Spacer controlled photo-induced intramolecular electron transfer in a series of phenothiazine-boron dipyrromethene donor-acceptor dyads

Article abstract of DOI:10.1016/j.jphotochem.2015.07.007

Abstract A series of phenothiazine (PTZ)-boron dipyrromethene (BODIPY) based electron donor-acceptor dyads, PB1, PB2, and PB3, containing PTZ as an electron donor and BODIPY as an electron acceptor, tethered together with spacers of varied sizes i.e., dir

Full text of DOI:10.1016/j.jphotochem.2015.07.007

Accepted Manuscript
Title: Spacer Controlled Photo-Induced Intramolecular
Electron Transfer in a Series of Phenothiazine-Boron
Dipyrromethene Donor-Acceptor Dyads
Author: Naresh Duvva Kolanu Sudhakar Deepak Badgurjar
Raghu Chitta Lingamallu Giribabu
PII:
S1010-6030(15)00263-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2015.07.007
JPC 9964
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
18-5-2015
27-6-2015
10-7-2015
Please cite this article as: Naresh Duvva, Kolanu Sudhakar, Deepak Badgurjar,
Raghu Chitta, Lingamallu Giribabu, Spacer Controlled Photo-Induced Intramolecular
Electron Transfer in a Series of Phenothiazine-Boron Dipyrromethene Donor-
Acceptor Dyads, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2015.07.007
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Spacer Controlled Photo-Induced Intramolecular Electron Transfer in a
Series of Phenothiazine-Boron Dipyrromethene Donor-Acceptor Dyads
Naresh Duvva,a Kolanu Sudhakar,a Deepak Badgurjar,b Raghu Chitta,b* Lingamallu
Giribabua*
*aInorganic& Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology,
Tarnaka, Hyderabad – 500007, Telangana, India.giribabu@iict.res.in.
*bSchool of Chemical Sciences & Pharmacy, Central University of Rajasthan, Bandarsindri,
Tehsil: Kishangarh, Distt. Ajmer, Rajasthan – 305817, India.raghuchitta@curaj.ac.in.
Graphical abstract
Highlights
 A series of phenothiazine (PTZ)-boron dipyrromethene (BODIPY) based donor-acceptor
dyads, tethered together with spacers of varied sizes are reported.
 Ground state properties indicates that there exist a moderate - interactions between
PTZ and BODIPY units.
 Excited state properties indicates that there is an intramolecular phtoinduced electron
from ground state of PTZ to singlet state of BODIPY.
 The electron transfer rates (kET) of these dyads were found to be solvent dependent and
were in the range 0.06 x 108 to 71.96 x 1010 s-1.
Abstract
A series of phenothiazine (PTZ)-boron dipyrromethene (BODIPY) based electron donor-
acceptor dyads, PB1, PB2, and PB3, containing PTZ as an electron donor and BODIPY as an
electron acceptor, tethered together with spacers of varied sizes i.e., directly connected (absence
of the spacer), phenyl bridge, and ethoxy phenyl spacer, respectively, are reported. Optical
absorption spectra of the dyads showed marginal ground state interactions between the
chromophores and, these interactions are found more pronounced in PB1 and PB2
.
Electrochemical studies on the dyads revealed minor shifts in the first oxidation of PTZ moiety
and the first reduction of BODIPY moiety, compared to their control compounds. In the newly
synthesized dyads, the free-energy calculations performed by employing the redox and singlet
excited state energies, in both polar and non-polar solvents, suggested the possibility of electron
transfer from the ground state of PTZ to the singlet excited BODIPY moiety (PTZ


1BODIPY*) to produce PTZ·+-BODIPY·─. Accordingly, steady state emission studies revealed
the efficient fluorescence quenching of the singlet excited BODIPY in the dyads and the
efficiency of quenching for all the dyads is higher in polar solvents. More importantly, these
emission studies also revealed that, in a given solvent, as the length of the spacer between the
chromophores is increased, the efficiency of fluorescence quenching is decreased. The degree of
electronic communication between the chromophores w.r.t. the spacer type and size was
supported by density functional theory (DFT) and time dependent-DFT (TD-DFT) calculations.
The electron transfer rates (kET) of these dyads were found to be solvent dependent and were in
the range 0.06 x 108 to 1.96 ×1010s-1.
Key Words: BODIPY, Phenothiazine, Dyads, Intramolecular, Electron transfer, Spacer type and
size, Time-resolved.
Corresponding authors: giribabu@iict.res.in, Phone: +91-40-27191724, Fax: +91-40-27160921;
raghuchitta@curaj.ac.in, Phone: +91-1463-238535.
1. Introduction
Studies on photo-induced electron transfer (PET) events occurring in artificial donor-acceptor
systems, that mimic the natural photosynthetic machinery, form as viable information to address
the mechanistic details of electron transfer in chemistry and biology,[1,2] to develop artificial
photosynthetic systems for harvesting solar energy[3-10] and, to build opto-electronic devices.
[11] Several molecular and supramolecular dyads, triads, tetrads etc. have been elegantly
designed and studied with an emphasis of generating long-lived charge separated states via
charge migration route and achieved longer lifetimes for the charge separated states.[12-18]
Systematic investigations exploring the role of photosensitizer (electron donor), nature of the
electron acceptor, the spacer connecting the donor-acceptor entities, the spatial orientation of the
donor-acceptor entities have provided a significant and valuable information necessary for the
direct utilization of these systems for various light harvesting applications. In order to build these
donor-acceptor systems, several porphyrins[19,20] or porphyrin like chromophores such as
phthalocyanines,[21] sub-phthalocyanines,[22,23] corroles,[24] etc., and non-porphyrin like
molecules, ferrocene,[25,26] triphenylamines,[27] phenothiazines[28,29] have been chosen as
electron donors and quinones,[30] perylenediimides,[31] and fullerenes (C60&C70)[32] have been
utilized as electron acceptors. Among the electron acceptors, fullerene (C60) has been extensively
utilized, compared to any other acceptor, due to its remarkable electrochemical and optical
properties and, more importantly, small re-organization energy in electron transfer reactions.[33]

