**2. "Turn on"sensors**

There are few publications on "turn on" materials with respect to "turn off" sensors, due to several factors: the rarity of processes accompanied by an increase in luminescence requires a rational design of materials, and at the same time, impurities in analytes can cause quenching processes that level the effect of luminescence enhancement. However, examples of materials with such a response have been growing in recent years, and the described mechanisms can serve as an inspiration for new researchers.

Like quenching, luminescence enhancement can be described by the Stern-Volmer equations, but the definition of the constants is given only in a small number of papers.

The most common "turn on" response mechanisms are:


An analysis of the literature shows that the SQD strategy (**Figure 1**) is the most often implemented. In this case, the organic solvent molecules or other ligands that do not have their own absorption in the same excitation region of the complex displace water molecules from the coordination sphere of lanthanide. Quenching of Ln3+ centered luminescence through interactions with OH, CH, and NH bonds is caused by the dissipation of the energy of the Ln3+ excited state into high-energy stretching vibrations of several neighboring molecules [1, 2]. The efficiency of vibrational quenching on these bonds depends on the energy of the excited state Ln3+ and the number of vibrational modes of the X-H bond that cover this energy (**Figure 2**).

For europium ions the required number of OH bonds vibration modes (4) is less than for terbium ions (5), which determines a much more efficient quenching of the luminescence of Eu3+. In the transition to O-D bonds, the number of modes for both ions increases (6 and 5, respectively). As a result, the observed lifetime of Eu3+ (aq) is

*Luminescent Materials with* Turn-on *and* Ratiometric *Sensory Response Based… DOI: http://dx.doi.org/10.5772/intechopen.109189*

**Figure 1.** *Substitution of the solvent quencher by the analyte (SQD: solvent-quencher displacement).*

shorter than that of Tb3+ (aq), and when the medium is replaced by D2O, the difference is noticeably leveled (see Part 1, **Table 1**). Even greater is the effect of vibrations of these bonds on the emission of IR-emitting ions. The difference in τobs of various REE ions in protic and deuterated water allows to estimate the number of water molecules in the near coordination sphere of lanthanide according to empirical formulas *qLn* <sup>¼</sup> *<sup>k</sup>* � <sup>1</sup> *<sup>τ</sup>H*2*<sup>O</sup>* � <sup>1</sup> *<sup>τ</sup>D*2*<sup>O</sup>* � *<sup>b</sup>* , where *qLn* is the number of water molecules in the coordination sphere of lanthanide, τH2O and τD2O are the observed lifetimes of excited REE states in water and D2O. k and b parameters are given in **Table 1**. This makes it possible to evaluate the difference in sensitivity for materials based on ions with lower slope values (Nd3+, Eu3+, and Yb3+) and larger ones (Tb3+, Sm3+, and Dy3+). Most of the papers are devoted specifically to europium derivatives, which is not accidental. Neodymium compounds have been studied as sensor materials with less SQD response, but appear to be very promising [7].

An interesting example of this type of sensor is presented in [8], which shows the selectivity of the response with respect to methanol against the background of ethanol and propanol-1. The effect is due to the lock-and-key matching of the channel diameters in the MOF sensor structure with the sizes of the indicated alcohol molecules.

#### **Figure 2.**

*Nonradiative deactivation of excited states of terbium and europium by multiphonon relaxation on O-H and O-D bonds.*


#### **Table 1.**

*Slope (k) and intercept (b) in the Ln3+ hydration number formulae.*

The QFR mechanism (**Figure 3**) has been often implemented using notluminescent copper-lanthanide heterometallic complexes capable of "donating" copper ions under the action of various analytes, especially those containing sulfur [9–12] and nitrogen [12–14] donor atoms with a high affinity for copper ions. This approach makes it possible to achieve greater selectivity with respect to background ions not strongly interacting with Cu2+ ions. In other cases, an analyte with strong oxidizing (ClO, [15]) or reductive (ascorbic acid, [16]) power removes the quencher fragment. Other analytes having the same redox properties are expected to show a similar effect. *Luminescent Materials with* Turn-on *and* Ratiometric *Sensory Response Based… DOI: http://dx.doi.org/10.5772/intechopen.109189*

**Figure 3.** *The quencher fragment removing mechanism (QFR).*

The interaction of some analytes with the sensor material leads to a change in the electronic structure, which increases the efficiency of sensitization of lanthanide ions. The detailed mechanism of such enhancement is difficult to determine, but a suppression of the non-radiative relaxation of the singlet and triplet states of the ligands [17], an increase in the energy transfer constant from the ligand to the metal, and a change in the position of the triplet level [18], which affects the efficiency of the reverse transfer, can also make a contribution. These sensor materials showed a response with respect to s- and p-metal cations, as they generally contain a crowncontaining fragment [17] or suitable coordination sites determining the material selectivity. The response to gases has been studied much less frequently than the response to analytes in solution; a paper [18] describing the turn-on sensor for NO2 is of particular interest, two luminescent Eu and Tb complexes being investigated: the Tb complex shows that a reversible sorption of NO2 leads to a "turn on" response, while for europium, a "turn off" response is observed. The interaction between the

#### **Figure 4.**

*Jabłoński diagram featuring the ground singlet (S0 ), first excited singlet (S<sup>1</sup> ), and lowest triplet (T<sup>1</sup> ) states of the ligand together with the relevant atomic levels of Gd3+,Tb3+, and Eu3+. Values for the energy levels are given in cm<sup>1</sup> . Represented with a permission from Ref. [18].*

sensor with the analyte leads to an increase in the energy of the triplet level by 260 cm<sup>1</sup> , which is favorable for energy transfer to a higher resonance level of terbium, but reduces the efficiency for the low-lying level of europium (**Figure 4**).

A sensor material with a positive luminescent response to Cu2+ ions has been recently described [19]. Upon addition of up to 4 equivalents of 3d-metal salts (especially Cu2+) an intense band appears in the excitation spectrum associated with intraligand energy transfer. The appearance of this band makes the luminescence excitation more efficient when using the corresponding wavelength, which leads to a more than twofold increase in the quantum yield of Eu3+. This work is a unique example of a turn-on sensor for a d-metal cation.

Finally, the direct antenna function of the analyte (**Figure 5**) can take place if the analyte contains a suitable conjugate system [20, 21]. A not-high selectivity has been showed by such sensors, but the use of a well-defined excitation wavelength, coinciding with the absorption of the analyte, can increase it (**Table 2**).

To determine the correct mechanism of the observed "turn on" response, we propose the following algorithm, which is also relevant for the rational design of such materials:

i. A chemical analysis of the "analyte + sensory material" system to answer the question "whether a new chemical compound is formed" during their interaction.

*Luminescent Materials with* Turn-on *and* Ratiometric *Sensory Response Based… DOI: http://dx.doi.org/10.5772/intechopen.109189*

**Figure 5.** *Direct antenna analyte function mechanism (DAAF).*



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