd:YAG lasers,
respectively; thus, UV-MALDI experiments in these instru-
ments necessarily require a matrix that efficiently produces

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
Figure 1. Photophysical properties for PVs derivatives. Dashed and dashed-dotted lines correspond to the laser wavelength (355
nm) and the PV core maximum absorption wavelength (381 nm), respectively

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
primary ions under these conditions. Analysis of molar absorp-
and -OCH3 groups do not disrupt compound planarity to a
significant extent.
tivity values at 355 nm (ε355nm) allow for comparison between
the UV-vis spectral properties of the PV series and established
MALDI matrices like DCTB and CHCA in liquid phase.
Experimental ε355nm values for CHCA (15,490 L mol-1 cm-1)
and DCTB (30,144 L mol-1 cm-1) are in accordance with
reported values [25, 49]. All PV derivatives, with the exception
of PV-OCH3, have ε355nm values above CHCA (>15,490 L
mol-1cm-1); while PV-CN, PV-COOH-CH3 and PV-OH have
higher ε355nm values than DCTB (> 30.144 L mol-1 cm-1).
These values indicate efficiently absorption of 355 nm photons
by the PV derivatives in liquid phase and perhaps a good mass
spectrometric behavior as MALDI matrices. However, this
observation is not meaningful from a MALDI mechanistic
point of view and only gives a general idea of the matrices’
absorption behavior trends in liquid phase. We have to keep in
mind that according to the coupled photophysical and chemical
dynamics (CPCD) model of UV MALDI, primary ionization
events (and also much of the secondary reactions) occur in a
condensed-like phase and that ion yields in MALDI strongly
depend on the solid state absorption properties of the matrix
and the laser characteristics [2, 49–51]. Hence, matrix
photophysical properties in condensed phase are the ones that
play a crucial role in primary ions generation through processes
involving excitation, migration and concentration of energy
[50].
Interestingly observations by Sierra et al. [19] and Heller
et al. [43] indicate the possibility of intramolecular hydrogen
bond formation in PV-OH and PV-OCH3 analogues, involving
the oxygen in the –OCH3 units in the terminal phenylene rings
and the ß carbon of the adjacent vinylene group. These H bonds
could reduce planarity while not dramatically, which could
explain the observed ΦF values. Also interestingly, an increase
on effective π conjugation length and increased planarity in
PV-Cl, PV-OCH3 and PV-OH explains both the observation of
moderate ΦNF and the bathochromic shifts in the UV-vis spec-
trum for these PV derivatives. Finally, PV-NO2, due to its red-
shifted absorption, did not show any fluorescence at the exci-
tation wavelength of 355 nm; for this reason, the ΦF value for
this derivative is not included in Figure 1.
Mass Spectrometric Characterization
Figure 2 shows positive mode LDI mass spectra for the tested
PV derivatives, using 2.5 nmol of each compound on target and
a laser power of 0.5 μJ per pulse. No clusters were observed in
the mass spectra, where the base peak always corresponds to
the PV radical cation (M+.). The registered mass accuracy for
the M+. species was always below 75 ppm. Upon LDI some of
the PV molecular radical cations lose a hydroxyl radical (.OH)
to produce a low abundance cation; as an example PV-COOH
(Figure 2b) shows signals at m/z 370.122 and 353.123 corre-
sponding to the molecular ion (M+.) and the fragment [M-
OH]+, respectively. A similar behavior is observed for both
PV-COOH-CH3 and PV-COOH-OCH3 derivatives (Figure 2c
and d) where the M+. ions, at m/z 398.163 and m/z 430.148, are
accompanied by the [M-OH]+ ion at m/z 381.156 and m/z
413.141, respectively. On the other hand, PV-CN, PV-Cl,
PV-NO2, PV-OH and PV-OCH3 derivatives (Figure 2a, e, f,
g, and h) do not fragment and their radical cations are observed
at m/z 332.156, 410.091, 432.138, 582.236 and 522.255, re-
spectively; the chlorine characteristic isotopic pattern is clearly
distinguishable for the PV-Cl. In addition, experimental isoto-
pic patterns for the PVs (see insets in Figure 2) coincide with
the calculated isotopic patterns for each derivative [41].