Among the electron donors utilized for the purpose, unarguably, porphyrin or porphyrin like
molecules have dominated the area of research as photoactive entity due to their resemblance to
the natural photosynthetic chlorophyll pigment, rich photo- and electrochemical properties, and
relatively easy synthetic modifications.[19,20] However, the tendency of porphyrins or
porphyrin like molecules to form aggregates and participate in self-quenching, has limited the
wide range of opto-electronic applications.[34] On the other hand, chromophores such as
phenothiazines (PTZ) have gained attraction as suitable electron donors in donor-acceptor
conjugates[28] and potential p-type material in organic devices[35] due to several interesting
structural, photophysical and electrochemical properties. In general, PTZ derivatives absorb in
the UV region, exhibit low oxidation potentials and possess a high propensity to form stable
radical cations.[36,37] One of the most striking structural features of phenothiazine is that the
ring is non-planar with a butterfly conformation in the ground state, which impedes the
molecular aggregation and inhibit the formation of intermolecular aggregation and self-
quenching.[38] Thus, because of its ground state intra-molecular charge transfer (ICT) and
excited state PET, PTZs are regarded as excellent donors and widely used in making organic
light emitting diodes,[39,40] acid-base dyes, and pigments[41] etc.
Towards the goal of constructing new artificial photosynthetic systems that possess the
ability to convert light into chemical energy, BF2-chelated dipyrromethene compounds,
4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (abbreviated as BODIPY)
and its structural analogue, BF2-chelated tetraarylazadipyrromethenes (aza-BODIPY),
have attracted much attention in recent years as antenna molecules, electron donors and
electron acceptors.[42] In particular, BODIPYs are versatile functional dyes with high
stability and favourable photophysical and electrochemical properties, which can be tuned
readily through chemical modification of the BODIPY core.[43] Generally, they exhibit
large molar absorptivities (ε ≈ 105 M-1cm-1), relatively high fluorescence quantum yields
(0.57 in CH2Cl2), and relatively long singlet excited state lifetimes (≈ 4-6 ns).[40] As a
result, they have been extensively used as energy-absorbing and transferring antenna
molecules in photosynthetic antenna-reaction center mimics,[44,45] and PET donor[46]
and acceptor entities.[42,47]
Despite many reports of PTZ and BODIPY containing them separatelyare published in
the literature, only a few cases, however, containing systems with BODIPY tethered to
either one or two PTZ moieties and amalgamating the properties of both these
chromophores have been reported. Some of these include, the photosynthetic antenna-

center mimic involving BODIPY as energy funnelling entity to porphyrazine and PTZ as
a sacrificial electron donor,[47] chiral BODIPY and aza-BODIPY-based D- -A

conjugated polymers involving PTZ as electron donor,[48,49] Recently, D’Souza et al
has reported PTZ-BODIPY-C60 triads as photosynthetic reaction center models and
studied the effect of solvents and the substitution on the photo-induced charge separation
and recombination.[50] However, the study was limited to only two dyads i.e., PTZ
connected to BODIPY either directly (no spacer) or via phenyl bridge and comprehensive
photo-induced electron transfer studies w.r.t. the spacer size and type were not presented.
In the present study, donor-spacer-acceptor systems involving BODIPY covalently
connected to PTZ moieties, especially with systematic variation of spacers, and photo-
induced electron transfer events w.r.t. the spacer type and sizes, in three different solvents
(hexanes, CH2Cl2 and acetonitrile) is reported. Such kind of study is quite important to
understand the variations in energetics of electron transfer w.r.t. the type and size of the
spacer, and utilize these concepts in systematically controlling the electron transfer in
various molecular and opto-electronic devices. To this, we have synthesized a series of
PTZ-BODIPY based electron donor-acceptor systems, PB1, PB2, and PB3, tethered
together with varied spacer types and sizes i.e., absence of spacer (directly connected),
phenyl bridge, and saturated ethoxyphenyl spacer, respectively, and studied the effect of
the spacer in photo-induced electron transfer events associated with these donor-acceptor
conjugates. The molecular structures of the dyads and the control compounds are shown
in Chart 1.
2. Experimental Section
2.1. Chemicals. Commercially available reagents and chemicals were procured from
Sigma-Aldrich. Analytical reagent (AR) grade solvents were used for the reactions while
laboratory reagent (LR) grade solvents were used for purifications and column
chromatography. CH2Cl2, chloroform, acetonitrile and N,N-dimethylformamide were
dried in presence of calcium hydride under nitrogen atmosphere. Hexanes were purified
using sodium metal added benzophenone by refluxing overnight, distilled under vacuum
and stored over 4Å molecular sieves. Triethylamine was distilled with NaOH pellets.
ACME silica gel (100-200 mesh) was used for column chromatography. Thin-layer
chromatography was performed on Merck-pre-coated silica gel 60-F254 plates. Either
gravity or flash chromatography was performed for purification of all compounds. All the

reactions were carried out under nitrogen or argon atmosphere using dry and degassed
solvents.
2.2. Synthetic procedure. N,N´-difluoroboryl-1, 3, 7, 9-tetramethyl-5-phenyldipyrrin
(
BODIPY),[51]
10-(
n
-hexyl)-10
H
-phenothiazine
(
1
),[52]
10-(n-hexyl)-10H-
phenothiazine-3-carbaldehyde (2)[53] were synthesized according to the literature
procedures.
2.2.1. 10-(4-Formylphenyl)phenothiazine (3): K3PO4 (0.53g, 2.5mmol) was added to a
glass vial containing phenothiazine (0.1 g, 0.5 mmol) and the 4-fluorobenzaldehyde (0.12
g, 1 mmol), in dry DMF solvent (5 mL). The vial was sealed with a Teflon cap and the
mixture was heated at 170°C for 48 hours. The mixture was cooled to room temperature
and then poured into 200 mL of water and the product was extracted using methylene
chloride. The organic layer was dried over anhydrous Na2SO4 and concentrated under
reduced pressure to give the crude product. The crude product was purified by silica gel
column chromatography and the desired compound was eluted with petroleum
1
ether/ethylacetate (10:2, v/v) as light yellow colour solid. Yield: 0.12 g (79%). H NMR
(500 MHz, CDCl3): (ppm) 9.75 (s, 1H, -CHO), 7.65 (d, J = 8.1 Hz, 2H, phenyl-aromatic
H
), 7.35 (d, J = 8.6 Hz, 2H, phenyl-aromatic
aromatic ), 7.0-7.10 (m, 4H, phenothiazine-aromatic
(M+, C19H13NOS requires 303.38).
H
), 7.15-7.25 (m, 4H, phenothiazine-
H
H
). MALDI-TOF: (m/z) found 304
2.2.2. 10-(2-bromoethyl)-10H-phenothiazine (4): To a suspension of sodium hydride
(60% mineral oil, 1.2 g, 50.2 mmol) in DMF was added a solution of phenothiazine (5g,
0.025 mol) in DMF and stirred at 0°C under nitrogen. After 30 min., the reaction mixture
was heated to room temperature and stirred for 1h under nitrogen. Then the reaction
mixture was cooled to 0°C, 1,2-dibromoethane (47 g, 0.25 mol) was added and the
temperature was raised to 90°C and stirred for 12 h under nitrogen. After cooling, the
reaction mixture was washed with water and extracted using ethyl acetate. The combined
organic extracts were washed with brine solution, dried over Na2SO4 and filtered. The
crude compound was purified using silica gel column chromatography and the desired
compound was eluted using petroleum ether:ethylacetate (20:1, v/v). Evaporation of the
solvent under the reduced pressure yielded the titled compound as a white solid. Yield:
1
3.8 g (49%). H NMR (500 MHz, CDCl3):