We also performed LDI experiments under acidic condi-
tions for the eight PV derivatives, using various amounts of
TFA, to assess the PVs ability of forming protonated molecules
or adducts. We did not observe protonated molecules or ad-
ducts of the type [M+H]+ or [M+Na/K]+, despite the presence
of polar functionalities in the PV structures. Matrix background
signals in the PVs mass spectra are negligible; there is no
evidence of clusters, or π-π stacking oligomer formation in
the gas phase (Figure 2). Traditional ET matrices such as
DCTB and 9-nitroanthracene exhibit extensive adduct forma-
tion in MALDI MS which interfere with the analysis of low
mass analytes. For instance, in positive ion mode DCTB is not
only observed as a molecular radical cation, but also as
[M+H]+, [M+Na]+, [2M+H]+ and several other charged spe-
cies, dominating the low mass range of the MS (See Figure S4 of
Fluorescence Quantum Yields in Solution
We found that non-radiative (ΦNF) and radiative fluorescence
(ΦF) quantum yields for the PV series also depend on molec-
ular structure. The core PV structure is reported to be highly
fluorescent in solution with ΦF of 0.9 [43]. Introduction of
substituents dramatically reduces ΦF for the PV derivatives as
Figure 1 clearly shows. CN and acid PVs (PV-CN, PV-COOH,
PV-COOH-CH3, PV-COOH-OCH3) exhibiting ΦF values of
0.27 0.29, 0.38 and 0.40 (ΦNF 0.80, 0.71, 0.62 and 0.60),
respectively, experience the highest reduction in ΦF of all the
tested PVs. Low ΦF, or high ΦNF in other words, is mainly
attributed by many authors to a severe deformation of the PV
planar structure [42, 43]. From a mass spectrometric point of
view both yields are important depending on the desired reac-
tion channel during the ionization step. Low ΦF suggests a
relaxation pathway predominantly thermal; an important effect
during the desorption step in the MALDI process because is
related to heating, expansion and increased matrix/analyte
ejection from the solid phase which will eventually improve
ion release into the gas phase [52, 53]. Interestingly, a reduction
on effective π conjugation length and decreased planarity in
PV-CN, PV-COOH and PV-COOH-CH3 explains both the
observation of high ΦNF and the hypsochromic shifts in the
UV-vis spectrum for these PV derivatives. On the other hand,
PV-Cl, PV-OCH3, PV-OH exhibit ΦF yields of 0.52, 0.53 and
0.55, almost equivalent to ΦNF. Although there is still a reduc-
tion in ΦF for these structures, apparently the presence of -Cl

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
Figure 2. LDI-TOF spectra for the PV derivatives: (a) PV-CN, (b) PV-COOH, (c) PV-COOH-CH3, (d) PV-COOH-OCH3, (e) PV-Cl, (f)
PV-NO2, (g) PV-OH, (h) PV-OCH3. Inserts show experimental isotopic distributions. See Table S3 of the Supplemental Information or
molecular ion (M+.) isotopic abundances and mass accuracy data

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
the Supplemental information) [25]. Likewise, 9-nitroanthracene
depends on the matrix performance. For the tested PV deriva-
tives m ranges from 5.5 up to 9.47; while for DCTB m corre-
sponds to 4.45. Clearly, under identical experimental condi-
tions PV derivatives have not only the ability to efficiently
absorb energy but also to produce more ions, when compared
to the standard DCTB matrix. In Figures S5 and S6 of the
Supplemental Information we include an analysis of signal to
noise (S/N) ratio and resolution for PV derivatives and DCTB
molecular ions as a function of laser pulse energy.
produces π-π stacking adduct ions detected in the m/z 200-800
range [5]. PVs, on the other hand could be potentially useful for
characterization of either low or high mass analytes.
Using the same amount of each PV derivative and the
standard matrix DCTB (2.5 nmol on target) we increased the
laser power from 0.14 to 2.5 μJ per pulse and determined the
detection threshold (the lowest laser power needed to get a
molecular ion signal with S/N>3) and the ion appearance curves
for all tested molecules as seen in Figure 3. Under the same
experimental conditions, the power threshold for ion detection
for all PV derivatives is always lower than that of the standard
DCTB matrix, while molecular ion abundances for the PV
derivatives are always higher than for DCTB. Molecular ions
for PV-OCH3, PV-Cl and PV-OH exhibit detection thresholds
of 0.14, 0.17 and 0.22 μJ, respectively; while PV-COOH-CH3,
PV-COOH, PV-NO2, PV-CN and PV-COOH-OCH3 show
higher thresholds of 0.27, 0.27, 0.34, 0.47 and 0.47 μJ, respec-
tively. Under our experimental conditions the DCTB detection
threshold was 0.58 μJ. For analytical purposes, a laser power of
two or three times over that of the matrix ion threshold is always
recommended. In the case of the PV derivatives those values are
extremely low when compared with the standard DCTB, this
fact translates into analytical applications using PVs as matrices
requiring less laser power. This could not only benefit the
instrument hardware, in terms of laser durability and internal
surfaces cleaning, but also its analytical performance in terms of
mass accuracy, sensitivity and resolution.