(ppm) 7.11-7.07 (d, J = 8.0 Hz, 4H,
), 6.91-6.86 (t, J = 10 Hz, 2H, phenothiazine-aromatic ), 6.80-
6.76 (d, J = 12 Hz, 2H, phenothiazine-aromatic ), 4.22-4.17 (t, J = 10 Hz, 2H, Br-CH2),
phenothiazine-aromatic
H
H
H

3.58-3.53 (t, J = 9.4 Hz, 2H, N-CH2). MALDI-TOF: (m/z) found 306.11 (M+, C14H12BrNS
requires 306.22).
2.2.3. 4-(2-(10
(0.1 g, 0.8 mmol) was dissolved in DMF (50 mL) and dry potassium carbonate (0.9 g, 6.5
mmol) was added and stirred under nitrogen for 30 min. Then (0.5g, 1.6 mmol) was
H-phenothiazin-10-yl)ethoxy)benzaldehyde (5): 4-hydroxybenzaldehyde
4
added and the reaction mixture was stirred at 90°C for 3 h. Then the reaction mixture was
cooled and the solvent was evaporated. Water was added and the mixture was extracted
using ethyl acetate. Evaporation of the organic extracts yielded the crude compound
which was purified by silica gel column chromatography using petroleum
ether/ethylacetate (20:1, v/v) as an eluent. Evaporation of the solvent yielded the titled
compound as light yellow solid. Yield: 0.4g (71%).1H NMR (500 MHz, CDCl3): δ (ppm)
9.88 (s, 1H, -CHO), 7.83-7.80 (d, J = 8.8 Hz, 2H, phenyl-aromatic
8.2 Hz, 4H, phenyl-aromatic 2H & phenothiazine-aromatic 2H), 7.00-6.93 (m, 6H,
phenothiazine-aromatic ), 4.38 (4H, s, O-CH2 N-CH2). MALDI-TOF: (m/z) found
348.02 (M+, C21H17NO2S requires 347.43).
H), 7.18-7.16 (d, J =
H
&
2.2.4. Synthesis of Dyads: 2,4-dimethypyrrole (0.64 g, 6.72 mmol) and corresponding
aldehyde (3.36 mmol) were dissolved in absolute CH2Cl2 (200 mL) (nitrogen gas was
bubbled through CH2Cl2 for 30 min.) under N2 atmosphere. One drop of trifluoroacetic
acid (TFA) was added and the solution was stirred at room temperature for 2 h. At this
point, a solution of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (0.77 g, 3.36
mmol) in 50 mL absolute CH2Cl2 was added, stirred for 1h followed by the addition of
Et3N (4.5 mL, 3.4 g, 33.62 mmol) and BF3.OEt2 (3.4 mL, 3.8 g, 26.9 mmol). After 30
min., the reaction mixture was extracted with CH2Cl2 and the organic layer was dried
over anhydrous Na2SO4. Evaporation of the solvent under reduced pressure gave the
crude product, which upon purification by silica gel column chromatography using
dichloromethane/hexane (1:1 v/v) as eluent gave the corresponding dyad.
N,N´-difluoroboryl-1,3,7,9-tetramethyl-5-[10-(n-hexyl)-10H-phenothiazin-3-
yl]dipyrrin (PB1): Yield: 310 mg (18%). 1H NMR (500MHz, CDCl3): δ (ppm) 7.19-7.16
(m, 1H, phenothiazine-aromatic
aromatic ), 7.01-6.99 (dd, J = 7.4 Hz 1.5 Hz, 2H, phenothiazine-aromatic
(m, 3H, phenothiazine-aromatic ), 5.97 (s, 2H, pyrrole ), 3.89-3.86 (t, J = 7.3 Hz, 2H,
N-CH2), 2.54 (s, 6H, pyrrole-CH3), 1.85-1.79 (m, 2H, aliphatic ), 1.51 (s, 6H, pyrrole-
CH3), 1.47-1.41 (m, 2H, aliphatic ), 1.38-1.34 (m, 2H, aliphatic ), 1.32-1.28(m, 2H,
H
), 7.11-7.09 (dd, J = 7.6 Hz 1.5 Hz, 1H, phenothiazine-
H
H
), 6.95-6.87
H
H
H
H
H

aliphatic
C31H34BF2N3S requires 529.49).
H), 0.89-0.85(t, J = 7.1 Hz, 3H, aliphatic H
).MALDI-TOF: (m/z) found 530 (M+,
N,N´-difluoroboryl-1,3,7,9-tetramethyl-5-[4-(10H-phenothiazin-10-
1
yl)phenyl]dipyrrin (PB2): Yield: 0.36 mg (30%). H NMR (500 MHz, CDCl3): δ (ppm)
7.51 (s, 4H, phenyl-aromatic
aromatic ), 6.92- 6.88 (m, 4H, phenothiazine-aromatic
Hz, 2H, phenothiazine-aromatic ), 6.04 (s, 2H, pyrrole-
H
), 7.11-7.09 (dd, J = 7.8 Hz 1.3 Hz, 2H, phenothiazine-
), 6.38-6.36 (dd, J = 7.9 Hz 1.6
), 2.58 (s, 6H, pyrrole-CH3),
H
H
H
H
1.56 (s, 6H, pyrrole-CH3). MALDI-TOF: (m/z) found 521.88 (M+, C31H26BF2N3S requires
521.43).
N,N´-difluoroboryl-1,3,7,9-tetramethyl-5-[4-(2-(10H-phenothiazin-10-
1
yl)ethoxy)phenyl]dipyrrin (PB3): Yield: 0.21g (26%). H NMR (500 MHz, CDCl3):
7.18-7.13 (m, 6H, phenyl-aromatic 4H& phenothiazine-aromatic 2H), 7.01-6.96 (dd, J =
7.4 Hz 1.2 Hz, 6H, phenothiazine-aromatic
H), 5.97 (s, 2H, pyrrole-H), 4.37 (s, 4H, O-
CH2 N-CH2), 2.54(s, 6H, pyrrole-CH3), 1.41 (s, 6H, pyrrole-CH3). MALDI-TOF: (m/z)
&
found 566.02 (M+, C33H30BF2N3OS requires 565.48).
2.3. Methods and Instrumentation.
2.3.1. Characterization: 1H-NMR spectra were recorded on a 500MHz INOVA
spectrometer. Cyclic and differential pulse voltammograms (DPV) measurements were
performed on a PC-controlled electrochemical analyzer (CH instruments model
CHI620C). All the experiments were performed with 1 mM concentration of compounds
in CH2Cl2 at a scan rate of 100 mVs-1 in which tetrabutylammoniumperchlorate (TBAP)
is used as a supporting electrolyte as documented in our previous reports.[54,55]
2.3.2. Absorption and fluorescence measurements: The optical absorption spectra were
recorded on a Shimadzu (Model UV-3600) spectrophotometer. Concentrations of
solutions are ca. to be 1 x 10-6 M. Steady-state fluorescence spectra were recorded on a
Fluorolog-3 spectrofluorometer (Spex model, JobinYvon) for solutions with optical
density at the wavelength of excitation (λex) ≈ 0.1. Fluorescence quantum yields (ϕ) were
estimated by integrating the fluorescence bands and by using N,N´-difluoroboryl-1,3,7,9-
tetramethyl-5-(4-carboxy)phenyldipyrrin (BODIPY-CO2H)[56] (ϕ = 0.57 in CH2Cl2),
when excited at 503 nm. Phenothiazine (PTZ) compounds were almost non-fluorescent,
when excited at ex 254 nm.[57] Fluorescence lifetime measurements were carried on a
picosecond time-correlated single photon counting (TCSPC) setup (FluoroLog3-Triple