Ionization Potential (IP) Calculations
Figure 4 shows PV derivatives IPs calculated using the Elec-
tron Propagator Theory - EPT. According to Ortiz EPT provides
one of the most reliable tools for accurate IP calculation; the
author demonstrates this fact comparing experimental IP values,
from photoelectron spectroscopy, with theoretical values from
EPT [39, 40]. For IP calculations of highly conjugated systems
such as PVs, EPT was proven to be a valuable tool. However, a
detailed description of the theoretical considerations involved in
IP calculations is out of the scope of this paper and will be
addressed as a separate contribution. It is well know that the ET
processes occurs through a two-step ionization mechanism
where the matrix (M) is first photoionized to produce a radical
cation according to (Equation 1 ), and then undergoes an ET
reaction with the analyte (A) to produce an analyte radical
Above the detection threshold ion abundances increase
rapidly with increased laser power and then level out, as
observed in the ion appearance curves for the PV series in
Figure 3. The linear section of the experimental ion appearance
curve can be fitted to a power function of the type Y~Hm [54],
where Y is the ion abundance, H the laser power and m
Figure 4. Ionization potentials for PV derivatives calculated
using Electron Propagator Theory – EPT (right). Analytes IPs
correspond to reported values (left) [13–15]
Figure 3. Appearance curves for PV derivatives and DCTB
molecular ions (M+.)

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
cation and a matrix neutral molecule in the gas phase (Equation
2) [7, 52].
due to its low solubility was not included and we used instead
PV-COOH-OCH3.
Figure 5 shows the structures of the standard compounds
(cobalt porphyrin, phthalocyanine, naphthalocyanine, zinc
phthalocyanine, rubrene, coronene) tested as additional
analytes, and the S/N ratios of their radical cations using PV-
Cl, PV-COOH-CH3, PV-COOH-OCH3, PV-CN, and DCTB
as matrices with different analyte to matrix molar ratios. All the
experiments were performed five times using a laser output of
0.8 μJ per pulse, a suitable working value for the PV deriva-
tives as seen in Figure 3, and the reported data correspond to an
average value of these measurements. These analytes readily
form radical cations and are commonly studied by LDI using
high concentrations. LDI requires harsh conditions such as
increased laser power, which commonly induces fragmentation
and decreases molecular ion survival yields.
M þ nhv →Mþ:
ð1Þ
Mþ: þ A →M þ Aþ:
ð2Þ
Limbach et al. [7, 53] suggested that analyte radical cation
formation only occurs if the recombination energy of the matrix
exceeds the recombination energy of the analyte. Approximat-
ing the recombination energy of both matrix and analytes to the
vertical ionization potentials, they proposed the well-known
criterion IP(M)>IP(A) for the ET processes to happen. Interest-
ingly they also observed that analyte ion abundances are directly
related to the IP differences between matrix and analyte, with
higher differences indicating a more favorable electron transfer
process [52, 53]. Consequently, knowing the IP for ET matrices
in MALDI is of fundamental importance for analytical applica-
tions. High IP values are desirable for ET MALDI matrices,
although not so high that the photoionization process does not
occur under MALDI conditions. Commonly IP values for com-
mercially available ET matrices range from 7.0 eV for 9,10-
diphenylanthracene [30] to 8.22 eV for DCTB [52]. Our calcu-
lations indicate that IPs for the PV derivatives range from
6.88 eV for PV-OCH3 to 7.96 eV for PV-CN. The highest IP
of the series for PV-CN is due possibly to a synergistic effect
between the presence of the -CN electron withdrawing group
which directly affects the π electronic conjugation system and a
considerable planarity disruption in this derivative, as discussed
previously. IP values for the tested analytes (see below) are also
included in Figure 4 and correspond to reported values [55–57].
The variation in the S/N value for the cobalt porphyrin (m/z
791.206) using PV-CN, PV-COOH-CH3, PV-COOH-OCH3,
PV-Cl, and DCTB as matrices and different analyte:matrix
molar ratios is shown in Figure 5a (see also Figure S9 of
the Supplemental Information). When the analyte:matrix ratios
vary, from 1:100 to 1:500 and 1:1000, the analyte amount on
target decrease from 25 to 5.0 and 2.5 pmol, respectively. No
signal for m/z 791.206 was observed by either LDI or MALDI
when using DCTB and PV-COOH-OCH3 as matrices. In con-
trast, MALDI experiments, using PV-CN, PV-Cl, and PV-
COOH-CH3, showed significant S/N values for the radical
cation m/z 791.206, ranging from 3217 to 348. Among the
tested PV derivatives, the PV-CN matrix displays the best
performance for the cobalt porphyrin analysis. For instance,
an analyte:matrix ratio of 1:100 (25 pmol of cobalt porphyrin)
afforded the highest S/N ratio (3217) for m/z 791.206 using
PV-CN, whereas no signal is observed at the same analyte
concentration using LDI. High analyte:matrix ratios, in the
order of 1:1 to 1:100, are commonly used on ET ionization
experiments [3, 6, 30, 53]. However, when using PV-CN, PV-
Cl, and PV-COOH-CH3 derivatives as MALDI matrices, low
analyte:matrix ratios (1:1000, 2.5 pmol on target) provide S/N
values of 1006, 614, and 348 for m/z 791.206, respectively.
Along the same lines, Figure 5b shows the variation of S/N
values for the phthalocyanine (m/z 514.165); no signal for this
compound was registered using LDI or MALDI with DCTB as
matrix (see also Figure S10 of the Supplemental Information).
However, phthalocyanine radical cation signals at m/z 514.165
were readily observed with high abundances for MALDI ex-
periments with PV-CN, PV-Cl, PV-COOH-CH3, and PV-
COOH-OCH3 as matrices. In a similar fashion to Figure 5a,
the use of PV-CN as matrix improves considerably the S/N of
the phthalocyanine molecular ion (m/z 514.165). In addition,
low analyte:matrix ratios (1:1000, 2.5 pmol on target) can be
used and still observe significant S/N ratios of 1579, 894, 197
and 114 for m/z 514.165, using PV-CN, PV-Cl, PV-COOH-
OCH3 and PV-COOH-CH3 respectively. Figure 5c shows the
S/N values for the naphthalocyanine radical cation (m/z
714.228). This molecule exhibits low ionization efficiency
and only the use of relatively high analyte:matrix ratios
Analytical Performance of PV Derivatives
as MALDI ET Matrices
In Figures S7 and S8 of the Supplemental Information we show
the results of preliminary experiments where we tested the
performance of the eight PV derivatives as electron transfer
matrices for the ionization of a single analyte NP
(naphtho[2,3-a]pyrene; M+. at m/z 302.109, IP:6.89 eV) using
a laser output of 1.3 μJ per pulse and analyte:matrix ratios of
1:10 (250 pmol), 1:100 (25 pmol), 1:1000 (2.5 pmol), and
1:10,000 (0.25 pmol). At the lowest tested NP concentration
(0.25 pmol), the best performance in terms of average S/N ratios
was shown by PV-COOH, PV-Cl, PV-COOH-CH3, and PV-
CN with values of 27, 29, 138, and 515 for the NP molecular
ion, respectively. Since the S/N ratio for NP with DCTB was 9,
we observed S/N net enhancements of 3, 3.2, 15.3, and 57 times
for NP when using PV-COOH, PV-Cl, PV-COOH-CH3, and
PV-CN. These S/N enhancements correlate well with IP differ-
ences between analyte:matrix as reported previously by
Limbach et al. [7, 53]. Three of these four matrices (PV-Cl,
PV-COOH-CH3, and PV-CN) were selected to carry out further
experiments with additional analyte molecules; the PV-COOH

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
Figure 5. Effect of different matrices and analyte:matrix ratios on S/N values for analyte’s radical cations of (a) cobalt porphyrin, (b)
phthalocyanine, (c) naphthalocyanine, (d) zinc phthalocyanine, (e) rubrene, (f) coronene
(1:100 and 1:500, corresponding to 25 and 5.0 pmol on target)
results on signal observation with S/N values of 75, 54, 37, and
21 using PV-CN, PV-Cl, PV-COOH-CH3, and PV-COOH-
OCH3 as matrices. No signal was registered for
naphthalocyanine, even at high analyte:matrix molar ratios
when using LDI or DCTB as matrix.