Illuminator, IBH Horiba JobinYvon) employing a picosecond light emitting diode laser
(NanoLED, λex=470 nm) as excitation source. The decay curves were recorded by
monitoring the fluorescence emission maxima of the dyads (λem=500 nm).
Photomultiplier tube (R928P, Hamamatsu) was employed as the detector. The lamp
profile was recorded by placing a scatterer (dilute solution of Ludox in water) in place of
the sample. The width of the instrument function was limited by the full width at half
maxima (FWHM) of the excitation source, ~625 ps at 405 nm. Decay curves were
analyzed by nonlinear least-squares iteration procedure using IBH DAS6 (version 2.3)
decay analysis software. The quality of the fits was judged by the χ2 values and
distribution of the residuals.
2.3.3. Theoretical calculations: Full geometry optimization of the dyads PB1, PB2, and
PB3 were carried out by Gaussian 09 (Revision B.01) ab initio quantum chemical
software[58] in the personal computer. Density Functional Theory (DFT) was used to
determine the ground state properties, while time dependent-DFT (TD-DFT) was
employed for estimation of ground state to excited state transitions. B3LYP method[59]
and 6-31G (d,p) basis set[60] were used to optimize the geometries of the dyads to be
genuine global minimum energy structures. The geometries were used to obtain the
frontier molecular orbitals (FMOs) and were also subjected to single-point TD-DFT
studies (First 15 vertical singlet–singlet transitions) to obtain the UV-Vis spectra of the
dyes. The integral equation formalism polarizable continuum model (PCM)[61] within
the self-consistent reaction field (SCRF) theory was used in the TDDFT calculations to
describe the solvation of the dyes in CH2Cl2. The software GaussSum 2.2.5 was
employed to simulate the major portions of the absorption spectra and to interpret the
nature of transitions.[62] The contribution percentages of individual units present in the
dyes to the respective molecular orbitals were calculated.
3. Results and Discussion
3.1. Syntheses of Phenothiazine-BODIPY dyads. The methodology developed for the
synthesis of phenothiazine functionalized BODIPY dyes, PB1
spacers is shown in Scheme 1, while the details of the syntheses are given in the
Experimental Section. The phenothiazine moieties bearing aldehyde functional groups,
and , required for the synthesis of BODIPYs are synthesized in one or two steps using
slightly modified literature procedures. Briefly, N-(n-hexyl)-phenothiazine, was
formylated at the 3-position by standard Vilsmeier-Haack formylation procedure[51] by
, PB2 and PB3, with varied
2
,
3
5
1

the reaction of
carbaldehyde,
phenothiazine in presence of K3PO4 at high temperatures in DMF produced 10-(4-
formylphenyl)phenothiazine, in good yields. However, 4-(2-(10H-phenothiazin-10-
yl)ethoxy)benzaldehyde, was synthesized by reacting phenothiazine with 1,2-
dibromoethane in DMF in the presence of NaH to obtain followed by its reaction with
4-hydroxybenzldehyde in presence to K2CO3 in DMF. Phenothiazine functionalized
BODIPY dyes PB1 PB2, and PB3, were synthesized by a one-pot reaction involving the
1
with POCl3 in DMF to yield 10-(n-hexyl)-10H-phenothiazine-3-
2
while the one-pot reaction of 4-fluorobenzaldehyde with pristine
3
5
4
,
condensation of the corresponding aldehyde with 2,4-dimethylpyrrole in presence of
trifluoroacetic acid to make dipyrromethane substituted at the meso-position w.r.t.
phenothiazine entity. Subsequent treatment of these dipyrromethanes with DDQ,
triethylamine, BF3-etherate afforded the crude compounds as a black reaction mixture.
Evaporation of the solvent and purification of these crude products by silica gelcolumn
chromatography afforded the target molecules as orange crystals.[51] The newly
1
synthesized compounds have been fully characterized by mass, H NMR and other
1
spectroscopic techniques. H NMR, MALDI-TOF mass spectra of the precursors and the
final compounds are shown in the supporting information Figures S1-S12. For example,
The peak at m/z 212.29 corresponds to “10-methyl-10H-phenothiazine”. The peak at m/z
1131 might correspond to the dyad of PB3
.
3.2. Optical Absorption Studies. Figure 1 show the optical absorption spectra of the
investigated compounds along with pristine PTZ and BODIPY used as the control
compounds. Spectroscopic data which include wavelength of maximum absorbance
(max), molar extinction coefficients (log ) of the dyads and the control compounds are
summarized in Table 1. The control compound, PTZ revealed two distinct peaks at 254
and 316 nm in dichloromethane which were assigned to the localized aromatic 
transitions.[63] The reference compound, BODIPY exhibited three minor intense
absorption bands at 227 nm and between 305-400 nm, and a major intense absorption
band at 501 nm corresponding to the electronic transition from the ground state to the first
excited state (S0-S1 transition). Tethering the phenothiazine moieties to BODIPY
chromophores did not reveal any drastic spectral changes except minor spectral changes
in the UV and visible region. Compared to pristine PTZ, the phenothiazine moiety in
thedyads PB1
, PB2, and PB3 having the substitution on the aromatic ring or on hetero
atom revealed the spectra with a split band in the UV region at 227-240 nm and 245-285