Figure 5d, e, and f show the variation in S/N of zinc
phthalocyanine (m/z 576.079), rubrene (m/z 532.219), and
coronene (m/z 300.094) (see Figures S11, S12, and S13 of
the Supplemental Information for mass spectra). Interestingly
these three analytes exhibit discernible signals under LDI con-
ditions (25 pmol on target), corresponding to S/N ratios of
1397, 557, and 284, respectively. Comparison of the LDI S/N
values for these compounds with the values registered when
using PV-CN gives us an idea of the role of this PV derivative
as ET matrix. Using also the same experimental conditions as
in the LDI experiment (25 pmol of analyte on target), the S/N
values for the MALDI experiment with PV-CN as matrix are
4246, 6497, and 5569, respectively, for zinc phthalocyanine,
rubrene, and coronene, values which correspond to an

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
Table 1. Detection Limits for Analyte Signals Using PV Derivatives as MALDI ET Matrices
Analyte
LDI
PV-CN
PV-Cl
PV-COOH-CH3
Co porphyrin
Rubrene
Coronene
125 pmol
12.5 pmol
2.5 pmol
125 fmol
2.5 fmol
2.5 fmol
12.5 fmol
2.5 fmol
2.5 fmol
125 fmol
2.5 pmol
12.5 fmol
12.5 fmol
250 fmol
1.25 pmol
1.25 pmol
Zn pphthalocyanine
enhancement in the molecular ions S/N ratio of 304%, 1166%,
and 1961%.
concentrations three orders of magnitude below the LODs with
LDI. PV-CN, in particular, allowed detection of low-fmol
amounts of the four analytes, in contrast with LDI where
analytes were observed in the range of pmol to higher fmol.
As a general conclusion from Figure 5, we can state that the
PV-CN derivative stands out as ET MALDI matrix since it
considerably increases molecular ions yield during the ET
process, as observed by a dramatic increase in S/N for all tested
analytes. We believe that this fact is a combination of the
excellent photophysical properties of PV-CN, such as the
highest molar absorptivity of all tested PV derivatives
(72,242 Lmol–1cm–1), low energy threshold (0.47 μJ), and
physicochemical characteristics such as high solubility (20
mM), and high ionization potential (7.96 eV). In addition, the
PV-CN is the derivative that will undergo the most energeti-
cally favorable ET process with the studied analytes because of
the energetic condition: IPM > IPA (M=matrix, A=analyte).
In Figure S15 of the Supplemental Information, we ana-
lyzed the ion appearance curves for four analytes (zinc phtha-
locyanine, cobalt porphyrin, coronene, and rubrene) and the
evolution of S/N ratios as a function of laser pulse energy using
different PV matrices. Under the same experimental condi-
tions, we observe that the M+.abundances for all analytes are
always higher when using PV-CN as matrix, even at low laser
power (0.4 μJ); this observation supports our conclusions from
Figure 5. Figure S15 also indicates that the ideal working
energies for the PV matrices lie around 0.8 μJ (where the
highest S/N ratios for the analytes are recorded), which is
almost a 40% decrease in working laser energy from traditional
matrices like CHCA, DCTB, and DHB, usually above 1.3 μJ.
A matrix that efficiently performs at low laser energy consti-
tutes a major advantage in MALDI because it can not only
increase laser lifetime and generate less source contaminations
but also hinder fragmentation of labile analytes, increasing ion
survival yields as will be illustrated in a future report.
Finally, LODs for four analytes in Figure 5 were calculated
using PV-CN, PV-Cl, and PV-COOH-CH3 derivatives as
MALDI matrices because of their high performance. For LODs
calculation, analyte concentration was varied from 125 μM to
2.5 nM (125 pmol to 2.5 fmol on target), while maintaining the
same matrix concentration (1.25 mM or 1.25 nmol on target);
the results are shown in Table 1. For Co-porphyrin analysis, PV
derivatives afford LODs six orders of magnitude lower than
LDI, and one order of magnitude lower that LDI for Zn-
phthalocyanine. Previously, LODs for phthalocyanines using
retinoic acid as MALDI matrix were reported at 24 pmol [58],
whereas for the PV matrices the LODs are as low as 2.5 fmol
(PV-CN) as reported in Table 1. For aromatic compounds anal-
ysis, PV derivatives allow detection of rubrene and coronene in
Conclusions
Unlike proton-transfer reactions in UV-MALDI, involving sev-
eral ion formation mechanisms, electron-transfer reactions al-
most exclusively rely on the formation of primary ions through
matrix photoionization. From the point of view of the CPCD
model [2, 50, 51] excitation and concentration of energy in the
matrix network causes Bpooling^ events that eventually induce
matrix ionization. For these pooling events to happen, energy
mobility within the matrix molecules is of fundamental impor-
tance; these Bmobile excitations^ are known as molecular exci-
tons. Molecular excitons are capable of energy transfer process-
es, depending on intermolecular distances between the molec-
ular species, and have lifetimes on the order of nanoseconds.
According to our observations, radical cation formation in
PV derivatives occurs at much lower laser powers than current
ET matrices such as DCTB. In addition, not only signal abun-
dances for both matrix and analyte are significantly enhanced
but also resolution and sensitivity are improved when using PV
derivatives in ET processes. We believe that the reason behind
these observations lies in the outstanding photophysical prop-
erties of PV derivatives. Several literature reports describe
exciton formation in high yields, with lifetimes of nanosec-
onds, when PVs interact with electromagnetic radiation [59,
60]. In addition, PVs forming π stacking structures with
interplanar distances of less than 8 Å can have an efficient
conjugation between their HOMO and LUMO states, making
the exciton formation process in solid phase more favorable
[61]. We have previously reported, by X-ray measurements, an
interplanar distance for PV-OCH3 of 4.1 Å [19]. Considering
the initial laser/matrix interaction in ET MALDI occurring in
condensed phase, we believe that an efficient π–π stacking,
combined with long exciton lifetimes, in the tested PV deriv-
atives, could explain the excellent performance of these mole-
cules as electron transfer matrices for UV-MALDI.
In addition, we also demonstrate the use of PV derivatives
(PV-CN, PV-COOH-CH3, PV-COOH-OCH3, and PV-Cl) as
electron-transfer MALDI matrices for the analysis of metal
complexes and aromatic compounds. The use of these matri-
ces, particularly PV-CN and PV-Cl, increase the S/N ratio of
model analytes two to three orders of magnitude compared

L. J. Castellanos-García et al.: Oligo p-Phenylenevinylene Derivatives
14. Díaz, C., Alzate, D., Rodríguez, R., Ochoa, C., Sierra, C.A.: High yield
with LDI measurements. Working laser energy for PVs lies in
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the 0.4–1.0 μJ range, much lower than the laser energy used for
traditional matrices like CHCA, DCTB, and DHB, usually
above 1.3 μJ. Lower laser energies in MALDI translate into
increased laser lifetime and decreased source contamination. In
addition, we observed limits of detection in the low fmol range
for PV-CN, whereas LDI provides LODs in the pmol to higher
fmol range. The observation of low LODs when using the PV
matrices supports the idea of ionization through electron trans-
fer from the analyte to the matrix, rather than from direct
photoionization. Finally, among the tested PV matrices, PV-
CN stands out as ET MALDI matrix because of its high
analytical performance.
15. Alzate, D., Hinestroza, J.P., Sierra, C.A.: High-yield synthesis of the
novel E,E-2,5-dimethoxy-1,4-bis[2-(4-ethylcarboxylatestyril)]benzene
by the Heck Reaction. Synth. Commun. 43, 2280–2285 (2013)
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17. Schenning, A.P.H.J., van Herrikhuyzen, J., Jonkheijm, P., Chen, Z.,
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19. Sierra, C.A., Lahti, P.M.: A photoluminescent, segmented oligo-
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20. Praveen, V.K., Ranjith, C., Bandini, E., Ajayaghosh, A., Armaroli, N.:
Oligo(phenylenevinylene) hybrids and self-assemblies: versatile mate-
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Acknowledgments
The authors acknowledge funding from COLCIENCIAS (Grant
0041-2013). They also thank Guatiguará Technology Park and
the Central Research Laboratory Facility at Universidad Indus-
trial de Santander for infrastructural support. L.J.C. acknowl-
edges the Vice Chancellor for Research Office at Universidad
Industrial de Santander (VIE-UIS) for a travel grant.
23. Srinivasan, N., Haney, C.A., Lindsey, J.S., Zhang, W., Chait, B.T.:
Investigation of MALDI-TOF mass spectrometry of diverse synthetic
metalloporphyrins, phthalocyanines, and multiporphyrin arrays. J. Por-
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