nm ranges.[57] This spectral change may be attributed to the loss of symmetry in the
phenothiazine frame work due to the introduction of BODIPY moiety. On the other hand,
in case of all the dyads, the absorption peaks between 300 – 400 nm are blue shifted (~5 –
9 nm) compared to the control compounds, PTZ and BODIPY, displaying very marginal
ground state interactions within the chromophores in these dyads. Also, in case of PB1
and PB2, the characteristic peak at 503 nm corresponding to the BODIPY moiety is red
shifted ~2 nm, compared to the control compound BODIPY (max = 501 nm), while
similar changes are not observed in case of PB3. This marginal shift may be attributed to
the better electronic communication between the two chromophores, PTZ and BODIPY
moieties, in the dyads (vide infra).
Such kind of weak ground state interactions generated between energy/electron donors
and acceptors that are tethered together have been reported earlier.[16,45] However,
under these conditions, it was difficult to isolate any weak band corresponding to charge-
transfer type interactions between PTZ and BODIPY moieties. More importantly, the
stronger absorption of BODIPY between 425–530 nm (max = 501 nm) has no overlap
with the PTZ absorption in this wavelength range indicating that irradiation of the dyad at
485 nm would selectively excite the BODIPY moiety of the dyad.
3.3. Electrochemical Studies. The electrochemical behaviour of the dyads was
investigated using cyclic and differential pulse voltammetric techniques. Figure 2
illustrates the differential pulse voltammograms of PB1, PB2, and PB3, and Table 2
summaries the redox potential data (CH2Cl2 and 0.1 M TBAP) of the donor-acceptor
systems investigated in this study along with the control compounds. Figure 2 and Table
2 indicate that the dyads investigated showed two oxidation and one reduction peaks
under the experimental conditions employed for the study. Wave analysis suggested that,
in general, the first two oxidation steps and the one reduction step are reversible (ipc/ipa =
½
0.9-1.0) and diffusion controlled (ipc/
1) one electron (
Ep = 60–70 mV;

= constant in the scan rate (
) range 50-100 mVs-


Ep = 65±3 mV for ferrocenium/ferrocene couple)
reactions. In contrast, in case of pristine PTZ molecule, we have observed quasi-
reversible oxidation process.
All the dyads exhibited two one-electron oxidation peaks, with the first oxidation peak
in the range of 0.76 – 0.80 V (PB1: 0.80 V; PB2: 0.77 V; PB3: 0.76 V) and a second
oxidation peak in the range of 1.18 – 1.25 V (PB1: 1.18 V; PB2: 1.22 V; PB3: 1.25 V)
under the employed experimental conditions. The first peak was due to the oxidation of

the PTZ moiety of the dyad (PTZ•+ ─ BODIPY). The origin of the second oxidation peak
was difficult to be assigned to a particular chromophore because the second oxidation of
PTZ (PTZ•+/2+: 1.18 V vs SCE) and the first oxidation of BODIPY (1.22 V vs SCE) in
the control compounds were displayed at almost the same anodic potential. It should be
noted here that, in case of all the dyads, the first oxidation peak is shifted ~ 170 mV
towards higher anodic potentials compared to the first oxidation of pristine PTZ and this
shift may be ascribed to the introduction of electron withdrawing entity, BODIPY, in the
dyad which makes the PTZ hard to get oxidized compared to the pristine PTZ. However,
the similar trend is not quite pronounced in case of the second oxidation potential of the
dyads.
The dyads PB1, PB2, and PB3 undergo single one-electron reversible reduction at
−1.16 V, −1.08 V, and −1.18 V respectively. The origin of this peak is due to the
reduction of the BODIPY moiety of the dyad (PTZ – BODIPY•─). In case of these dyads,
it was observed that the reduction peak was shifted ~160 mV towards lower reduction
potentials compared to the control compound, BODIPY, centered at -1.24 V. This
cathodic shift may be ascribed to the introduction of electron donating entity, PTZ, in the
dyad which makes the BODIPY easy to reduce compared to the control compound.
The driving forces for charge separation (GCS) in three different solvents of varying
polarity, hexanes, CH2Cl2 and acetonitrile, were evaluated using the Weller-type
approach[64,65] utilizing the redox potentials, center-to-center distance (Rc-c), dielectric
constant of the solvents and the E0-0 values and listed in the Table 3. Generation of the
charge separated state (PTZ•+ - BODIPY•─) was exothermic via the singlet excited states
of the BODIPY in all the three solvents and followed the trend
acetonitrile>CH2Cl2>hexanes for a given dyad (vide infra). It should be mentioned here
that no apparent shifts in the redox potentials w.r.t. the variation in the spacer type or
sizes between PTZ and BODIPY moieties was observed in these dyads. Cyclic
voltammograms of the dyads and the controlled compounds are shown in the supporting
information S13.
In order to further assign the redox potentials of these dyads, we have carried our
spectroelectrochemical studies. Figure 3 indicates in-situ spectral changes of PB3 in
dichloromethane solvent during the applied potential. When applied potential at 0.90V,
the absorption band at 254 nm (absorption belongs to pristine PTZ) reduces its intensity
with formation of new at 270 nm and finally it disappears with an isosbestic point at 265

nm (Figure 3a). In contrast the absorption bands at 233 and 500 nm (absorption belongs
to pristine BODIPY) remain unchanged. This clearly suggest the oxidation at 0.76 V
belongs to PTZ moiety. Spectroscopic changes during the controlled potential application
at -1.30 V are shown in Figure 3b. The absorption band at 500 nm reduces with intensity
with the formation of isosbestic points at 260, 360 and 465 nm.[66] Whereas the
absorption bands at UV regions remains unchanged. This clearly suggestion the reduction
potential at -1.18 is belongs to BODIPY moiety. Similar spectral changes were also
observed in case of PB1 and PB2 dyads.
3.4. Computational Studies. To visualize the geometry, electronic structure and optical
properties of the dyads, computational studies were performed at the DFT and TD-DFT
studies using B3LYP/6-31G(d,p) level.[60] For this, the dyads were optimized on a Born-
Oppenheimer potential energy surface, and a global minimum for each dyad was
obtained. The energy-optimized structures and the molecular electrostatic potential
(MEP) maps for the dyads, PB1 and PB2, are shown in Figure 4a & b, while for the dyad,
PB3, is shown in Figure S15a & b in the supporting information. For all the investigated
dyads, it is observed that the PTZ moiety, in its neutral state, is bent around the N-S axis
with an angle of ~147.1°, which is in agreement with the value reported in the
literature.[67] The edge-to-edge (Re-e) and center-to-center distances (Rc-c) between the
PTZ and BODIPY moieties in the dyads were estimated and summarized in Table 4. In
the MEP maps, for all the dyads, the positive electrostatic potential was at the
phenothiazine and spacer connecting the chromophores, while the negative potential was
concentrated at the pyrrolic –NH and –BF2 groups of BODIPY. Frontier highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the
dyads, PB1 and PB2 are shown in Figure 4c, & d. In case of PB1, the HOMO was
located on the PTZ and BODIPY -system, suggesting strong interaction between the
donor and acceptor entities, while the LUMO was located on the BODIPY moiety. In
PB2, the HOMO was located on the PTZ moiety while the LUMO on the BODIPY
moiety. Importantly, parts of the HOMO and LUMO were also located on the phenyl
spacer, suggesting a notable interaction between donor and acceptor entities in PB2
However, it should be noted from the electronic distribution of frontier orbitals that the
interaction between donor and acceptor is weaker in case of PB2 as compared to PB1
.
.
The locations of the HOMO and LUMO also suggested the formation of the charge-
separated state (PTZ•+ - BODIPY•─) due to photoinduced electron transfer process.

In the case of dyad PB3, an increased donor-acceptor distance and less spreading of the
frontier orbitals were observed (see Figure S15c-f in the Supporting Information). In
contrast to the above dyads, in PB3, the HOMO and LUMO are located on the BODIPY
moiety, while the HOMO-1 and LUMO+1 are located on the PTZ moiety. This may be
attributed to very low energy difference between HOMO and HOMO-1 in this dyad.
However, the electrochemical studies revealed that PTZ moiety is oxidized at a lower
potential than BODIPY and, hence, as seen in case of PB1, PB2, and PB3, it can be
expected that PTZ moiety would act as electron donor and BODIPY moiety as the
electron acceptor. The calculated gas phase HOMO–LUMO gap was found to be in the
range of 2.99 eV and 2.65 eV for the dyads.
Based on the experimental observations, TD-DFT studies of these molecules were
carried using B3LYP energy functional with the 6-31G (d,p) basis set in order to gain a
deeper understanding of the excited-state transitions with the framework of the
polarizable continuum model (PCM) in CH2Cl2 as the solvent. These results are in
reasonable agreement with the experimental values. The singlet state properties of
maximum wavelength absorbance, oscillation strength (f), excited state energy (E) in eV
and the percentage contribution of molecular orbital of all three dyads by means of
absorption spectra are presented in Supporting Information. Theoretical absorption
spectra of PB1, PB2, and PB3, (see in supporting information (Fig. S16) of each dyad
segment has been computed from the frontier molecular orbitals by using the GaussSum
software.
3.5. Fluorescence Emission Studies. The photochemical behaviour of the phenothiazine-
BODIPY dyads was investigated, using steady-state fluorescence measurements. As can
be seen in Figure 5a-c, the emission spectra of all the three dyads, PB1, PB2, and PB3,
along with their reference compound, BODIPY, were measured in hexanes, CH2Cl2, and
acetonitrile solvents by exciting them at = 485 nm, wavelength at which BODIPY
moiety absorbs, and corresponding wavelengths of emission maxima and quantum yields
are presented in Table 5. Attempts to record the emission of the dyads and reference
compound PTZ by exciting the PTZ moiety at max = 254 nm, did not yield any
noticeable emission in case of the dyads and yielded very low intensity peak for pristine
PTZ (see Figure S14 in the Supporting information).[57] Hence, in the present
discussion, all the dyads were excited at
 = 485 nm and the resultant emission changes
are presented. It should be mentioned here that, in order to ensure that the BODIPY

moiety is selectively excited at this wavelength (ex = 485 nm), excitation spectra of all
the dyads in three solvents were recorded by monitoring the emission at 510 nm
corresponding to BODIPY and compared with their absorption spectra (see Figure S17 in
the Supporting Information). By overlapping both the spectra, it was found that the
excitation spectra of the dyads did not show any characteristic peaks pertaining to PTZ
and hence, it was confirmed that the BODIPY moiety was predominantly excited at this
wavelength. The emission of all the dyads was quenched when equi-absorbing solutions
of dyads, in three solvents, were excited at ex = 485 nm, where BODIPY moiety absorbs
predominantly. The possibility of self-aggregation of the chromophores, either BODIPY
or PTZ in a dyad, which may lead to fluorescence quenching, can be ruled out as all the
experiments were performed in very dilute solutions (10-6 M). For the dyads, the
quenching efficiency was found in the range of ~3% to 39% in hexanes, 71% to 99% in
CH2Cl2 and 84% to 99% in acetonitrile. The quenching efficiency is more in case of PB1
(no spacer directly connected) and PB2 (phenyl bridge) compared to PB3 (ethoxy phenyl
spacer).
Various radiative and non-radiative intramolecular processes can be responsible for the
excited state decay of the dyads. Among these, an excitation energy transfer (EET) or
photo-induced electron transfer (PET) can be conceived in the quenching of fluorescence
emission intensity. For EET to occur, the emission spectrum of the energy donor should
be well matched with the absorption spectrum of the acceptor. It is evident from Figure 1
& 4 that, as the emission of BODIPY (em = 512 nm) does not overlap with the
absorption of PTZ moiety (abs = 260 nm & 310 nm), the possibility of EET from
1BODIPY* moiety to PTZ can be eliminated. Also, as PTZ is very weakly emissive (see
Figure S14 in the Supporting Information) and there is no apparent emission in the region
where BODIPY absorbs (vide supra), the possibility of EET from 1PTZ* to BODIPY
can also be eliminated. Hence, no experiments demonstrating this phenomenon are
performed in this report. Alternative pathway for emission quenching is the
intramolecular PET from the ground state of phenothiazine moiety to the singlet state of
̇
BODIPY to produce PTZ•+-BODIPY•─. However, it should be mentioned here that the
1BODIPY* produced by excitation could undergo inter system crossing (ISC) to populate
3BODIPY* (~1.76 eV),[54] alternatively it could undergo electron transfer to produce
PTZ•+-BODIPY•─.[50] E0-0, energy due to 0-0 electronic transition, of BODIPY moiety of
dyads was found to be ~ 2.44 eV (estimated from intersecting wavelength of the

absorption and emission spectra of BODIPY) for all the dyads, which is comparable to
E0-0 value of control compound BODIPY. The change in the free energy due to PET from
the ground state of phenothiazine to the singlet excited BODIPY moiety can be
calculated by using the Eq. (1).
GCS(PTZ →1BODIPY*) = ECT( PTZ·+-BODIPY·─) ─ E0-0(BODIPY)─X
(1)
Where the term X accounts for the finite donor-acceptor separation (RC-C), ionic radii (r+,
r─) and solvent dielectric constant (ϵs),
e2
e2
1
1
1
1
(2)
X 

(

)(

)
40S RC 80 r r ref S
PTZ•+-BODIPY•─ is the difference in the oxidation and reduction potentials of PTZ and
BODIPY moieties of the dyads in three solvents. The radii of the molecular ions used
were r+ = 3.623 Å and r─ = 3.372 Å from computational calculations. The centre-to-centre
distance for the dyads PB1
12.001 Å respectively, from energy minimization studies (Table 4). The driving forces for
charge separation ( GCS) were found to be exothermic (negative) in the solvents,
hexanes, CH2Cl2, and acetonitrile, for all the dyads when excited at 485 nm. The solvent
, PB2, and PB3 were found to be 7.256 Å, 8.771 Å, and

polarity dependent negative
1BODIPY*.
G indicates that there is a possibility of PET from PTZ to
Fluorescence quantum yields of the dyads and the individual components in the three
solvents, have been estimated (Table 5) by comparing the emission curves of reference
compound (i.e., BODIPY, Φf = 0.98 ± 0.01% in CH2Cl2) with those of the dyads. It was
found that the fluorescence quantum yield (Φf) of PB1, PB2, and PB3 were found to be
0.01, 0.01, and 0.289 respectively in CH2Cl2. The fluorescence emission quenching
efficiency of the dyads was calculated using Eq. (3).
%Q =
(3)
Where Φ(BODIPY) and Φ(dyads) refer to the fluorescence quantum yields of control
compound BODIPY and the dyads, respectively, when excited at ex = 485 nm.

In addition, the evidence of the PET process of these D-A systems was obtained
from the excited state fluorescence decay measurements. Any excited state non-radiative
process would expected to decrease fluorescence emission intensity and there by
fluorescence lifetime. Figure 6 represents excited state decay curves PB1, PB2 & PB3 in

CH2Cl2 solvent and corresponding decay parameters are collected in Table 6. From the
data, it is clear that the fluorescence lifetimes, ex = 485 nm and em = ~510 nm) of the

(


dyads are decreased in all the solvents when compared to the reference compound
BODIPY.[68] All dyads except in hexane solvent, decay curves are fitted with a bi-
exponential expression: shorter lifetime component is attributed to the deactivation
(quenching) of the excited BODIPY by phenothiazine. Minor longer lifetime component
perhaps might be due to the unquenched decay of BODIPY unit of dyad or different
conformers that included face-to-face originated from donor-acceptor-donor type of
molecular arrangement. Also, as the length of the spacer increases, as in case of PB3
,
closed and extended conformers may be possible, which may be responsible for the minor
longer lifetime component.[69]
Further, rate constant (kET) for the charge transfer state PTZ•+─BODIPY•─ have
been calculated using eq. (3) and summarized in Table 3.
kET = (1/f) -k
Where is the reciprocal of the lifetime of the BODIPY
(3)
k
,
f is the lifetime of dyads in
respective solvent. The solvent-dependent kET values are in the range of 0.71 x 108 s-1 to
1.96 x 1010 s-1 for PB2 and 0.06 x 108 s-1 to 1.81 x 109 s-1 for PB3. The observed general
increase of the kET values with increasing polarity of the solvent is consistent with the
participation of a charge transfer state in the excited state deactivation of BODIPY
component of these D-A system. On the basis of these calculations, an energy level
diagram involving the photo-induced electron transfer events can be depicted for all the
dyads. For example, an energy level diagram depicting the photochemical events in all
the solvents for dyad PB2 is shown in Figure 7. Similar diagrams for the dyads PB1 and
PB3 are shown in the Figure S18a & b in the Supporting information.
3.5.1. Effect of solvent polarity on PET. As the static dielectric constant of the solvent
was increased, quenching of the fluorescence intensity was increased (see Figure 4). In
other words, with increasing the polarity of the solvent, a gradual decrease in the
fluorescence quantum yield was observed (Table 5), which further demonstrates the
excited state electron transfer mechanism. In addition to this the excited state decay also
follows polarity of the solvent. For a particular dyad, the quenching efficiencies are less
in hexanes compared to CH2Cl2 and acetonitrile and followed the trend hexanes<CH2Cl2<
acetonitrile. These results are in good agreement with the literature information that, as

the charge separated state (PTZ•+ - BODIPY•─) is relatively more stabilized in the polar
solvents, the PET process is accelerated in polar medium rather than non-polar
medium.[70]
3.5.2. Effect of spacers on PET. In addition to the effect of solvent polarity on PET
processes, the effect of proximity of PTZ and BODIPY moieties in the dyads on PET
events was also studied using the steady state fluorescence studies. In detail, it was
observed that, for a particular solvent, as the length of the spacer between PTZ and
BODIPY moieties is increased i.e., no spacer (directly connected) in PB1, phenyl bridge
in PB2 and saturated ethoxy phenyl spacer in PB3, the efficiency of fluorescence
quenching is decreased and followed the order PB1>PB2>PB3. For example, in hexanes,
the efficiency of PET and hence, fluorescence quenching is high for PB1 (39%)
compared to PB2 (8%) and PB3 (3%). Similar results were also observed for the dyads
in CH2Cl2 and acetonitrile. This can be attributed to a fact that the efficiency of PET
depends on the overlap of the electron clouds between the donor and acceptor
moieties.[71] In PB1, as PTZ (donor) and BODIPY (acceptor in excited state) moieties
are directly connected i.e., no spacer, the overlap of the electron clouds and the electronic
communication between PTZ and BODIPY is expected to be more (see section 3.4.) and,
thus, the fluorescence quenching efficiency is high compared to other dyads in all the
three solvents. Similarly, in case of PB2 and PB3, as the length of the spacer is increased,
the degree of electronic communication between the PTZ and BODIPY may be
decreased which resulted in the lower fluorescence quantum efficiency in the solvents
investigated. Hence, from the above discussion it can be concluded that these dyads
enabled us to control the efficiency of PET w.r.t. different spacers and served as excellent
models to study the effect of type and size of the spacer on the PET events.
Conclusions
This work presents a series of electron donor-acceptor systems, PB1, PB2, and PB3,
consisting of phenothiazine as an electron donor and BODIPY as an acceptor that are
connected via spacers of different types and sizes. All the dyads were synthesized and
characterized using various spectroscopic and electrochemical methods. Optical
absorption studies showed marginal interactions between the chromophores in the ground
state, while the electrochemical studies have revealed minor shifts in the redox potentials,
probably, due to tethering of electron donating PTZ moiety to an electron withdrawing
BODIPY moiety. DFT and TD-DFT studies using B3LYP/6-31G(d,p) calculations

showed stronger electronic communication in PB1 compared to PB2 and PB3 in the
ground level as well as excited level. Similar effects were observed in the steady stateand
time resolved fluorescence studies revealing that, upon photo-excitation of BODIPY, an
intramolecular PET was occurred from the ground state PTZ to 1BODIPY* generating
PTZ•+-BODIPY•─. More interestingly, the extent of quenching is greater for PB1 and PB2
i.e., in the dyads containing donor and acceptor at a relatively closer distance compared to
PB3 and the quenching was more efficient in polar solvents compared to non-polar
solvents.
Acknowledgements
We are grateful to Department of Science and Technology (DST, SB/S1/IC-14/2014) and
Central University of Rajasthan for financial support of this work. The authors ND and
DB acknowledges Council of Scientific and Industrial Research (CSIR) for Junior
Research Fellowship (JRF) and Central University of Rajasthan for Research Fellowship
respectively.
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Table 1: Absorption Data
amax, nm (log , M-1 cm-1)
Phenothiazine moiety BODIPY moiety
Compound
PTZ
254
316
(4.77)
(2.78)
227
(4.49)
320
(3.78)
363
(3.76)
501
(5.14)
BODIPY
PB1
232
260
309
503
(4.72)
(4.96)
(4.38)
(5.23)
234
253
258
310
412
503
PB2
(5.82) (5.00) (4.36)
(4.34)
(5.14)
235
256
307
354
501
PB3
(4.73)
(4.72)
(3.95)
(3.96)
(5.07)
aSolvent CH2Cl2, Error limits: λmax, ± 1 nm, log ε, ± 10%
Table 2: Electrochemical Data
Potential V vs. SCEa
Compound
PTZ
Oxidation
Reduction
0.63 1.18
-0.82
1.22
0.80 1.25
0.77 1.22
-0.94, -1.24
-1.16
BODIPY
PB1
-1.08
PB2
0.76 1.18
-1.18
PB3

aCH2Cl2, 0.1 M TBAP; Glassy carbon working electrode, Standard calomel electrode is reference
electrode, Pt electrode is counter electrode. Error limits, E1/20.03 V
Table 3: First oxidation and reduction potentials (E, V vs SCE) and free-energy changes for
charge-separation (GCS) for the dyads in hexanes, CH2Cl2 and acetonitrile.
1
-GCS(PTZ- BODIPY*)c/eV
Compounda E(PTZ0/·+)b/V E(BODIPY0/·ˉ)/V Hexanes CH2Cl2 Acetonitrile
0.80
0.77
0.76
-1.16
-1.08
-1.18
-0.451
-0.554
-0.464
-0.488
-0.596
-0.514
-0.507
-0.614
-0.531
PB1
PB2
PB3
b
aSee Chart 1 for structures. Redox potentials were obtained from DPV recorded in CH2Cl2.
cGCS = (Eox– Ered)E0-0 – X where, Eox and Ered are the one-electron oxidation and reduction
potentials of phenothiazine and BODIPY moieties respectively,
e2
e2
1
1
1
1
X 

(

)(

)
40 S Rctct 80 r r ref S
Where 0, S refers to vacuum permittivity and dielectric constant of the solvent, Rct-ct is the
donor-acceptor center-to-center distance, and r+ and r- are the ionic radii (PTZ·+= 3.623 Å and
1
BODIPY·─= 3.372 Å), E0-0 is the energy of the lowest excited states (2.45 eV for BODIPY* in
CH2Cl2).

Table 4: B3LYP/6-31G (d,p)-Optimized Distances between PTZ and BODIPY moieties and
related orbital energies in the investigated dyads.
Dyads
edge-to-edge
distance, Å
center-to-center
distance (Rct-ct), Å
HOMO-1, HOMO,
LUMO,
eV
LUMO+1,
eV
eV
eV
1.494
5.744
9.004
7.256
8.771
-5.332
-5.28
-2.291
-2.487
-2.294
-0.6533
-0.9981
-0.5260
PB1
PB2
PB3
-5.747
-5.354
-5.133
-5.293
12.001
Table 5: Fluorescence Dataa
em, nm (Φf, %Q)
Compound
Hexanes
CH2Cl2
Acetonitrile
PTZb
-
-
-
512
(0.875)
514
512
(0.980)
514
509
(0.814)
509
BODIPY
PB1
PB2
(0.532, 39.2)
514
(0.803, 8.2)
(0.010, 98.9)
514
(0.010, 98.9)
(0.008, 99.0)
510
(0.010, 98.6)
512
(0.848, 3.1)
512
(0.289, 70.5)
509
(0.133, 83.5)
PB3
aSpectra were measured at 293  3 K. Error limits: em,  1 nm; f,  10%; bex at 254 nm.

Table 6: Fluorescence Decay Parametera and electron transfer rate constants (kET, s-1)b
τ, ns (A, %), kET, s-1
Compound
BODIPY
PB1
Hexanes
CH2Cl2
CH3CN
2.74
3.26
3.18
3.00
3.01(40)
4.89(60)
2.24(20)
4.73(80)
0.07(56)c
3.30(43)
0.05(53)c
3.18(46)
2.29
PB2
PB3
0.71 x 108
1.39 x 1010
1.96 x 1010
0.86 (85)
1.96 (15)
8.56 x 108
0.47(88)c
3.36(12)
2.69
0.06 x 108
1.81 x 109
b
aAll life times are in nanoseconds (ns), at λex = 485 nm. Error limit of τ and kET, ~10%. Values
c
in parenthesis are relative amplitude of corresponding decay component. Short lifetime
components are limited by the instrument response function (IRF).
Figure Captions
:
Chart 1. Molecular structures of dyads PB1, PB2, PB3, and control compounds, PTZ and
BODIPY.
Scheme 1. Synthetic route for the preparation of phenothiazine-BODIPY dyads and control
compound, BODIPY, employed for the study.
Figure 1. Absorption spectra of the indicated compounds in CH2Cl2 at RT.
Figure 2. Differential pulse voltammograms (a) oxidation (b) reduction of the indicated
compounds in CH2Cl2 containing 0.1 M (n-C4H9)4NClO4. The concentrations of the dyads were
held at 1 mM; scan rate = 100 mVs-1.

Figure 3. In-situ UV-Visible absorption changes of PB3 at an applied potential of (a)
0.90 V (b) -1.30 V.
Figure 4. B3LYP/6-31G(d,p)-calculated (a) optimized structure, (b) molecular electrostatic
potential map, (c) frontier LUMO, and (d) frontier HOMO of the dyads PB1 and PB2. The red
and blue colours in (b) indicate the negative and positive potentials.
Figure 5. Fluorescence spectra (λex= 485 nm) and (OD λex = 0.1) of equi-absorbing solutions of
PB1 (black), PB2 (red), PB3 (green) and BODIPY (magenta) in (a) hexanes, (b) CH2Cl2, and (c)
acetonitrile.
Figure 6. Fluorescence decay curves of BODIPY, PB1, PB2, and PB3 (λex = 470 nm, λem
~510 nm) in hexane, CH2Cl2 and acetonitrile
=
.
Figure 7. Energy level diagram depicting the photochemical events in non-polar and polar
solvents for the PTZ-BODIPY dayd PB2.